Building Research Establishment

Wind driven rain: assessment of the need for new guidance

Date: March 2011

A1533015

Report prepared by: John Reid and Dr Stephen Garvin, BRE BRE Scotland Scottish Enterprise Technology Park East Kilbride G75 0RZ The opinions expressed in this report are those of the author. Report commissioned by: Directorate for the Built Environment Building Standards Division Denholm House Almondvale Business Park Livingston EH54 6GA Tel: 01506 600 400 Fax: 01506 600 401 e-mail: buildingstandards@scotland.gsi.gov.uk web: www.scotland.gov.uk/bsd © Crown Copyright 2011 Applications for reproduction of any part of this publication should be addressed to: BSD, Directorate for the Built Environment, Denholm House, Almondvale Business Park, Livingston, EH54 6GA

This report is published electronically to limit the use of paper, but photocopies will be provided on request to Building Standards Division.

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Contents

1 Executive Summary 4

2 Introduction 6

3 Current guidance in the Technical Handbooks 7

4 BS8104 and data used to assess site exposure 9

5 Recent research and data on wind driven rain 12

6 Climate change 15

7 Discussion 23

8 Conclusions 29

9 References 31

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1 Executive Summary The Building Research Establishment (BRE) has been commissioned by the Building Standards Division of Scottish Government to undertake a review of the guidance referenced for wind driven rain within the Scottish Building Regulations. The guidance that supports Building Standard 3.10 (Precipitation) provides compliance details on methods of preventing rain penetration to the internal surface of buildings by referencing British Standard 8104: 1992 – ‘Assessing exposure of walls to wind-driven rain’. This British Standard divides Scotland into seven different zones as part of the methodology for calculating expected rainfall driving against vertical surfaces. The wind and rainfall data used in BS8104 is however based upon a 33 year period from 1959 to 1991. BS8104 is based on a series of previous research papers developing various approaches to assessing exposure rating with a view to understanding and predicting the risk of water penetration of building construction details.

The aim of the research is to inform whether or not the guidance provided in BS8104, is still an appropriate document to reference for determining the likely exposure of buildings to wind driven rain, across all areas of Scotland.

A number of research reports, a sample of which is given in section 4, have been reviewed. The reports are relevant to recent rainfall and wind data as well as wind driven rain. The climate change projections as set out by UKCP09 have been reviewed and the implications for wind driven rain in Scotland assessed. The weather in winter generally produces the highest levels of rainfall and the highest winds. It is also when the maximum coincidence between high winds and rainfall occurs. Therefore buildings which can resist water penetration in winter will generally be fit to face the other seasons of the year.

The following points are concluded from the research:

• BS8104 was published in 1991 and the rainfall data on which it was based are becoming increasingly out of date. There is no indication that the observed increases in rainfall have yet reached a level that renders the British Standard irrelevant in the short term, but as further changes occur then this becomes an increasing risk. The observed increase in rainfall between 1991 and 2011 therefore needs to be recognised within Section 3 of the Technical Handbooks.

• BS8104 was created after many years development looking at the impact on buildings when rain and wind occur together. The suggested weather changes can be expected therefore to lead to an increase in the quantity of water impacting on vertical surfaces of buildings during rainfall.

• UKCP09 reports on climate change indicate that the weather in the UK, including Scotland where it is differentiated in the papers, will become significantly wetter in most locations and marginally drier in some others. These changes are based upon annual periods. Changes to wind speed, loads and storms are less certain and are likely to be insignificant over most of Scotland. No specific estimates of wind driven rain are given by UKCP09, but as most of the increase in rainfall will be experienced in winter periods the combination of existing wind patterns and rain will result in increased amounts of wind driven rain.

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• A new Standard partly based on the methodology developed for BS8104 was issued in 2009 as BS EN ISO 15927-3:2009. This mirrors some parts of BS8104 without incorporating the weather maps and introducing methodology which permit calculations from hourly figures. This potentially increases the current relevance of the new standard as changes in climate occur and permits its continued use in changing conditions.

• BS8104 appears to remain a useful Standard, in the short term, but this should be reviewed at intervals. However, this report sets out a number of reasons why consideration should be given to updating the guidance for Standard 3.10.

• Four options are set out for consideration in relation to the guidance for Standard 3.10. In all options it is recommended that the technical handbooks provide references to observed rainfall changes since 1991, and climate change projections as published in the UKCP09 reports.

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2 Introduction The Building Research Establishment (BRE) has been commissioned by the Building Standards Division of Scottish Government to undertake a review of the guidance referenced for wind driven rain within the Scottish Building Regulations. The guidance that supports Building Standard 3.10 (Precipitation) provides compliance details on methods of preventing rain penetration to the internal surface of buildings by referencing British Standard 8104: 1992 – ‘Assessing exposure of walls to wind-driven rain’ (BS8104). This British Standard divides Scotland into seven different zones as part of the methodology for calculating expected rainfall driving against vertical surfaces. The wind and rainfall data used in BS8104 is however based upon a 33 year period from 1959 to 1991.

In the years since BS8104 was published, a number of issues such as the publication of new standards, the observed changes in rainfall over the past 20 years and climate change have arisen that make it prudent to question whether or not the guidance within the technical handbooks is still applicable. The current climate change projections indicate that there may be changes to rainfall across Scotland to 2080. It is therefore reasonable to infer that the quantity of driving rain will also increase. The information provided on the wind driven maps within BS8104 may no longer be an appropriate basis on which to determine the level of wind driven rain on building walls.

The aim of the research is to inform whether or not the guidance provided in BS8104, is still an appropriate document to reference for determining the likely exposure of buildings to wind driven rain, across all areas of Scotland. The research does not involve writing any new guidance, but is instead intended to indicate what direction the guidance could take. The project has assessed whether or not masonry walls constructed in accordance with the historical rainfall data and mapping in BS8104, can still achieve the level of performance required by the Technical Standards and the supporting guidance for preventing precipitation penetrating to the inner face of buildings.

The research has the following objectives:

• To determine whether or not the information provided on the wind driven maps within BS8104 still adequately reflects the wind driven rain received in Scotland;

• To assess the need for updated building standards guidance to reflect possible increases in wind driven rain in Scotland;

• To address the need for a simplified up to date exposure map that adequately reflects current or projected wind driven rain across Scotland;

• To review current the rating of exposure zones in Scotland, BS8104 and any other appropriate guidance that could be cited in the Technical Handbooks.

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3 Current guidance in the Technical Handbooks

The current guidance in the Technical Handbooks is set out in Section 3, Environment, within which the issue of precipitation and wind driven rain is covered in Standard 3.10.

Standard 3.10 requires the following:

“Every building must be designed and constructed in such a way that there will not be a threat to the building or the health of the occupants as a result of moisture from precipitation penetrating to the inner face of the building.”

The guidance states that a floor, wall, roof or other building element exposed to precipitation, or wind driven moisture, should prevent penetration of moisture to the inner surface of any part of a building so as to protect the occupants and to ensure that the building is not damaged.

