Matt89

The plan is to keep the 50mm gap the rafters create between the breather membrane and the insulation

Thanks for your input everyone! To fill in a few details - the footprint of the house that the roof covers is about 7m x 9.3m. Some details on the current roof: Rafters are 50mm deep, the bitumen is laid directly on top of them The purlins fixed beneath the rafter are between 100mm and 110mm deep (very old timbers so a mix of depths) Plasterboard is fixed directly to the rafters, and buts up to the purlins so you can see the purlins inside the room That leaves a 50mm air gap between the top of the plasterboard and the bitumen for ventilation The cross beams and hips are about 300mm deep There is absolutely no insulation present in any part of the roof, and no eaves trays - when the wind picks up you feel it blow through the room (so I guess it's well ventilated at least!) In terms of the amount of insulation to put in - given there is nothing at all and it's extremely draughty, putting in anything is going to make a huge difference. Having measured again in feels getting in 100mm PIR should be totally doable by having the plasterboard fixed to the level of the bottom of the purlins. 150mm is going to get a bit more complicated as it would mean re-doing all of the cupboard doors and likely not leaving enough clearance for the radiators. I will need to check in with the regulations - if it's mandated that 150mm has to go in it's going to be a bit more complicated.

I live in a semi detached Victorian house with a loft room. It’s a pitched hipped roof with 2 dormers. Currently there is absolutely no insulation in the roof and it is very draughty, the room is completely unusable in the winter and there’s no door so getting it insulted is a priority. There isn’t much space between the roof tiles and the plasterboard so the ceiling needs to be dropped to accommodate 75mm PIR boards. Turns out the tiles are asbestos so need replacing as well. I have received a quote for £60k including VAT to get it sorted - this is way higher than I was expecting. It covers scaffolding, skip hire, new roof batten, new tiles, eave protection trays, new breather membrane, new ridge tiles, 75mm PIR plus gapotape, dropping ceiling and replastering, re-rendering the chimney stack and redoing the roof on a bay window. That is quite a bit of work but still seems like a lot - is this in the ballpark of reasonable?