I've just insulated my back bedroom by taking most of the ceiling down and putting 50mm Xtratherm batts between the joists/rafters.
I'm now intending to put up 40mm insulated plasterboard to increase the insulation. The bedroom's directly under a pitched roof and from the top of the inside wall (the one near the eaves) the plasterboard will be a straight slope up for 5 foot and then for another foot it will need to be flexible enough to bend a bit to then meet the rest of the flat ceiling. I'm just wondering if 40mm backed PB will be flexible enough to bend like this.
Also, I'm wondering what the best fixings are to fix the PB to the joists. I think they'll need to be about 75mm long but I've read that if you use metal screws, this allows thermal bridging.