U Value Calculations

U-Values have a huge role in determining the energy performance of a building.
The U-Value of a property shows how hard and how well your insulation is working.

What are U-Value Calculations? And how do they relate to SAP?

A U-Value Calculation measures the thermal transmittance (transfer of heat) of each material used within a building.  Put simply the lower the total u-value the slower the heat is transmitted in and out of the building (therefore indicating it’s better insulated).

In terms of SAP Assessments, U-values relate to all exposed;

  • Floors
  • Walls
  • Roof
  • Doors
  • Windows
  • Roof-lights

U-value information for openings (i.e. doors, windows and roof lights) must come from the manufacturer or supplier.  The required u value is that of the whole opening including the frame, not just the glass itself.

For the floors, walls and roofs, confirmation of U-values of each element is required to confirm compliance with Part L U-value thresholds and must be provided as evidence before an EPC can be lodged for a dwelling. 

‘Bedtime reading’ – the technical bit of U-Values

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 considers the material thickness and conductivity (K-Value). Layer by layer, it looks at the materials, thicknesses and conductivity within the different exposed elements of a property, ultimately determining how efficient your property is at retaining heat.

The u-value of an element depends on all materials within it, using a cavity wall as an example, this would include the plaster, plasterboard, blockwork, insulation, cavity and brickwork.

A U-Value is the sum of the thermal resistances of these layers that make up an entire building element. As in our cavity wall example above, each of these materials will have a thickness and conductivity value, which combine to make a thermal resistance value. 

All of these materials’ resistance values are added up to give the total thermal resistance for that element.  Then simply dividing 1 by this total resistance gives us the U-Value, therefore, a larger total resistance will give a smaller U-value, which means less heat loss.

A good example below; (this needs editing as outdated but good base)

Useful U-Value information…

These are the U-Values used in the notional SAP assessment which determines the TER (Target Emission Rate) that you need to meet for compliance.  You should aim to always meet or improve these values or they will need to be compensated elsewhere.

Insert u value minimum table in here for England and Wales

How do I get a U-value calculation?

The majority of U-values for standard construction types are available via the manufacturer’s online calculator tools, see Celotex and Kingspan websites.  If you have a more bespoke construction build up that requires a personalised U-value calculation, please get in touch.

Using specialist software, our in-house technicians can produce u-value calculations for any type of construction make up. 

U-Value Calculations & Extensions

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

Having U-Value Calculations carried out 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 element.

For U-value guidance, support and calculations please contact us here.

Get in touch

Melin Energy Consultants


01554 571010

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