General recommendations are given in the guidance as follows:

• “masonry walls of bricks and/or blocks incorporating damp-proof courses, flashings and other materials and components constructed in accordance with the relevant recommendations of BS5628: Part 3: 2005. The construction used should suit the degree of exposure to wind and rain in accordance with Clause 21 and as described in BS8104: 1992;

• masonry walls incorporating external rendering which conforms to the relevant recommendations of BS5262: 1991, to suit the degree of exposure and the type of masonry;

• masonry walls of natural stone or cast stone blocks constructed in accordance with the relevant recommendations of Section 3 of BS5390: 1976 (1984) and to suit the degree of exposure to wind and rain as described in BS8104: 1992;

• masonry cavity walls incorporating insulation material, either as a complete or partial cavity fill, where the insulating material is the subject of a current certificate issued under the relevant conditions of an independent testing body. The walls should be constructed in accordance with the terms of the certificate and to suit the degree of exposure to wind and rain as described in BS8104: 1992 and the relevant recommendations of the following British Standards:

o Urea formaldehyde (UF) foam BS5617: 1985 and BS5618: 1985

o Man-made mineral fibre (slabs) BS6676: Parts 1: 1986

o Assessment of walls for filling BS8208: Part 1: 1985

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• roofs with copper, lead, zinc and other sheet metal roof coverings require provision for expansion and contraction of the sheet material. In 'warm deck' roofs, in order to reduce the risk of condensation and corrosion, it may be necessary to provide a ventilated air space on the cold side of the insulation and a high performance vapour control layer between the insulation and the roof structure. It may also be helpful to consult the relevant trade association.

• walls or roofs incorporating cladding materials constructed in accordance with the recommendations of the following British Standards or Codes of Practice:

o Aluminium wall or roof CP 143: Part 15: 1973 (1986)

o Galv. corrugated steel wall or roof CP 143: Part 10: 1973

o Lead wall or roof BS 6915: 2001

o Copper wall or roof CP 143: Part 12: 1970 (1988)

o Slates and tiles wall or roof BS 5534: Part 1: 2003

o Zinc wall or roof CP 143: Part 5: 1964

o Non-loadbearing walls wall or steep roof BS 8200: 1985

o PC concrete cladding wall BS 8297: 2000

o Natural stone cladding wall BS 8298: 1994

o Flat roofs roof BS 6229: 2003

o Bitumen felt roof BS 8217: 2005

o Mastic asphalt roof BS 8218: 1998.

Although BS8104 is still current, other standards quoted in the guidance as set out above have been superseded by European standards in recent years. For this reason alone there is some need to update the guidance to Standard 3.10. The guidance gives some specific examples of wall and roof constructions. However, this research is more concerned with how to assess wind driven rain rather than the construction details to avoid the risk of rain penetration occurring.

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4 BS8104 and data used to assess site exposure

BS8104 is based on a series of previous research papers developing various approaches to assessing exposure rating with a view to understanding and predicting the risk of water penetration of building construction details. The development of wind driven rain guidance up to the publication of BS8104 is briefly set out in this section.

4.1 Climate and Building in Britain, written by R. E. Lacy

This book was based on work undertaken at the Building Research Station supplemented mainly by specially commissioned studies by the Meteorological office. The aim of the project was to develop an understanding of the interaction between weather and building. The book was published in 1977 and was a collation of the best information available at that time on weather conditions in the UK and how weather affects buildings.

Rainfall In the reports discussed by Lacy, Weather stations across the country had been recording the rainfall at 6400 specific locations and the map collated from the data shows the average annual rainfall pattern across the UK between the years 1916 to 1950. This created a basic database into which future rainfall data could be inserted to modify and improve the predictions for specific locations.

Note: none of the maps referenced in Lacy are included here since they are no longer directly relevant in terms of quantities but simply part of the progress towards BS8104.

Wind Speed Wind speed varies across the UK and for any specific location the main factors that affect the recorded wind speed are the height of the anemometer and the surrounding topography or obstacles. The records of wind speed used to prepare the national maps were based on anemometers at 10m above ground level with no appreciable obstacles within a radius of 200m.

Obstacle free surroundings are not possible in all locations but results are adjusted back to equivalent values.

The influence of gust speeds is also important; both in relation to the sustained effect on vertical surfaces and to the sensitivity of the recording equipment that is required to record the winds at short enough time intervals to be meaningful.

The shortest gusts recorded for this purpose last for around three seconds and consist of fast moving quantities of air a few tens of metres across. Longer gusts generally represent larger masses of air, but moving more slowly and therefore with a different impact on vertical surfaces.

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Driving rain Assessing driving rain in any location involves estimating a combination of the quantity of rainfall and wind speed. It is relatively easy to design a building to resist water penetration due to vertically falling rain. It is more difficult and more realistic to design water resistance when the wind acts on the rainfall and changes the angles of impact with the building. The result of this interaction is that water runs towards joints and vulnerable details both vertically and from directions that differ from those generally assumed in the basic design.

Combining the frequency and quantity of rain and the strength and direction of wind for any specific location is complex since there is a continuum between rain falling during periods when there is no wind to a combination of heavy rain and severe winds occurring together.

This most severe combination of wind and rain was considered during the period this book was written as a rare occurrence and therefore to design as if it were a frequent event would lead to over-design and increased cost, not justified by the risk of dampness in the building fabric. The projections of climate change suggest that this risk is increasing.

4.2 DD93: 1984 Methods for assessing exposure to wind-driven rain. Preparation of Sites and Resistance to Moisture

This Draft for Development was prepared over a number of years and aimed to produce a Driving Rain Index, which would operate as the standard method of assessing the impact of rainfall on the external fabric of a building.

DD93 continued to be used as a design aid for a number of years until it was fully evaluated and developed into British Standard BS8104 in 1992. The approach that was developed in DD93 and formalised in BS8104 has continued to be the basic method of assessment for exposure for buildings in the UK until the present day.

4.3 BS8104 and data used to assess site exposure

The methodology for this code was developed over many years with directional driving rain maps for the UK being produced by Prior (1985) and these were incorporated into BS8104. These maps provided a basis for consistent calculations across the UK permitting the exposure of different locations to be assessed comparatively. The resulting value allowed buildings and building details with a successful track record in particular exposures to be chosen as appropriate for similar exposures in other areas of the country.

This Standard divides Scotland into seven different zones as part of the methodology for calculating expected rainfall driving against vertical surfaces. Guidance is then provided on the effect of orientation, terrain roughness, and topography. An obstruction factor based on the height of the wall relative to other objects or buildings can then be calculated from tables to provide an assessment of the location of the building. Finally a wall factor then provides an adjustment for each specific wall related to simple building forms and heights.

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4.4 BS EN ISO 15927-3:2009 This standard (BS EN ISO15927-3 Hygrothermal performance of buildings, Calculation and presentation of climatic data, calculation of a driving rain index for vertical surfaces from hourly wind and rain data) has been developed from BS8104 and adopts a similar methodology to utilise the spell index and annual index to predict the exposure of walls. The major difference between this standard and BS8104:1992 is that the CEN standard does not have the wind-driven rain maps and directional roses which are at the heart of BS8104. This reduces it to a methodology without the standard data from which to calculate the quantities. Guidance is also given however on assessing the Airfield and Spell indices from hourly wind and rain data.

