BRE BR497 PDF

Introduction

Global warming, with the need to limit CO₂ emissions into the atmosphere, is the principal driver for conserving fuel and power in buildings. Confirmation that this is very much part of the government agenda is found in the successive changes to Part L of the Building Regulations in England and Wales, with similar changes to the equivalent sections in Scotland and Northern Ireland.

The introduction of more highly insulated buildings, that has resulted from this need to save energy, has also led to the need for more sophisticated methods for calculating heat loss and surface temperatures than were previously felt to be adequate. Two changes are particularly important:

• While U-values of the building fabric could previously be calculated by assuming that an element was made up of a series of parallel layers each with uniform thermal resistance, it is now recognised that features such as mortar joints, timber studs or the metal spacers in built up roofs cause thermal bridging of the insulation layer(s) and so contribute significantly to the heat loss. A more detailed calculation method for U-values, as defined in BS EN ISO 6946 (ref. 1) and known as the combined method, has been introduced to take account of these repeating thermal bridges.

• It has also been recognised that thermal bridging at the junctions between the various plane building elements - walls, roofs and floors - of a building can add significantly to the fabric heat loss. The higher heat flows that occur, because of complex geometries or the use of materials with a high thermal conductivity, also cause localised reduction in the internal surface temperatures, which in turn can lead to condensation and mould problems.

Although various simplified calculation methods have been developed to take account of the effects of thermal bridging in certain situations, two- or three- dimensional heat flow calculations continue to be required for some U-value and for most (non-repeating) thermal bridge calculations. These calculations of two- or three-dimensional heat flow require the use of numerical modelling software. Several packages are available but, whereas most software packages themselves are validated as being able to produce correct and consistent results, many important decisions are left to the user regarding the input to the modelling software and the determination of certain quantities from the output of the software, both of which can have a very significant effect on the results. This guide gives the information needed to carry out these calculations, so that different users of the same software package and users of different software packages can obtain consistent results. However, before using the conventions given in this publication it is important for the numerical modeller to demonstrate that their numerical modelling software can model the validation examples in BS EN ISO 10211 (ref. 2) with results that agree with the stated values of temperature and heat flow within the tolerance indicated in the standard for each appropriate validation example.

This publication has been prepared to complement the outline methodology for the treatment of thermal bridges given in BRE's Information Paper IP 1/06 (ref. 3). It can be used by designers who wish to develop novel solutions for the detailing at junctions for which a simplified approach is not adequate. In the body of the text the application of the results of calculations for building regulation purposes is signalled.