Warm roof construction with insulation between joists?

[greenlikeapples]

Hi everybody!

This is my first ever DIYnot post after many many months of very useful sifting - thanks everybody!

I am doing a dormer conversion. We are really pushing to get maximum head height. For this reason i would prefer to have insulation between the joists as with a cold flat roof construction.

BUT - as we are pushing head height, our dormer meets the roof ridge right at ridge level, so any vents - and Building Regs seem to require a lot of them - will be a little too exposed in my opinion. I have read a bit about them leaking, which I would like to avoid!

So - a solution I would like to propose is: Making a WARM FLAT ROOF construction, but instead of putting the insulation above the structure/joists as seems to be normal, I propose to include the joists effectively as insulation (I believe wood is -quite- good for that) and install a proper vapour control layer underneath the joists.

It seems to me that the essence of a Warm Roof Construction is that there is insulation above anywhere where condensation could collect - while a cold roof lets condensation collect and escape above the insulation. So why not just move the joists (am I repeating myself?) to WITHIN the insulation layer, and the moisture control to below the joists, rather than just above it?

I can't find anything about this anywhere else online and I'm not sure why.

I look forward to hearing what you have to say...

Jon

 

[freddiemercurystwin]

Its only a warm roof if there is insulation above the joists.

If its in between and no vents its a hybrid and may or may not be accepted by Building Control. This type of detail gets accepted on garage conversions and the like but they are conversions (not new build) where there is little scope to achieve a proper warm roof as the existing roof finish is being retained.

 

[hardmetalking]

The vapour control layer is the most critical point in any warm roof construction. It is often overlooked by architects, builders and even roofers!
Although its important what is used its more important how its installed. We install some of the most sophisticated and expensive roof systems on the market and offer guarantees for hundreds of years so I'm sure the methods we use stand up to the most stringent of tests.
Above the rafters we would install a marine grade plywood sheet with a minimum thickness of 18mm.
We would then prime the plywood and install bituthene, lapping joints and sealing against abutments and penetrations.
On top of this we would install between 100mm and 350mm insulation. Again sealing all joints, abutments and penetrations.
On top of this would be a separation layer, usually a breather membrane (tyvek or klober).
Then we would install zinc, fixing down through the insulation into the 18mm ply with stainless steel fixings through stainless steel clips with a plastic plug to stop thermal transfer.
Each stage is important and cannot be skipped, rushed or corners cut. A specialist job and typically around £200 a square metre.
Hope this answers some questions.