It can also be referred to as an ‘overall heat transfer co-efficient’ and measures how well parts of a building transfer heat. This means that the higher the U-Value the worse the thermal performance of the building envelope. A low U-Value usually indicates high levels of insulation. They are useful as a way of predicting the composite behaviour of an entire building element rather than relying on the properties of individual materials. U-Values are measured in W/m²K.

U-Values are important because they form the basis of any energy or carbon reduction initiative. In practice, nearly every external building element has to comply with thermal standards that are expressed as a maximum U-Value. Knowledge of how to simply calculate U-Values at an early stage in the design process, avoids expensive re-working later in a project. It allows the designer to test the feasibility of their project at an early stage to ensure it is fit for purpose and will comply with regulatory frameworks.

Now we know what a u-Value is, let’s look at how each individual part of Document L influences the amount of insulation required to meet building regulations.

This is measured with TER (Target Emissions Rate) and is based on elements such as insulation, heating, lighting, building materials and windows. This value is calculated via a system known as Standard Assessment Procedure or SAP which calculates the CO2 emissions and in the case of England, the energy demand of the proposed specification. This procedure includes stating specific U-Values of each element used in the house.

The below drawing shows the U-Value requirements set out for new properties in the Building Regulations for each element (roofs, walls and floors). The actual specification required for a compliant building and necessary building fabric U-Value target may vary depending on the actual overall proposed specification and the outcome of the energy assessment.

a whole building Target CO2 Emissions Rate (TER, see New Dwellings above). However, existing dwellings do have to meet certain target U-Values for building elements (roofs, walls and floors); each of which are shown in the below drawing.

differing U-Values to the standards that apply in England. The drawings below show the U-Value requirements for both new and existing domestic buildings.

Section 6 (Energy) of the Scottish standards which states that every building must be designed and constructed in such a way that an insulation envelope is provided which reduces heat loss. The drawing below shows the maximum U-Values for each building element.

The information shown in the U-Value drawings shown is provided for guidance only. You should consult your building control authority and building designer regarding compliance with building regulations. Now we know what a U-Value is and what target U-Value we need to achieve, let’s look at what information is needed to calculate a U-Value.