Ventilation space in insulated roof

15 Dec 2008
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United Kingdom

I'm in the midst of getting a builder to build a hip to gable loft conversion.

As our house has solid wall construction and is not very airtight we are keen that the new construction performs well with respect to energy efficiency.

I've been reading various websites about roof construction and I have some questions.

Some sites states that you must maintain a 50mm air space between the insulation and the roof tiles for ventilation.

Another site states that with a breathable roof membrane, building control will allow this air space to be reduced to 25mm. This would allow for increased insulation.

Anyone able to confirm?

The rafters in our roof are 89mm x 44.5mm so there would be less space for ventilation compared with the rafters that would be used for the dormer construction which I understand to be 145mm x 47mm.

So the air space in the pitched section of roof will significantly affect the amount of insulation that could be fitted between the rafters.

Are these breathable membranes good enough to allow a reduction in the air space without issue? Any particular brand I should insist on being used?

Previous experience has taught me to spec as much as possible to prevent the tradesmen fitting a worse performing alternative.

Many thanks
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I am not a trained engineer so this is in my opinion only.

From my research, you need to achieve U=0.18W/m2K for your roof sections, 0.28 for wall sections including the dormer cheeks (and fronts I suppose) and dwarf walls which are inside an uninsulated roof space.

I believe that with a breathable membrane you can have fully filled rafters (source: ecotherm) - however my engineer has specified air gap + breathable membrane.

My roof is 150x50 rafters, just like your new dormers, and there I will need 120mm-130mm insulation and 37.5mm insulated plasterboard to hit the 0.18. That will be on paper only, as there are double and triple rafters and small gaps/spaces, which are impossible to insulate other than with rockwool or foam maybe (which I am in the process of investigating).

The dormer cheeks are 100x50, and there I need 75mm insulation and 37.5mm insulated plasterboard to hit 0.23 (better than required 0.28). Obviously I would rather reduce the 0.23 to whatever I can, eg maybe 80mm or 85mm insulation, if they make it even. As my dormer cheeks and fronts are very large and I can imagine increased heat losses.

If you create space inside the untouched section of the old roof with the 90x50 rafters, I'd use the 75mm insulation + 37.5mm insulated plasterboard, that gives 0.23 more or less, which is miles away from 0.18, but better than nothing. I might even stick some rockwool below membrane, air can still travel in it for humidity purposes. But you'd have to replace the old thick felt with new membrane which means stripping the roof of old tiles.

You can also increase the insulated plasterboard to say 52.5mm to approach 0.20 or better for your 90x50 rafters.

And I would use a private building control company, not the council.

All in my opinion :)

Oh, the brand is tyvec I believe, this is what I am getting.
25mm clear air gap is fine. You just have to be more careful when fitting, I.e. don’t allow any ex-foam etc to expand into the void.
And I would use a private building control company, not the council.
Thanks for your reply. I just wondered if you could expand on this point? I’m new to building works so not aware of any pitfalls.

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Many years ago I hired a builder to make some foundations for me and he used a private building control firm for inspections etc, and since then I stuck with them. They are more amenable, I am told. Although it also depends on the person visiting, they can sometimes be awkward and ask you to do extra things.

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