Failing an air tightness test at practical completion can be costly – it delays handover, adds the expense of remedial work and means paying for a retest. There’s a lot you can do to avoid it and options if it happens anyway.
In this blog you’ll learn:
- The difference between In Construction and Final Phase testing
- What happens if a dwelling fails its Final Phase test
- Whether a failed score always means failing Part L
- What to do next if your SAP calculation isn’t passing
If you’re not yet sure what an air tightness test involves, our air tightness test guide covers that in full – this post picks up from there, on what happens when a test doesn’t go to plan.
Two-Stage Testing: In Construction and Final Phase
If you want to adopt a best-practice approach, you’ll need to carry out two air tests – an ‘In Construction’ test and a ‘Final Phase’ test.
In Construction Air Tightness Testing
An In Construction Air Test happens after the building envelope is complete but before additional fittings go in (kitchen units, skirtings, flooring, and similar). This early-stage test shows how the house is basically performing. We identify the majority of major air leaks at this stage, and clients find it much easier to fix issues before later fit-out work covers them up.
This is exactly what caught a problem before it could spread across an entire site. On Woodstock Homes’ nine-home Baily Court development, a single diagnostic test on one plot ahead of final testing identified leakage risks at typical junctions and service entries – giving the site team time to implement preventative measures across the remaining plots. All nine passed first time, with no retests.
Final Phase Air Tightness Testing
The Final Phase Air Test is undertaken at the last stage of construction, once everything except final finishes (such as carpets) has been fitted. If the issues from the In Construction test have been addressed, the dwelling should achieve its air tightness score or lower.
If the dwelling fails the Final Phase Air Test, there are a couple of options. Our testing specialist will go around the dwelling to find all air leakage sources. Sometimes we can fix issues on the spot by sealing small apertures and resolving minor issues – our testers will stay while you address what they’ve identified, then perform another test within the allocated site time, which will hopefully bring the score down to a level that passes SAP requirements. This retest is free of charge provided it’s completed within the time allocated for the visit.
If there are multiple leakage issues requiring remediation, your tester will prepare a report showing everything that needs fixing before the dwelling will pass. Most failures trace back to the same handful of weak points – worth checking against before you’re staring at a report:
We’ll then arrange a retest once the issues have been resolved.
What Happens If It Fails?
If a dwelling fails to achieve its design air permeability score, even after sealing any quick fixes identified on site, that doesn’t necessarily mean it’s failed Part L requirements. The maximum air permeability allowed varies depending on the building type and which edition of the regulations applies – your SAP assessor can confirm the actual threshold for your project, and it’s worth asking them to input the higher air permeability score into the SAP calculation to see if it still passes.
If the SAP calculation is failing, your SAP assessor will require a further reduction in the air tightness score. Sometimes that isn’t practical, so you’ll need to look at improving other building elements instead. Once the building’s fabric energy efficiency has been met, the next step is usually additions – Solar PV or mechanical heat recovery ventilation, for example. If fabric energy efficiency hasn’t been met, you’ll need to improve either the insulation or the air tightness score.
Taking these additional measures lowers the required air leakage target, making it more likely your property will pass the dwelling emission rate set by SAP and Part L.
What’s Next?
Failing a test doesn’t have to mean a failed project – with the right remediation approach, most dwellings get there. Our experienced team has extensive experience getting dwellings to meet the air tightness thresholds required and we work hard to go beyond mere compliance to help clients pass first time.
If you’ve had a failed result and want advice on next steps, get in touch – we’re happy to talk it through.
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