What changes between design and as-built
The as-built model has to reflect the building as it stands. That means the tested air permeability rather than the design figure, the products actually installed rather than those specified, any dimensional changes made on site, and the heating, hot water and ventilation equipment as commissioned. Where the build followed the design, the change is small. Where it did not, the model shows exactly how much margin was lost and where it has to be recovered.
The evidence pack at completion
Part L 2021 expects more than a calculation at completion. The building control body will look for the as-built calculation, the air tightness test results, commissioning records for the fixed building services, and photographic evidence of key construction details taken as the work was covered up. Photographs cannot be recreated after plasterboard, which is why the schedule for them is issued at design stage rather than requested at the end.
- As-built SAP calculation for each dwelling
- Air tightness test results for the tested sample
- Commissioning records for heating, hot water and ventilation
- Photographic evidence of the specified construction details
- On-construction EPC for each new dwelling
When the air test comes back worse than modelled
A worse result than the design figure does not automatically mean failure. It means the model has less margin, and the shortfall has to be made up or the result improved. Depending on the scheme, that can mean remedial sealing and a retest, a change to controls or a services parameter that is still open, or in the worst case a specification change on the remaining plots. The sooner the first test happens on a site, the more of those options exist, which is why we push for an early test on a first plot rather than testing everything at the end.
Product substitutions
Substitution is normal on site and is not a problem in itself. The problem is a substitution treated as equivalent when its thermal performance, efficiency or control capability is not. Insulation with a different lambda value, a boiler or heat pump with a different seasonal efficiency, a different glazing unit, a different MVHR unit with a different specific fan power: each of these moves the result. Send us the change when it happens and it is a five-minute check. Send it at completion and it can be a redesign.
On-construction EPCs
Every new dwelling needs an on-construction EPC, produced from the as-built calculation rather than from a survey. Because it comes from the same data, the rating on the certificate follows directly from the as-built model, and errors in the model show up on the certificate the buyer sees. We lodge the certificates once the as-built evidence is settled.
