Kelvin Posted June 30, 2023 Share Posted June 30, 2023 Due to yet another error the insulation in the flat roof of our dormer bathroom isn’t as deep as it should have been so have tried to address by utilising what I have on-site. However, I’m now worried I’ve caused myself a condensation problem. Is this ok? outside - Alwitra VSK roof - 18mm plywood - ventilated cavity - vapour control roofing membrane - 9mm OSB - space stud 140 wool insulation - 9mm OSB - vapour control layer - 75mm Celotex PIR (i’ve added this) - service void - 12.5mm plasterboard (should have been 53mm insulated plasterboard) Inside Link to comment Share on other sites More sharing options...
saveasteading Posted June 30, 2023 Share Posted June 30, 2023 Sometimes you can worry too much. This stuff happens in what is a very complex 3d assembly. The heat loss through 2m2 or so of dormer with slightly reduced insulation, could be regained more easily in a simpler area of the building. Or replace the mineral wool with pir. I wouldn't go checking dew points here... Link to comment Share on other sites More sharing options...
Kelvin Posted June 30, 2023 Author Share Posted June 30, 2023 I thought about replacing the wool with PIR but I really couldn’t stomach taking the roof apart to get to it. I should have done when it was being erected but I had so many other issues to deal with I never got to it Link to comment Share on other sites More sharing options...
Iceverge Posted June 30, 2023 Share Posted June 30, 2023 That's a U value of about 0.15. Absolutely sufficient. 95mm is a beefy service cavity, is this 95x44 mm timbers on their sides to creat this? Link to comment Share on other sites More sharing options...
Kelvin Posted June 30, 2023 Author Share Posted June 30, 2023 2 hours ago, Iceverge said: That's a U value of about 0.15. Absolutely sufficient. 95mm is a beefy service cavity, is this 95x44 mm timbers on their sides to creat this? It is. It seems to be the standard build up for HH flat roof. I’ve not done the service void yet so can make it smaller. I am using the mountain of material that HH over ordered. I just fitted the PIR then had a moment of doubt when the last board was in. 😂 Link to comment Share on other sites More sharing options...
Iceverge Posted July 1, 2023 Share Posted July 1, 2023 Wool between joists is better than PIR in my opinion anyway. It can move to accommodate shrinkage etc and you'll avoided thermal bypass long term Link to comment Share on other sites More sharing options...
Kelvin Posted July 1, 2023 Author Share Posted July 1, 2023 So we’re quite happy with the build up. Specifically the wool, osb, vapour control layer (pro clima), pir? Link to comment Share on other sites More sharing options...
Dave Jones Posted July 1, 2023 Share Posted July 1, 2023 you have a cold roof by the looks so as long as your ventilation space is open both ends for air flow then you can mix and match insulation as it will be able to dry out. Link to comment Share on other sites More sharing options...
Kelvin Posted July 1, 2023 Author Share Posted July 1, 2023 Thanks all Link to comment Share on other sites More sharing options...
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