Insulating between rafters

[Brett James]

I've recently had a new roof (tiles + membrane) put on and now I'm looking at the easiest way to insulate it from the inside.

I've heard that putting Kingspan - or similar - between the rafters and ensuring that you leave an air gap between that and the membrane is one approach.
The plan is to then wood clad the rafters (looks better than plasterboard - unless there are any watch-outs?)

Has anyone done similar - and have any guidance or advice?

 

[cdbe]

It's fairly straightforward - 50m air gap, ventilation at top and bottom of roof (check that mineral wool isn't interfering at the eaves). Try to get at least 50mm celotex between (if the rafters are only 3" deepen them with a bit of timber), put another layer of celotex under, if it's not being done to building regs then another 50mm is reasonable and will allow you to fix your cladding without too much trouble. Foil tape the joints to form a vapour barrier. You could form a small ceiling in the apex and use mineral wool insulation above it - bit cheaper.