Meeting the Challenges of Part L
As the dust is starting to settle on the latest revised changes of Building Regulations Part L1A, we review the preferred methods and challenges in meeting the requirements for new homes.
Through our experience of meeting the current standards we have been able to identify points of focus which enable beneficial methods of meeting the target emission rates (TER).
Finding a balance between fabric performance, heating system and thermal bridging
Besides the fabric changes, the other areas of variation are those to thermal bridging and the use of assumed (not calculated) values in the SAP assessment. Previously the use of accredited details enabled developers and builders to use an assumed bridging value of 0.08 W/m2K. Within the SAP 2009 assessment the assumed bridging jumps to 0.15 W/m2K. This results in a much higher dwelling emission rate (DER) requiring further improvements to meet the TER.
We recommend SAP assessments include a calculated thermal bridging, measuring the various elements of the design. In most cases this will give a thermal bridging value similar to 0.08 W/m2K, enabling specification optimisation and ultimately reduced build cost.
It is important to not underestimate the impact of the heating system and its controls. There is only so much the DER can feasibly reduced with improved building fabric. As an ongoing generator of emissions the heating system and its controls can have a big sway on the dwelling Co2 emissions. Fitting an efficient system can enable optimisations and saving to the build cost elsewhere.
Builders and developers should be well converse with ensuring air tight construction and the relevant forms of detailing, however more rigorous attention in this area can give DER reductions, allowing a further optimised specification. We would most likely recommend reduced design air permeability for developers or builders with many property completions per year who are comfortable with the consistency of their pressure tests.
In our experience of meeting the current requirements a jointly considered approach to detailing, specification and SAP assessment offers the best opportunity to create an efficient cost effective build with the performance of the fabric balanced with the heating system and thermal bridging.