Unit IVS T R A W B A L E S

Under unit IV……. Straw as a building material Physical aspects Fire, moisture, insects and pests proof. Plastering with straw Straw bale walls Straw bale roof.

Introduction: Straw – the stem material left as a by product of modern grain harvesting. Compressed and tied with string/ wire – bales stacked in running bond, plastered with mud. HAY is grown for livestock feed and harvested green, while straw has no nutritional value

and is therefore not as susceptible to rot or insects. When properly designed and constructed, straw bale is a viable and sustainable

building system. Straw has been used as a building material for thousands of year either as an additive to

clay In the form of adobe or Cob, or as a water resistant layer in the form of thatch. Straw – low environmental impact construction for…

- Straw has excellent thermal insulation properties.- Straw has excellent sound insulation properties.- The production of straw is a low energy process compared with other building materials.- Straw stores carbon within its structure thereby reducing atmospheric CO2.

Straw is inherently structural and maintains its form because of its tubular shape. The material’s microscopic waxy coating makes it slightly water resistant. A straw bale is a bundle of straw bound with bailing wire or polypropylene twine. Sisal has also been used for binding in the past but is not rot resistant. Straw varieties include wheat, rice barley, hops and oats. There are two types of bales… 2 string and 3 string. Bale sizes varies according to the Baler and practices. Bale weight may also vary depending on density and moisture content. Straw is an organic material that carrier particular risks with it in the context of living

accommodation. These risks included….- Fire : Straw is highly flammable.- Rodent and insect infestation : Straw can contain protein and carbohydrates which can sustain life. - Decay : Under the right environmental conditions straw is susceptible to both aerobic and anaerobic decay.- Structural instability : Straw bales have low compressive and flexural strength and stiffness.

Bale loaded: Bale loaded “flat” are loaded perpendicular to their largest face parallel to the

plane of the tie hoops, and generally perpendicular to the straw fibre. Bales loaded “on edge” are loaded parallel to their largest face – perpendicular

to the plane of the tie hoops, and generally parallel to the straw fibres.

Different methods of building with straw bale: Load bearing Light weight frame and Load bearing Infill and timber frame Hybrid design

1. Load bearing: Nebraska Load bearing straw bale, is constructing straw bale walls in such a way

that it supports all roofs. This is the original method of building, pioneered by the Nebraskan settlers

in the USA. In this method, the bales takes the weight of the roof without any

structural framework. They are placed together like giant building blocks, pinned to the

foundations with an wooden roof plate on top. The roof plate is fastened to the foundations and the bales with coppiced

hazel and strapped, the roof is constructed in the manner on top of the roof plate.

Windows and doors are placed inside structural box frames, which are pinned into the bales as the walls go up.

This is the simplest method, ease of design, minimal use of timber, and the opportunity it affords for a modern day wall raising.

Advantages of Load bearing: A simple, straight forward and accessible building method. Easy for non-professional to design, following comprehensible basic

principles. Designs from one home to two-storey homes can be created using a

simple, step by step approach. Curves and circles are easy to achieve for little extra cost. Ideal for self-builders for its simplicity, accessibility, ease of design and

low cost . Great versatility of design shape can be achieved.

Disadvantage of Load Bearing : The straw must be kept dry throughout the whole building process until

it is plastered. This can be very difficult on a large building, or being constructed slowly. Openings for windows and doors must not exceed 50% of the wall

surface area. Maximum unsupported (unbraced) wall length is 6M (20f)

Infill style / post and beam / timber frame: Infill straw bale involves building some independent structural system - Stud walls,

post and beam, etc, - to support all structural loads, and then straw bales are in-filled between or around that structural system to create the enclosure.

Milled lumber is the most frequent material for supporting elements, although concrete block piers and concrete or steel columns have been used.

The advantage of this system is greater design flexibility, including the possibility of designing for more than one storey in height.

The system has greater long-term stability and can be custom engineered, in contrast with the prescriptive approach used in the Nebraska style

It is a more sophisticated system requiring skilled labour, and is, therefore, more costly. The infill construction system has posts and beams that extend up to the top of the walls. The ceiling and roof are mounted on the upper beams or ring beam. Consequently the weight

of the ceiling and roof is transferred through the posts and beams down to the foundations. Traditionally, infill buildings have had a timber or steel structure that is completely self-

supporting. More often, there are 16 mm steel bracing rods fitted to the structure to provide the lateral

stability to the building.

