SAP10.3 Oddities: Low Air; Don't Care

The new SAP10.3, which checks homes are compliant with the Future Homes Standard, does a very strange thing with air leakage results. You could have an air test of 1.5 or 4.5, and your fabric performance (FEE) won't change.

The updates to SAP10.3 methodology have created a quirk where low air tightness isn't rewarded with better results.

Logically, a house with a better air tightness will lose less heat, so you’d expect the fabric performance to score better. That’s always been the case in previous versions of SAP.

So why does SAP10.3 ‘bottom out’ around 4.5, regardless of the actual air test result, and regardless of which type of ventilation has been installed?

The Fabric Energy Efficiency (FEE) doesn’t follow the usual Notional specification. It has its own stripped back version which just focuses on building fabric.

In SAP10.2, the FEE Notional is based on intermittent extraction fans (System 1 in old money; iMEV in new money) with an air test of 5.0.

The actual ventilation system isn’t considered in this calculation (it’s not a ‘fabric element’) but designers can offset other fabric parts to keep everything balanced, so a higher air test could be offset with an improvement like triple glazing.

In SAP10.3, the FEE notional uses a stricter air test of 4, and the ventilation is changed to decentralised MEV (previously known as System 3).

This change of notional ventilation is the cause of the weirdness.

SAP isn’t clever enough to accurately calculate air change rates in a dwelling. It makes assumptions based on the air permeability, ‘sheltered sides’ and how many flues/fans/chimneys the house has.

When SAP calculates the air change rate through the ventilation system, if it's below 0.5 an hour, it sticks at 0.5.

This also happens in 10.2, but the change to the 10.3 Notional ventilation system brings this quirk into sharper focus.

It means the ACH in the Notional FEE calculation is routinely bottoming out at 0.5 which has a knock-on effect to the final SAP results. Dwellings with high-performing air tests no longer show the FEE improvement you may expect.

Annoyingly for us SAP assessors, this takes away a very useful tool to deal with issues caused by unforeseen things happening on site… maybe there’s a block shortage, and the alternative block increases the wall U-Value and worsens the SAP results.

In SAP10.2, we can lower the air test target to make everything alright again.

In SAP10.3, if the FEE results fail, lowering the air test beyond 4.5 isn’t going to help.

This feels like an unintended consequence being caused by SAPs historical methodology not keeping up with the times. It should resolve itself when we move to the Home Energy Model, because HEM will calculate air change rates at a level of accuracy that SAP can't comprehend).

But while we continue to use SAP10.3, this peculiar quirk of the new methodology is going to continue to annoy every SAP assessor out there.