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An air tightness test measures how much air leaks through the fabric of a building, so that Building Control, a SAP assessor or a retrofit coordinator can see the number they need. Inter Infinity carries out blower door and Pulse tests and background ventilation assessments across London and Kent, with the certificate lodged with the scheme. Prices are per dwelling, and you can book a date through the form below.
What an air tightness test costs
| Test | What it is for | From |
| Air tightness test (blower door) | New dwellings and conversions for Part L; SAP; PAS 2035 retrofit | £195 |
| Pulse air tightness test | Occupied and retrofit homes where a low-pressure test is wanted, before and after window or door replacement | £325 |
| Background ventilation assessment | Part F after window or door replacement; PAS 2035 ventilation checks | £275 |
One price covers the visit, the test, the certificate and the lodgement. Multiple plots on one site are priced per dwelling on one visit, so ask for a site rate when you book. Retests after a failed result are priced below.
Book a test or ask for a site rate
Which test do you need
Both tests measure air permeability, the volume of air that leaks through each square metre of the building envelope per hour. The air permeability test and the air tightness test are the same test under two names. The difference is how the pressure is applied.
| Blower door test | Pulse test | |
| How it works | A fan in a doorway pressurises and depressurises the building to 50 Pa and measures the airflow needed to hold it | A compressed-air unit releases short pulses inside the building and measures the response at about 4 Pa, then converts the result to the 50 Pa figure |
| Accepted for | Part L testing of new dwellings, SAP, PAS 2035, conversions | Part L testing of new dwellings under CIBSE TM23 (2022), PAS 2035, before-and-after retrofit checks |
| Suits | Any building that can be sealed for the test; the method most Building Control officers have seen | Occupied homes and retrofit work, where sealing the building and running a fan is disruptive |
| Time on site | About an hour for a house | About 30 minutes for a house |
| Limit | None in practice | Less reliable for very leaky buildings, where the conversion to 50 Pa is less certain |
Both methods are set out in CIBSE TM23 (2022), which Approved Document L cites as the test method, so either produces a certificate Building Control accepts. If your SAP assessor or coordinator has specified one method, use that one.
When a test is required
Air testing for new builds
Every new dwelling in England is pressure tested before completion under Approved Document L, volume 1. The limit is an air permeability of 8 m³ per hour per square metre at 50 Pa. The design figure in the SAP calculation is usually lower, between 4 and 5, so the building has to match its design, not only the limit. The 2026 edition of the Approved Document applies to building regulations applications from 24 March 2027; the 2021 edition, as amended in 2023, applies before then. The limit is the same in both.
Conversions
A conversion that creates a new dwelling is tested on the same basis as a new build.
Window and door replacement
Replacement windows in an existing home come under Part F. Approved Document F, volume 1, requires the home to be no less ventilated after the work than before, which in practice means background ventilators in the new windows at the sizes the document sets, or evidence that the home already has enough background ventilation. A background ventilation assessment provides that evidence and can show that trickle vents are not needed where the home is already leaky enough.
PAS 2035 retrofit
Retrofit projects under PAS 2035 use airtightness and background ventilation testing to check the home before and after measures are installed. Elmhurst’s retrofit guidance requires both tests to be lodged with the scheme, carried out by a registered tester to CIBSE TM23 for air tightness and to the IAA method for background ventilation.
What result you need
The number you need is on your SAP calculation. The SAP assessor models the dwelling with a design air permeability, and the test has to come in at or below it. A tested figure above the design figure means the SAP is recalculated, and if that takes the dwelling over its target, the fabric has to be improved and retested. Inter Infinity also carries out SAP calculations, so the test and the calculation can be done by the same firm.
Where buildings leak
A test finds the leakage paths with smoke or a thermal camera while the building is under pressure. On new builds the usual ones are the loft hatch, service penetrations where pipes and cables pass through the ceiling and walls, the boxing behind kitchen units and baths, window and door reveals, and the junction between floor and wall. On retrofit work it is more often the floor, the chimney and old vents. Each is sealed with the right material for the junction, and the report lists what was found and where.
Preparing for your test
The test is done when the building is airtight by design, which for a new build is after plastering, with windows, external doors, loft hatch and all service penetrations in place, and before the final fix hides them. Before the visit:
- All external doors and windows fitted and closable, with seals in place.
- Trickle vents, extract fans and flues fitted so they can be temporarily sealed for the test.
- Traps in sinks, basins, baths and toilets filled with water, or the waste pipes capped.
- Loft hatch fitted and closable.
- Mains power available for the fan, or tell us if it is not.
- The floor area and envelope area from the drawings, which set the figure the result is calculated against.
If the test fails
A failed test is a number above the design figure. The tester shows you the leakage paths while the building is under pressure, so sealing can start the same day. A retest on the same visit, once the sealing is done, is charged at the retest rate; a return visit is charged as a new test. Most failures on new builds are at the loft hatch and service penetrations and can be sealed in hours.
Background ventilation assessments
A more airtight home holds moisture as well as heat. Without enough background ventilation, condensation, damp and mould follow. A background ventilation assessment measures the air leakage of the home at the low pressures normal weather produces and compares it with what Approved Document F requires, so you know whether the home needs background ventilators and how much. It is the evidence Building Control asks for after window replacement, and the ventilation check a PAS 2035 coordinator needs before measures are installed.
Check your tester
Inter Infinity is registered with the Elmhurst Airtightness Scheme, one of the two certification schemes for air tightness testers in the UK, and for background ventilation with the Installation Assurance Authority, whose method the assessment follows. You can look us up on the Elmhurst Airtightness Scheme tester search and the IAA approved assessors list, and check any certificate we issue on the scheme’s validation page. A certificate from a scheme-registered tester is what Building Control and a PAS 2035 coordinator accept, and the certificate is lodged with the scheme after each test. The firm is also TrustMark registered.
What happens after you book
We confirm the date, the method and the price by email, and ask for the drawings. The tester arrives at the agreed time, prepares the building, runs the test and shows you any leakage paths before leaving. The certificate and report are lodged and sent within two working days of the test.
Sources
- Approved Document L, volume 1: dwellings, 2021 edition incorporating 2023 amendments, and the 2026 edition applying from 24 March 2027, gov.uk
- Approved Document F, volume 1: dwellings, gov.uk
- CIBSE TM23: testing buildings for air leakage (2022)
- Standards for airtightness and background ventilation testing for retrofit, Elmhurst Energy, 3 December 2024
