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Upcoming Changes to Part L - Wall Thickness


Yes, the thickness of our walls, and the amount of insulation in them, looks set to increase significantly when the new Regulations go live.

Currently, the majority of houses being put up in the UK are to a cavity construction with a 100mm cavity, either fully filled with mineral wool or partially filled with 50mm insulation board.


This will get you a U-Value (how we measure thermal efficiencies) of around 0.25 – 0.29 depending on type of block work. Current regulations say the maximum U-Value allowed for a wall in a new house is 0.30 (England and Wales) or 0.25 (Scotland), so that’s fine for the moment.

But we’re expecting the maximum U-Value for walls to be lowered by quite a way – the various consultation and draft documents which are currently circulating indicate a maximum figure for a wall U-Value of around 0.20 / 0.23.

In real terms, to build a cavity wall to a U-Value of 0.20, you would need to install at least 90mm of PU board insulation, plus a cavity (which is usually recommended at 50mm minimum). Alternatively, fully fill a 150mm cavity of mineral wool to achieve a similar result... either way, this means you’ll need to design your building to have thicker walls to be able to fit all this extra insulation in.

SAP Target Emission Rate

Also you may need to go further than this... one draft paper shows the new SAP Target Emission Rate in Wales will be based on an external wall U-Value of 0.15. It’s important here to differentiate; this isn’t a maximum allowed figure, it is what the calculation target is based on. In other words, you don’t need to build to 0.15, but you’ll need to balance the equation by making improvements in other areas of the SAP.

A U-Value of 0.15 would need a more thermally efficient internal block as well as 200mm of mineral wool... or a minimum of 125mm of PU insulation board.

U-Values for floors, roofs and windows are all expected to be reduced also, but not by such an extent.

Look out for other mini-blogs in this series this week which will cover more rumours and discussions that are currently circulating around the Part L / Section 6 changes.  

Jon Ponting

Author: Jon Ponting

This article was published by Jon Ponting on 14.01.2013.