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.
