flat roof insulation

11 Sep 2007
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United Kingdom
Urgent help needed please.
We are having a new flat roof to our kitchen extension done next week , it joins already existing flat roof .
We cannot use Ply with insulation stuck on to it as it will be different thickenss to the existing roof ,what other type insulation can we use that will meet current building regulations?
Can we use insulation between the joists and if so what?
The ceiling will be plasterboarded.
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you can use quilt or fibreglass insulation - provided the joists are 270mm deep!

or you can solid fill the space with celotex and maybe underdraw with 25 or 50mm insulation board, dependant upon joist depth.

leaving any kind of airspace may require cross ventilation, which can be awkward with flat roofs.
Thanks your reply, the joists are only 8" deep ,we will have soffit upvc ventilation , is rockwool ok ? we need to leave an air cap for ceiling recessed lights . It seems to be very difficult to know what we can put between the joists .
you have not got the required depth for mineral or glass fibre type insulation.

you could seek advice from the b.c.o. :?:

use thermodeck and just have a cocking/angle fillet to deal with the change in heights.
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Thanks for your reply , there will not be any difference on the levels of the flat roof ,we will ask the building inspector to see what he suggests , what is thermodeck? and where to get it?
If you don't have the depth for mineral wool, then you will have to insulate with solid board between and under the rafters. This may mean that the ceiling ends up lower if you can't adjust the joist depth.

The other way would be to have reduced insulation in the roof, but compensate by increasing insulation elsewhere, but this would require a SAP calculation to prove the overall house insulation values are no worse off.

Or just renew the existing flat roof and thermal board the whole lot
Thanks for all your help, as we only have 8" joists the building inspector said we can put thermal board like celotex 150mm and leave aircap of 50mm above it .

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