Airtightness and Blower Door Tests
How blower door tests measure air leakage, what the result means, how to find leaks, and what a good or poor result implies for retrofit.
The short answer
A blower door test measures how much air leaks through a home's fabric. A fan is fitted temporarily to an external doorway, all other openings are closed, and the fan depressurises or pressurises the home to a standard test pressure. The airflow needed to maintain that pressure shows how leaky the building is, expressed as air changes per hour or as leakage per square metre of envelope. A lower figure means a more airtight home.
The test does two jobs. It gives a number, allowing comparison with standards and with retests after draught-proofing, and it lets the tester find leaks, by feeling for air movement, using smoke or thermal imaging while the fan runs. The number alone is less useful than the number plus the leak locations, because leaks, not the average, are what retrofit work addresses.
What it means in practice
Preparation matters. Before testing, windows and external doors are closed, internal doors are opened, trickle vents and extract fans are sealed or noted, and heating and fires are dealt with according to the tester's instructions, because open flues and combustion appliances need care under depressurisation. The tester records building volume and envelope area, so the result can be expressed on a comparable basis, and notes wind and temperature conditions.
During the test, the tester will walk the home feeling for draughts at loft hatches, window and door frames, service penetrations, skirting boards, floorboard gaps and electrical fittings. Smoke pencils or thermal imaging can make small leaks visible. That leak hunt is often the most valuable part of the visit, because it turns an abstract leakage figure into a specific list of gaps to seal, with ventilation that must remain clearly identified.
Checks before you act
Check safety before testing. Tell the tester about open fires, stoves, gas appliances, and any flues, because depressurisation can affect flue operation. Check that the tester follows a recognised method and records conditions, building dimensions, and what was sealed for the test, so the result can be compared with a retest on the same basis.
Check what the result will be compared with. New homes in many jurisdictions must meet an airtightness standard, while existing homes have no single pass mark, and a result should be judged against the home's age, construction and ventilation strategy, not against a new-build standard alone. Ask the tester what a realistic improvement target is for this home, and what ventilation provision that target requires.
How this differs by place
In the UK, blower door testing is required for many new homes under building regulations, with results recorded as part of compliance, while for existing homes it is a diagnostic tool used in audits and retrofit projects. Testing methods and reporting follow recognised standards, but existing-home results are interpreted as diagnostics, not as pass or fail against a new-build limit.
In EU member states, airtightness testing is embedded in new-build compliance in many countries, with national standards and, in some cases, mandatory testing for all new homes. For existing homes, testing is used in audits and deep retrofit programmes, often alongside thermal imaging, and national programmes may set improvement targets for renovated homes that differ from new-build standards.
Mistakes that waste money or time
One mistake is sealing every leak found without a ventilation plan. A home made significantly more airtight needs designed ventilation to supply fresh air and remove moisture, and sealing without that provision can create condensation, mould and poor indoor air. Airtightness work and ventilation work are a pair, and the test report should address both.
Another mistake is comparing tests carried out on different bases. A test with trickle vents sealed, a test with them open, a pressurisation test and a depressurisation test can give different figures for the same home. Retests after work should replicate the original test conditions and sealing arrangements, or the comparison will reflect method change rather than building change.
Evidence to keep, and what to ask
Keep the test report with its method, conditions, building dimensions, sealing arrangements, result, and leak location list, with photographs where provided. That report is the baseline for retesting after draught-proofing, and it allows a future tester to replicate conditions for a fair comparison.
Ask the tester which leaks matter most, which should not be sealed because they provide necessary ventilation or appliance air, and what ventilation provision is needed if leakage is reduced. Ask what retest result would show success, and what work would achieve it. Those answers turn a number into a plan.
Sources and evidence on the record
- CIBSE, Chartered Institution of Building Services Engineers - professional guidance on heating, ventilation and audit practice.
- BRE Group, BRE - building science research behind UK assessment and audit methods.
- Energy Saving Trust, Home energy advice - independent UK guidance on audits, insulation and heating.
Evidence basis: Educational summary from the official sources below. It describes general audit practice only; a specific home needs its own audit, and no saving or performance outcome is promised. Applies when: You want to understand airtightness testing before commissioning one or reading a result