U-Values explained…….

A U value is a measure of heat loss in a building element such as a wall, floor or roof. 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 it is a way of predicting the composite behaviour of an entire building element rather than relying on the properties of individual materials.

Why use U values?

U values are important because they form the basis of any energy or carbon reduction standard. 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 on 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.

The table below gives typical U-values for different areas of a modern house and shows the effect of added insulation. The effect of adding insulation to a house is to reduce the U-values of the different areas.

U-value / Wm-2K-1
Roof Without insulation 2.3
With insulation 0.4 – 0.16
Cavity Wall Without insulation 1.6
With insulation 0.6 – 0.21
Floor Without insulation 0.9
With insulation 0.6 – 0.25

Source SEAI Note these are typical values. Actual values depend on factors such as the shape, size and location of the house, materials used in its construction and the amount and type of insulation added

GD 2011 Part L Requirements

In general all domestic buildings where work has commenced on or after December 1st 2011 are required to comply with the revised Technical Guidance Document Part L 2011. If you have applied for planning permission or received approval on or before 30th November 2011 you are required to comply with the TGD 2008 Part L.


Building Regulations 2008 – Part L Conservation of Fuel and Energy – Buildings other than Dwellings relates to residential buildings, places of assembly, offices, shops and industrial and storage buildings an includes both new build and change of use / material alteration works.
The chart below illustrates the U-Value requirements for relevant buildings based on the elemental method where work commenced or took place on or after July 1st 2008.


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