97mm Joists What thickness of Rigid Foam Insulation?

[bluewire]

I want to put rigid foam foil backed insulation between my rafters and probably on top at a later date + boardout.

I've measured my joists and there actually 97mm so my question is do i buy 100mm or 95mm board.

Reason I'm not sure is that if I use 100mm and board I'm worried it might push the plasterboard down. 95mm will obviously sort this but then leave a small gap were condensation might form.

Hope you can help. Thanks

 

[freddiemercurystwin]

So you are insulating a roof, is this as part of a legal conversion or just making the loft a warmer place?. Do you have felt beneath your tiles or nothing or a breathable membrane.

I suspect you may not like the answers following your replies.

 

[bluewire]

Sorry I ment joists as in the title. No its not a conversion. I just want to replace my glass fibre loft insulation with foam board. So I don't have to have 270mm depth.

Its a ventiled loft space eaves and ridge tiles with felt.