Hi all, I'm part way through a kitchen renovation and wondering what to do about insulating the ceiling (between kitchen and cold loft space). I've got plenty of depth between the K&L, so 300mm of loft roll will fit fine (not compressed), but wondering what the best approach is for vapour control here. I'll use non-hygroscopic loft roll so not overly concerned about mould forming, but should I be using a pure barrier, e,g, polyethene, or a vapour control layer that actually lets some moisture through in high humidity situations? Obviously the barrier option is less costly, but neither are out of scope so want to just get it right. I would staple it to the underside of the ceiling joists and lap the seams so it's got good coverage. FYI, for the adjoining walls I'll have a continuous layer of foil-faced PIR, with the seams taped, which will provide a vapour barrier here. The VB/VCL on the loft would be taped to this to provide continuity, but I guess I would expect most of the moisture to want to travel up to the loft if it's being allowed to... Also, any recommendations on brands etc appreciated. Thanks for your help!
Vapour barrier is the only thing to use. It prevents any moisture from below from getting into the insulation. If moisture were able to get into the insulation then it would condense at the condensation point (usually somewhere inside the insulation) and would accumulate as water, which would then render the insulation less effective. If you have the choice, I would use rigid insulation in preference over the woolly insulation. Firstly you get more insulation per millimetre of thickness and secondly it is less likely that moisture will enter it to any significant depth from the loft above it. If you have removed the ceiling in your kitchen, then the best place to put your vapour barrier is directly behind the plasterboard.
Lighting isn't mentioned so just making you aware that perhaps Loftlid / Thermahood might be something you'd be interested in.