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Damp Issue

  • 28-10-2022 9:31am
    Registered Users Posts: 30 rigamincedeal

    Hi just looking for opinions on these damp patches showing up on my wall. Rear extension done around 2 years ago, wall is cavity block and has 110mm internal insulation, wall has a fibreglass parapet capping too. See pictures attached of damp patches, the external render and the parapet capping. Not sure what way this pictures will show but one patch is above the other and not side by side. Thanks


  • Registered Users Posts: 2,980 ✭✭✭chooseusername

    The circular damp spots look like condensation on the steel mushroom fixings used to fix the internal insulation. Cavity blocks would be cold and the steel fixings would form a cold bridge to the inside, unless plastic insulating caps are fitted.

  • Registered Users Posts: 2,758 ✭✭✭MicktheMan

    As above, this has nothing to do with the roof or external render/finish and likely thermally unbroken mechanical fixings used when installing the insulated pb. Perhaps followup with the designer / contractor as to what was used and why.

    Nevertheless ...

    ... reducing moisture load where possible, improving heat retention & maintaining a good internal temperature and above all else having appropriate ventilation will be key here because the damp circles are mere symptoms of an excessive moisture issue in the house.

  • Registered Users Posts: 30 rigamincedeal

    Hi thanks for your replies and I definitely thought that the issue was cold bridging with regard to the mushroom head fixings for the insulated plaster board. I have followed up with the contractor and I am waiting for someone to inspect it. However what concerns me now is the fact that the first damp circle is getting bigger each time and as the heavier rain persists over the last few days the circler damp patch which it started at 10cm in diameter is now at 16cm, the work done was it’s a large enough extension approx 32sqm and this is the only area of the extension that appears to have the issue. Thanks again.

  • Posts: 0 Josie Green Pest

    I've used a ton of mushroom fixings into cavity blocks in my house, and I've no issues like that, so I can't see it being cold-bridging.

    I find it extremely difficult to believe that a 1mm thick plastic cap on the mushroom fixings can magically solve cold-bridging issues....or that cold-bridging is only an issue when it's raining & gets worse the more it rains...or that the standard fixing for rigid insulation is such an incredibly good conductor that it creates such a cold disk on the plasterboard that the plasterboard becomes visibly damp (but only when it rains)....and only in one place in a largish extension...or that cold bridging focused on the small metal disc at the end of a mushroom fixing would cause a damp patch 16cm wide....

    So nuts to those people who are claiming all of your problems are being caused by the absence of a 1mm plastic cover at the end two of your mushroom fixings.

    I would guess that there's water ingress into the part of the wall into which those particular mushroom fixings were hammered. There's near zero overhang on that outside wall, so not so much protection from driving rain which will penetrate the render after a while. On top of that, the visible crazing in the render is a clear indication that it wasn't done right (something to to do with the ratios of cement-sand-water, or allowed to dry out too quickly or something).

    A mushroom fixing is basically a tube with a slot running along it. If it's horizontal, then a small amount of free water entering it at the end in the block could travel back as far as the plasterboard causing the visible issue you can see, expanding more or less uniformly from where it hits.

    In the photos, it looks like one damp patch is bigger than the other? I'm guessing that's the bottom one? Because it seems likely that water penetration from driving rain would be worse lower down because gravity.

    Solution is the same: better render (even decent paint would probably improve things) to help keep driving rain out of the blocks when it hits the render, combined with something overhead to keep rain off the render in the first place. The something would ideally be a decent overhang which you will see on virtually every exterior wall for a reason, but I'm guessing that's not gonna happen...

    Also good luck convincing the contractor who said "Roof overhang? Exterior render? Bollocks to that..." back in to fix the issue his crappy workmanship has caused in the first place.

    (PS, "reducing moisture load where possible, improving heat retention & maintaining a good internal temperature and above all else having appropriate ventilation"— are all excellent suggestions all of the time; I just don't believe they'll do anything to address this particular issue).

