How many airbricks needed?

31 Oct 2010
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United Kingdom
Attached is a sketch of my extension built a year ago by not-very-good builders. Floor is suspended timber and I am unsure if there is oversite or a DPM.

Can anyone advise how many airbricks I should have? (Note the one on the left is old, and has always been there. May not even be under the new extension).

Also, how big a job is it to have new ones fitted? Thanks a lot.
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A response from JohnD to a similar request many years ago:

If that old airbrick is to a solum that has not been opened, e.g. by honey-combing the wall, then it's of no asistance to your extension. You do need some crossflow ventilation.

It's only a little job to install extra airbricks, just knock out one brick and cement in an airbrick, possibly with a duct across the cavity.
I have just built a similar extension and the spec was airbricks at 2 metre centres all round.
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As RedHerring2 has mentioned, putting in additional airbricks should be fairly straight forward. Just find out where your floor joists are first because you don't really want to be taking out bricks on the end of one. Ideally position them in the space between joists.

Your floorboard fixings should indicate where the joists are.

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