Gap between layers of celotex

[Frogeye]

Hi,

I'm currently insulating between my rafters with celotex, I am also insulating across the rafters with a further 30 or 50mm sheet of celotex to reduce cold bridging. I have a bit of query though as to whether it is okay to have a small void (25mm) between these layers as I have got diagonal bracing on the rafters and have added extra batten to the remaining sections of rafter to give me a flat (even) surface to fix to.

The insulation between the rafters is, in places, sitting behind the diagonal bracing and the second layer will be sitting on top of the diagonal brace - this is one of those confusing posts that would really benefit from a photo!! (I'll try and add one in the morning). I'm also considering a vapour membrane if taping the 'inner layer' of celotex boards doesn't look sufficient.

[ProDave]

Make SURE the ends of the gap are not open to cold air, otherwise it negates the insulation of the outer layer.

Perhaps 2 layers of over boarding, the first being just 25mm to match the thickness of the braces, then a further layer covering the whole lot.