Photo by Quattro Design Architects

U-values, cause a surprising amount of stress.

Usually, someone has asked for a U-value, and there’s a lack of clarity about what it is.

A crucial element of SAP and SBEM calculations, U-values quantify the heat transfer through floors, walls and roofs and openings.

  • The amount of heat transfer depends on the thickness and thermal conductivity of each building material in a cross-section of the thermal envelope.
  • Part L contains limiting backstop U-value thresholds for all floors, walls, roofs, windows and doors. Low U-values are essential for all domestic projects to meet the Target Fabric Energy Efficiency (TFEE) threshold outlined under Part L1A 2013 as well as the mandatory averaged U-values in Wales now and expected in Part L 2020.

“Usually we can estimate heat loss in a 2D plane without a complex thermal model,” says Brandon Wipperfurth, senior sustainability and energy consultant at Darren Evans.

“The only time we do 3D U-values is for rainscreen cladding systems, or other bespoke walls usually involving abnormal repeating metal punctures through the insulation lines.

“Normal U-values are usually done as part of a SAP calculation, and are included in our service,” he adds.

Top tips

  1. Is it a standard or non-standard U-value? This is the most important thing to know. “If you’re not sure, we can help,” says Brandon.“We’ll look at the specifications in the architects’ drawing and tell you which U-value you need and why.”
  2. Don’t forget about condensation risks Correct positioning of the vapor control layer is absolutely critical.In the correct place, it reduces risk, in the wrong place it can create a risk. It needs to be on the warm side of the insulation in most cases.“Also, don’t confuse a vapor control layer and the breather membrane,” says Brandon.“Try and avoid double lines of insulation. This can create lukewarm surfaces within the element for water to condense upon.”
  3. Think about rain screens “Try and keep the railing from cutting into the insulation line,” advises Brandon. “This normally means a minimum 50mm gap between the back of the cladding and the insulation.”The thicker the wall the longer the brackets and an increase in bracket density is seen, increasing cost.“Don’t try and get Passivehaus U-values. There are thermal limits to these systems,” adds Brandon.

Watch out for: 

Heat loss targets are set to become tighter and tighter under changes to Part L. “This makes certain U-values very difficult to build. We can advise you on what you need to do,” explains Brandon. “For example, you can’t keep adding more insulation in the hope the calculation comes out the same.”
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