Air Tightness Testing

Air tightness testing (also known as air permeability testing) measures the amount of uncontrolled air escaping from or entering a building, identifying leaks in the building fabric that cause heat loss, higher energy bills and poor comfort. It’s a mandatory requirement under Building Regulations for most new constructions and significant renovations, and is also often a required part of the evidence trail for grant-funded retrofit projects, domestic and non-domestic alike. A good result means reduced energy consumption, lower carbon emissions, fewer draughts and – when combined with proper ventilation – better indoor air quality.

Best practice is two tests: an In Construction test, carried out once the building envelope is complete but before fittings like kitchens, skirtings and flooring go in – catching major leaks while they’re still easy to fix – and a Final Phase test at the last stage, once everything except final finishes (like carpets) is in place. On Woodstock Homes’ nine-plot Baily Court development, an In Construction test on a single plot identified leakage risks at typical junctions and service entries in time to fix them across the whole terrace, and all nine plots passed their Final Phase test first time.

If a dwelling fails, our tester identifies the leakage sources on site – sometimes issues can be sealed on the spot and retested free of charge within the visit. Where more remediation is needed, we provide a report setting out exactly what to fix before a retest. Most failures trace back to the same handful of weak points – skirting boards, window and door surrounds and service penetrations are frequent culprits – and even a failed score doesn’t automatically mean failing Part L, since your SAP assessor can confirm the actual threshold that applies to your project. We also offer SAP calculation services in-house, so where we’re delivering both, your air test result and SAP assessment stay fully joined up rather than requiring back-and-forth between separate suppliers.

Blower Door Testing: The most common and standard method. A powerful fan is installed into an exterior doorway to create a pressure difference (typically around 50 Pascals) between inside and outside, and the airflow needed to maintain it gives the building’s overall air leakage rate. Excellent for verifying construction quality and for detailed diagnostics pinpointing specific leak locations – effective for new builds and retrofit evaluations alike.

Pulse Testing: An alternative method that releases a quick, precise burst of air and analyses pressure decay over a short period, at a much lower pressure range than blower door testing (around 4 Pascals). Particularly suited to occupied buildings or quick, preliminary assessments in smaller, simpler layouts – faster and less intrusive, though less able to pinpoint specific leak locations than blower door testing.

Smoke Testing: Often used alongside a blower door test: harmless smoke is introduced while the building is pressurised or depressurised, visually revealing the precise location of air leaks and gaps for targeted sealing.

Most failures come down to gaps that are easy to reach before finishes go in and hard to reach afterwards – skirting boards, service penetrations, loft hatches and similar. Our engineers can advise on preparation ahead of the test itself, and we provide a pre-test checklist covering the areas worth double-checking before we arrive, so you’re not relying on catching problems on the day.

While not a direct MEES requirement, improving air tightness is a highly effective way to raise a property’s EPC rating and help landlords meet MEES for existing rented buildings. Standard EPC assessments default to pessimistic air permeability values when no real test data exists – for two Bristol warehouses, an actual air test replaced those defaults and took one from a failing F to a high D, and the other from a near-miss B to a full A.

Most retrofit assessors in the UK aren’t qualified air testers, so this normally needs an additional supplier; we cross-train our staff so they can undertake both assessments and air tests in a single visit, reducing disturbance for residents and the overall cost of the project. Our typical approach across a retrofit programme is to pulse test every property to establish an air permeability baseline, blower door test a sample of archetypes for more detailed diagnostics, and carry out further post-installation testing once works are complete to verify the retrofit has performed as intended. We’ve delivered retrofit air testing on this basis for clients including the Centre for Sustainable Energy across Somerset and Wiltshire, and St Mungo’s and Runnymede Borough Council on Bell Group-led PAS 2035 programmes.

All new dwellings need an air tightness test to calculate the Dwelling Emission Rate as part of the mandatory SAP process, normally just before practical completion with the result determining much of the final EPC rating. We also offer preliminary air tightness tests, giving valuable data on potential leakage points before final testing to avoid costly retests, schedule delays and construction friction – as on Woodstock Homes’ Baily Court development, where a single early diagnostic test led to all nine plots passing first time.

All new commercial buildings have needed an air leakage test to comply with Building Regulations since 2003. Our engineers are experienced testing a wide variety of commercial properties – office blocks, accommodation blocks, schools, hospitals, industrial units, laboratories and more. On a large-scale new-build data centre campus in London, we used multi-fan blower door testing to certify each data hall against Part L requirements as it was completed in phases, keeping the construction partner’s commissioning schedule on track.

Yes – these tests are designed to meet the stringent standards set by the Passivhaus Institute. The process is more complex, with multiple stages, and each project requires at least three tests across the design and build phases. We recently delivered Passivhaus testing for The Sheds, a North Cotswolds new build featured on Channel 4’s Grand Designs as it aims to become the UK’s first residential Passivhaus Premium.

Yes – for any existing home where you want to improve energy efficiency, address comfort issues like draughts, reduce bills or prepare for future upgrades, an air test gives you a clear, evidenced starting point rather than a guess. For Bristol homeowner David Butler, a blower door test (funded through a Retrofit West voucher) revealed his home was leaking well outside the typical range for existing UK homes, giving him a prioritised list of fixes rather than a vague sense that the house was “just draughty.”

Yes – heritage and commercial buildings often need a non-invasive approach that won’t risk historic fabric or disrupt an occupied or trading building. For the Churches Conservation Trust, we used multi-fan air testing with depressurisation to establish an air permeability baseline for St Paul’s Church, a Grade I listed Georgian church in Bristol, without any invasive testing that could damage the historic stone and timber.

Isolation room air testing is undertaken in line with technical standard BTS 03 to validate air permeability in critical isolation facilities (healthcare, laboratories, data centres). Achieving low air tightness is vital in these environments for controlling airborne infections and maintaining design pressurisation levels.