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U-Values

18 Mar 2022

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AD L changes will a 100mm wall cavity be enough?

In just a few months time, the energy efficiency targets in English Building Regulations are changing.

This will be the first upgrade in nearly a decade, and sets the foundations in place to steer the country’s construction sites on course for the UK’s 2050 zero carbon pledge.

The new Approved Document Part L introduces far more stringent energy and carbon emission requirements for developments, sets new rules on amounts of insulation, requires photographic evidence and includes clearer reporting processes.

AD L takes effect from June 15th 2022, with similar changes to follow in Wales and Scotland.

Can I still build with a 100mm cavity?

One of the Frequently Asked Questions we’ve been receiving at Energist is from designers wanting to know if a traditional 100mm cavity in their external wall will be good enough to meet the new targets.

This is a commonly used thickness for new homes in England. The heat loss will vary slightly depending on the types of products used, but typically a fully filled cavity with wool insulation and a low density block will achieve U-Values around the 0.25 W/m2K mark.

Under current requirements, an external wall U-Value cannot be higher than 0.30. When the regulations change, this limit is being reduced to 0.26.

So, as long as the wall construction achieves 0.26 or lower, the wall will meet the heat loss requirements of the new regulations.

But, and there’s always a ‘but’, the new AD L sets three mandatory targets for energy use, carbon emissions and fabric performance. These targets are much stricter than the current requirements, and means a house constructed with a high wall U-Value, even one that meets the 0.26 limit, will need to include significant offsetting elsewhere in the specification to meet overall compliance.

There’s no single answer to this, as results vary from house to house, but a dwelling with a 100mm cavity wall may need to include triple glazing, or may require mechanical ventilation with heat recovery, to make up for the heat losses through the wall.

Why let us build to a high U-Value if this won’t comply?

The rules in England allow for design flexibility in the fabric build-up, so you can build a wall to just meet the U-Value limits, but in return for this flexibility, you must make additional improvements to other parts of the building.

Interestingly, Wales and Scotland don’t take this approach. In Wales, for example, the current wall U-Value limit for houses is 0.21, and this is expected to drop to 0.18. The days of a 100mm cavity on Welsh construction sites are already a distant past!

We all know the life lesson that just because you can do something, doesn’t mean it’s a great idea. Continuing to build with a 100mm wall cavity after June will still be allowed in England, but it will be far more viable to increase the cavity and add more insulation, as this will save having to make significant compromises to the rest of the specification.

Have a wall U-Value of 0.18 in mind, even if you don’t quite achieve this, as this will reduce the amount of offsetting required elsewhere in the specification.

Can I keep my wall thickness as it is, but stick some solar panels on the roof to offset?

Sadly, that isn’t going to give you the answer you’re looking for. The Fabric Energy Efficiency Target is a mandatory requirements in AD L. This sets a maximum heat loss target for the building shell.

You can stick as many PV panels on as you like, but that isn’t going to change the result of the fabric performance one bit.

You can only control this target through increasing insulation levels, improving solar gains through windows, and by reducing the air leakage.

This article has focused on the performance of an external wall, but the same logic can be applied to roofs, windows and ground floors. All U-Value limits in AD L are being made stricter under the new regulations, and although design flexibility still exists, insulation thicknesses will need to increase throughout your design to help meet overall compliance.

If you have any questions about these Building Regulation changes, feel free to contact the Energist team. Otherwise keep an eye out on our ‘Articles’ page for more guidance about the incoming changes.

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