This standard also incorporates a second method which does not demand weather maps and is based on average wind and the Present Weather Code (PWC). It is stated that the PWC is obtained from a World Meteorological Organisation (WMO) guide, the second edition of which was published in 1983. The third edition is complete and will soon be publicly available on the internet. A brief review of the second edition however did not clearly identify any PWC and therefore the connection between the guide and the BS EN ISO 15927 is not clear.

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5 Recent research and data on wind driven rain

The reports discussed in this section address changes in precipitation levels across Scotland and the UK. Some identify seasonal changes showing the difference in all four seasons. Where this occurs, the focus of this report will, given the scope of this study, remain on winter conditions since they generate the greatest risk of water penetration and water damage to buildings.

The weather in winter generally produces the highest levels of rainfall and the highest winds. It also suffers from the periods of maximum coincidence between high winds and rainfall. Therefore buildings which can resist water penetration in winter will generally be fit to face the other seasons of the year.

5.1 Changing intensity of rainfall over Britain (1900 to 2006) Tim Osborn and Douglas Maraun [Climatic Research Unit Information sheet 15]

This paper reviews the rainfall over the UK over approximately 100 years. The collated statistics show a gradual increase in the yearly rainfall. The seasonal changes however show a greater increase over the winter period and in some locations a decrease in summer rainfall.

The increase is discussed in terms of the number of days of rainfall and the average quantity of rain falling during these periods. There was an increase in the number of days when rain falls in western areas of the UK but the change is small. The more significant change is the increase in the average quantity of precipitation on days when rain falls. This shows an increase across the majority of the UK.

The assessed changes are considered from the period 1961 to 1995, which is based on the best data set obtained over the recorded period.

The data clearly identifies an increase in the average rainfall per day when rain is recorded, in some locations by as much as 80%. This trend decreases in Aberdeenshire and Caithness over that period. There is no indication of change in precipitation in the Western Isles, Orkney or the Shetlands but this is possibly due to a lack of recording stations in these locations included in this study.

The assessment of the trend was based on what was considered as normal precipitation for each area and therefore a large increase (80%) in the average rainfall in an already wet area is a significant increase. This change is shown to be occurring on the west coast of Scotland. There is also an 80% increase shown occurring between Dundee and Edinburgh but this starts at a lower initial rainfall of less than half that assumed for the locations indicated on the west coast. A greater increase in the quantity of rain does not necessarily translate into a greater impact and risk of rain penetration of a building. If a building has been designed assuming a relatively dry location, even a small increase in wind driven rain may overwhelm the water resisting features designed into the external fabric. However, a building designed to meet the

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demands of a very exposed location may be more capable of resisting significantly more wind driven rain due to the increased focus of the designer on this type of problem.

In addition, there is no differentiation in the report between increased rainfall due to storms lasting longer than average and storms which are more intense with a greater quantity of rain falling per minute. This could be significant since the quantity of wind driven rain is affected by the size of the raindrops during precipitation. If rain is falling at greater quantities within the same time period and the drops are bigger there may be little effect on the quantity of rain impacting on vertical surfaces and therefore wetting up the walls.

This study does not include any reference to wind and the resulting horizontal movement of rain while falling. However, assuming no changes to the pattern and degree of wind across Scotland, the increasing rainfall would, all other factors being equal, lead to an increase in driving rain.

5.2 Patterns of climate change across Scotland – CC03 March 2006

This is a report produced by the Scottish and Northern Ireland Forum for Environmental Research (SNIFFER), supported by The Scottish Government, The Scottish Environment Protection Agency (SEPA), Scottish Natural Heritage (SNH), and the Forestry Commission.

The data set used for this report, including temperature and precipitation, which was recorded over a similar time period to that outlined in Section 5.1 (1914 to 2004). The objective of the report was not simply to identify nationally averaged changes but to identify regional patterns of change.

The key findings in terms of precipitation are as follows:

• Scotland has become wetter since 1961, with an average increase of almost 60% in winter months in northern and western Scotland. For the majority of the country there has not been a large-scale significant change in average summer rainfall although some parts of north-west Scotland have become up to 45% drier in summer. Contrary to the Scottish national trend, Aberdeenshire has seen little change in precipitation in winter months although this is compensated for in this region by a significant increase in precipitation in autumn (September – November).

• Heavy rainfall events have increased significantly in winter, particularly in northern and western regions.

• The snow season has shortened across the country since 1961, with the season starting later and finishing earlier in the year. The greatest reductions have occurred in northern and western Scotland.

The other aspect of this report which is of note is that: “The Scottish Mainland and Scottish Isles warmed by 0.690C and 0.640C respectively, over the period 1861 to 2000” (Jones and Lister, 2004).

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This warming trend could, potentially, aid drying out of buildings following periods of rain. However, the increase in rainfall is more significant than the increase in temperature and therefore the relative warming can be ignored at the present time.

The trends identified here are broadly similar to those discussed in section 5.1 and again wind is not a feature of the report. The pattern of the increases is similar and the influence of this particular report on the present study will also be broadly similar.

5.3 Influence of wind-driven rain data on hygrothermal performance (Achilles N. Karagiosis, Mikael Salonvarra, Andreas Holm and Hartwig Kuenzel)

This report is focussed on the hygrothermal performance of wall systems but also considers the calculation of wind driven rain on the walls.

The main issues identified within this report relating to the present study notes that previous work by Lacy [1965], Sanders [1996], Kuenzel [1994], Karagiozis and Hadjisophocleous [1998], and Straube [1998] developed approaches giving results which vary by up to 25%. This however is deemed acceptable since experimental measurements of rain falling on a vertical surface have been shown to vary by up to 60% [Hugo Hens, 2002]. This is a difficult area to obtain reasonable measurements and therefore the basic methodology will continue to retain credibility until we are able to refine the measurements necessary to prove the system.

The report also identifies the importance of relating the wind driven rain to the orientation of the building. The methodology of BS8104 encapsulates this in both the spell and annual rose values which show the variation of the driven rain against twelve orientations.

This report infers that the basic methodology of BS8104 has not been superseded but does not comment on the specific values relevant in Scotland.

5.4 Wind-driven rain distribution and its hygrothermal effect on two different types of building geometry [Kumaraperumal, Sanders, Baker, Galbraith, Essah]

This report looks at two buildings in the West of Scotland and wind-driven rain (WDR) is discussed as the major cause of dampness in the external fabric of walls.

Earlier research is referenced with regard to different techniques designed to predict the quantities of WDR impacting on building surfaces. It also concluded that: “From these studies it is well understood that, out of all exterior environmental loads, the effect of WDR causes more than 90% of critical damage to the buildings” (Karagiozis et al. 2003) and that “the important consequences of climate change for buildings in the UK are associated with increased moisture stress”.

The paper highlights the importance of wind driven rain in terms of predicting the hygrothermal performance of a sandstone building and a concrete building.

