Flat roof insulation

Celotex is good, if expensive. Make sure you have a ventilation gap and adequate air flow to prevent nasty rot and stuff.

You might consider fibreglass instead of felt - can last a lot longer if done correctly.

HTH Dave R

David W.E. Roberts wrote

It's common these days to have a warm deck roof, where the insulation board is interleaved in between the layers of felt, with a layer of felt underneath acting as a vapour barrier. That way there is no risk of condensation on the roof timbers or decking, and no need for ventilation. The insulation board would typically be Celotex type foam board, although usually a cheaper brand. This method marginally increases the risk of thermal shock to the top layers, which could cause splitting, so it's advisable to use a polyester based high performance cap sheet. The roofing company will know all this.

Peter