ELMHURST ENERGY MATTERS EDITION 7.qxp_Layout 1 12/08/2021 11:40 Page 14
Energy Matters | Issue 7 | 2021
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TECHNICAL CORNER www.elmhurstenergy.co.uk
What do
U-value? Dr. Richard Jack, Product Manager at Build Test Solutions
U-Values are used to define how easily heat transfers through building elements like walls, floors, roofs, windows and doors. A low U-Value means low heat loss, and hence good thermal performance.
U-Value measurement can be used to provide quality assurance on new or newly retrofitted buildings, or better understand the real performance of walls, floors and roofs. But until now U-Value measurements have been few and far between, Build Test Solutions are trying to address this by making tools that are more accessible and easy to use including the exciting new Heat3D iOS app.
In an existing building, it can be very difficult to determine the build-up of a construction in a non-destructive manner. For that reason, U-Values are often estimated based on the age and approximate construction type of an element or building, as in RdSAP. These estimates are determined from lookup tables, with the tables themselves informed by calculations for similar walls and previous measurements studies for samples of similar walls. U-Value calculations and lookup tables are valuable tools to help understand and design buildings, but are naturally limited in how accurately they will work for an individual building. Error and uncertainty is introduced through a number of sources. This means that the actual U-Value of an element is not as predicted or designed. Unexpected performance can mean unintended consequences,
and incorrect U-Values are no different. If the U-Value of an element is higher (worse) than expected, this means additional heat loss and could in turn lead to issues with condensation or mould growth. If the U-Value of an element is lower (better) than expected prior to a retrofit, it could mean that the expected benefit in energy and cost savings are not delivered in practice. All of these factors make in-situ measurement a valuable and insightful tool. By knowing the actual performance the building owner and user can get quality assurance on works carried out, the thermal comfort and energy demand of the space can be better understood and controlled, and designers can create more informed retrofits and feedback can be delivered on what materials and processes work in practise. Measurement has generally been carried out using ‘heat flux plates’. These are coaster-sized sensors which measure heat flow through them and the surface they’re attached to by very accurately measuring the temperature difference between one side and the other. A complication is that heat is stored in the material of
