Blocking Vent into Cavity Wall (w/Blocked Air Brick) - Advice Needed!

16 Aug 2019
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United Kingdom
Hi all,

Just had our upstairs box room re-plastered - old vent taken off, so now an ugly hole into a void... it's a 1940s property - cavity wall - (filled with hay/straw?) and there is an air brick in there.

The air brick appears to be blocked from the outside - can't see through it! - perhaps when it was re-rendered by the previous owner? Anyway, there is a draught that you can feel coming from the hole... I guess from the eaves?

My question is, is it OK to block off the hole in the wall (inside), as we can vent the room via the windows and is it OK for there to be a draught in the cavity wall or would that create damp... my guess is that it would be OK as the rest of the outside wall will no doubt be constructed in the same way and they don't have any wall vents (no damp present and survey done).

Previous owners had stuffed the inside vent with something - may have been more hay - so by blocking it off we're essentially back to how it was. (Not keen on the idea of cavity wall debris blowing into the room!)

Thinking of doing a temp fix and monitor for any issues with blocking it.

Any thoughts? My first post, so go easy! ;)
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Basil_the_Brush, good evening.

if no one else does, welcome to the board with your first post.

A draught from a cavity is not uncommon and indeed to be welcomed, air flow will assist in the removal of any dampness and condensation, not to mention rain driving through the external leaf of the wall. Down side of air movement in the cavity is that heat can and will be [slowly] removed from the inner leaf? Air flow is probably down to the walls cavity being open direct into the loft area??

The straw is odd, but as you mention probably from previous owners efforts, not an altogether good idea, straw tends to absorb moisture??

As for filling the internal surface, yes go for it, the ventilation to the room is achievable Via. the window.

Those in my house were blocked up mid-1980's, on the inner leaf. I don't remember how they blocked, I suspect they were vent removed and bricked up, but all walls were replastered. Mine was built in the latter days of coal fires and coal fires need good ventilation.
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Hi Ken and Harry - Thanks for your replies... yes, I think the draught is to do with the loft, so I'm pretty sure that blocking the interior wall will be fine as the air can circulate around the cavity and loft anyway... on closer inspection it appears to be twig like stuff in the cavity - wicker? I was told straw/hay but it doesn't seem to be. Probably will do so on a temp basis just to make sure and stick a 'fake' vent to cover my bodge! Cheers!

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