What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Close, not identicalairbrick
also air brick, telescopic vent, periscope vent, subfloor vent
A TELESCOPIC or periscope vent is the version that carries the opening through a cavity wall and up over a raised external level, which is what the plain unit cannot do.
Australia
Close, not identicalsubfloor vent
also air vent, airbrick, vermin-proof vent
Vermin-proofing is a standard requirement rather than an option, and the required free area is prescribed against the subfloor's plan area.
United Statesyours
Same word, different thingfoundation vent
also crawlspace vent, automatic vent, flood vent
A FLOOD VENT looks similar and does something else entirely: it lets floodwater in and out of an enclosure so hydrostatic pressure does not collapse the wall, and it is sized by enclosure area under flood rules.
Canada
Close, not identicalcrawlspace vent
also foundation vent, air vent
As the United States, and frequently omitted entirely where the crawlspace is sealed and heated instead of ventilated.
The governing standard, by market
United Kingdom
Approved Document C / BS 5250
Resistance to moisture including the ventilation of under-floor voids, and management of moisture in buildings.
United States
IRC R408.1 / ASCE 24
Under-floor ventilation openings and their required net area, and flood-resistant design where flood openings are required instead.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Why does blocking one rot a floor?
- Because the void under a suspended timber floor has a continuous supply of moisture from the ground and only one way to lose it. Water evaporates from the earth beneath the floor all year; the airbricks let outside air pass through and carry it away, and the flow depends on there being openings on at least two opposite sides so air crosses rather than stalls. Block them — with a raised patio, a render coat, a new path, a flower bed, an extension across one elevation, insulation pushed against them from inside — and the humidity in the void rises to the point where timber takes up water faster than it dries. Rot follows over a few seasons, and it starts at the joist ends where they enter the wall, which is the last place anyone looks. It is the single most common way a sound floor is destroyed by an improvement.
- What is the free area, and why is it not the hole size?
- Free area is the actual open area for airflow, after the perforations, the grille and any mesh are taken off the nominal size — so an airbrick's free area is a fraction of the rectangle it occupies, and two units of identical external size can differ substantially. Guidance is expressed as free area per length of wall, or as a proportion of the floor area, which is why a specification counts free area rather than counting bricks. It also explains a common failure: replacing a clear old cast-iron airbrick with a fine-mesh plastic one for vermin-proofing can cut the free area sharply, and doing that to every opening on a house reduces the void's ventilation without a single opening being blocked.
- What does a telescopic vent do that an ordinary airbrick cannot?
- Two things, and both are about geometry rather than airflow. It carries the opening ACROSS a cavity without letting the cavity connect to the void — a plain airbrick in a cavity wall opens into the cavity, which both loses the airflow and lets cavity moisture into the floor void. And it carries the opening UP, so the external face can sit above a raised ground or patio level while the internal face stays at the void. That second use is the common one on an older house, where paths and patios have been laid over decades and the original airbricks now sit at or below the new external level. Fitting one is the correct answer to an airbrick that has been buried; digging a small trench in front of the old one is the answer that fills with leaves.