Vaulted ceiling damp patches

15 Nov 2006
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United Kingdom
Hi, had a tiled, vaulted ceiling, single storey extension added to the kitchen about four years ago. Have noticed dark patches on ceiling which are damp to the touch. Don’t appear to be as a result of roof leak? as fairly uniform in shape following roof timber lines and no appreciable difference in heavy rain conditions. We Have underfloor heating in the extension and two roof lights. There’s no obvious apparent air circulation provision and no vaulted ceiling access. Have added some pics to help. Before contacting a contractor would like some idea of what I’m facing. Any advice most welcome.
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Shouldn't be happening as much now it's warmer but is warm moist air from your kitchen passing the down lights and condensing in the ceiling void?

Those down lights will have broken any vapour barrier you may have had in the ceiling.

I'd drop the lights and have a feel around.
Hi thanks for reply. Prob fair to say that issue has lessened with better weather. Interesting you mentioned the down lights as there is a second row which I did not photograph and there is more patching along that line but not as bad. Not sure I fully get the comment re moist air from kitchen coming down lights? The original kitchen wall was not breached so there is no openings to cause that BUT we did have an extractor built in (exiting the roof) so I could see that possibly that may provide the moist air in the void? Whilst space very limited I’ll try dropping a few of the lights and having a look/feel around. Thanks
Classic thermal bridge.
Was there insulation installed across the rafters?
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Yes as far as I’m aware, Cellotex? I believe. If not or if badly installed presumably that would mean taking down the ceiling or lifting the roof tiles?
Insulated plasterboard probably 50mm would can go over the existing ceiling.
Ok, so I’ve had one of the light fittings out and as access is too tight to get my hand in I’ve managed a few pics which might be useful. Nothing seems particularly damp, but that’s maybe due to the weather change, and what is left is previous condensation shaddowing? As far as I can tell the problem is where the insulation blocks have been cut out to allow for the lighting instal? Would that be logical? I’m aware that the pics, without scale, are hard to identify but you clever chaps might be able to see what’s what. Any observations welcome please?


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There is no underdraw installed, i still think its thermal bridging.
Very grateful for your input. Sorry to be a thickie but could you explain what ‘underdraw’ is and what you’d suggest to rectify the problem? Thanks again.
‘Alastairreid’, just looked back at your previous post and note your suggestion of second plasterboarding with insulation. prob 50mm. Having seen the pics, is that still your suggestion? Cheers.
I'm wondering if you could pack some fire resistant mineral wool in to the void created by the cut out. Those lights if LED wont be producing much heat, the cut-outs seem a bit excessive.
I vaguely remember a building control issue with cutouts larger than 150mm2. What is the width of the celotex in the void? It doesn't look like 100mm

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