Air Bricks: What They Do and Why You Shouldn't Block Them

Air bricks are easy to overlook. They look like a spare hole low down in the wall, and on a lot of houses they get treated that way — rendered over, buried under a new patio, or stuffed with foam to stop a draught. On a house with a suspended timber ground floor, that's a mistake. The air brick is part of the building, not a leftover, and blocking it lets damp air sit under your floor with nowhere to go.

What Air Bricks Actually Do

Many older UK homes have timber joists and floorboards suspended over a void, rather than a solid concrete slab. Air bricks fitted low down on opposite external walls let outside air move through that void, carrying moisture away from the ground and the masonry before it can do any damage.

Not every vent low down on a wall is doing the same job, though. Some ventilate a room, a cellar, a gas appliance flue, or a masonry cavity rather than a floor void. If you're not certain what a particular vent is for, don't cover, alter or remove it until someone competent has checked it.

What Happens When They're Blocked

Cut off the airflow and moisture takes longer to escape from under the floor. Over time, that tends to show up as:

• Musty smells near the floor
• Mould below the boards or around cold edges of the room
• Soft or bouncy floorboards
• Timber decay in the joists and boards themselves

The air brick didn't cause the moisture — it was the route that helped the moisture get away. Opening a blocked vent back up can improve the airflow, but it won't repair a leaking pipe, a bridged damp-proof course, or timber that's already rotten. If you're seeing several of these signs together, get the underlying cause properly diagnosed rather than assuming the vent is the whole story. Our damp diagnosis tool is a reasonable place to start working out which type of damp you're actually dealing with.

How They Usually Get Covered

Almost nobody blocks an air brick on purpose. Most blocked vents have simply disappeared behind later work:

• Raised ground. A new patio, driveway or path has been laid above the old level, burying the vent or leaving it below ground.
• Render and paint. Thick coatings have bridged or clogged the slots without anyone noticing.
• Garden changes. Soil, bark, leaves, pots and raised beds have built up against the wall over the years.
• Extensions. New construction has cut off a vent without carrying the airflow through to the new external wall.
• Well-meaning "fixes." Foam, rags or dense mesh have been pushed into the opening to stop draughts or keep pests out.

Check Yours in Five Minutes

1. Walk the outside walls. Look low down, often below what should be the damp-proof course line, and note every air brick or vent you can see.
2. Check the openings. Clear loose leaves and cobwebs from the face by hand. Don't force tools deep inside — you can't see what's on the other side.
3. Look at the ground level. As a general rule, outside ground should sit around 150mm — roughly two brick courses — below the damp-proof course. The right detail varies by property, so ask a builder if you're unsure.
4. Check opposite sides. A floor void ventilates best with cross-ventilation — air moving in one side and out the other — not a single clear grille at the front and blocked ones everywhere else.
5. Take warning signs seriously. Musty smells, springy boards, staining, mould, or a floor that's started to dip deserve a closer inspection, not a rug over the top.

If Yours Are Blocked

Start by clearing loose leaves, soil and flaking paint from the face carefully, by hand. If paving or a driveway has been raised over the vent, a builder will usually need to lower the local ground level or fit a proper ducted or telescopic vent to carry the airflow through to a clear opening. Don't leave a small pocket in the paving that will simply collect rainwater against the wall instead.

Don't:

• Seal vents because the floor feels draughty
• Fill them with expanding foam or mortar
• Assume one new vent will solve an existing damp problem on its own
• Cut new openings without checking what services and structure are behind the wall first

If the floorboards feel soft, bouncy or are visibly sinking, avoid loading that area and get it inspected. Finding the cause early is what keeps the repair small.