Warm roof ratio

Ak84

Hi all.

Originally planned on 80 mm phenolic above rafters on my extension, but it's probably not possible now due to truss roof size and height restrictions, joining to original house.

If I only had 20 mm insulation above the rafters, would this still work as a warm roof, or hybrid roof. Obviously the rafters will be filled with insulation and VCL under it again.

10-10-20

Some of this is covered off by Steve when talking about flat roofs. If the insulation is above and between the rafters then it's a hybrid roof.

https://www.youtube.com/watch?v=RthYYgzhMWU&t=180s

You'd need to get the roof engineered so that the DP is identified and you can be assured of a working solution.

Ak84

The above is from the changeplan.co.uk calculator.

Dew point is in the middle of the rafters. I dunno if this would be a problem.

Anyy thoughts.

I know I will probably have to get an engineer anyway.

10-10-20

Is that small cavity ventilated? Presume not. Plus are the rafters factored into that simulation or is that an insulation-only simulation? DP is supposed to be in the insulation. As long as it's well sealed at the membrane then there should be little moisture to condense but bear in mind that a VCL still allows some vapour to pass so that has to be matched at the external finish so that any buildup can escape.

(Not a building engineer btw, just a try-hard 😀 )

MicktheMan

If the top 20mm layer of insulation is phenolic, then, imo, this is an unsafe buildup.

Ginger83

https://www.boards.ie/discussion/2058327464/warm-roof-ratio

Can you give us any more information? Height restriction in which way? Rafter size?

We built an extension with a flat warm roof. Upper window restricted the insulation thickness but a structural engineer speced it and the surveyor was able to fit enough to comply.

Ak84

New roof butt's up to original and has to be slightly lower as per drawings. Tight for head height inside the room as well.

Rafter size is looking like 9 inch. Was expecting 5 inch.

I should say this is a pitched roof.

Reason for warm roof was for a continuous insulation layer to meet external insulation.

But kinda looking like a "cold roof" now.

I think the hybrid roof is too risky in any case.

Have done a bit more reading since I first posted.

sydthebeat

you say its a trussed roof, have you asked the manufacturers of the trusses were a closer centers, can that give you a smaller rafter size?

Ak84

https://www.boards.ie/discussion/comment/121521358#Comment_121521358

Hadn't thought of that. Will say it to the builder.

Thanks Syd