Hello,
Today I saw a construction I felt a bit uncertain about, it's regarding the air gap for a shed roof. They haven't used the traditional tried-and-tested solutions with air gap boards made of masonite, corrugated cardboard, polystyrene, etc. Instead, they have rolled out a strip of underlay paper over the insulation, centered between the rafters (about 20 cm open on each side of the rafter). This means that the weight of the underlay paper presses down the insulation from the sheathing and creates about a 7cm air gap; the paper is not fastened anywhere but lies loosely on top.
My thoughts are that they have used a material that is not intended/adapted for that construction. Could there be a risk of condensation forming on the underside of the paper against the insulation since it is completely sealed and vapor pressure remains against the paper and then drips down into the construction? Or is the dew point further down in the construction, about 390mm of insulation? Is it really good for the insulation to be exposed openly so close to the sheathing, is there a risk for future moisture issues? Will the underlay paper really stay in place since it is not anchored anywhere?
What do you think, am I completely off base, or is there a point to the whole thing?
/Pimme
