Insulating an old victorian Dormer

I'm renovating our top bedroom and half the room is dormer style. I've no idea how to insulate this, could use some advice. The roof felt appears to be the old bitumen type. I'd obviously like the room to retain heat better but just as importantly, I want to reduce noise as we're on a main road. Its never been insulated before so no signs of damp/condensation and I'd like to keep it that way. Currently I think air can flow in under the eaves between rafters.

Do I need to keep a gap between the felt and insulation? Do I insulate between the pitched rafters up to the flat ceiling level and stop there or keep going? I guess it should be ok to insulate the flat ceiling part between the joists with loft roll above that. Do I use rigid type or Rockwool or a mix?

The rafters are only 75mm deep. I was thinking metal furring channels fixed across the rafters might be useful to give me another 25mm depth. Do I insulate between the metal channels? I thought 2 layers of plasterboard screwed to that would help down. Should one of the plasterboards be the insulated type with a vapour barrier? Thanks in advance all

keep the noise

 

Wow! that looks like a great room when its finished
Tricky to insulate thou. insulated plasterboard would be the easiest after fitting some kingspan/celotex between the studwork/rafters leaving at least an inch gap for ventilation.
I would consider putting an inch baton on top of all the studs and rafters to increase the depth.

 

50mm between 75mm across then foil back boards using long screws. Foil tape and foam joints/gaps.

 

Thanks for the replies gents, that's given me something to chew on. Do I need vapour barriers if I'm using Celotex? Best strategy for downlights? Build out a void space?

 

If you are using twin foil sandwich Celotex and ensure robust fitting of the foil tape to BOTH layers of Celotex and use foil back board, then I'd say no to additional VB.

 

No need to cut big holes in your insulation anymore with the new 22mm deep led downlighters

 

Brilliant gents thanks for that. I'm going to abandon downlights tbh and go with ceiling mounted lights. Great looking job noseall; I was wondering how to run the lighting power cables through the insulation - is that just a wooden block?

 

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