Roof Insulation - Air Gap Needed with Breather Membrane

I'm shortly going to be insulating a the upstairs of my dormer. The rafter sizes on the roof are only 6 x 2's, which doesn't leave me a great amount of dept for insulation(150mm). I'm going to be insulating between the rafters with a rigid insulation board like xtratherm/kingspan etc. Someone has advised me that if a breather memebrane if used on the roof then the recommended 50mm air space between the breather and insulation is not needed. Does anyone know if this is true, or should the airspace still be left even with a breather membrane?

thanks,
dj01

dj01 said:

Does anyone know if this is true, or should the airspace still be left even with a breather membrane?

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The short answer is "It depends on the (breather) membrane".

How certain are you of the membrane type installed? I suggest you give the tech support dept of the insulation manufacturer a call - they will advise you.

Personally, I'd be inclined to leave an air gap and ensure good cross-ventilation regardless. Additional board thickness can be fixed under the rafters (cross-boarded) to get extra R value and protentially a vapour barrier (with the right product). If you go with a between-rafters (only) solution then each rafter counts as a cold bridge, reducing your calculated insulation effectiveness. Call the companies, give them your element dimensions, they will give you an authoritative answer.

hth

Thanks Mysticcoil. The breather is Solitex Plus which has come recommended as one of the better products available. I will contact them and see what they advise - thanks again.