Damp wall - caused by air brick at ground level?

9 May 2004
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United Kingdom
Hi all,

I have damp on my dining room wall. The paint is peeling off and the plaster is soft to touch.

The house was built in 1930 and the wall is a cavity wall which is currently not filled. I'd like to get the cavity insulated but clearly must resolve the damp issue first. The right hand wall that can be seen in the photo is a chimney breast and there is a tiny amount of similar damp on the opposite side of the chimney breast also (but barely noticeable).

Outside, the wall is South-facing with a narrow alley way to the back yard. There is an air brick right underneath the centre of the window and I believe that this may be the cause. As you can see in the photo below, the concrete pathway outside has been laid right up to the level of the bottom of the air brick so any standing water is almost certainly entering the floor void through the air brick.

I've lifted a floorboard and there are no obvious signs of damp, but wanted to get your views in case I've missed something obvious and what the best way might be to resolve this.

I'm thinking that cutting a channel alongside the wall and installing a linear drain would be the first thing to do. Is there anything else that I should do?


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The air brick is most unlikely to be the cause of your dampness.
You need to look for other things, such as non-existant/failed dpc; path level too high relative to dpc; cavity filled with rubbish near the bottom; leaking gutter etc.
Are you sure you have cavity walls not solid.Tony has covered everything so have a check.

If you external level is too high then rain water may be splashing against the wall making it damp.Failed Dpc will add to water being sucked in through the bricks.
It's nearly always condensation in the chimney breast.
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as you mention a similar damp patch on the other side of the chimney breast, is the chimney in use or capped?

rain water could be coming down the chimney and causing the damp.
The concrete in the alleyway is too high - water is penetrating above any DPC in-situ.

Maybe remove a few bricks and check the cavity for blockages?

Perhaps remove skirting either side of the c/breast and check for decay.

The DPC should sit below the joist tails.

This appears to have been a persistent difficulty because you can see that the brickwork has been painted with black bitumen in an attempt to prevent dampness.

The air-brick(s) needs cleaning out to allow full ventilation.

Examine your joist ends for fungal damage, likewise with the wood plate that the joists are sitting on.
Crawl the sub area if possible and examine all wood work esp. along that wall and chimney breast.

Chimney flue issues have been mentioned above - perhaps Search on here for similar issues and what remedies to take for flues and damaged plaster.

Until you have resolved your present difficulties you should be cautious about injecting cavity fill, and even then you should note all the pre-injection precautions. Search on here for details.
Thanks everyone for such comprehensive replies (as usual).

There appears to be a slate dpc 2 courses above the air brick so i don't think the problem is caused by splashback. Even if it was, wouldn't i see it elsewhere on other walls too?

There are no leaks in the gutter above here but water does tend to pool a little on the concrete path.

The wall is definitely a cavity wall but i'm not sure if it has excessive debris in it.

The chimney has a gas fire connected to it. The previous owners said it wasn't lined and I've never used the fire. I'm not sure what kind of capping is on the top, so rain ingression is certainly a possibility. No damp is visible on the face of the chimney but I suppose that may be due to how it runs down the inside of the chimney.

This is starting to sound well beyond my capability to remedy so I think it's time to get someone in to look at it. Any advice on what to ask to avoid having lots of unnecessary work done that won't help fix it? I'm not so worried about the cost, more about resolving the problem rather than just masking it.


For God's sake whatever you do,do not invite any damp proofing company in the house.They will make the whole job look like it is a massive problem.Your best bet would be to ask a local builder to have a look at it and he might be able to guide you in the right direction..

You have lot of useful advice here so start with the least expensive first.Most of the time the problem is caused by something very small which is overlooked.How about capping the chimney's and removing the plaster which will be required regardless what action you take.Good luck.
It's got nothing to do with the capping. It's condensation in the chimney.
I've taken a closer look and the chimney is vented at the bottom and the top. The bottom vent is clear but I can't tell about the top without getting onto the roof for a closer look.

Contrary to what the previous owners told me, the chimney is lined as I accessed it via the (external) sweeping point. There was a lot of dust/fine rubble here which I've now cleared but it was bone dry and looked like it had accumulated over a very long period.

The floor void is too small to crawl but from lifting floorboards i cannot see any evidence of rot on the joists.

I'm feeling clueless :oops:
How many times do I have to tell you. It's condensation.

Go out and look at end terraces with a chimney and you'll find loads of examples. Just do it.
Thanks Joe, I'll buy the diagnosis but how do I fix it? All the research seems to point to venting the chimney, but that's already done. So what am I missing?

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