Damp proofing around windows

Rowan S-Bradley · 5 Aug 2018

In my house (which was built 25 years ago) the windows have been fitted so that part of the outer leaf of the cavity wall is exposed inside the window (plastered over, of course). We have had damp coming through around the windows, which I have assumed is because the outer leaf of the wall is damp from rain etc., and there is nothing to stop this coming through inside the windows through the plaster. We have just had all the windows replaced, and one of them needs the plaster around the window replaced.

It seems to me that if we just have the plaster repaired, we will again get the same damp problem. Questions:

Should the window be fitted so that part of the outer leaf of the wall is exposed inside the house?
If so, what damp proofing (if any) should be used?
What can I do now, before plastering, to prevent ingress of damp? Is there some kind of spray-on or paint-on damp proofing that could be applied before plastering (and then plastered over)?
Or am I worrying about a problem that will not occur (like my plasterer says)?

Thanks - Rowan

stuart45 · 5 Aug 2018

Is your window almost flush with the external skin so that you have no outer reveal?

bobasd · 5 Aug 2018

1. no.the inner leaf should returnto meet the outer leaf. but cant tell from your photo.show photos of full inside an fulloutside reveals.
some dont return an a cav closer is installed.

2. you have a vertical DPCin the photo.

3. wait for photos

4. strong possibility of happenin again - wait for photos

Rowan S-Bradley · 6 Aug 2018

"The window is about 14mm in from the surface of the bricks, and a plastic trim has been fitted to the join between the PVC and the bricks, about 7mm think, so it is about 7mm from the brick surface to the surface of the plastic trim. As shown in the first photo above, there is an inch or more of the outer leaf of the wall exposed behind the plaster to the inner reveal.

Yes, I know there is a DPC there, but I can't see how to use this or to extend it to stop the passage of damp through the outer leaf bricks and through the plaster to the surface inside the room. Unless it can be painted on the inside with some kind of silicone or bitumen sealer. And if I use bitumen, will it bleed trough the plaster and the paint?

Thanks - Rowan

bobasd · 6 Aug 2018

the frame in the photo has been fixed in a well wrong position.its to far forward.

cut back any plaster on the inner leaf reveals,more or less like youve done,for about another 25mmback. to keep the cavity clear of bridginplaster.

nock off any plaster on the outer leaf, like you have done.
then use lengths of trim to cover the gap.
but first pack up the trim on the outer leafwith plastic off-cuts to stay on a level plane.
you could run a bead of silicone where the framemeets the brick?it may help a bit.

you didnt show the inner reveal in full depth so i cant say much about if thers any insulation in ther.
but the inner leaf looks maybe its returning to the outer leaf as it should.
shine a light into the cacity an look for cav insulation, cav ties an anycav blockages.

Rowan S-Bradley · 6 Aug 2018

Here is the full width of the inner reveal:

I have tried to look into the cavity, but to do so would require me to remove a lot more plaster, which I am loath to do.

Is there an authoritative document saying how windows should be mounted in cavity walls? I am going to have to fight the windows installer over this, and unless I can produce something authoritative, he will probably say he knows a lot more about installing windows than I do, and that he did it correctly.

Re fixing the problem, is there no way to do this with plaster, rather than plastic trim? It seems to me that it ought to be possible to paint the exposed brickwork in the outer leaf with some kind of damp proofing, maybe silicone or bitumen, and prevent the ingress of moisture in that way.

Thanks for all your help - Rowan

stuart45 · 6 Aug 2018

Looks like a 100mm cavity, probably insulation boards on the inner skin leaving 50mm clear cavity, which would be typical for it's age.
Don't know why the fitters put them so far forward leaving part of the inner reveal exposed to damp.
When timber windows were built in as the walls were built the 6inch DPC was fixed to a groove in the frame and the frame was set flush with the back of the facework.

bobasd · 6 Aug 2018

FENSA guide based on Buid Regs says 75mm set-back from face of buildin is needed min.
but your old frame were probly set in that same photo position so the fitter just copied an stuck on a length of silltrim to make it respectable?.

dont use paints an bitumens or youl give youself more problems in long run.

Rowan S-Bradley · 7 Aug 2018

>but your old frame were probly set in that same photo position so the fitter just copied an stuck on a length of silltrim to make it respectable?

Yes, that is exactly what I think happened.

>FENSA guide based on Buid Regs says 75mm set-back from face of buildin is needed min.

How can I get a copy of this FENSA guide? Or can someone point out where in the building regs it says this?

Thanks - Rowan

josejaspy · 10 Dec 2022

Hi Rowan,

I realise this is an old thread, but did you ever come up with a solution to this? We're having the same issue with one of our windows and are keen to hear if you found a fix that worked.

Thanks,
Joe

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Damp proofing around windows

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