Renovating an old cottage - previous owner re-slated the roof well but unfortunately used an impermeable membrane (Protek A1). Clearly this causes condensation as the rising warm air is trapped.

My question is - if I insulate well, 100mm between rafters and also say 50/70mm wood fibre board then plaster - does that negate the requirement of replacing the impermeable membrane with a proper vapour permeable version? - which would be a very expensive option. My thinking is that with enough insulation no air is escaping upwards to cause condensation...

Any help appreciated. Thanks.

Thanks Mike. Roof is slate on wood sarking as is usual in Scotland. So not really possibility to introduce air gap. For info, the house is currently unoccupied i.e. no heating. My thought was that, properly heated and insulated, would mean no condensation (and therefore no real need for vapour permeable membrane).

Just for clarity, is this Glidevale Protect (not Protek) A1, like this: https://www.roofingsuperstore.co.uk/product/pitched-roofing/non-breathable/glidevale-protect-a1-roofing-underlay-15m-x-30m-pallet-of-49.html ?

Posted By: jackcarnan: “Roof is slate on wood sarking as is usual in Scotland. So not really possibility to introduce air gap.�

Can still have an airgap between the rafters below the sarking. E.g., some sort of board across between the rafters 50 mm below the sarking with insulation stuffed in below. Could be wood fibre board to double up as a bit of insulation. How deep are your rafters? You mentioned 100 mm insulation between but it's not clear if you meant full depth.

Yes, if that 50 mm gap is properly ventilated and there's a vapour barrier inboard (between the woodfibre and the plaster, presumably) then I think that's sensible and consistent with Mike and Tony's advice.

Thanks Ed and George but one thing still puzzles me - how can the 50mm gap can be sufficiently ventilated if there is a impermeable membrane above and solid insulation/vapour barrier/plaster below...

Apologies if I'm missing something.

Hi Jackaran. The ventilation path is from the eaves to the ridge between each of the rafters.

There are various ways of venting the eaves.

Eg. over-fascia ventilators or disc type soffit vents. Shown here https://www.google.co.uk/search?q=eaves+ventilators&espv=2&biw=1745&bih=868&tbm=isch&imgil=U_r0ElO1snFHLM%253A%253B0aWAyuNUjZ8-pM%253Bhttp%25253A%25252F%25252Fwww.monier.co.uk%25252Fproducts%25252Fproduct-catalogue%25252Fproduct-range%25252Fproduct%25252Fredvent-eavesvent-3.html%25253Ftx_monierproduct_single%252525255Btab%252525255D%2525253Dinfos&source=iu&pf=m&fir=U_r0ElO1snFHLM%253A%252C0aWAyuNUjZ8-pM%252C_&usg=__zbkQfR-qI_pUXaYlcQa5_jtBLFg%3D&dpr=1.1&ved=0CFQQyjc&ei=a65YVf6uC6qU7Qa05oHAAQ#imgrc=U_r0ElO1snFHLM%253A%3B0aWAyuNUjZ8-pM%3Bhttp%253A%252F%252Fwww.monier.co.uk%252Fuploads%252Fpics%252FRedVent-Eaves-Profile-CAD_01.gif%3Bhttp%253A%252F%252Fwww.monier.co.uk%252Fproducts%252Fproduct-catalogue%252Fproduct-range%252Fproduct%252Fredvent-eavesvent-3.html%253Ftx_monierproduct_single%25255Btab%25255D%253Dinfos%3B571%3B562

The ridge needs to then be ventilated with a dry ridge ventilation system.

Eg https://www.google.co.uk/search?q=dry+ridge+ventilation+system&espv=2&biw=1745&bih=868&source=lnms&tbm=isch&sa=X&ei=PK5YVYGKKem67gaChIPYCA&ved=0CAcQ_AUoAg