Warm roof detail

Rock12

Hi, I just want to run my warm roof make up by you to see if you have any comments on this make up. It is a 15 degree pitch roof which will have 150mm PIR between joists and an airtight layer on the underside.

It will be:
-metal external roof
-OSB3
-vapour membrane
-60mm PIR
-225mm joists

Also, if there were an 18mm gap between the 60mm PIR on top and the 150mm PIR between the joists would the air gap act as insulation or would it make the 60mm insulation obsolete? I have battens to fit on the jousts and may lay them cross ways if it doesnt cause an issue with the insulation layed on top of them.

Thanks for any info.

Poor Uncle Tom

What is the purpose of the 18mm gap?

Rock12

It's a structural detail, due to long spans and a cantilever I have to fix 18mm ply to a section of this roof to strengthen it. This leaves me with a need to batten the rest of my roof with 18mm battens to make up the difference to keep the roof flat on top to lay my metal roofing.

Every change ripples through the build.

Poor Uncle Tom

Rock12 wrote: »

It's a structural detail, due to long spans and a cantilever I have to fix 18mm ply to a section of this roof to strengthen it. This leaves me with a need to batten the rest of my roof with 18mm battens to make up the difference to keep the roof flat on top to lay my metal roofing.

Every change ripples through the build.

Indeed it does, but the 18mm is no longer a 'gap' as it has a mass within it. If the volume of the '18mm ply/batten' is sealed (airtight) then it should not cause any problem.

I'm just interested as to why you are not putting all the insulation between the joists. Is it just to stop the element of cold bridging through the joists?

Rock12

Poor Uncle Tom wrote: »

Indeed it does, but the 18mm is no longer a 'gap' as it has a mass within it. If the volume of the '18mm ply/batten' is sealed (airtight) then it should not cause any problem.

I'm just interested as to why you are not putting all the insulation between the joists. Is it just to stop the element of cold bridging through the joists?

I have a timber edging the height of the insulation and battens put along the perimeter of the roof and I will seal this so hopefully this will negate any air movement in the 18mm gap.

Yes, the joists are in at 400mm centres so this would prove quiet a substantial cold bridge. I hope the 60mm will stop this, any more thickness in insulation on top of the joists would require an expensive fixing to be used.

If I fit the breather membrane on the insulation and then fit my OSB ontop of this, is there a need for a ventilation gap for the membrane in your opinion?

BryanF

roof finish, 60mm PIR, ply, full fill between timbers with Mineral wool or cellouse then AT variable layer