I am currently finishing off a flat roof dormer internally. The flat roof deck has been constructed using Kingspan roof deck on top of 150mm deep joists. I have some 90mm thick insulation left over from constructing the timber walls and cheeck of the dormer. Can anyone tell me if it is ok to install this 90mm insulation between the joists beneath the Kingspan deck insulation. If it is possible should i push the insulation up to touch the deck insulation or should i leave the 50mm gap between the two? Not sure if this would lead to any condensation problems?
Flat roof dormer insulation
- Thread starter AndrewSchofield
- Start date
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How much insulation do you have on top? If you have used Kingspan TR31 then it should be 96mm above the joists and 25mm between the joists pushed up tight to the deck insulation
Whatever insulation you put between the joists, you want it together with no air gap between the insulation
If the roof is vented, then have the void between the insulation and the ceiling board
BTW, dormer cheeks are normally 100mm Kingspan/Celotex - has the 90mm board been approved?
Whatever insulation you put between the joists, you want it together with no air gap between the insulation
If the roof is vented, then have the void between the insulation and the ceiling board
BTW, dormer cheeks are normally 100mm Kingspan/Celotex - has the 90mm board been approved?
The insulation on top is Kingspan insulated deck where the plywood deck is bonded to the insulation board. The insulation is 110mm thick giving 116mm overall including the plywood. I don't need any between the roof joists to get my U-value but i've got this 90mm spare so thought i'd put it in there to increase the insulating properties of the roof (as this is where i'll be losing most heat). My logic was that if i put the 90mm tight up against the roof deck insulation then there will be no air between the two layers and therefore no condensation issues - and you seen to concur with this?? I've been told i can't vent the void between the joists as if i do this the insulated board on top will be doing nothing?
The timber studs used in the cheeks are only 100mm wide so i was advised to only use 90mm between them in case the timber width varied and it was slightly less than 100mm meaning the insulation board wouldn't fit in. I've installed an extra 25mm on the whole of the inside of the cheeks to get me the required U-Value which has been calc'ed by Celotex and approved by my independant BC officer.
The timber studs used in the cheeks are only 100mm wide so i was advised to only use 90mm between them in case the timber width varied and it was slightly less than 100mm meaning the insulation board wouldn't fit in. I've installed an extra 25mm on the whole of the inside of the cheeks to get me the required U-Value which has been calc'ed by Celotex and approved by my independant BC officer.
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