Insulating between garage rafters.

[HJC1972]

I've got to insulate a detatched garage flat roof that I'm turning into a studio/workshop. At the moment the roof is just felt covered ply, sitting on top of the 8x2s. I have aquired enough 100mm celotex to insulate the whole roof but I'm unsure wether to place the boards so that they are flush with the underside of the ply roof, leaving a cavity below the boards, or wether to place it so that it is flush with the bottom of the rafters (which I an going to dry line on the underside) leaving the cavity above. Your thoughts please?

 

[freddiemercurystwin]

In an ideal world you would insulate the joists at ceiling level so leave a gap under the ply and then ventilate this void between each joist. If you cannot be bothered to ventilate this void then you should push the insulation tight up to the underside of the ply so that you leave no void at all. Use foil backed plasterboard.

 

[HJC1972]

Hi,

Thanks for taking the time to reply.

Assuming that the celotex boards are fitted flush with the bottom of the rafters, if I was to use a vapour barrier, would this negate the need to have the cavity above the insulation ventilated?

Or alternatively, If I do ventilate it, would I have to ventilate each and every cavity at the facia board, or could I do, say, every third one and then ventilate between the rafters internally with a series of small holes drilled in the rafters?

Once again, thanks for the reply.