What are air bricks for, and why must underfloor ventilation stay clear?
How cross-ventilation under a floor actually works, the everyday things that stop it, and why blocked air bricks are one of the most common causes of rot in British houses.
- Difficulty
- beginner
- Time
- 8 min
- Read
- 5 min
- Safety
- caution
Short answer
Air bricks let air enter the void under a suspended floor on one side of the house and leave on the other. That moving air carries away moisture evaporating from the ground. Blocking them, rendering over them, planting against them or raising the ground level above them stops the flow, and the timber over a still, damp void rots. Clear them, do not close them.
Air bricks look like a draught problem and get treated like one. They are the ventilation system for a part of the house nobody sees, and the damage from closing them takes years to appear and thousands to put right, which is exactly why the mistake keeps being made.
Safety
Step by step
- Work out whether you have a suspended floor at all.Air bricks below floor level on an external wall are the giveaway. A solid ground floor has no void to ventilate, and vents in that wall are serving something else — often an appliance.
- Walk the outside of the house and find every one.Count them and note where they are. Cross-ventilation needs openings on opposing walls, so a run of air bricks along the front and none at the back is a problem in itself.
- Check each one is actually open.Clay air bricks silt up, plastic ones fill with cobwebs and leaves, and both get painted over. Holding a hand or a tissue to one on a windy day tells you whether anything is moving.
- Look at the ground level in front of each one.A raised path, a new patio, a flower bed or gravel piled up the wall will submerge an air brick. The vent needs to sit clear above the outside ground, and the ground should also fall away from the wall.
- Look for internal obstructions.Where the void is deep or the wall is a cavity, the air brick is connected across with a duct or a telescopic vent. If that has been disturbed, the outside vent can look perfect while nothing reaches the void.
- Check that internal walls have not sealed the void.A later solid wall built off the oversite, or a sleeper wall rebuilt solid, splits the void into compartments that have no way out. This is why rot is often confined to one room.
- Restore rather than replace.A blocked air brick usually needs clearing, not replacing. Where one has been rendered over, cutting it back open is straightforward. Where one is missing entirely, adding vents is a real improvement and a job with a definable scope.
Tips
- The cheapest damp survey you will ever do is a walk round the house counting air bricks and looking at what is in front of them.
- If cold draughts through the floor are the actual complaint, the fix is sealing the gap between the floorboards and the skirting inside, not closing the vents outside.
- Vermin guards and mesh covers are fine as long as they are the right free area and are cleaned. A fine mesh that clogs is worse than no mesh.
Common mistakes
- Blocking air bricks in winter to keep the house warmer — It makes almost no difference to the room and it stops the ventilation the floor structure depends on. Sealing the perimeter inside achieves the comfort without the rot.
- Laying a patio or path up to the wall over the air bricks — It drowns the ventilation and often bridges the damp proof course as well, so you get a damp wall and a rotting floor from the same job.
- Assuming any grille in the wall is an air brick — Some are combustion air vents for a boiler, stove or fire. Blocking one of those is a carbon monoxide risk rather than a rot risk, and it is a much faster kind of harm.
If it doesn't work
Air bricks are below the level of a new patio
Cause: The external ground level has been raised over them — Fix: The correct remedy is to lower the ground or form a drained channel so the vents sit clear. Extending vents upwards through the wall is sometimes possible but it is a design decision, not a bodge.
Vents on one elevation only
Cause: Original construction, or later work that removed them — Fix: Air needs a way in and a way out. Adding vents on the opposing wall is what restores cross-flow, and it is a job worth specifying properly rather than adding one more vent to the side that already has them.
An air brick has been rendered or painted over
Cause: Redecoration that treated it as a blemish — Fix: Cut and clear it back to full opening. Then check inside the void for what the years of no ventilation have done to the nearest joist ends.
Questions people ask
How many air bricks does a floor need?
It is a free-area requirement per length of wall rather than a count, set out in the building regulations guidance, and it depends on the floor and the layout. Practically, what matters is that air can enter on one side and leave on the other, and that every opening is clear.
Can I fit a cover to stop draughts and mice?
Vermin mesh is normal. Anything that closes an air brick, including a hit-and-miss vent turned shut or a fitted cover, is not. Mice come in through much bigger routes than a properly guarded air brick.
Do air bricks cause damp?
The opposite. They remove moisture from under the floor. Damp near an air brick is more often driving rain or a bridged damp proof course, and the answer is to find that cause rather than to close the vent.
What is a telescopic vent?
A duct that carries the ventilation across a cavity wall from the outer air brick to the inner one, keeping the path open and stopping debris falling into the cavity. If the outer vent is clear but nothing is moving, the telescopic unit is a good suspect.