Damp beside a window. Where to start?

[Jemster]

First off hello all! First posting here, hoping it's a bit more active than some of the forums I've tried. Looks like a great mine of information

We've got a 1920's house with solid, rendered, brick walls. Over the years of past owners it's had a few issues and patch-ups. Dodgy UPVC windows fitted 30+ years ago, extensions built, chimney breasts half-removed. Things like that. We've been in it 18 months or thereabouts and have fixed a pile of damp-related problems, mainly coming down to things like badly sealed frames, poor airflow, chimneys without vents etc... and there's only maybe 1 or 2 left to sort out. One of them has me stumped as to where to start looking as I don't want to leave a trail of destruction hunting this down!

The problem damp patch lies to the side of a window in a first floor bedroom on a gable wall.

IMG_4662 (1).jpg (68.08 KiB) Viewed 2937 times

Things we do know:
* There's no external guttering
* The slate roof is in good condition (my trusted roofer has been up to take a look above the window area) and although there is no underfelt and long-term we plan on replacing it, we don't believe it to be leaking
* The outside of the house was re-painted last year with all visible cracks filled and looks to be holding up well - one crack is showing signs of starting again, but it's down to the bottom left of the window and I don't see that causing damp 2/3 of the way up.
* The silicone sealant around the window frame is in great condition and was largely replaced with Tec-7 by the painters as well. I've had a good poke around and there's nothing notable
* I replaced the silicone sealant and trims on the inside of the window frame as well
* Condensation is pretty well controlled in the building - Nuaire Drimaster fitted last year just to keep this in check but windows barely got any condensation beforehand and now, none. Windows are usually left on the vent position.
* There *may* have been sash boxes at some stage as the plaster changes, presumably from lime to gypsum around 6" or so in from the reveal in line with some of the damp.

The damp meter is showing around 10% to the right of the patches, up to about 25% max on the damp areas. Above the damp areas (i.e. top third of the frame) the damp levels are back down to under 10% and similar to the left hand side of the damp it tails off, making me think it's not coming from above.

I've tried taping clingfilm to the wall to see if it gathered moisture but after a week it didn't seem to have picked anything up, inside or outside.
We re-decorated the room 6 months ago after the cracks were filled and I used an alkali-resist primer on the area as it had obvious salts. This has started to push outwards from behind, along with a few fresh salts, again making me think it's not condensation.

I'm at a bit of a loss as to where it's from. My next thoughts are to hack away the plaster on the left of the frame and take a look in, but that's also my last-resort option as I don't want to do this unnecessarily if there's anyone has any idea what else I could be looking at?

Any advice appreciated. Even a few wild guesses might get the cogs turning and lead to a fix.

(Outside shot for completeness, I can take more of course if anybody wants to see)

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[toolbox]

Might be the plaster that is absorbing moisture, if it has been damp for some time previously it becomes hydroscopic that is why you re-plaster after treating long term damp.
I have linked to an article here.
https://www.britishdampproofing.co.uk/n ... e-plaster/

[Jemster]

Might be the plaster that is absorbing moisture, if it has been damp for some time previously it becomes hydroscopic that is why you re-plaster after treating long term damp.
I have linked to an article here.
<snipped cos I'm not allowed to post http links to external sites...>

Thanks for the reply, interesting article, I did think that once the source of the damp had been eliminated, the salts would eventually stop coming through as there is no more moisture to pull the salt out of the plaster.

If a plaster becomes hydroscopic, wouldn't it still need a moisture source? I noticed it seems to come from behind the paint as the paint is starting to bubble off the wall, and as the emulsion is sitting on a skin of alkali-resist primer does this indicate that the moisture is within the wall rather than condensation based?

The concern I would have is that I'd knock off this plaster, re-plaster and the damp would still be there within the wall (due to whatever leak may be present) and that I'm patching the symptom rather than treating the cause. How would I determine that?

Sorry for all the questions, it's all a learning experience for me :)

[toolbox]

My understanding is it simply draws moisture from the air, like those silica crystals you put on window sills to collect moisture.

[Jemster]

Ok, but to draw it from the air it would have to draw it through the Alkali-resist primer layer to get to the plaster?

Maybe that is what happens, I thought the moisture was coming from within. If it were soaking wet it would be easy to tell with the clingfilm / foil stuck to the wall test, but there doesn't appear to be enough moisture for that to work conclusively.

One damp-proofing "expert" who wouldn't come out and take a look or provide a quote (it's not what we do, we do homebuyer surveys blah blah...) suggested that the window may have been badly fitted and this area has been filled with bonding and isn't waterproof. Sounded like a possibility, except again, for the bonding to let damp through, it has to be letting it through from somewhere...

[ayjay]

the salts would eventually stop coming through as there is no more moisture to pull the salt out of the plaster.

