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NPL and BRE Collaborate to Investigate Book Values for Thermal Conductivity of Building Materials

The National Physical Laboratory (NPL) is currently working in collaboration with the Building Research Establishment (BRE) on a project to investigate discrepancies between new in-situ techniques for measuring the thermal performance of brick walls and the values obtained from calculations based on long-established reference values for building materials.

The work is part of a larger project investigating the thermal performance of solid wall properties pre- and post-insulation across the UK, funded by the Department of Energy and Climate Change (DECC). Although making this type of thermal measurement is extremely challenging, this project has identified that there are also problems with the established material property values on which calculations of thermal performance are normally based.

Many of the standard 'book values' for the thermal properties of some basic building materials are based on historic measurements produced using the guarded hot-plate technique. Some of these measurements are very old - possibly 60 years old or more.

Thermal conductivity measurements made by NPL on bricks from the DECC project have produced much higher values than would have been expected based on historic values. This was not a surprise to the NPL experts, as the measurement technology and instruments have improved significantly, and it is extremely unlikely that many historic measurements dealt with thermal contact resistance issues in a way that would be considered acceptable within the current framework of international measurement standards.

These shortcomings would have led to the historic measured values being significantly lower than the true thermal conductivity values. As a consequence, the book values for thermal conductivity of basic building materials may be incorrect.

We need to ensure that models used to evaluate the thermal performance of buildings are based on up-to-date and accurate data, provided by measurement techniques which are traceable to national standards, such as those developed by NPL. If we are to see improvements in the thermal performance of the UK's buildings and the development of in-situ measurement techniques to evaluate them, then a great deal of work is needed to improve understanding of the thermal properties of construction materials.

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