Building Regs Wall U-Values
There are two building regs wall u-values in Approved Document L for new builds and extensions.
- 0.26 W/m2K – Maximum
- 0.18 W/m2K – Notional Target
It is best practice to design external walls for new builds to the notional target of 0.18 W/m2K or better. All new build dwellings require a SAP Calculation at both design and completion. As part of this process a ‘notional’ building of the same size and shape is created which uses the notional u-value.
In practice, to achieve a u-value of 0.18 W/m2K in a modern development, a full fill cavity of 120mm PIR usually makes this grade. Exact materials for the inner and outer leaves can affect this as well as any thermal bridging through wall ties.
It is possible to pass SAP using a wall with a higher u-value than the notional target, however, the difference in performance will need to be made up elsewhere.
The limiting u-value of 0.26 W/m2K is the maximum that a wall can be under Part L. It’s in place to avoid mandating a ‘recipe’ for new builds and to give design freedom. A building designed to the limiting targets will not pass SAP. Where the notional targets are exceeded, performance will need to be made up elsewhere.
In a dwelling where a wall is built to the limiting u-value, the floor and roof will need to deliver exceptional performance to make up for this.
About the Author
Ian Kay is an Elmhurst accredited On Construction Domestic Energy Assessor (OCDEA), Overheating Consultant and the founder of SAPgen. He specialises in Overheating Assessments and SAP Calculations for new builds, conversions and extensions across the UK, helping architects, builders and homeowners achieve building regulations compliance quickly and with zero stress.
When he’s not helping clients, Ian writes clear, practical guides to help the construction industry understand energy compliance, Part L / O rules and best practice build specifications.
Learn more →This article offers general guidance based on current SAP 10 and Part O practice. Every project is different, so always check the exact specification and requirements with your assessor before making design or build decisions.