Help, I’m a girl and definitely not a builder, possibly taken on more than I can chew but here goes. we are self building a kitchen extension and due to pitch 15 degrees and no knowledge we chose to sark the lean to roof with plyboard before coving with rubber shield and then a rubber slate called Eco slate. This is a warm roof with 3 velux windows. On advice from the building regs we have cut a gap in the top of the sarking under the flashing and and used a hedgehog type plastic layer to allow an air flow under the flashing, the rafter have holes cut in them to allow an air flow from the soffits to the top. We’ve put 100mm celotex in between rafters leaving around 75 mm gap above between insulation and sarking. Our intention was to put a further layer of 50 mm insulation over the whole lot the plasterboard but we hasn’t anticipated this damp on the sarking ply which h is only appearing now I’ve put the insulation in. We screed the floor about 3 weeks ago and haven’t turned on the underfloor heating yet but I guess there may be a lot of damp in the air due to this. Saying that it is a kitchen so will be a fairly moist room. Will it help to have a polythene sheet between the plasterboard and insulation to stop the air getting up there in the first place? Do we just crack on and seal it with plaster and hope it’s ok??? I’m really hoping this isn’t an expensive set back as we are totally out of money.