Insulating pitched roof

[tommyleestaples]

We are just about to insulate our roof space which will be used as a useable loft room. 

 

The new roof will be tiled with a breathable membrane. The rafters are 100mm deep.  I plan to install Celotex PIR insulation or something similar in-between the rafters and then over the rafters to get the required thickness and U Values. 

 

I have ready conflicting things regarding the air gap required between the felt and the insulation. I have read it should be 50mm gap but as we are using breathable membrane could we get away with 25mm gap?

[Redbeard]

I have seen examples with a 25mm gap and no problems. I have also seen horrendous condensation and mould on breathable membranes with a 25mm gap (and admittedly some other poor practice too). The Gov't Best Practice Guidance for rooms-in-roof says 50 (https://assets.publishing.service.gov.uk/media/61d727d18fa8f50594b59305/retrofit-room-in-roof-insulation-best-practice.pdf) and that seems to me to be a good a**e-covering 'belt and braces' (to mix my metaphors).