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Sweating warm flat roof
Hi all – got a warm roof installed – fully vapour barriered with alutrix600. Under the bottom deck has the bottom of the fascia sat on the roof joists and the cavity is open. We had some rockwool which I’ve removed in this area – noticed it was sweating. What’s recommended to stop condensation forming on the roof deck in the picture? Does it need filling with PIR to bridge the cavity right upto the fascia and expand foam or will simply 150mm PIR filling the gap on the inside wall skin foamed in be enough to solve the problem? I assume it’s Cold fitting the fascia and warm air from the inside mixing?
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9 Replies
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That cavity shouldn’t be open. It will carry the moisture that gets through the outer skin up and into the roof structure. It’s been built wrong in a number if ways tbh. I guess you are pretty much stuck with trying to seal the cavity with ‘somthing’ foam I guess. And then insulating (probably with PIR) as it should be.
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So would I best to fill and foam PIR right upto the back of the fascia and close all the gaps bridging the back of the fascia all the way to the inside wall? Essentially making a PIR sandwich that’s tight front to back and top to bottom?
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I think that’s all you really can do in that case. Cut PIR and foam it in the gap. Joists should’nt go all the way through to facia.
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cheers – would it help if I ran some wide dpc over the cavity gap before filling with PIR and foam?
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Insulation should go from cavity to the deck and seal the the inner to stop the cold bridging
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I get that bit but what out the area in front of the cavity insulation (120mm upto the fascia) won’t that just seal it up leaving a warm ceiling as it’s a warm roof?
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that’s how I do it do effectively u end up with 2 layers of 100mm. First 100mm extents for cavity to underside of the osb deck the 100mm from wall plate to bottom of deck perfect job for all the offcuts then beam of foam around or tape stop
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found this which was my idea Tucking in your roof – London Flat Roofing
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ye will be fine over thinking it providing provided insulated right and blocked off internally will be fine
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