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6 Climate change The climate is changing, both globally and in the UK. The fourth assessment report from the Intergovernmental Panel on Climate Change in 2007 said that:

‘it is likely that anthropogenic greenhouse gas increases caused most of the observed increase in global temperatures since the mid 20th century.’

The changes that have been projected by climate change models are likely to significantly impact on the UK. Previous greenhouse gas emissions have already committed the world to a changing climate no matter what degree of mitigation takes place.

The UKCP09 climate projections are the latest outputs in a set of studies that allow the decision makers and planners to address adaptation needs. The approach in UKCP09 differs scientifically from that in UKCIP98 and UKCIP02. It has sought to reduce uncertainty in the projections, or to allow a greater understanding of the uncertainties involved. The climate projections in UKCP09 cover a range of weather factors such as temperature, rainfall and wind. However, the confidence in the projections changes across these factors.

It should be noted that UKCIP98 and UKCIP02 refer to scenarios, whilst UKCP09 uses the term climate change projections.

6.1 Scottish Climate Change Programme

The Scottish Climate Change Programme sets out the steps being taken in Scotland now and in the near future with regards to adaptation for climate change. The programme recognises that Scotland, cannot by itself, address climate change as it is a global issue and a coordinated international approach is essential.

No matter how successful global efforts to reduce emissions are, some degree of climate change is inevitable. Much of the change in climate over the next 30 to 40 years is already determined by past and present emissions. Adapting to the impacts of climate change is largely a national and a local issue and the Scottish Government has full control over devolved policy levers to encourage appropriate Scottish responses at a range of levels.

The world is already witnessing changes in the climate. Global temperature has risen by about 0.7°C over the last 100 years, with about 0.4°C of this warming occurring since the 1970s. There was widespread flooding across mainland Europe in 2002 and a heatwave throughout mainland Europe in 2003 caused an estimated 35,000 extra deaths.

Findings from research into historical climate trends in Scotland show that over the last 40 years temperatures have increased in every season and in all parts of Scotland since 1961, and that in the North and West, rainfall has increased by almost 60% in winter months.

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6.2 UKCP09: Approach and data

There are a number of publications that support the UKCP09 climate projections. These projections replace the previous UKCIP climate change scenarios as set out in 1998 and then 2002. The Briefing Report to UKCP09 provides the most helpful guidance to readers on the subject and should be read with more detailed guidance on wind and precipitation. It summarises the information in the four main technical reports for those requiring a general understanding of UK Climate Change. Some key points are as follows:

o The climate projections are based upon the use of the global climate model HadCM3, but comparison is made with other models.

o Probabilistic projections are made over 25 km square grids across the UK.

UKCP09 gives probabilistic projections for a number of variables with different temporal and spatial averaging, by several future time periods, under three future emissions estimations.

In the Briefing Report, precipitation is a total of precipitation of all types, rain, snow and hail, and is given as a rate, in millimetres per day. Variables include two measures of temperature extremes (high and low percentiles) and one precipitation extreme. For most variables, changes are given for three temporal averaging periods: month, season and year. Additional projections at daily and hourly resolution, consistent with the probabilistic projections, are available from a weather generator, described later in this report. The spatial resolution of the projections over land areas is 25 km including islands that are large enough to be seen at such a resolution.

Probabilistic projections of change over the oceans surrounding the UK are not available at 25 km resolution from UKCP09, but instead are averaged over nine marine regions.

Projections given are averaged over each of seven future overlapping 30 year time periods, stepped forward by a decade, starting with 2010–2039. The use of 30 year time periods reduces the effect of uncertainty due to natural internal variability. These future time periods are referred to for simplicity by their middle decade, starting from the 2020s (2010–2039) and ending with the 2080s (2070–2099). All changes are expressed relative to a modelled 30 year baseline period of 1961–1990.

Note that, by 2009, a significant proportion of the time between the baseline period and future time periods has already elapsed, so the changes should not be referred to as “from today’s climate”.

For some variables, UKCP09 also makes available probabilistic projections of future climate over land areas, also at 25 km resolution, in addition to those of the change in climate. This is done by combining probabilistic projections of climate change with the corresponding baseline (1961–1990) climate taken from observations. For marine regions, only climate change projections are available, and not projections of future climate.

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Projections are given corresponding to three future emissions estimations, i.e. Low, Medium and High. In UKCIP02 four emissions scenarios were used; two of them (Low and High) are the same as the corresponding estimations in UKCP09. Factors such as inertia in the climate system mean that climate change over the first two or three decades from now is relatively insensitive to emissions. However, after the 2040s, projections based on different emissions estimations increasingly diverge.

The provision of probabilistic projections is an improvement for UKCP09 compared to UKCIP02. Probabilistic projections assign a probability to different possible climate change outcomes, recognising that a single answer is not possible and a range of possible climate change outcomes is better, and can help with making robust adaptation decisions.

UKCP09 describes the probability of climate change being less than or greater than a certain value, using the Cumulative Distribution Function (CDF). This is defined as the probability of a climate change being less than a given amount.

Climate change at the 50% probability level is that which is unlikely to be exceeded. The CDF (a hypothetical example at a certain location, by a certain future time period, for a given month of the year, under a particular emissions scenario) shows that there is a 10% probability of temperature change being less than about 2.3ºC and a 90% probability of temperature change being less than about 3.6ºC.

There is variation in the confidence of climate projections, as follows:

o There is very high confidence in the occurrence of global warming due to human emissions of greenhouse gases.

o There is moderate confidence in aspects of continental scale climate change projections.

The 25 km scale climate change information is indicative to the extent that it reflects the large scale changes modified by local conditions. There is no climate change information in the 5 km data beyond that at 25 km. The confidence in the climate change information also depends strongly on the variable under discussion. For example, there is more confidence in projections of mean temperature than mean precipitation. The probabilities provided in UKCP09 quantify the degree of confidence in projections of each variable, accounting for uncertainties in both large scale and regional processes as represented in the current generation of climate models.

6.3 Climate change data and advice on wind driven rain

UKCP09 data related to wind driven rain As has been stated above in section 5.2 the confidence levels in climate change projections for temperature are greater than those for precipitation. A comprehensive set of pre-prepared maps and graphs can be seen on the UKCP09 website.

Precipitation The central estimates of changes in annual mean precipitation are within a few percent of zero everywhere. In winter, precipitation increases are in the range +10% to +30%

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over the majority of the country. Increases are smaller than this in some parts of the country, generally on higher ground. In summer, there is a general south to north gradient, from decreases of almost 40% in SW England to almost no change in Shetland.

The changes at 10%, 50% and 90% probability levels have different magnitudes and may show an increase or a decrease. For example summer precipitation at the 10% level shows a decrease, but at 90% an increase.

The Met Office Hadley Centre models, as used in UKCP09, tend to project the storm track weakening slightly and moving further south. The alternative models show, in general, less change in the position of the track, but a wide range of changes in strength. Furthermore, a comparison of each model’s current storm track with observations shows an equally wide range of differences between model simulations and reality. These differences between individual models, and also between different types of model ensemble, indicate that robust projections of changes in storm track are not yet possible.

