The log cabin has a pent roof with low pitch and 18mm T&G roof boards on which I will lay 50mm standard PIR insulation. My question is, is there any reason to then lay OSB before applying shed felt or could I just felt onto the insulation using 65mm clout nails which go through into the wood underneath? There seems to be conflicting advice: Dunsterhouse website says I should lay waterproof membrane before felting on top. SimplyLogCabins says that I need either OSB or expensive Kingspan TR26 which provides a sanded surface for the felt to apply to. So the question is, can shed felt be laid over polythene or PIR foil or not? Thanks!