  • Posts: 0 Josie Green Pest

    It's probably the photo angles, but that roof looks unusually flat for a tiled roof? And it looks like there's a bit of a valley (where the cable is strung along for some reason) between the tiles & the fibreglass bit?

    Please for the love of god tell me that valley doesn't exist, and it isn't bringing rainwater from the tiles to the gutter?

    Because if it is, I'm pretty sure I've identified your water ingress route, and also that it will get much worse. Or more accurately, I'm pretty sure it's already much worse, because what you see as a couple of damp patches is only the water which has traveled from the outside through a 225mm block & 100mm of insulation and actually reached the plasterboard— doing untold damage to your roof, walls, and insulation on the way.

    I would prepare to lawyer up to be honest.

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  • Registered Users Posts: 30 rigamincedeal



    I absolutely take all that onboard 100% and agree with you on every point, I think I just needed to get someone else’s opinion on it too. The only thing you pointed out that I didn’t even consider an issue is the lack of an overhang. Photos of roof don’t show its pitch, I am no engineer and I’am  far from it but I’d be fairly certain that the pitch of the roof is fine. It’s actually the top damp patch that is bigger than the other and has continued to get bigger to a max of 16cm in diameter, while the second patch below it appeared once and was approx 6/8cm in diameter and has since dried out. The second patch didn’t appear again even though the driving rain was worse the last two nights. The valley is fibreglass and a parapet capping type wall (I think) bringing the rain water from the roof to a hooper and a down pipe. I am thinking the water is getting in through the render as if it was through the valley it would have possibly showed higher up at this stage. Do you think there a possibility of drilling the wall from outside right through to the hollow of the cavity directly behind the damp patch then putting a small camera inside to see what’s happening in and around the gap in the cavity block? Thanks

  • Registered Users Posts: 8,146 ✭✭✭Furze99

    Proper layer of render and preferably painted as well is critical with concrete block walls where you have wind driven rain in particular. I learnt this on a wee piece of construction where I actually built a standard cavity wall with 100mm gap between standard 4"/ 100mm blocks. I hadn't time to render after roofing before autumn rains came and was surprised to see how much rain soaked through the outer leaf. A couple of coats of render and a bit of paint and divil a bit since. Cavity block as you have, is even less substantial - so if this wall is exposed to rain blowing in, that's where I'd look first. Might only be an apparent issue when wind is from a certain direction. So could be other very wet days and less of an issue.

  • Posts: 0 Josie Green Pest

    I'm not about to start recommending anyone starts drilling random holes in their outside walls.

    It's difficult to get a sense from those photos of how the roof is put together, but to help keep water out everything should overlap— to keep your feet dry in the rain you should have an umbrella over a raincoat over leggings over wellies. If you skip the umbrella and tuck your raincoat into your leggings and your leggings into your wellies, there's a fair chance the water will run into your crotch right down your legs and your toes will soon be wet.

    Same principle with a house. It doesn't look (from the photos at least) that that principle has been applied with that roof. And again, the crazing of the render tells its own story— and it's not the story of a contractor who pays close attention to all of the important details. Same story as told by the lack of overhang— overhangs aren't decorative, they serve a purpose.

  • Registered Users Posts: 30 rigamincedeal

    I found a few pictures from when the work was done, the extension was done by squaring off the back of a standard 4 bed house with and Apex Roof. So the thrusts are sitting inside the wall as is the valley. I am sure you’ll get the gist from these pictures. Thanks

  • Registered Users Posts: 2,980 ✭✭✭chooseusername

    "this is the only area of the extension that appears to have the issue."

    What's the area used for; utility, shower room, Kitchen? and where on the wall are they?

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  • Registered Users Posts: 556 ✭✭✭SC024

    That valley looks way wrong. Even more so with the earlier pic you posted of tiles. some thing way off there.