A question that comes up on almost every job: what can you actually seal before an air tightness test, and what counts as cheating? The answer comes down to a single principle — and understanding it removes most of the uncertainty on test day.
Under ATTMA TSL1 — the current Technical Standard for measuring air permeability of dwellings, referenced alongside CIBSE TM23 (2022) in the Building Regulations — temporary sealing of intentional ventilation openings is permitted. These openings are designed to be controlled: they operate in a closed state in normal building use, and sealing them for the test reflects how the building performs when the ventilation system is managed correctly. What is not permitted is sealing the building fabric itself: gaps, cracks, and penetrations that should not be there.
The rule is the same regardless of building type
Yes — the principle is the same whatever the building type. Controllable ventilation openings can be sealed; building fabric cannot.
The standard changes depending on size: ATTMA TSL1 covers houses, flats, and smaller buildings; ATTMA TSL2 applies to larger and more complex buildings — schools, hospitals, offices, industrial units, and anything tested in sections. But in both cases, the question to ask about any opening is the same: can the people using this building open and close it as part of normal use? If yes, it can be sealed for the test. If no, it cannot.
In larger buildings there are more types of openings to consider — big extract louvres, commercial ventilation terminals, loading bay doors. The rule still applies to each one. A loading bay roller door, for example, is building fabric: its perimeter seal is there permanently and cannot be taped up to make it perform better on the day. A supply air terminal that can be opened and closed by the facilities team is a controlled opening and can be sealed.
Fire dampers and smoke dampers are a special case — these are safety devices, not ventilation openings, and are not sealed for the test. If you are unsure about a specific element, it is always worth a quick call to the tester before the day rather than a judgement call on site.
What you can seal
The following openings can all be controlled by the building occupant in normal use, so can be temporarily sealed for the test:
- Supply and extract terminals — including those served by HVAC systems, fan coil units, and mechanical supply air systems
- MVHR grilles — both supply and extract grilles on mechanical ventilation with heat recovery systems
- Trickle vents — background ventilators in window or wall frames; these must be sealed closed, not left open
- Closeable airbricks and hit-and-miss vents — only where there is a functioning closeable cover
- Fireplace dampers and throat plates — where a working damper or closure plate seals the flue completely
- Mechanical extract fan backdraught shutters — where the fan has a motorised or gravity-close shutter in working order
In all cases, temporary sealing must be documented by the tester and noted in the test report. If you are uncertain whether a specific element qualifies, ask before test day — it is easier to resolve in advance than mid-test.
What you cannot seal
The following cannot be sealed under any circumstances, as they represent the actual leakage the test is designed to measure:
- Gaps around window and door frames — if the frame-to-reveal seal is absent or failed, this requires permanent remediation before the test, not tape on the day
- Gaps around vehicle access doors — especially common in mixed-use schemes with internal garage access
- Service penetrations through the building fabric — holes where pipes, cables, or ductwork pass through external walls, floors, or ceilings should be permanently sealed before test day
- Gaps between construction types — junctions between blockwork and timber frame, slab edges, or structural elements
- Any building fabric defect — holes, cracks, or unfinished sealing that are not the result of a deliberate, controlled ventilation strategy
Common questions
Can you seal a letterbox?
A letterbox with a functioning self-closing draught excluder flap may be left as-is — the flap operates in a closed state in normal use. A letterbox with no effective seal is a fabric defect and should be fitted with a permanent draught excluder before the test, not taped on the day.
Can you seal a chimney or open flue?
Yes, if there is a functional closure in place — a throat damper, a chimney balloon, or a purpose-made sealing plate. An open flue with no functioning closure cannot be temporarily sealed: it requires a permanent draught excluder installed before test day. This is a common failing on plots that have a decorative fireplace opening installed without a damper.
Can you seal an open airbrick?
Only if it has a functioning closeable cover. A standard open-face airbrick providing background ventilation with no close-off mechanism cannot be temporarily sealed. It would need a closeable cover installed before the test.
What to do if defects are found on test day
On test day there is a temptation to tape every gap that is losing air — including genuine fabric defects. This is not permitted, and experienced testers will flag it. But more importantly, sealing a defect to get through a test does not solve the problem: the same gap will be leaking heat and moisture in normal building use, and the as-built SAP or SBEM compliance figure will not reflect reality.
If a defect is found during the test, the right approach is to note its location, stop the test, carry out proper remediation, and retest. On many residential sites this can be done the same day once sealant is applied — a partial retest over the affected area is often sufficient when the defect location is known. Air Tightness consultancy at design and pre-completion stage significantly reduces the chance of defects surfacing on test day in the first place.
Getting the building ready before test day
The best time to think about preparation is weeks before the test, not the morning of. A pre-test inspection walkthrough — working from a site-specific checklist — catches the majority of issues while they are still straightforward to fix: unsealed skirting perimeters, missed service penetrations, open back boxes, or incomplete masonry joints at window reveals.
If you are approaching a test and unsure about your site’s readiness, speak to the tester before the day. A brief conversation about specific details often prevents a failed test and a return visit. For a broader look at how the test works and what targets apply to your project, see our guide to air permeability testing.
