Loft room insulation

[bjmhughes]

Hi all.

I’m after some advice on loft room insulation and additionally required ventilation.

I plan to strip all existing lathe & plast from our Victorian detached loft room and re board & skim. Figured it’s worth insulating the roof from inside whilst at it. I’ve read a lot about how best to go about this with PIR inside and over rafters and happy with details re 50mm air gap and ensuring all gaps are sealed/taped to avoid thermal bridges. But the sticking point is that the roof currently has an impermeable bitumen felt installed and I’m reading mixed advice as to whether installing insulation under this (even with 50mm air gap) is a good or bad idea. Can anyone offer any insights or advice on this, grateful for any info you can offer. 

Also to note - the cold parts of loft space ( loft storage at either end and at top of roof are already ventilated by air bricks at either side in the gable walls. 

thanks in advance.

BH