Anticyclones can persist over the UK for days or even weeks. They are associated with low wind speeds. As with storm tracks, model projections do not give a clear picture of changes to anticyclones. There is no compelling evidence that the frequency, duration or intensity of those affecting the UK will change markedly either way, but it cannot be ruled out.

Wind speed Future changes in mean wind speed, and indeed in other climatological metrics relating to storms or anticyclones can potentially be influenced by several aspects of the forced response to increases in greenhouse gases or aerosols. In particular, a number of factors likely to affect future changes in mid-latitude cyclones have been identified in previous research. For example, increases in atmospheric moisture content in a warmer atmosphere provide an increased source of energy for the intensification of storms through condensation and precipitation, once the storms have formed.

Probabilistic projections of changes in 30-year mean wind speeds relative to 1961–1990 have been produced for UKCP09, based on the same methodology previously used to produce other variables available online. As for other UKCP09 variables, the probabilistic climate projections of changes in wind speed reflect current scientific understanding.

In winter for the 2050s, the ranges of projected changes in wind speed are approximately symmetric about near-zero change. The largest uncertainty ranges are over Scotland, where the 10% to 90% probability range is typically −0.5 ms−1 to 0.5 ms−1. This range equates to –1 to 1 knots, however these changes in 30-year mean winter wind speeds are small compared with the typical winter averages for present day climate, which are 10 to 14 knots over the Lowlands and 18 to 24 knots over the Highlands.

In the 2080s the 50% probability level shows small reductions in wind speed of about −0.1 ms−1 over the UK, except for the Highlands where small increases are found. The results again reflect the range of competing mechanisms.

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There is no evidence within the UKCP09 projections of an increase in the frequency, nor intensity of storms in the UK. In HadAM3H (the climate model used in UKCIP02) there is an increase in the future number of UK winter depressions because there are more depressions overall and not because the probability of the most intense storms increases. This is attributed to a southward shift in the north east end of the North Atlantic storm track in this model giving more UK storms.

In HadCM3 (a climate model used in UKCP09) there is little change in the frequency of storms over the UK in winter. Although there is a southward shift in the North Atlantic storm track in this model the increase in frequency occurs to the southwest of the UK giving little change over the UK. There is also little change in the intensity of UK storms in this model. There are uncertainties associated with both the position and strength of the present day storm tracks and these contribute to the large uncertainties in the future predictions of storms. The different results between the two models illustrate the lack of any robust changes in UK storms.

The UKCP09 data does not give a strong indication of significant changes to either mean wind speeds or the number and severity of storms. As driving rain is likely to be at its worst during storms it may be supposed that the contribution from wind to driving rain is not significantly changed. The data that is available would tend towards increased precipitation as being the main issue to consider with regards to wind driven rain.

6.4 Advice on wind driven rain and climate change

BRE Scotland published a study (Impact of climate change on building) in 1998 (BRE, 1998) that gave a broad view of the potential impacts of climate change on building. This was further advanced in a Foundation for the Built Environment (FBE) Report in 2001 (FBE, 2001). The purpose of that report was to quantify some of the impacts and review possible adaptation strategies. Both these reports used the UKCIP98 climate change scenarios as the basis of the estimates of both impact and adaptation.

Impacts of climate change were expected to include changes to precipitation where an increase and decrease will affect water tables (foundations and basements); cleaning costs will be increased in winter, with associated redecoration requirements; durability and risk of water ingress will be affected by combination of precipitation increase and gales.

The report states that changes in wind speed, wind direction and rainfall are key to an understanding of how driving rain will change in the UK. The results of this study suggested that both wind speed and winter rainfall will increase in the UK, therefore, it was reasonable to infer that the quantity of driving rain would also increase. The relationship between rainfall and wind speed had not been developed in the climate change projections produced by UKCIP. One of the suggestions inferred from the UKCP98 climate change data was that individual rainfall events may become more intense than those historically experienced. This would imply that driving rain may also be more intense as heavier rain across the country combined with the same or greater wind speed would result in increased amounts of driving rain on a building facade. It should be noted that the FBE Report was based on UKCIP98 climate change scenarios and therefore they do not represent the state of the art in climate change.

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Assuming no significant change to the relationship between precipitation and wind, estimates of future driving rain using established algorithms (developed by Lacy, 1977) are detailed in table 3.1.

The Foundation for the Built Environment FBE Report indicates that currently most driving rain in the UK comes from the south-west direction. UKCIP98 predictions for wind direction suggest that during the summer, autumn and winter the predominance of south-westerly winds will increase, but in spring there will be increased winds from the north and east. This would suggest that for most of the year south-westerly driving rain will remain the direction of most exposure, although in spring there may be increased driving rain from the north and east. The change in direction of driving rain in the spring could lead to new rain penetration problems being identified in structures where problems have not previously been reported.

The FBE Report provides an adaptation strategy for new buildings’ design that will allow rain penetration problems to be avoided and the maintenance requirements to be set to practical levels. In this report it was suggested that traditional construction from regions currently with high exposure may offer good design practice.

Increases in driving rain will change the areas of the UK that are subject to the four different exposure zones shown in figure 5.4 of the FBE Report (reproduced from BRE Report BR262). The FBE Report stated that this figure should be redrawn to show future exposure zones. One way of designing to future exposure would be to use the calculation procedure defined in British Standard BS 8104:1992 to evaluate the current exposure level for any UK site. The calculated current exposure can then be adjusted. It was recommended that the changes in winter driving rainfall be used for this adjustment. By designing to the estimated future exposure level the building should, if properly maintained, continue to be weathertight in the future.

The FBE Report also gives a simple but pragmatic way of allowing for climate change by modifying the exposure map illustrated in figure 5.4 to increase the exposure rating of all properties. This would allow climate change effects on the exposure of properties to be considered in design, as illustrated in table 5. However, the use of some construction techniques currently limited to use in exposure zone 1 would no longer be recommended. Much of the existing housing stock is relatively sheltered, i.e. within zone 1 or zone 2 exposure areas. Overall this method would tend to overestimate the impact of climate change for a large number of properties.

Examples of this are as follows:

o London currently in exposure zone 1 would, for the purposes of climate change, be rated as being exposure zone 2. According to BRE Report BR262, ‘Thermal insulation: avoiding risks’ this would result in some types of facing masonry with full cavity fill no longer being recommended for future use.

o Glasgow currently in exposure zone 3 would become zone 4. From Thermal insulation: avoiding risks this would mean that no facing masonry with full cavity fill would be allowed, and some render finish solutions would also not be recommended.

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The FBE Report was based on UKCIP98 scenarios of climate change, which indicated greater changes in wind loads and storms than the recent UKCP09 projections. It is therefore necessary to review the type of changes and recommendations given in this report.

The NHBC Foundation report on climate change and innovation in house building from 2007 addresses a number of risks related to climate change impacts on housing. This includes storm (wind and rain) as well as other issues. The report was written to highlight issues such as potential changes to wind and rain and the adaptation measures that could be undertaken. The report was based upon the UKCIP02 climate change scenarios, where it is stated that more extreme weather in terms of wind and rain will be one of the key impacts on construction. Increased costs due to wind driven rain result from an impact across the whole building. It is inferred from this conclusion to the NHBC Report that increased incidents of rain penetration would result from climate change with costs of repairs across the country increasing.

