FHS Launch - Fabric performance

Now the Future Homes Standard and Part L (2026) have been published, we can begin unpicking what compliance looks like for new building sites across England.

Changes to the fabric performance targets are under the spotlight this morning, or lack of changes may be more appropriate...

To recap, SAP creates a notional specification that it uses to calculate its targets.

If you copy the specification precisely, you pass Part L.

If you move away from the notional, you have to offset a worse change with a better change to keep everything balanced overall. ⚖️ 

Homes built to the Future Homes Standard should never need be upgraded for better energy performance. But the new rules on fabric performance are fairly similar to the previous ones.

In the new notional, all of the U-Values remain unchanged:

Wall: 0.18 – Floor: 0.13 – Roof: 0.11 – Windows: 1.20 – External doors: 1.00

The Part L backstop values aren’t changing either. These are the worst-case values you’re allowed to build to (keeping in mind the offsetting note above):

Wall: 0.26 – Floor: 0.18 – Roof: 0.16 – Windows: 1.60 – External doors: 1.60

But there are a couple of adjustments to be aware of…

🟠 The new notional uses an air test target of 4.0.

It was previously 5.0, so that’s going to make the Fabric Energy Efficiency Target (TFEE) a bit tougher to meet.

🟠 Glazing specification details are also changing. Currently, the notional uses a g-value of 0.63 and frame factor of 30%.

If a site has a low g-value (usually required as an overheating mitigation), the SAP results are penalised due to the reduced levels of solar gains.

Under FHS, both the g-value and frame factor are being uncoupled from the notional, which means SAP reports won’t fail on account of having low g-values.

This change could work in favour of some sites, especially those that need to install glazing with low g-values.

⭐ We also need to consider the HEM factor.

Some of us long-in-the-tooth SAP assessors have worked with the methodology long enough to understand how it will react to, for example, offsetting a 100mm cavity wall with triple glazing.

But a fabric specification that works under SAP may not behave as well when we move to the Home Energy Model.

It’s true to say the two calculation models are aligned, but only when you consider the notional specification.

The more you move away from the notional, the more likely you'll get different answers from SAP and HEM - it could be enough of a discrepancy to jump between a pass and a fail.

That’s something we’ll need to explore later in the year when we properly begin modelling in HEM.