U-Value Calculations

A U-Value Calculation measures the thermal transmittance (transfer of heat) of each material used within a structure/ building. The lower the total u-value is, the slower the heat is transmitted in and out of the building (better insulated).

Building Regulations/Standards continue to tighten the requirements of U-Values, with lower limits provided to ensure that building elements, materials and fabric are considered throughout the design and construction process.

Elmhurst Energy Consultancy provides U-Value Calculations to assist with Building Regulations/Standards Compliance- particularly important for extension projects (extension calculations).

What is a U-Value calculation?

A U-Value calculation is a computer-based assessment that measures the total thermal resistance of a structure. The unit of measurement for U-Values is given as W/m²K (Watts per square metre degree Kelvin); these are calculated through an approved U-Value Calculator which takes into account the material thickness and conductivity (K-Value).

When do I need a U-Value Calculation?

With the amount of terminology thrown around by Building Regulations/ Standards it can be tricky to identify what calculation is required for compliance and when.

Where compliance is required for a whole-building energy performance (completely new buildings), U-Values are used within Standard Assessment Procedure (SAP) calculations for new dwellings, and also Simplified Building Energy Modelling (SBEM) calculations for new non-dwellings.

However for some extensions, U-Value calculations should be undertaken for compliance.

U-Value Calculations and Extensions

The big concern when adding an extension to an existing building is the potential heat loss that could occur from poor construction. Therefore efforts should be made to limit the heat loss. This can be achieved through the thermal elements (wall, floor or roof) and other parts of the building fabric.

U-Value Calculations can demonstrate compliance for new extensions, as they provide 'reasonable provision' for newly constructed thermal elements introduced as part of an extension.

The newly constructed or replacement thermal elements would simply need to be better than the maximum u-values for the building elements.

These standards were taken from Part L1b of the Building Regulations. The type of building and region may offer different standards.

Whole Dwelling Calculation Method

For residential extensions with glazing that exceeds 25% of the extension floor area, it is recommended that a whole dwelling calculation method (SAP Calculation) is undertaken, as design elements can be relaxed through compensating measures elsewhere.

Similarly, larger commercial extensions that have a useful floor area greater than 100m² and greater than 25% of the total useful floor area of the existing building, should be treated as a new building and will therefore require SBEM Calculations.

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