Breathable membranes don't like the back being touched by anything, or they can become leakable membranes. I didn't understand how that's supposed to work when they rest on a rafter (except that the natural sag between rafters will direct any water away from them), so I used non-breathable when I needed to replace some of the old tar ones (which were non-breathable until they wore out). In my case, there's a vent formed at every sarking overlap, so there's no shortage of loft ventilation.
Breathing isn't the same as (or the opposite of) windproofing. Breathing allows a low flow rate with only a small pressure differential, windproofing prevents the flow rate becoming excessive even when the pressure difference is high (i.e. the wind is blowing). A breathable roof still shouldn't be draughty. Although an unsarked roof would be breathable, it will also be unacceptably draughty.
Be careful with ventilation above roof insulation. Condensation in that insulation layer will cause rot, rot you don't notice, and by the time you do the repair costs get pretty serious.
Andy Dingley coughed up some electrons that declared:
Ah, but the celotex and taped joints would prevent the wind from entering the loft/room space, surely. Not discounting what others have said aboutwater blowing in, but this was my original line of thought...
Indeed. Given the abysmal lack of effort to ventilate anything there right now, I'm surprised I haven't more rot than there is... But a roof rafter would be very difficult to replace.
When I eventually get round to this (it's in phase 2, much later...) I will put some little probe holes (like a short bit of 8mm pipe glued in) and plugs in various locations in the ceiling so I can stick an RH probe in for occasional monitoring.
Cheers
Tim
OK. What are you disagreeing about?
In article , Andrew Gabriel writes
A relative who is a building surveyor (working in Scotland) has reported that some contractors have expressed a preference for using tougher, non breathable sarking even though it requires a greater ventilation space.
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