The report states that storms can lead to rain penetration through external masonry walls. Walls without cavities are most at risk, but cavities may also allow water to track across to the inner leaf if the workmanship is poor (e.g. mortar adhering to wall ties or the presence of debris in the cavity). The effectiveness of well designed and constructed cavity walls should not be underestimated.

Guidance on using a higher driving rain index is discussed in the report. The basis of exposure assessment is taken from the distribution of wind driven rain map in BRE Report BR262 (and other documents). Currently the exposure map has four zones as follows:

• Zone 1 – sheltered – less than 33 l/m2 per spell

• Zone 2 – moderate – 33 to less than 56.5 l/m2 per spell

• Zone 3 – severe – 56.5 to 100 l/m2 per spell

• Zone 4 – very severe – 100 l/m2 per spell, or more.

The proposal in the report was to create only two zones, effectively combining Zones 1 and 2 and then Zones 3 and 4. The cut off figure is 56.5 l/m2 per spell between the two revised zones, which splits the country roughly by the north west (higher than 56.5 l/m2 per spell) and the south east (lower than 56.5 l/m2 per spell). There is of course a need to consider the local exposure factors that define the actual exposure and therefore building techniques to use on a specific site.

Other actions given by NHBC to consider included the following:

• Increasing minimum roof pitch for different types of covering. There is a relationship between the pitch of the roof and the overlap of different coverings: tile, slate, metal sheet, etc, and this must ensure wind driven rain does not penetrate the overlap. Designers specify roofs at pitches as low as 17.5° using interlocking slates for single storey low rise dwellings: these can perform successfully as long as the correct overlap is used but problems may arise when incorporating roof windows because some flashings may be at a lower pitch and

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• Extending the roof overhang at the eaves can help to protect the supporting wall from the combined effects of wind and rain. This, though, may increase the forces on the roof in high winds, so it is important to ensure that adequate restraint (such as holding down straps) is specified.

• The type of finish applied to mortar joints in masonry walling can influence the weather tightness. The best performance is achieved from ironed (‘bucket handle’) joints whereby the action of ‘ironing’ the joint with a convex tool compresses the surface of the mortar to leave a concave surface finish. Other finishes should be reserved for more sheltered locations.

• Walls in severe exposure areas can be protected by render or rain screen cladding systems. Avoid fully filling cavities with insulation in cavity masonry construction in very severely exposed areas (NB – currently Section 3.10 allows fully filled cavities only in sheltered locations). Recessing windows in a check reveal can reduce the incidence of leakage on window-to wall joints, and sub-sills with stooling (i.e. a raised portion to prevent water flowing into the building) can improve rain penetration problems at sill level. Sub-sills should also have a projecting weather drip, flush sills should be avoided. Cavity trays over openings should have stopped ends and adequate provision (e.g. weep holes) made to discharge rainwater to the outside.

Both the FBE Report and the NHBC Foundation use the exposure maps in the BRE documents, which are also used within some British Standards (e.g. the national annex to the BS EN 13914-1:2005) as opposed to BS8104.

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7 Discussion Wind driven rain is an issue for the building standards system, and is currently covered by the requirements and guidance to Standard 3.10. The basis of the guidance is to use BS8104 to assess the exposure of any one site. The buildings on that site then need to be designed and constructed appropriately in order to resist the effects of driving rain. BS8104 is a current standard and has not been superseded even by the more recent BS EN ISO 15927-3.

7.1 Current guidance

The current guidance in the UK is perceived to be BS8104, the presence of BS EN ISO 15927-3:2009 is not well known and not as easy to use since the basic figures need to be generated rather than simply read from a table. It is possible that the requirement to obtain information from outside the standard will create a barrier to the up take of this new Standard. However, BS8104 was not well used despite being self contained in terms of information. Some designers appear to have used the simplified tables in BR262 and some may not have calculated exposure. The fact that BS8104 has been retained as a current Standard indicates that BS EN ISO 15927-3:2009 has not completely replaced it in scope.

BS8104 remains a usable standard in the short term although recent research indicates that the figures contained within it, are becoming progressively less appropriate, particularly for winter conditions. If the results of climate change continue to follow a similar trend, the unmodified figures may eventually provide a calculated result which will mislead designers in their approach to design and specification. It would be possible to continue to use BS8104 as it stands over the next few years while it remains in force as a British Standard but some thought should be given to the process for directing the industry towards the new Standard, or to a modification of the new Standard to a more practical and generally accepted methodology.

7.2 Outcome of recent research

It is clear that the majority, if not all of the current research, indicates that wetter winters with slightly increased wind speeds will be the future trend for Scottish winters. The changing impact on buildings due to the changing weather patterns is not simple and some of the impacts may be counter-intuitive. The increasing intensity of rain predicted may result in larger raindrops, which may be less affected by the wind and therefore potentially lead to less rain impacting on vertical surfaces.

There are still gaps in the research in these areas which do not permit this type of analysis and therefore assessments are limited to the predictions of increasing winter rainfall and uncertain changes to wind in UKCP09. If the wind patterns and intensity do not change significantly from those on which BS8104 was based, then the increasing rainfall in winter must lead to an increase in the rain reaching the vertical external surfaces of all buildings. This will be absorbed into porous materials and test the adequacy of the joints between components. This increase and additional impact from the weather changes is currently the most rational conclusion from the recent research.

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The creation of BS EN ISO 15927 can be viewed as an outcome of recent research since the approach appears to remove the calculation methodology from set tabulated figures within the Standard which, due to global warming, become dated. The calculation methods in the BS EN permit the calculation to be linked with potentially updated figures and guidance.

7.3 Climate change

The impact of climate change on buildings has been estimated in various research projects to be most likely to affect the ability of buildings to resist wind and rain. Most observers have been of the opinion that there will be an increase in problems with water leakage through walls, roofs and windows (and the junctions between these elements) unless design and construction changes are made.

The latest climate projections from the UKCIP, termed UKCP09, have been described in the previous section. The projections consider changes to precipitation over 30 year periods covering effectively 2020, 2050 and 2080. Under all estimations of emissions the changes tend to show an increase in winter precipitation, but a decrease in summer precipitation. The overall annual change for 2080 is likely to be no greater than +/- 10%. However, winter precipitation in 2080 under the medium scenario may be increased by at least 30% in some locations. These significant changes are at the higher level of what is likely to occur, whilst the median (central estimate) indicates only a change of around +/-10% in winter.

Total rainfall and the exposure spell index data from either BS8104 or the Zones 1 to 4 of the exposure map cannot be readily transposed on top of each other. Therefore, some degree of estimation or calculation is required in order to assess how climate change would impact on the wind driven rain. No conclusion on this matter is given in the climate change reports from UKCIP, but in general an increase in the severity of rainfall might be considered to result in the overall increase in total rainfall. Therefore, changes in the exposure spell index for a site may be considered as the outcome of climate change.

