I have built a large workshop and the builders have used gavanised sheets on top of plywood (untreated.) they have put a permeable builders membrane underneath but have left an area of 500mm at the front as they ran out. It has already got alot of mildew at the front. I will of course put some now along the front area but I am concerned re the word breathable permeable as i would have thought that a non permeable would be better. The barrier is only there to protect the timber from condesation from the galvanised sheeting isn't it.
I saw 'Protect A1 roofing underlay cheap in Screwfix. Is this better?
