Evening MP1.

 

I used the Firestone EPDM 1.5mm thick, a little more expensive that the thinner stuff (1.2mm) but a failure here could let water into the warm roof. If this happens it will not get back out easily, the OSB would be particularly vulnerable. Considering what I was spending on the insulation, the labour effort, all the seam tapes and work to dress the second storey cills over the EPDM then I was ok going for the heavier EPDM.

 

If you are going to do this detail you show for the roof then you must make sure you get a good draft blowing through the 50mm air gap and that can be hard to achieve.

 

 

Edited April 21, 20215 yr by Gus Potter

Thank you @Gus Potter

 

 

 

One question, my roof buildup currently is a combo of what we all said here:

 

GRP Fibreglass
OSB 18mm
200mm PIR insulation
A Vapor  Layer
OSB 18mm 
Joists I beam (TBC, opinions welcome?)
15mm Plasterboard.

 

But that adds up to 18+200+18+15 = 251mm (or am I ignoring e.g. the joists somehow..). Isn't that super thin? For comparison, my main external walls are 435mm. Or is really the insulation sufficient (that's about the same) since the roof obviously has to carry very little load?

 

Yes, add probably at least 180mm+ for the roof joists.

Edited April 26, 20215 yr by pudding