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Part L: Will U Values become stricter?

13 Nov 2019


In this series of articles, we aim to answer some of your questions about the upcoming changes to Approved Documents L and F, SAP methodology and the Future Homes Standard.

Current proposals suggest these regulations will be coming into force in England from October 2020. Contact us with your own questions about the regulation changes, or to discuss our training seminars and workshops.


Will I need to use more insulation?

These new regulations are setting out the strictest targets yet to push us towards building homes which are cheap to run and use much less energy. That said developers will keep design flexibility, providing they stay within the boundaries of compliance.

Building Regulations in England set minimum thicknesses of insulation which can be used on building projects. Developers use U-Value calculations to prove their specification reaches the required targets.

Under the proposed changes to Approved Document L1a the maximum allowable U-Values are being lowered which means designers will need to plan for thicker and better insulation products.

As is the case under current regulations, meeting these maximum U-Values alone will not be enough to comply with the Model Design in SAP (We have other articles explaining this in more detail).

We would encourage designers to aim for the Model Design values where possible, and only scale back their design once they are confident their overall specification will comply with AD L’s tough new targets.

These proposals only affect new build dwellings. We’re expecting additional information to cover renovation works and non-residential projects in the New Year.

Starting with limiting wall U-Values, the proposed maximum target is 0.26. This reduction from 0.30 is still more relaxed than what is expected from building sites in Wales and Scotland, and most developers already achieve this new value.

Using a traditional brick and block cavity you can achieve 0.26 with 100mm mineral wool or 60mm rigid insulation.

However, this will generate higher losses than the Model Design. A U-Values around the 0.18 mark would be a safer bet. This can be achieved by increasing the cavity to facilitate 150mm mineral wool or 90mm rigid insulation.

For roofs the U-Value is being reduced to 0.16 from 0.20. Most developers we work with already exceed this target. The proposed Model Design will use 0.11 which should be achievable in a cold roof using 350mm mineral wool, or by using 220mm rigid insulation in sloping areas.

The limiting U-Value for floors is reducing to 0.18 from 0.25. Again, not a drastic change and still lagging behind other parts of the UK. But as the Model Design will use a floor U-Value of at least 0.13 you should look to increase insulation levels – somewhere in the region of 140mm rigid insulation in a solid ground floor.

Windows and doors also have maximum U-Values to meet. Under the proposed changes these would need to achieve at least 1.60 (including frames).

How windows are treated in the Model Design is still up for debate. The Model may use triple glazing with a U-Value of 0.80. If that makes it into the final publication, developers who stick with double glazing will be at a disadvantage.

The proposed changes to AD L give emphasis to fabric performance and begin to push the industry away from traditional building designs. Thicker walls, modular frames and triple glazed windows will all need to be considered as developers piece together a new specification to meet AD L’s new requirements.

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