Salts can continue coming out for years: funny story, my mate lived in a maisonette, he'd been there for at least ten years when we did some work for the small local(ish) company that built them. My mate talked to the son of the company owner who was running the job we were on about where these salts were continuing to come through.

When my mate described exactly where it was, the answer was " That's where everyone went for a wee when we had no toilet on site"

[Jemster]

That'd be some contest going on at my place, peeing 20 feet up a wall

Sounds like hacking back the affected plaster may be the only sensible option. Do I need to go right down to the blockwork or is removing the skim down to whatever bonding layer may be there going to be sufficient? I've only ever done small amounts of plastering, left that to the experts but as this isn't a large area, I may go for it myself if I find the time.

[OchAye]

* The outside of the house was re-painted last year with all visible cracks filled and looks to be holding up well - one crack is showing signs of starting again, but it's down to the bottom left of the window and I don't see that causing damp 2/3 of the way up.

Just taking a complete guess. If the exterior cracks were filled using an interior filler (i.e. mainly gypsum based stuff) that will absorb moisture etc etc etc. It may be the case of stick your head out of the window (don't fall out and do not drop the phone/camera), look for any bubbling paint around about where the crack is and above it if possible. Are other houses in the street painted?

[Jemster]

I cannot be 100% on what the cracks were filled with. I've used the painters that did the job on another house and they've always seemed to know what they are doing. Initially we were looking at getting a re-render on the front and gable walls as there were a few hairline cracks, <2mm at the absolute worst case. The render was sound though and I didn't want to start hacking it off if it wasn't necessary. These guys said they'd seen and repaired much worse. Given their experience, it's unlikely they'd have used anything interior-rated.

Won't the few coats of masonry paint over the filler give a decent level of waterproofing regardless of filler type?

All the houses around our way are fairly unique. Some are roughcast render and painted, others are red brick and others are painted smooth render. I honestly have not yet worked out what the render is. Being a 1920's build, I suspected it was lime based, but it's absolutely smooth and seems very hard which leads me to thinking it's gypsum. The other gable wall has been re-rendered at some point in it's life and has a slight texture to it. I'm no expert on renders obviously and don't know if these differences mean something or if it's just the way they are.

If the weather allows, I'll pop up on the ladders at the weekend and take a closer look. I bought a tub of Toupret Fibacryl with the intention of hacking out the crack that has re-appeared a bit and then filling it again as deep as I can before re-painting. From reading around, this seems the best filler for exterior work where there could still be a little movement.

[OchAye]

Toupret Fibacryl

That would be suitable for "moving" cracks (flexing cracks). As (for over 45 years) I have not had the chance to use exterior fillers, I do not really know anything to recommend. Murex seems to be the one easily available and commented on, but have a look here and make up your own mind http://www.toupret.co.uk/business-customers/products/exterior-fillers.html, you sound capable enough

[Jemster]

Toupret Fibacryl

That would be suitable for "moving" cracks (flexing cracks). As (for over 45 years) I have not had the chance to use exterior fillers, I do not really know anything to recommend. Murex seems to be the one easily available and commented on, but have a look here and make up your own mind [url=<span class="skimlinks-unlinked">http://www.toupret.co.uk/business-custo ... html</span>]<span class="skimlinks-unlinked">http://www.toupret.co.uk/business-custo ... html</span>[/url], you sound capable enough

I figured if the crack was filled and it is showing a hairline 6 months later then there is a little movement going on. which I've I put down to the 1920's smaller foundations - it's a large wall and the cracks are superficial (said the structural engineer who surveyed the property prior to purchase) so having read around a few sites and comments this came up a couple of times. Also gets mostly decent reviews on Amazon.

It's meant to be easy to work with, bonds well and is over-paintable. I see Toupret list it as a "specialist" filler rather than an "external" filler but it is still exterior-rated so I'm hopeful it'll hold where others haven't - Murex is described as "rock hard" which, to me, says, "will crack"

I have a small crack under a window sill in one of the front bays that's easy to get to. I may give that a go first to get some practice with it before hitting an obviously visible area :)

[Jemster]

Just as an incomplete follow-up and because I hate threads that disappear into 'what happened next' as they're no use to nobody...

I took a closer look at the crack in the render externally and yes, it had opened a little. Didn't take much screwdriver prodding to open it wider. On one side the render sound boast but sound on the other side. I think long term we will have to get a bit hacked off and re-rendered. Really don't want to as I doubt that can be done invisibly. For now I have widened it out to 1-2mm and filled it as deep as I could with the Fibacryl and a coat of masonry paint. It may be drying out on the inside but I guess I really am going to have to take off the internal plaster and re-fill the area. Current situation has put a hold on these activities and left me too much time to poke other areas of the house. But that's another thread... literally... :)

[toolbox]

Thank you for updating