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Blocked Or Painted-Over Air Bricks
The vents low down in an external wall are blocked with soil, paint, insulation or a new patio.
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What's happening?
Air bricks are not decoration. On a house with a suspended timber ground floor there is a void under the floorboards, and that void needs a through draught to stay dry. Air bricks on opposite or adjacent walls let outside air cross the void, carrying away the moisture that evaporates from the ground beneath. Block them and the void becomes still, humid and cool, which is exactly the environment timber decay and furniture beetle need — the Property Care Association points to keeping timber dry as the practical control for woodworm, and a sealed sub-floor void does the opposite. This is one of the most common faults created by well-meaning work: a new path, a raised border, external wall insulation, a conservatory or a coat of masonry paint can each close the vents without anyone realising they mattered.
What usually causes it?
- A new patio, path or driveway laid up to or over the vents
- Soil, mulch or a raised flower bed banked against the wall covering them
- Masonry paint or render applied straight over the air brick, filling the slots
- External wall insulation or cladding fitted without sleeving the vents through it
- An extension, conservatory or porch built across an elevation, sealing the vents that served that side
- Vents deliberately blocked to stop draughts, mice or wasps getting in
- Leaves, debris or old cement rubble filling the void behind an apparently open grille
Is it dangerous?
Not an immediate hazard, but two consequences are serious and slow. First, timber decay: an unventilated damp void rots joist ends and floorboards from underneath, and the damage is invisible until a floor feels soft. Second, where a gas appliance draws combustion air through that void or through a vent on that wall, blocking it interferes with something safety-critical, and permanently sealed vents near an appliance are a Gas Safe question rather than a bricklaying one. Approved Document C treats keeping the sub-floor space ventilated as part of resisting moisture from the ground, so this is a designed feature that has been switched off.
Can I fix it myself?
Often, yes, and it is one of the best value jobs in the house. Walk the perimeter and find every vent, including ones now behind bins, decking or shrubs — count them on each elevation, because cross-ventilation needs vents on more than one side. Dig soil back from the wall, clear the slots with a screwdriver and a brush, and replace broken or paint-clogged grilles with new ones. Fit a plastic vent liner rather than mesh so the opening is not restricted. What you must not do is unblock a vent near a boiler, fire or flue and assume that is the whole story, or leave a vent open into a void full of rubble.
When should you call a bricklayer?
Call a bricklayer where the vents have been rendered or built over, where a patio or path needs cutting back and a duct forming through it, or where the wall needs new air bricks cut in because there are too few. Also call if the floor above already feels soft or springy, or if you can smell a musty cellar smell in the room, because the void will need inspecting as well as venting. Anything involving combustion air for a gas appliance goes to a Gas Safe registered engineer.
What will the bricklayer actually do?
- Survey every elevation to count existing vents and work out whether air can actually cross the void
- Cut in additional air bricks where the existing provision is too small or all on one side
- Form ducts or periscope vents through raised paving, render or insulation so a blocked vent works again
- Clear the sub-floor void of rubble and debris that is stopping air moving even where vents are open
- Lower ground levels back below the damp proof course where paving or soil has bridged it
- Report soft joist ends or decayed floor timbers found in the void rather than closing it back up
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