Unvented "cold" flat roof insulation question.

[7doughnuts]

Hi All,

I'm just completing a loft conversion and had to change the initial warm deck flat roof design of the dormer to a cold deck design at the last minute due to height issues. The building control officer and architect both said this would be fine and we'll sort out the insulation later....and now it's later and I'm a little confused as to what to do for the best.

The roof in question is a flat foof dormer, around 4.5m x 3.5m, constructed with 6" x 2" timber, 18mm plywood and a 3 layer waterproof topping. The rest of the roof is pitched with clay tiles and a breather membrane.

The BCO today said it would be OK to use the multifoil insulation as a vapour barrier and he would be happy to sign that off. I want to make sure I make the right decision and have been thinking about the alternatives.

A few questions:

1) If I sprayed the spray foam insulation between the joists on to the underside of the plywood will that stop any condensation forming if moisture penetrates the insulation? I have no idea what people think of that stuff!

2) If I were to pack rockwool against the plywood, then kingspan/celotex between the joists and thin foil over would that have the same effect?

3) Should I leave a 25mm gap which vents in to the pitched roof space and allow moisture to escape that way? Or will moisture from the pitched roof space enter and condense of the flat roof ply?

4) Is there a better solution?

I'd really appreciate any advice anyone can give me as I've put my blood, sweat and tears in to this project so far and just want to do the best I can.....and not have to reroof in 10 years time!

Thanks
7d

 

[noseall]

I have never heard of an unvented cold flat roof, though i have seen a cold system where all the air is occupied with insulation.

However, this system is not completely reliable nor is it sanctioned by some authorities.

 

[7doughnuts]

Is what you describe not unvented? What would be the point in having vented soffits etc if there was no air gap for air to flow.

Do you know why this system is not reliable and what the issues are?

Thanks for your reply.

 

[noseall]

The 'cold' system i described is not vented.

The theory is that the roof void is completely filled with insulation thus removing all the air and any potential for water vapour penetration and subsequent condensation.

The above scenario also needs to be fitted with a robust vapour barrier as well as being extra vigilant with snug fitting insulation.

Some bco's will argue that moisture laden air is still able to penetrate the void and subsequently condense, but he ho.

Wherever possible a warm deck system is simpler and the most economically sensible.

 

[7doughnuts]

I think that may be the best option for me as venting the space is going to be impractical and I can't insulate on top. I found this link regarding spray foam insulation and would be happier with the thought of no gaps when the insulation is bonded to the plywood and under the furrings.

http://www.renotherm.co.uk/trade/pdf/Data sheet - New flat roof.pdf

Does anyone have any experience of this application, renotherm or an idea of costs? I'd only use this for the flat roof section so around 15m2.

7d

 

[henhead]

What did you do in the end? I have the same dilemma