How best to insulate in and around rafters

[Beau]

One side of the room is vented to a lower loft and the other side has no vents at the eaves but zero evidence of damp issues. The rafters are 4x2 and I'm wondering if I can part insulate the gap between the rafters as well as under the rafters. The roof has a non breathable felt but plenty of air comes through the laps of the felt. I'm planning to just use mineral wool between and above the ceiling joist. Whats peoples thoughts? 

[Beau]

I will ask another way. How big a gap is advisable between the roofing felt and a PIR board on the underside of the rafters to allow ventilation?

[Susie]

I think it’s 25mm air gap minimum but our BC suggested 50mm for us to allow plenty of air in at the eaves and out at the ridge. 
 

[Beau]

9 hours ago, Susie said:

I think it’s 25mm air gap minimum but our BC suggested 50mm for us to allow plenty of air in at the eaves and out at the ridge. 
 

Thanks Susie

 

I've hunted around the internet for info and found others suggesting 50mm so I will go with that. 

[Super_Paulie]

50mm gap was required up here in the North East.

Could always increase the rafter size with battens and install rigid PIR which has twice the U value? or install PIR anyways, pretty simple to do.

[Beau]

3 minutes ago, Super_Paulie said:

50mm gap was required up here in the North East.

Could always increase the rafter size with battens and install rigid PIR which has twice the U value? or install PIR anyways, pretty simple to do.

Yes I was intending to use PIR between and below the rafters. Mineral wool above and between the ceiling joist as no presure on space there

 

Thanks