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Hello, Looking for some advice with insulating a vaulted ceiling. The rafters are 100mm in thickness, with 2 vented roof tiles at the top. Is it as simple as insulating it with 100mm PIR board and then using insulated plaster board, or are there other layers that need building up, such as a vapour barrier etc? Many Thanks Joe

I have a vaulted roof, and wondering how best to insulate it as there is a section of the roof that won’t have any ventilation at the bottom of the rafters as there are no eaves. Difficult to explain, so let’s for the sake of keeping things on topic assume it’s a case of semi-detached houses – one has vaulted roof as in the pic below. No eaves therefore along the central valley. My construction so far is: Slates battens (50x 25mm) Breather membrane 175mm rafters My plan was to: fill in between the rafters with insulation (full fill with 200mm), leaving no ventilation gap as I have no ventilation pathway at the bottom of the rafters (i.e. valley). Taped joins. VCL Insulated Plasterboard NB: I don’t need a service void (will be using uplighters) If I fully insulate between rafters and ensure the VCL is carefully taped up along all joins, should there be much condensation risk? I have breather membrane on top of rafters and the ridge is ventilated, so warm air beneath the tiles has an ‘outflow’ at the top at least. Internally, I will have MHVR. As for insulation, I need to check u-values but was thinking of going for PIR as my rafter spacings are irregular in any case so would have to cut whatever I use. May as well improve u-values and use PIR rather than Frametherm. Probably a bit more of a pain cutting PIR to size, but would foam in every crevice/gap, then add VCL carefully. I have considered using foam (icynene) but concerned contractor will bodge it up with too much uplift of breather membrane. And also the cost is an issue! TIA!