HEM Deepdive: Colours

[2 of 3]: I’ve been digging into areas of the Home Energy Model which us humble SAP assessors have never had to bother ourselves with previously.

One of the new questions HEM asks is:

🟠 What 🟡 colour 🟢 is ⚫ the ⚪ house?

In the early days of HEM development, every wall, roof and external door had to be graded from 0 to 100, where 0 was the ⚪est of ⚪, 100 the ⚫est of ⚫, and every other colour fitted somewhere in between.

I’m happy to report that’s been simplified to a three option selection of Light, Medium or Dark, but why does HEM want to know at all, and what difference does it make?

☀️ It’s all to do with solar absorption.

Black surfaces absorb more heat than white surfaces.

Logic follows that a black painted wall will get hotter in the summer and cause the internal temperature of the house to rise. This is why most homes built in hotter climates are painted white.

I’ve built a HEM model in Python, using the FHS 1.0.7a wrapper* and run it with all external surfaces set to light, and again with everything set to dark. ⚪⚫

The heating and cooling demands of the same dwelling change depending on the colour of the external surfaces.

HEM agrees with the above logic… just about! The black house has a lower annual heating demand, but only by 1% compared to the same house in white. ⚫✅

If a cooling system is added to HEM, the white house has a lower cooling demand by 9% compared to the same house in black. ⚪✅

If a cooling system isn’t included, that benefit is removed (no cooling is no cooling regardless of the colour of the house).

In this example of a typical two storey semi with no cooling, darker surfaces have a slightly lower energy demand, but only marginally. There’s no need to bulk buy black masonry paint just yet!

When we get to model in the real world, it will be interesting to see when/if the inclusion of a cooling system starts to advantage lighter coloured dwellings.

This would be more noticeable in heavily glazed dwellings and penthouse-style apartments, and if a hotter weather file was used (such as 2026 with its prolonged heatwave season).

For Part L compliance, we won’t be allowed to switch weather files ⛅, but this feature will be available outside the FHS Wrapper, and could prove useful for checking overheating risk in extreme scenarios.

*FHS 1.0.7a isn't the finalised wrapper and can't be used for checking compliance with the Future Homes Standard.