Advantages: The roof can be constructed before the straw is placed, giving secure

weather protection. Framework and posts can be constructed off site. Provides greater stability for window frames than in the load bearing

style. In conjunction with a steel frame, can create large warehouse space

(and gives an even temperature throughout the year).

Disadvantages: It is more complicated than the Nebraskan style to construct. It requires a high level of carpentry skill (or metalwork experience in the

case of a steel frame) to construct the frames. It uses a large amount of timber.

Light – weight frame and load bearing: It uses a timber framework that is so light-weight that it cannot stand up alone. It

requires temporary bracing to give it stability until the straw is in place. The straw is an essential part of the structural integrity of the building, more so than

the timber, and it works together with the timber to carry the load of floors and roof. Timber posts are located at corners and either side of window and door openings

only, and are designed such that the timber wall plate at first floor and / or roof level can be slotted down into them once the straw is in place allowing for compression on the bales.

Compression of the straw bale infill walls is essential for stability. To increase stability, the bales are pined externally, and the pins are secured onto the

base and wall plate of the framework once all the settlement of the walls is complete. It is constructed in such a way that the wall plate and roof are kept 100mm above the

finished straw wall height whilst the wall is being built, allowing for compressive settlement of the straw wall once the bracing and props are removed.

Advantages: The roof can be constructed before the straw is placed providing secure

weather protection. Framework and posts can be constructed off site. Provides greater stability for window and door frames than in the load

bearing style. Vastly reduces the amount of timber required compared to the more

traditional post and beam method.

Disadvantages: It is more complicated than the Nebraskan style to construct. Greater technical ability is required to make the structure stable whist

the straw is being placed

Straw bales as a building materialsFire Resistance:

The tight packing keeps the available oxygen needed for combustion very limited. Also the high silica content in straw (3-14%) is said to resist fire – as it begins burning a layer of char develops, which insulates the inner straw.

Straw bale walls are naturally fire resistance. While loose, dray straw is combustible, but when it is compacted into bales, there’s not enough air from the straw to burn well.

Combining plaster with the compactness of a bale enhances fire resistance. The plaster coating effectively seals the already fire resistance bales inside a

non combustible casing.

Insects and other pests: Moisture and rot in straw buildings is the concern about vermin. There are accounts of major problems with fleas in early hay – bale

building in Nebraska that were not plastered on the interior. But with careful plastering, you can easily keep out rodents and other

pests. In areas with significant rodent problems, using a finer woven mesh than chicken wire for plastering might be a worthwhile precaution.

Moisture problems and rot: Few organisms are able to decompose straw. High moisture level in straw bales – over 70% - can provide a habitat for fungi and lead to decomposition. Keeping bulk moisture away from walls – for example, using wide overhangs, sloping the ground away

from the building, and installing a good capillary break between the foundation and bale walls. The main source of potential moisture is liquid penetrating into the wall cavity. This can happen in many

ways such as wind blown rain, drifting snow, splash back from dripping roof, plumbing leak, floods, hoes and breaches of the walls protective layers in leaky roofs and window sills.

A bale wall is raised on a wooden curb on the foundation or floor will ensure that any spills or floods inside house will not soak into the walls.

Plastic or tar paper placed along the top of walls will protect against any water that may come through your roof.

Windows and doors also can be installed with proper flashing and drip edges that shed water away from the walls.

The other moisture can damage a building is vapour migration through walls. During the heating season, the house functions similarly to the balloon, when heat is added to your living

space, your relatively airtight house is filled with warm, moisture-laden air. Extra moisture is added by breathing, cooking, bathing etc and that air will naturally search for a way out of the house and into cold, dry air outside.

As the warm, moist air tries to travel outdoors, it will begin to cool. As it cools, the water vapour it carries will condense back to liquid.

If liquid deposited in our walls and allowed to remain there without drying, it will reduce the efficiency of your insulation and eventually lead to mold and rot.

In hot climates, the process can happen in reverse, especially if you use air conditioning.

The plaster coating on bale walls is an effective barrier against moist air leakage. If properly tied in with conventional polyethylene vapour barriers installed in the ceiling and under the floor, a bale house can be made almost airtight.

Conventional building practice-prevents the moisture through the use of plastic vapour barriers in the walls.

Conventional frame construction often relies on a plastic film, vapour retarder paint or other vapour diffusion retarder to help keep moisture out of walls.

Climate specific moisture detail-in cold climates use a vapour retarder paint on the interior, in hot climates use a vapour retarder paint on the exterior, in other climates the walls probably be left permeable.