Part L1, SAP and TFEE

Thermal Performance: Part L1A

The 2013 edition of Part L1A (new dwellings) came into force in April 2014. Key changes include a 6% reduction in CO₂ emissions across the build mix, and a new ‘elemental recipe’ approach for producing compliant dwelling specifications. This provides a straightforward, non-prescriptive means for house-builders to meet performance targets, whilst maintaining a reasonable degree of design flexibility. Another significant change is the introduction of a fabric energy efficiency target, which addresses a limitation of the previous edition of Part L1A that permitted a comparatively low standard of fabric performance to be offset through the use of renewable and/or low carbon technologies.

Relatively minor changes have been made to the Standard Assessment Procedure (SAP) in the current 2012 edition. In contrast, there were more significant revisions to the previous version (2009), which were largely in response to the European Energy Performance of Buildings Directive. These revisions improved the tool’s ability to account for year-round fabric performance and included better account of a dwelling’s thermal mass. The changes have also facilitated the new Target Fabric Energy Efficiency standard (TFEE). The TFEE accounts for a dwelling’s combined, annual heating and cooling load measured in kWh/m², for which a limit is set by SAP based on the performance of a notional dwelling of the same size and shape as that being assed, but with fixed values for the fabric performance e.g. U-values, thermal bridging, air leakage etc. A 15% margin is then added, giving the designer some additional design flexibility.

The performance of the notional dwelling is also used to set the CO₂ emissions target (TER), but without a margin being applied. It is possible that the next edition of Part L1A, will have a reduced margin added to the TFEE, or it may be removed entirely. Overall, Part L1A compliance can be achieved without the need for renewables, but if used, the fabric performance can be relaxed slightly providing the TFEE is still achieved.

Whatever approach is adopted in meeting Part L1A requirements, the beneficial qualities of concrete and masonry construction are becoming increasingly important as we move towards ever higher standards of fabric energy efficiency. These include the durable air barrier that can be provided, helping ensure a good standard of airtightness is maintained over the building life. In addition, the concrete and masonry sector has developed a broad range of high-performance construction details that are freely available and provide a low-cost means of minimising heat loss from thermal bridging. The level of fabric performance now required by Part L1A makes thermal bridging a key consideration in the overall design; the difference between older construction details and newer, more efficient details can be quite significant in terms of dwelling performance.

Also of importance is the thermal mass provided by concrete and masonry, which has a useful role to play as architects and designers tackle the growing problem of summertime overheating in dwellings. The SAP overheating check continues to be the official assessment tool, and remains largely unchanged following the 2010 and 2013 revisions to SAP/Part L1A. However, the Department for Communities and Local Government (DCLG) have indicated that overheating and space cooling are “anticipated for consideration” in the next SAP review. From a concrete and masonry perspective there has been one important revision since the tool was first introduced, which is recognition of night-time ventilation and the added effectiveness it brings to thermal mass as a means of helping to limit or prevent overheating.

More detail on changes to Part L1A, SAP and concrete/masonry homes are included in our publication entitled Thermal Performance: Part L1A 2013.