Workshop roof

[The Bear]

Hi to all

I need some advice on insulating the roof of the workshop I'm building. The workshop itself is built of 9 inch aircrete (thermalite) blocks.
The roof is slated. underneath is a breathable roof felt that the local roofing supplier recommended. This is over 4 inch rafters.
I intend to fill the voids between the rafters with insulation and then put 6 mm ply on the underside to finish the inside.

I have 2 questions which I think go hand in hand.

Firstly do I need to leave an air gap between the top of the insulation and the bottom of the felt? The roof suppliers said I didn't. A builder friend said I should to prevent the roof sweating and condensation forming, even with a breathable felt.

Secondly should I put a vapour barrier (ie polythene sheet) behind the 6mm ply, ie underneath the insulation?

Insulation wise I want to use rockwool/fibre glass to keep costs down. However as this expands so much I could use cavity bats (fibreglass that doesn't expand when unrolled) if I need to keep a space below the felt. Failing that I know a supplier of half price kingspan/celotex if I need to go down this route but this would still be expensive.

Any thoughts or experiences greatly appreciated.

Mark

 

[OLD]

Your proposals are spot on no gap required and poly vapour barrier required any insulation type no gaps. Condensation will only form on under side of tiles .

 

[George_N]

When I had an attic convertion done some years ago the building control inspector made my builder rip out the rock-wool insulation and replace it with foil- backed PU foam board (Kingspan IIRC) presicely because there was no air gap between the insulation and the sarking. I'm pretty sure he wanted a 50 mm gap and all this after the plasterboard was up. The builder was not happy and his apprentice even less so as he had to crawl round the eaves to do it.