warm roof construction

[matt73]

sloping roof single storey with roof lights for dining room.
need to construct and insulate to form warm roof. can anyone help me out as to how a warm roof is formed without suffering condensation.

thanks mattt

 

[noseall]

usually 120mm celotex between rafters then underdraw with 25mm celotex then 12.5mm foil backed plasterboards and skim.

 

[matt73]

just been on knauf site. they have loads of suggestions using ... knauf products...

i was thinking of rock been the rafters with a breatable felt on top of the rafters to let out vapour. just not sure how i woould finish under the rock wool? would i need another layer of vapour barrier or just ply and plasterboard or foil backed as u said?

 

[matt73]

also will this do way with the need for ventilating the roof

 

[PatrickD]

Matt

I think the idea of rockwool between the joists etc stands a very good chance of going tits up.

The idea of the warm roof principle, by Knauf or anyone, is that the joists are at room temperature. Yours won't be. If you do it this way, your vapour barrier is going to have to be exceptional; and that's a very hard thing to achieve.

Cold will be transmitted to your joists from outside and, with even the smallest amount of moisture coming up from below, interstitial condensation will form on them. This in turn will soak into the Rockwool and, as you have virtually no ventilation, it won't evaporate readily.

I don't suppose I need to go on but; you'll have provided a very nasty environment between the PB's and the Breathable felt!
HTH

Patrick

 

[BOB NUTS]

For a roof to be a 'Warm Roof' as PatrickD says the rafters need to be at room temperature (not bridging between cold (outside insulation) and warm (inside insulation) environments.

So the insulation needs to be entirely above the rafters.

www.celotex.co.uk may have a solution - i know they have warm flat roof solutions.

for a true warm roof -no ventilation required.

If you have trouble ventilating then consider a breatheable felt with conventional insulation as an alternative

 

[matt73]

ok then... thanks but i've got this wrapped round my neck bear with me...

tiles- breathable membrane stretched over tile battens- battens laid over insulation fixed to cross battens-insualtion between rafters-plasterboard finish...

how does this sound (obviously i will have to chose the correct materials but it seems to be the order in which they are laid is vital)

and definatelty no ventilation as the breathable membrane will allow any vapour out but no leaks in.....

have i got it right guys. i am a novice as you see

 

[chappers]

bobnuts is correct it is no longer acceptable to solid fill bettween rafters, all insulation must be above the rafters usually from the top down would be

roof covering
ply
insulation
vapour barrier
ply
joists/rafters
plasterboard
ceiling finish

there are ready made products by celotex, kingspan etc which are both roof deck and insulation in one

 

[noseall]

building control will accept celotex between the rafters with underdraw as long as there is 50mm airflow above the celotex. the reason i suggest this method is it saves using very long fixings for the counterbattens. i realize too that any velux windows on a rafter run provide an obstacle for the air flow, b.c. were happy to let a good breathable membrane such as tyvec solve that issue.

 

[constructionbuyer]

You'll probably need foil backed plasterboard for the ceiling to make the breathable membrane work - I know Tyvek insist on this. But I'm not as familiar with warm roofs!