The climate projections have been assessed over 25 km square grids for the whole of the UK, including Scotland. Therefore it would be feasible to estimate the likely increase or indeed decrease in exposure zone (using the type of map shown in figure 5.4). The zone or exposure index for a specific location would be used as the starting point. The winter projections would then be selected and the exposure recalculated as a first approximation. Either the median point of the exposure zone band would be selected or BS8104 would be used to assess the site exposure for the specific site.

For example by selecting Glasgow which is Zone 3 on the current map and then considering the probability of a change occurring then the exposure can be recalculated, as follows:

• 10% probability, which is equivalent to an increase of no more than 10% in the total winter rainfall, results in turn in the exposure increasing from 78.25 l/m2 per spell to 86.075 l/m2 per spell, i.e. it remains within Zone 3.

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• 50% probability, which is equivalent to an increase of no more than 20% in the total winter rainfall, results in turn in the exposure increasing from 78.25 l/m2 per spell to 93.9 l/m2 per spell, i.e. it remains within Zone 3.

• 90% probability, which is equivalent to an increase of no more than 40% in the total winter rainfall, results in turn in the exposure increasing from 78.25 l/m2 per spell to 109.55 l/m2 per spell, i.e. it moves to Zone 4.

At the 90% probability the exposure zone changes and the resulting guidance on construction methods would change for the site. The key issue is how to select the appropriate probability level. In accordance with UKCP09 the 90% level is unlikely to be exceeded, but there is no specific guidance on whether to select such a high value or not. The central estimate is likely to be selected by most observers, but this may ultimately underestimate the exposure to wind driven rain.

The approach set out here takes climate change into account. However, it assumes that the climate projections of precipitation results in rainfall that increases the exposure spell index directly and by the same amount. There is no evidence to suggest that this will occur within the UKCP09 reports and it should therefore be viewed as a worst case scenario with regard to the likely change to wind driven rain. A more realistic estimate may be to half the increase in total rainfall and then proceed to assess the change to wind driven rain and the effect on the exposure zone.

Climate change cannot be ignored as an issue for the assessment of exposure to wind driven rain. Whilst the worst effects are likely to be experienced during intense periods of rainfall that is combined with high winds, there is likely to be a greater risk over winters in the years up to the 2080 period. The UKCP09 data does not specifically deal with wind driven rain and therefore a simple and pragmatic approach is required to take account of climate change and therefore address future risk.

7.4 Future Options for assessing exposure

A number of specific issues have been addressed as part of the research undertaken on wind driven rain. These issues are discussed in this section. The issues relate to the specific objectives of the project, which are as follows:

• To determine whether or not the information provided on the wind driven maps within BS8104 still adequately reflect the wind driven rain received in Scotland;

• To assess the need for updated building standards guidance to reflect possible increases in wind driven rain in Scotland (i.e. the guidance in Section 3.10);

• To address the need for a simplified up to date exposure map that adequately reflects current or projected wind driven rain across Scotland;

• To review the current rating of exposure zones in Scotland, BS8104 and any other appropriate guidance that could be cited in the Technical Handbooks.

Adequacy of BS8104 wind driven rain maps, exposure maps and other guidance BS8104 is a current British Standard and to retain the reference to it within the Technical Handbooks without modification is a possible option. However, there is

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evidence that rainfall has increased in winter across much of Scotland and that this trend will continue with climate change. No significant changes to wind loads, mean speed or number of storms can be adequately determined at this stage.

The approach to design in terms of exposure has, in practice, been based on the voluntary calculation of the exposure rating in accordance with BS8104. In practice designers often rely on their experience to guide their detailed designs in relation to weatherproofing buildings. It would be possible to use the regulations to impose the most onerous details for all new buildings with designers able to relax this provided they can show that the risks are lower than assumed for severely exposed areas.

The exposure maps that have been used in various British Standards, BRE publications (e.g. BRE Report BR262) and industry guides are based on four exposure zones across the country. The definition of each of these zones in terms of exposure will not change, i.e. a low and a very severe exposure zone will form the bottom and top category respectively. However, the areas of greater exposure may increase in the future.

The research has determined that an international standard has been published that sets out a framework for determining the exposure to wind driven rain in any European country. This standard does not give enough information to determine the actual exposure on a site as it lacks specific data. Users would still have to refer to BS8104 or the four zone exposure maps for such data. The result is that the exposure zone and spell index for a site would probably be the same as for that currently calculated with BS8104.

The use of BS8104 and similar maps may be considered to adequately cover buildings in the present day. However, as buildings have a lifetime of 60 years or more, those being built today should take into account the potential increases in wind driven rain. BS8104 can, until revised, form the initial basis of the assessment to wind driven rain, but additional consideration of increased winter rainfall in the past 20 years and projections climate change would be advisable.

The need to update the Section 3.10 The research has shown that a number of factors mean that the guidance to Standard 3.10 needs to be reviewed. The factors are summarised as follows:

• A number of standards that are currently cited (not BS8104) are either out of date or have been withdrawn in favour of European standards. It is necessary to update these references and to check that the replacement standards are ‘fit for purpose’.

• A reference to BS EN ISO 15927-3:2009 should be added and its relevance explained.

• The guidance should provide a consideration of changes to rainfall as observed in research since 1991, and take account of projections of climate change over the rest of the century.

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The latter point is the most difficult to address as there is uncertainty over the changes to rainfall in the last 20 years and the projections for future changes. Any change to the technical guidance should be sufficiently robust to manage the implications of climate change, whilst being sufficiently measured so as not to result in excessive costs as a result of over-design for winter weather. A pragmatic and clear approach is encouraged, based upon the use of existing standards and guidance followed by making an allowance for the increasing amounts of rainfall, both observed and projected.

Updated exposure map for Scotland There are two options with regard to creating an updated exposure map for Scotland. The first would simply consider the evidence of observed changes in winter rainfall over 1991 to 2011 in order to update the exposure map as set out in BR262 and other documents. This map could be used as an alternative to using BS8104. The second would update the map using climate change projections and possibly the observed changes over 1991 to 2011.

BS8104 could also be modified by increasing the calculated values and enhancing them by possibly one zone to encourage more robust design against wind driven rain. This however could not be applied as a blanket solution since the latest rainfall predictions suggest that Aberdeenshire will become drier in the winter and therefore it would be no justification for increasing exposure ratings in that area. The majority of the country however is predicted to become wetter and in some locations significantly wetter (up to 80%) the slightly modified BS8104 method therefore would be a practical solution in appropriate areas.

Wind projections in UKCP09 are determined on a spatial range that is greater than Scotland and do not indicate significant change. Therefore, it is the changes to rainfall that should form the basis of any new simplified exposure map. The key issue is whether or not to assume that all additional winter rainfall is as wind driven rain, resulting in an increased spell index for a site, or does only a certain amount fall as additional wind driven rain.

The exposure maps (in BS8104 and BR262) have a different basis from those presented in the UKCP09 reports. The former give a spell index, whilst the latter give projections of increases to rainfall on 25 km grid squares. Layering the climate change projection maps on the existing exposure maps would be possible, but the spatial distinctions would need to be aligned. As a number of maps are given in UKCP09 that are based upon probabilities and timescales then there is a need to deal with this issue within the guidance.

