Get a Quote

What is a Y-Value?

Before reading this blog – check out the other blogs in the series on U-Values and Ψ-Values. A Y-Value is used within the SAP assessment to show how much heat is lost through the junctions of a building. It is usually a very low figure – 0.15 is about as high as a y-Value will go… 0.02 is a very good result, which means very little heat is lost through the building junctions – in other words the building has minimal thermal bridges. To calculate it, you first need to gather all of the Ψ-Values for the individual thermal bridges of a building. For a basic house there may be around 10 Ψ-Values. More complicated structures could have up to 30!

The different Ψ-Values are listed within SAP methodology and include areas where the external wall insulation is interrupted (such as window frames, corners etc). Next, you need to multiply each Ψ-Value by the length of each thermal bridge. Some are quite straightforward (such as where the wall meets the ground floor… take the ?-Value for this junction and multiply by the perimeter). Others are more complicated… (the Ψ-Value for window sills must be multiplied by the total length of all window sills in the dwelling. There’s a separate one for window jambs and a third for lintels… when you have a large house with 50+ windows, this gets quite monotonous!) When you have all of your Lx ? calculations, add them all together. This gives you the total thermal bridging loss.

There’s still more to come…

Work out the total heat loss area of the dwelling. To do this, add up the areas of any element where heat can escape to an unheated space (usually outside, but can also include garages and lofts). This will include the area of all heat loss walls, floors, roofs and windows. This is based on the insulation envelope, not the fabric envelope. Divide the total thermal bridging value by the total heat loss envelope area, and you’ll get your y-Value. If you are building in line with Accredited Construction Details, you can expect your y-Value to be around 0.08…

Enhanced Construction Details are around 0.04. If you are not adopting any thermal bridging guidance, the SAP assessment will be based on a worst-case value of 0.15. An alternative approach is to have Ψ-Values calculated for your specific junctions instead of using the one-size-fits-most approach – this has clear benefits, but is only viable if you use the same construction time and time again. The Target Emission Rate is calculated using a y-Value of approx. 0.05, so it’s of benefit to use values which are lower than this for the purposes of meeting Part L 2013 targets.

Rhiannon Edwards

Author: Rhiannon Edwards

This article was published by Rhiannon Edwards on 27.10.2014.