The importance of a common method and correct calculation ......EN 13789 – Heat transfer...

Lund University / Energy and Building Design / 2011-05-30, NSB 2011 - Tampere

The importance of a common method and correct calculation of thermal bridges

Björn BerggrenLund University, Energy and Building Design

Founded by:

Agenda

• Introduction• Theoretical background• State of knowledge• Example of consequence• Conclusions

INTRODUCTION

Introduction

• Recast of the Energy Performance of Buildings Directive, EPBD– By the end of 2020 all new buildings shall be “nearly zero-

energy buildings”• Approach to design robust and energy efficient residential

building in a Nordic climate:

Source:SINTEF Byggforsk - Husbanken

Introduction

• EPBD, recast:– The methodology for calculating energy performance should

be based not only on the season in which heating is required, but should cover the annual energy performance of a building. That methodology should take into account existing European standards.

EN ISO

EN ISO 6946 EN

ISO 7345

EN ISO

10211EN ISO

EN ISO

EN 15242

EN ISO

THEORETICAL BACKGROUND

EN 13789 – Heat transfer coefficients

• Transmission heat transfer coefficient:

• Where– HD direct heat transfer coefficient– Hg ground heat transfer coefficient– HU heat transfer coefficient, unconditioned spaces– HA heat transfer coefficient to adjacent buildings

EN 13789 – Heat transfer coefficients

• Direct heat transfer coefficient:

• where– Ai area of element i of the building envelope [m2] – Ui thermal transmittance of element i [W/m2K]– lk length of linear thermal bridge k [m]– k linear thermal transmittance of thermal bridge k [W/mK]– j point thermal transmittance of point thermal bridge j [W/K]

Application of EN ISO 13789

• Clearly divide the building envelopes into different elements:

Application of EN ISO 13789

• EN ISO 13789 allows for measuring of elements according to one of the three methods

EN ISO 10211 – Thermal bridges in constructions

• Linear thermal bridges

• Point thermal bridges

Important

• The sum of transmission losses through building elements, the term AiUi, will vary depending on the chosen measuring method

• Consequently, the thermal bridges, k-values and j-values will vary

• Subscripts to clarify used method for measuring

STATE OF KNOWLEDGE

The survey

• Sent to 100 engineers and architects with experience of building projects with focus on energy efficiency

• The survey– Four questions about measuring methods used to

define thermal transmitting area in energy calculations– Six questions, qualitative assessment of different

junctions– Eight questions about professional background

Junctions

EXT. INT.

EXT.INT.

Insulation Concrete Wood

EXT.INT.

JUNCTION 1 – J1 JUNCTION 2 – J2 JUNCTION 3 – J3

JUNCTION 6 – J6JUNCTION 5 – J5JUNCTION 4 – J4

Results

EXAMPLE OF CONSEQUENCE

Heat transfer is not 1D

3,00 5,00 7,00 9,00 11,00

Heat resistance in wall [m2·K/W]

%-Diff J7

%-Diff J8

CONCLUSIONS

Conclusions

• State of knowledge is not satisfying• Important when we are increasing the use of BIM tools –

which may provide quantity take offs for energy simulations• Subscripts (i, oi, e) should always be used• It seems as consultants does not always understand that a

thermal bridge occur due to difference between internal and external areas

Thank you for the attention!

And thanks to everyone who took the time and answer the questionnaire.

The importance of a common method and correct calculation ......EN 13789 – Heat transfer...

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