U-values measure the rate of heat loss through elements of the building fabric, such as floors, walls, roofs, windows and doors. The lower the U-value of an building element or component, the lower the rate of heat loss through it.
U-values are essential for designing energy efficient buildings and for assessing whether a building will comply with the energy efficiency requirments of Building Regulations and Building Standards. They are used:
- as inputs into SAP and SBEM calculations for new buildings.
- to assess thermal performance in extensions and refurbishment projects.
How are U-values calculated?
In the UK, U-values are calculated using the combined method as defined in BS EN ISO 6946:2007 (together with BS EN ISO 13770 for ground floors and basements) and following the guidance contained in BR 443 Conventions for U-value calculations.
A U-value calculation takes account of the thickness and thermal performance of the materials and air spaces in the construction, and the effect of any thermal bridging. The initial result is then adjusted to allow for the effects of mechanical fixings (such as wall ties and fasteners) and air gaps.
This post gives some more background on U-values.