How do I rodent-proof airbricks and pipe entries without blocking ventilation?
The two details that account for most rodent entry into a house, done properly: a meshed airbrick that still ventilates, and a service pipe sleeve that is actually sealed.
- Difficulty
- intermediate
- Time
- 2 hr
- Read
- 4 min
- Safety
- warning
Short answer
Cover airbricks with a fine stainless steel mesh guard — never filler — keeping the free air area as close to the original as possible. For a pipe entry, pack the annulus tightly with stainless or copper wool, then finish with mortar outside or intumescent-grade sealant where it passes through a fire compartment.
Two details on a house account for a large share of rodent entry, and both are usually left as the original trade finished them. An airbrick is deliberately a hole, so the fix has to keep the airflow. A service penetration is a hole drilled oversize, and the ring of space around the pipe is a permanent open door until someone fills it properly.
Safety
What you'll need
- Stainless steel mesh airbrick covers, or a roll of fine stainless mesh
- Stainless steel or copper wool
- Exterior-grade sealant, or mortar and a small trowel
- Masonry drill, plugs and stainless screws
- Tin snips and gloves
- Expanding foam, only as a backer (optional)
Step by step
- Identify what every vent is for before you touch it.A subfloor airbrick ventilates a suspended timber floor; an appliance vent supplies combustion air. Blocking either causes serious problems, and blocking the second can be lethal. If in doubt, do not restrict it.
- Clear the airbrick before covering it.Rake out cobwebs, leaves, soil and old paint. A blocked airbrick is already causing damp, and clearing it is worth doing whether or not you have mice.
- Fit a mesh cover, not a solid one.A purpose-made stainless steel airbrick guard, or fine stainless mesh cut oversize and screwed to the masonry through plugs. Aperture of roughly 5mm or less stops mice while keeping most of the airflow.
- Fix into the mortar joints, not through the airbrick.Drill the surrounding mortar and use stainless screws, so the airbrick itself is not cracked. Stainless matters — plated screws rust and stain the wall within a season.
- Check the free air area afterwards.Mesh reduces airflow. Where a floor void is already marginal, fit larger guards or add an extra airbrick rather than accepting a restriction — damp is a slower but more expensive problem than mice.
- Move to the service pipes and expose the gap.Rake out crumbling mortar, old foam and dried mastic from around the pipe until you can see the actual hole. Working on a half-filled gap achieves nothing.
- Pack the annulus with wire wool.Push stainless or copper wool tightly all the way round the pipe and as deep into the hole as you can. Tight is the operative word: loosely stuffed wool is simply pulled out.
- Finish the outside face with mortar or sealant.Mortar for a masonry wall, an exterior-grade flexible sealant where the pipe moves or is plastic. The finish holds the wool in place and keeps the weather out — it is not the barrier itself.
- Do the inside face too.The same pipe usually has a gap where it emerges into the cupboard, the floor void or behind the boiler. Sealing both ends stops movement inside the fabric of the building.
- Use the right sealant where fire compartmentation matters.A penetration through a compartment wall or floor — in a flat, or between a garage and a house — needs an intumescent or fire-rated sealant. Ordinary silicone is not a substitute there.
- Check the waste and soil pipes for their own gaps.The hole behind a soil pipe boss, the gap where a waste pipe leaves the wall, and an unused, uncapped waste stub are all routes. Cap anything redundant properly.
- Inspect annually.Mortar cracks, sealant perishes and mesh corrodes if it is not stainless. Add it to the same round as clearing the gutters.
Tips
- Stainless mesh costs a little more than galvanised and lasts many times longer on a wall that gets wet.
- Photograph each penetration before and after. It makes the annual check a two-minute comparison.
- If a floor void smells musty after meshing the airbricks, the free air area is now too low. Add vents rather than removing the mesh.
Common mistakes
- Filling an airbrick with mortar or foam — It stops the ventilation the airbrick exists to provide, which causes rot in a suspended floor and, where a fuel-burning appliance relies on it, creates a carbon monoxide risk.
- Using expanding foam alone around a pipe — Rodents chew through it easily and often nest in it. Foam is only a backer behind packed wire wool, never the seal.
If it doesn't work
Mesh guard rusted and stained the brickwork
Cause: Galvanised or plated mesh and fixings — Fix: Replace with stainless. The staining can be reduced with a proprietary brick cleaner, but prevention is the only real answer.
Mortar around a pipe keeps cracking out
Cause: The pipe moves with temperature and mortar cannot flex — Fix: Use a flexible exterior sealant over packed wire wool instead, or a proprietary pipe collar.
Damp smell under the floor after meshing
Cause: Free air area reduced too far, or the airbricks were already partly blocked — Fix: Clear every airbrick fully, use larger-aperture stainless guards, and add extra vents if the void is still under-ventilated.
Questions people ask
What mesh size stops mice but still ventilates?
Apertures of about 5mm or smaller stop adult mice while retaining most airflow. Finer than that reduces the free air area significantly and clogs with dust, so it is a balance rather than the finer the better.
Can I just put a plastic airbrick cover on?
Plastic vent covers are for ventilation, not rodent proofing — rodents gnaw them. Stainless steel mesh over the top is what does the work.
Does this help with draughts and heat loss?
Sealing service penetrations properly does reduce draughts and improves comfort noticeably. Airbricks must stay open, though, and should never be sealed for warmth.