Options for Section 3.10 Four options are presented for potential amendments to Section 3.10.

Option 1: to retain the status quo based upon the use of BS8104, but to update other references. The guidance could also inform users of the evidence base for changing rainfall and its impact on wind driven rain, as well as climate change projections. The international standard on wind driven rain should be referenced and an explanation of its usefulness given.

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Option 2: to retain BS8104 as the basis of the exposure assessment, but to require an additional assessment or consideration of increased rain as observed across Scotland from 1991 to 2011. This should be carried out using a risk based approach, where designers would be required to take decisions on the exposure and design based upon risk on construction and over the lifetime of the building. The guidance should also update references to other standards and inform users of climate change projections and where to source information. The international standard on wind driven rain should be referenced and an explanation of its usefulness given.

Option 3: in this option the user would consider both climate change projections and observed changes over 1991 to 2011. This could be done by either of the following:

• Use BS8104 or other exposure maps as the starting point for the wind driven rain assessment. A process would then be developed in which the site exposure would be adjusted for increasing rainfall amounts and therefore increased wind driven rain. The approach set out in Section 6.3 may form the basis of such an approach to account for climate change.

• Creating a new and simplified exposure map for Scotland, which takes into account both observed changes (1991 to 2011) and projections of future rainfall.

• The international standard on wind driven rain calculations should be used as the basis of the site calculation. Although at present reference to BS8104 or other exposure maps will also be required.

This approach could be supported by software programmes that allow users to enter site location data, and site specific factors relevant to adjacent buildings and topography. The software would provide a calculation of the wind driven rain exposure for the site. The calculations could be either simplified or more complex in nature, however, they should be in accordance with standards and be independently assessed. For this third option updated references will also be required.

Option 4: this would take a different approach where all buildings would be assumed to be within the highest exposure category (Zone 4 equivalent). The designer would then need to work back in order to determine if a lower exposure zone is appropriate. This option may encourage designers to design good weathertightness details as standard rather than allowing a range of designs to come across. In this option the guidance would be much simplified with the emphasis being on the designer to prove that the highest exposure zone is not appropriate to the verifier. The guidance could also inform users of the evidence base for changing rainfall and its impact on wind driven rain, as well as climate change projections. The guidance should include reference to specific standards (e.g. BS8104, BS EN ISO 15927-3), but leave the methodology of assessment to individual designers.

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8 Conclusions BRE has undertaken research for Building Standards Division of Scottish Government relevant to Standard 3.10 and specifically wind driven rain. At present the guidance to Standard 3.10 is based upon exposure derived from BS8104, which was published in 1991. A number of factors have been determined within the research that means that reliance on BS8104 is not sustainable into the future, in particular observed changes to rainfall and climate change projections in UKCP09. Changes to wind are less certain than rainfall and therefore new approaches to assessing wind driven rain should be based upon changes to precipitation only.

The following points are concluded from the research:

• BS8104 was published in 1991 and the rainfall data on which it was based are becoming increasingly out of date. There is no indication that the changes have yet reached a level that renders the British Standard irrelevant in the short-term, but as further changes occur then this becomes an increasing risk. The observed increases in rainfall amounts over 1991 to 2011 therefore need to be addressed within Section 3.

• BS8104 was created after many years development looking at the impact on buildings when rain and wind occur together. The predicted changes to the climate can be expected to lead to an increase in the quantity of water impacting on vertical surfaces of buildings during rainfall.

• UKCP09 reports on climate change indicate that the weather in the UK, including Scotland where it is differentiated in the papers, will become significantly wetter in most locations and marginally drier in some others as a result of climate change. These changes are based upon annual periods. Changes to wind speed are less certain and are likely to be insignificant over most of Scotland. No specific estimates of wind driven rain are given by UKCP09, but as most of the increase in rainfall will be experienced in winter periods the combination of predicted wind patterns and rainfall will result in increased amounts of wind driven rain.

• A new Standard partly based on the methodology developed for BS8104 was issued in 2009 as BS EN ISO 15927-3:2009. This mirrors some parts of BS8104 without incorporating the weather maps and introducing methodology which permit calculations from hourly figures. This potentially increases the current relevance of the new standard as changes in the climate can potentially be incorporated in the calculations, permitting its continued use as weather conditions change.

• BS8104 appears to remain a useful Standard, in the short term, but this should be reviewed at intervals. However, this report sets out a number of reasons to review the guidance to Standard 3.10, which includes adding further consideration to the exposure determined in BS8104 for a site to account for observed changes and/or climate change projections.

• Four options are set out for consideration in relation to the guidance within Standard 3.10. In all options it is recommended that the technical handbooks

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9 References Building Research Establishment (BRE), Report BR262, Thermal insulation: avoiding

risks, IHS Press, Watford, 2002.

Building Research Establishment (BRE), Report BR (authors - Garvin S L, Phillipson M C, Sanders C H, Hayles C S and Dow G T. Impact of climate change on building, IHS Press, Watford, 1998.

British Standards Institution (BSI), British Standard Code of Practice for Assessing the Exposure of Walls to Wind-driven Rain, BS8104, British Standards Institution, London, 1992.

British Standards Institution (BSI), DD93, 1984 Methods for assessing exposure to wind-driven rain, Preparation of Sites and Resistance to Moisture, BSI, London, 1984.

British Standards Institution (BSI), BS EN ISO 15927-3 Hygrothermal performance of buildings, calculation and presentation of climatic data, calculation of a driving rain index for vertical surfaces from hourly wind and rain data, BSI, London, 2009.

British Standards Institution (BSI), BS EN13914-1, Design, preparation and application of external rendering and internal plastering, external rendering, British Standards Institution, London, 2005.

National Housebuilding Council Foundation (NHBC Foundation), Climate change and innovation in house building, NHBC, 2007.

Kumaraperumal, Sanders, Baker, Galbraith and Adu Essah, Wind driven rain distribution and its hygrothermal effect on two different types of building geometry, Proceedings of the 9th International Conference and Exhibition, Healthy Buildings 2009, Syracuse, NY, USA.

Osborn T and Maraum D, Changing intensity of rainfall over Britain, Climate Research Unit Information sheet No. 15, 2008.

Mansell M G, The effect of climate change on rainfall trends and flooding risk in the West of Scotland, Nordic Hydrology, 28, 37-50, 1997.

Karagiozis et al, Influence of wind driven rain data on hygrothermal performance, 8th International IBPSA Conference, The Netherlands, 2003.

Sanders C, Indoor and outdoor environmental conditions, IEA Annex 24, Task 2, 1996.

Straube J F, TRATMO2, VTT Finland, 1998.

Scottish Government, Patterns of climate change across Scotland, CC03, March 2006.

Lacy R E, Driving rain maps on the onslaught of rain on buildings, Current Paper No. 54, Building Research Division, Garston, UK, 1965.

Lacy R E, Climate and building in Britain, BRE Report, 1977.

Prior M J, Directional driving rain indices for the UK – computational and mapping, BRE, Watford, 1985.