Thermal performance testing

Thermal modelling

Building regulations require that thermal bridging be taken into account in SAP and SBEM calculations. The junctions that need to be accounted for include wall-floor junctions, wall-roof junctions, lintels, jambs, cills, intermediate floors, balconies, corners, party walls and other significant junctions. Their effects are expressed in terms of Ψ-values, or linear thermal transmittance values, and, unless they are recognised Accredited Details, they should be evaluated using thermal simulation software, following  agreed conventions and standards.

The Certified Thermal Details and Products Scheme provides independently assessed thermal junction details.

BRE Scotland specialise in undertaking analysis of the thermal performance of buildings, building components, junctions, thermal bridge analysis and associated services leading to legislative compliance and improved energy performance calculations.

Conventions for U-value calculations
(BR 443, 2006 Edition)

There are various thermal standards dealing with the calculation of U-values of building elements. The conventions provide guidance on the use of the standards, indicating the methods of calculation that are appropriate for different constructional types, providing additional information about using the methods and providing data relevant to typical UK constructions.

The U-value conventions were original published in 2002 and the 2006 edition is an update which provides additional information and reflects changes in the underlying British Standards. It can be downloaded as a PDF using the link on the right and it is also available from BRE Bookshop.

In-situ U-value Measurement

A U-value can be established by several theoretical calculation methods described in the BRE publication “Conventions for U-value calculations” and the U-value calculator software available from BRE Bookshop.

In existing dwellings theoretical evaluation of the thermal resistance (R value) or thermal transmittance (U-value) can be difficult for the following reasons:

• Materials traditionally used in buildings are not homogeneous and their thermal conductivity values are not always available,

• Establishing the exact dimensions of layers of materials require destructive methods which may be not possible in the case of existing dwellings.

This document describes an alternative method of establishing U-values of walls given in BS ISO 9869. The procedure applies only where a defined number of measurements are undertaken for a development at different times to calculate an average U-value figure.

This method can be applicable of establishing representative U-values of walls for wider developments assuming that measurements and analysis are carried out in accordance with BS ISO 9869. It should be remembered that it may lead to long periods of measurement and may give erroneous results in certain cases. Note that this method would not be suitable for establishing U-values for a single dwelling for the purposes of EPC.

BRE publications

• Conventions for U-value calculations

• U-value calculator

• SAP assessment procedure 2009

Updates to BR 497

• Amendments to BR497 to clarify certain text and make corrections. – Amendment No. 1

• New conventions on separating wall/ground floor junctions, where the separating wall penetrates the insulation layer of the ground floor.