Warm Flat Roof ?

[noseall]

Woodys' idea is based on the theory of fully filling the joist void with celotex thus removing the air completely. You would still need to underdraw the joists with further insulation and this method is not always accepted by building control.

You could argue that all the air has been removed so no need for venting, but i suspect you would need to be super-diligent with the foam gun and/or foil tape, as there are likely to be fitting gaps.

 

[RobFurn]

It is tempting I must admit to try that route. Im not trying to cut conrners too much but come on how much condensation are we talking about here if a minisucle gap is missed? is it such a serious problem. Appologies for my ignorance but hey its a DIY site so shoot me down if im talking rubbish

 

[noseall]

So long as the moist air can find its way to the underside of the (uninsulated) deck then it will condense in low external temperatures.

If you can guarantee that all fitting is air tight then b.c have not got an argument. It is the equivalent of fitting a thick slab of celotex to the roof that just happens to have joists running through it.

However, you may still be vulnerable to cold bridging, so a 25-50mm underdraw may be necessary also. How deep are the roof joists?

 

[RobFurn]

The joists are 125mm deep

 

[noseall]

The joists are 125mm deep

You may need to fit a 40mm thick (insulation board) underdraw prior to fixing the plasterboards.

 

[dizz]

If what you have is a basic flattie then there is no reason why you can't still have an unvented warm deck.

You can remove the current felt covering then fix ply backed (97mm) insulation sheets over the existing. You can then fit the 25mm sheets to the underside of the osb.

The only downside to a warm deck is a rather deep fascia at the roof edge detail.

Do you have a link or specific name for the insulation sheets you'd use for a flatroof please noseall?