In SAP (Standard Assessment Procedure) calculations, U-values and solar g-values for windows and doors play a critical role in determining the energy performance of a dwelling—especially the DFEE (Dwelling Fabric Energy Efficiency) score.
Let me explain.
In the design calculation its common for the windows and doors to not be finalised, often due to the client not having decided as to what windows and doors they would like. If this is so we can input window data based on the SAP default values for solar g values, and a maximum u value to meet the current Building Regs.
It’s often the situation where an overall u value of say 1.4 is input, and then the solar g we input is one of the lowest from the drop-down menu in SAP, usually Double low E soft coated glazing with the solar g of 0.63.
If the calculation passes, and it’s the Dwelling Fabric Energy Efficiency Figure (DFEE) we are most interested in, then its fine. If not, there is a lot more to achieving this pass than just the windows and doors, but assuming for now those other inputs that may affect the DFEE are all at a good level, then the target u value of the windows and doors may be reviewed. If everything is passing and the client/architect are ok with the values input that’s fine.
However, the problem may occur when the windows and doors are confirmed, ie decision made and they are about to be ordered, and the U value and solar g value are different to those targeted, which is all too common.
The best results in SAP come from when the overall u value is low, and the solar g figure is high. This is because the lower the u value obviously means lower heat losses and a so a more energy efficient home. The solar g figure determines the amount of solar gain the glazing will let through. The higher the solar g, the more solar gain. This works well from an energy efficiency perspective in that more solar gain in winter means the heating works less, so a better overall result. However, more solar gain means that in summer there is a risk of overheating, which is not directly considered in the SAP calculation. With the preference in many new builds for large areas of glazing, particularly south facing, this could be an issue. So, a balance must be found.
Often the u value for a window does meet the target, but the solar g is often much lower, even as low as 0.37, and if that happens the effect on the overall SAP result can be very significant and often puts the DFEE at a fail.
It’s important therefore that once the windows and doors are determined, the SAP calculation is updated with the overall u values and solar g values and the result checked as soon as possible. If it passes, fine, if it doesn’t, there will need to be some changes in the inputs that make up the DFEE to account for this. This can be any of the input that goes into the DFEE figure, u values for all the thermal envelope, a change in the windows and doors themselves to a different type, lower air permeability, possibly the source of thermal bridging details.
The worst thing to do is to ignore the design SAP and leave it to the final as built checks. I’ve had many houses where they are complete, and it only becomes known as we undertake the as built calcs that the window and door u values and solar g values are different to those targeted at the design, and because the u value may be a bit higher and the solar g value lower, the calc fails overall. And at this point there is very little can be done to rectify this.
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