Correct caulk to use internally around uPVC unit

[Dorset Dabbler]

I am looking for informed advice on the right product(s) to use around a uPVC window unit to bridge the gap between its frame and the plasterwork of the window reveal.  Here's the situation:

 

House is a 1952-built bungalow. As part of a general major renovation, the owner before us replaced the original wooden windows with white uPVC - this would be 10 years ago - employing builders to do the work.  We have lived here 8 years, and for probably the last 5 there has been an increasing problem with staining and then black mould on the window reveal around a narrow strip (1/2 inch - 13mm - ish) next to the window. 

Here and there the staining leaches into the plaster too.  It seemed clear that the wrong filler had been used to seal the internal gap between window and plaster and so this year I dug some out to have a look; it was indeed slightly damp, according to my multi-meter (but not to the touch) and so was the exposed mortar and brick behind it, but after a day or so of exposure to air it was dry again.  The filler was ordinary grey powdery mortar, and was applied directly to the outer skin of bricks surrounding the window.

Despite not being Sherlock Holmes I was able to deduce that the mortar was acting as a wick to the slightly damp outer skin of the house (is this right?) and that the artisan who installed the windows has bridged the gap by (a) not installing any sort of membrane and (b) using the wrong filler. (Again - is this right?).

 

So, O wise people of the DIY forum,  what product should I use to replace the mortar when I have dug it all out, please?

 

 

[scottishjohn]

yes that would be damp you are seeing -- so you have to go to the outside first and check there -the damp will be coming from the outside 

does this house have a cavity wall?,should have at that age 

 air bricks clear -can you see the dpc on the outside  

 timber floor  or concrete

can you get under it?

pictures of outside?

[Oz07]

I see screwys or toolstation doing an overpaintable siliconised caulk. No idea what it's like but sounds good for something like that

[Dorset Dabbler]

Nothing untoward on the outside. The house is rendered from the windowsill level up, bare bricks below. The PVC unit is well sealed to the render, and the render is apparently sound onto the brickwork. The DPC is clearly visible and not bridged. The air bricks are good, and apart from this one location (on several windows) the house exhibits no other evidence of damp whatever. So yes, the outer skin of bricks is (very slightly) damp when it is wet outside, but isn't this just the natural absorption of damp from the air - which is why there's a cavity?  Happy to be put right if this is not the case.

[ProDave]

If this had been fitted by a window company, they would not have filled that gap with filler. instead the usual practice was to stick a strip of white upvc trip over the missing plaster.

 

You may not like that and think it is a cheap detail, but it does work.

 

I would look at doing similar though perhaps with something better than a bit of white upvc.

[Russell griffiths]

Are you looking for a cure, or just want to cover the gap and make it pretty to look at. 

[Temp]

Has the cavity  been filled with insulation? Sometimes that can cause damp to bridge across. Another source is a corroded or badly installed intel above the window. It should divert any water on the inside of the outer leaf to the outside, typically through weep holes on a modern house. Sometimes if these are blocked or missing it runs left and right then over the ends and down the side of the window frame into this area. Bit hard to tell what's going on without window out and/or a camera into the cavity.

 

Think I'd look at a damp resistant plaster or perhaps expanding foam and a uPVC cover strip as others have mentioned. If it reappears more investigation needed.

[Dorset Dabbler]

Thanks all for your informed comments.

The only place where there's damp staining and mould is that 1/2 strip immediately inside the window. I've no reason to believe that the cavity is breached though part of the previous owner's renovation was to have cavity wall insulation injected, however there's no damp beyond the immediate surround of the window.

I can see the appeal of the PVC strip, also of gap filling foam; would I have to look for some special type of foam or will any general purpose foam be damp proof?

[Dorset Dabbler]

PS, I see I didn't answer Russell griffiths, and I should have said that for it to look pretty is almost enough as the problem is confined to that 1/2 inch strip immediately inside the window - though I'd not want to simply cover it up, at the very least I will dig out the mortar from that strip to permanently get rid of the mould. It's what to put back in the gap (if anything) which I need advice on.

[Russell griffiths]

If it’s just to tidy it up, I would fill the gap with a low expansion foam, let it dry, trim it back and cover with a small flat pvc trim. 

[scottishjohn]

sounds like the cavity insulation is tracking moisture across  to inside

[Dorset Dabbler]

John, I don't think so, I believe the stain problem I have is all adjacent to the exterior brick skin of the house, and is entirely outboard of the cavity.  I will do some measurement though to verify this.

 

Conventionally, should a damp barrier of some sort be installed around the window aperture to separate the plaster of the window reveal from the outside skin of brickwork?  (in my case, I can see there isn't one).

[Temp]

4 hours ago, Dorset Dabbler said:

I believe the stain problem I have is all adjacent to the exterior brick skin of the house, and is entirely outboard of the cavity. 

 

Sounds like the window might be set too far outboard? Eg So far outboard that the plasterboard contacts the outer leaf? It should be inline with the cavity so that doesn't happen. Typically metal straps/brackets that clip into the sides of the frame are used to support it in that position.

[scottishjohn]

9 hours ago, Dorset Dabbler said:

Conventionally, should a damp barrier of some sort be installed around the window aperture to separate the plaster of the window reveal from the outside skin of brickwork?  (in my case, I can see there isn't one).

If as most window fitters do --they had sprayed builders foam in the cavity right next to where the window was to be fitted -then that would be your barrier ,

there is usually agap around the window after its fitted as they usually use 5mm-spacers where the screw fixings for the window are .

this leaves a gap they can fill with spray foam -trim and then finish with plaster or trim plate.or caulking

 outside brickwork whould be no where near the palseter finish -as the plaster finish is applied to inner skin --not the outer one 

the whole point of the cavity is to be a space ,in your old type house where the moisture from out side can evaporate or move up the wall and out of the wall head where roof starts

 if you seal the top of the wall then you trap any moisture in there --

your wall looks like hard plaster - If not plaster board so there would be no vapour barrier -so filling cavity can be a bad idea if any moisture gets through your pebble dash or the pointing in the outer brick work and cannot evaporate out of the cavity --eg no air flow 

I am guessing at that age of house it is a cold loft --which means wall head must not be blocked 

very easy to happen if someone gets excited stuffing insulation right into the eaves and blocking top of wall cavity 

all I have just  is guessing as your pictures do not show enough 

 

Edited June 20, 2020 by scottishjohn

[Dorset Dabbler]

On the window I am looking it - which I believe is typical of the four affected by a damp joint around its frame - I can see that there is hardly any foam. Most of the gap between frame and brickwork has been filled with grey mortar, which is continuous from near the outside of the bricks to around half an inch inside the frame, where it meets the plaster. It's this mortar which has been getting slightly damp.

 

The plaster itself is not board, but hard plaster applied to the brickwork of the house. It has not been suffering from damp staining in the same way. Although it clearly must span the cavity I can't see how the cavity is closed off without making a bit more of a mess of it than I need to (why make more work!) but I would suspect it's the original method, whatever that was in 1952, and seems to be perfectly sound.

 

Thanks to all contributors - plenty to think about, and I'll be back on if I find I am still stuck. This was my first post and I must report that I really like this forum and the quality of its help!

[scottishjohn]

2 hours ago, Dorset Dabbler said:

On the window I am looking it - which I believe is typical of the four affected by a damp joint around its frame

this is the first mention of it being on 4 windows the inference  of your post was that the problem was just with one window 

 this does indeed suggest you have a damp problem in the cavity 

my guess caused by poor or wrongly carried out insulation job on the cavity ,which is now holding water 

time for a professional to insepct  

 

 

Edited June 22, 2020 by scottishjohn