Warm/cold flat roof deck

[Bobanek5]

Hi all,

I have a question , perhaps someone here can help to answer it,
I'm not a huge fan of warm decks as they take an additional 100mm plus to the roof thickness and in most cases high is very critical on build, what I was wondering , if let's say instead of fitting 120mm celotex you put only 25mm on the top and 100mm between joist very tight to 25mm above , for example : fit 2x1 batten 98mm below top of joist ,100mm celotex on it 25mm over entire area then tight fix with wbp plywood (2mm compression should be good to achieve tight fit)
Can this method be good as a warm deck ? Or there is something that I'm missing and should be aware off ?
Many thanks if anyone can help to answer

 

[noseall]

How do you prevent cold bridging through the joist itself?

It would need to be a good fit also and have to take into account that the joists are likely to shrink.

BC don't like it here in Staffs, prolly for the above reasons.

 

[Bobanek5]

Thanks for your replay

I was hoping that 25mm celotex over entire area would help with cold bridging issue through joist, also joist shrinkage - that's why it would have to be a very tight fit between joist , also 2" joist wouldn't shrink much in width but a fare bit in depth, if 2" joist shrinkage was an issue then why spray foam is good for that ? Perhaps it's glued itself to joist and works with it? , in that case what if we leave let's say 10mm gap between joist and insulation that gets fitted between joist and fill this gap with appropriate foam ?

 

[tony1851]

Where would your vapour barrier go?

 

[Bobanek5]

It would go underside of joist

 

[StevieV1]

On Celotex's website they state they do not support having insulation above and then between joists, only all above or between joists and below.

I have just fallen a bit foul of this because I misread my plans. My architect spec'd for 120mm Celotex warm roof i.e. all above, but I put the 120mm between the joists. Result is my U value would not meet part L.

So I went on the Celotex website calculator and worked out that if get more insulation (30mm) and place under joists then Im good to go, so that is what I have done.

Whilst it could be said that this mistake has cost me money, I probably would have had to do it this way anyway because Ive got a crown roof so not completely flat so if I had put the 120mm above I still would have had to put more below where the pitched part of the roof was, which I think BC would have frowned upon.

 

[Bobanek5]

Thanks for you input here, but I think you have created a " cold deck" instead which technically needs to be vented now, is that right, warm deck didn't need any ventilation

 

[StevieV1]

Yes, its vented (cold roof) with 50mm air gap minimum above the insulation between the joists.

I think my point is Celotex either recommend a warm roof i.e. insulation all above, or a cold roof, insulation all below the deck with ventilation. I dont think they support a hybrid warm and cold roof which is what I think you were thinking.

 

[Bobanek5]

Thanks, I understand that, also , does anyone know why for dormer cheeks it's ok for bc to have celotex packed between joist but same can't be applied to ceiling ?

 

[noseall]

Thanks, I understand that, also , does anyone know why for dormer cheeks it's ok for bc to have celotex packed between joist but same can't be applied to ceiling ?

Condensation
Dormer cheeks are wrapped in breathable membrane. Flat roof coverings are impervious and trap vapour.

Water does not hang around on vertical surfaces like it does on flat roofs so this adds to the cooling effect.

Warm air rises too.