Building a warm flat roof

[trevorbayliss]

I understand the void between the joists (125mm in this case) is left empty and all insulation is placed above the joists.

Is there any reason not to fill the void with insulation and put SOME insulation on top ?

I was thinking of doing 125mm between and 50mm over the top. I could go for more over the top if needed. It has been mentioned you can put insulation in the void, so long as you have a good amount above.

 

[^woody^]

None at all. But your proposal is effectively a cold roof

You will need a greater u-value above than below, unless you can do a condensation risk calculation.

 

[tomfe]

It's not a cold roof or a warm roof, it's a hybrid roof.
They can work but at the moment they are normally found upon due to interstitial condensation risks.

 

[trevorbayliss]

I cannot see how that much condensation can get in there if i do the following:

Plastic membrane below joists.

Joists filled with 125 insulation.

silicone all joints where kingpin sits next to each joist.

50mm kingspan over the joists.

OSB board and fibreglass.

Make sure where roof and cavity walls meet attention is paid here.

Spoke to BC today and he said this roof would be fine.

How do you do a condensation risk calculation ? Is there any info on the web about this ?

 

[tomfe]

Kingspan will do yo one for free. So will celotex as well probably, not sure if kingpin does it though, he's too much of the boss.

I guess you won't be having any down lighters or speakers in the ceiling then?

 

[trevorbayliss]

No lights at all in ceiling, all wall mounted. No speakers etc.