Some years ago we had a problem with mice getting into our property through the under floor air bricks. We had them covered with a fine stainless steel wire mesh which has been highly effective - no more problems. However I’m now starting to worry that the mesh may be restricting ventilation of the subfloor void. It is very fine, 1mm holes. There appears to be conflicting advice on line about this. The mesh is clean and not blocked with debris so the question is whether or not it is likely to be preventing sufficent airflow to the under floor void. Thanks.
Hi
I wouldn’t worry
If there are no damp smells at that level and the subfloor is dusty it’s fine.
You can put bigger mesh on there even up to 3mm it would be fine.
Any bigger might invite bees or wasps.
I think if the brick is a flat holed version it will give more flow than the angle slatted type. 
Hi thanks. There aren’t any damp smells but i don’t know what it looks like under the floorboards re dust etc. It’s a 1930’s bungalow. I think the airbricks are the flat ones. I did buy some larger hole mesh with 4.5mm holes to possibly replace the finer ones. This would geve better airflow but from what I’ve read on line it may allow ingress of juvenile mice or smaller adult mice so from that point of view I’m not happy. The existing stuff has been in place for around 8 years or so now and there aren’t any obvious problems so I’m inclined to leave well alone as you suggest.
I’m sure you are fine.
Nothing to get in and no smell so great.
You’d be surprised how much of a flow there actually is and the better the floor is sealed and insulated it creates a good venturi underneath.
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The size of the mesh just needs to be sized to stop what ever it is you want to stop. In this case mice. So, why not replace with a mesh with holes that are a bit smaller than a mouse size?
On line recommendations on hole size of mesh to stop mice vary, with no definitive size as far as I can see. It’s confusing and it seems that mice, particularly juveniles, can squeeze through unfreasably small holes. Hence my reluctance to go for a larger hole size. However, I would obviously do that if I thought the currently fitted mesh was causing a ventilation issue.
I wonder if you’re referring to the rules regarding ventilation for woodburning stoves and the like. These must be used, and you are not allowed to add any mesh or other restrictions because they may become clogged with windblown leaves, cobwebs or other debris, potentially resulting in fatal buildup of toxic fumes or carbon monoxide. Many people disregard this rule and block the hole anyway once they discover that the hole provides free access to their house for rats, mice, wasps etc. and that the howling gale that blows through it removes all the heat your stove produces.
As far as I’m aware there is no such strict regulation covering airbricks which provide ventilation beneath suspended floors. I think it’s still important to ensure that these are maintained and mesh can certainly increase the chances of them becoming blocked.
You’re overthinking this ……
Is the mesh an approved rodent barrier type.
If it’s not, then get correct mesh fitted.
Properly installed 2mm–5mm stainless steel mesh for use with air bricks is designed to provide sufficient airflow.
Then all you need to do is periodically check and clear/clean debris
Thanks. Yes I know the rules about vents with boilers, stoves etc. and there is clear advice not to ever fit any kind of mesh over such vents as you say. It’s just that there doesn’t appear to be any definitive guidance on the best mesh to fit on sub floor airbricks regarding ventilation. Obviously any mesh must be kept clean to avoid blocking airflow.
Hi Mike. Thanks - yes I probably am over thinking it. The 1mm mesh I’ve fitted is designed for airbricks to prevent rodent access and it has worked perfectly in that regard. I checked all vents the other day and they are all clear from any dirt or debris. The only thing I found is that one at the front seemed a bit gummed up with paint that the painter had (stupidly in my view) put on when he painted the house. I hadn’t noticed this before as I confess I hadn’t ever given it a close look (too concerned about hi-fi!). I used a wire brush to clean off the paint and it seems much better now. So perhaps I’m worrying needlessly - as I know I am inclined to do with many things. I think this is partly a knock-on effect of retirement. Now being much less busy there is lots more time to think about things - which isn’t always beneficial.
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