Insulation air gap needed with no roof membrane ?

[wwwebber]

Hi, the roof on our house has no membrane and thus you can see the tiles from within the loft space. Current insulation is simply 50mm polystyrene on top of the 75mm deep rafters which I intend to remove.

My question is - as there is no membrane can I get away with using 50mm PIR between the rafters and leave a 25mm air gap ?. I'll then put some additional insulation below the rafters - any suggestions here ?

I don't want to batten the rafters or I'll lose valuable height - thus the question. I'm not intending to involve building control and simply want to improve the existing insulation.

Thank you.

 

[^woody^]

You can insualte the full rafter depth with no air gap required.

But there will be potentially significant air leakage via the tiles, so ensure a really snug fit of the insulation between the rafters and fill any gaps with expanding foam or flexible caulk plus silver foil tape all joints. Leave no gaps.

But insulating rafters over a cold loft is a bad idea. Not only inefficient, but it will increase the condensation risk within the loft.

 

[wwwebber]

You can insualte the full rafter depth with no air gap required.

But there will be potentially significant air leakage via the tiles, so ensure a really snug fit of the insulation between the rafters and fill any gaps with expanding foam or flexible caulk plus silver foil tape all joints. Leave no gaps.

But insulating rafters over a cold loft is a bad idea. Not only inefficient, but it will increase the condensation risk within the loft.

That's good news - it sounds like I could go with a full 75mm PIR insulation as an option and then a further 30mm insulated board under the rafters should do it. Thanks for the tips regarding gaps.