Part L U-values, otherwise known as thermal resistance is the rate of transfer of heat through a material, structure or matter and it is expressed in W/m²·K. The lower the U-value is, the better is its insulation properties, meaning a lower rate of heat transfer. Materials with low U-values, most especially lower than the standards set in Part L are highly encouraged as they are better insulators. The U-value of materials are varying depending on its thickness, thermal conductivity, and density. Upon installation, the U-values may be compromised due to thermal bridging, installation quality, and temperature difference between the inside and outside.

The Part L U-values standards set in Part L Building Regulations are applicable to all types of buildings, both new and existing dwellings, and both new and existing non-dwellings. Architects and designers should ensure that the insulation materials in the buildings should at least meet be lower or more than the standards.

There are, however, exemptions in the strict U-values to be followed in Part L regulations. Buildings that have special cultural or architectural importance have more relaxed U-value requirements in order to keep their integrity. They are usually the buildings officially listed in the special or historic ones, buildings that are in conservation areas, monuments, and buildings that were constructed before 1919 that used traditional materials and methods. Relaxed values mean that if these buildings were to be renovated or upgraded, developers don’t need to meet the U-values indicated in the Part L regulations if it compromises the following: (1) the building’s appearance; (2) historic fabric that may cause damping or condensation damaging the original materials, or; (3) windows, doors, and other decorative areas that are part of the heritage. Prior to construction, check with the Building Control bodies for exemptions.

Part L is periodically updated so it’s important to stay updated with the latest regulations as the U-values might get stricter too to align with the UK’s environmental targets.