U-value of insulation between rafters (loft conversion)

Condensation risk at rafter level

Insulate along the rafters and the cold face of the insulation sits just below the underlay, so the layers either side of it — the vapour control inside, the underlay outside — decide whether moisture gets trapped. As pre-filled, with foil-faced PIR between and under the rafters, U-Monkey’s free BS EN ISO 13788 check, with the roof surface taken 2 K below the air for night-sky cooling as the standard asks, finds no condensation in any month, though the margin is tight, at about 98% of saturation inside the outer board in December. It calls that a conditional pass, for a reason worth knowing: BS 5250 treats an underlay above sd 0.05 m as high-resistance, which needs a ventilated void beneath it, and the generic breather is entered at exactly 0.05 m. Enter your underlay’s declared value; most modern breather membranes are well under it. The pass also relies on the under-rafter PIR acting as the air and vapour control layer, so every joint needs taping and every penetration sealing.

Swap the PIR for mineral wool and the membranes take over. Under a vapour-permeable (LR) underlay with nothing on the warm side, the check will not call it a pass: BS 5250:2021 expects an air and vapour control layer on the warm side of a warm pitched roof, and the calculation itself predicts heavy seasonal condensation under the underlay. Add a vapour control layer behind the plasterboard and that drops to under 2 g/m² that dries out each summer. Under a traditional 1F bitumen felt with no VCL, condensate builds up year on year on the underside of the felt. A ventilated 50mm gap between felt and insulation clears the condensation, but BS 5250 wants the VCL as well, and only with both does the check pass it.

It is a screening to BS EN ISO 13788 (the Glaser method), honestly labelled as one: it does not model air leakage, rain or moisture stored in materials, so for high-risk or heritage constructions a full hygrothermal simulation (BS EN 15026) is the right next step. Figures above use the app’s defaults — Manchester (Ringway) climate, humidity class 3 (dwellings with high or unknown occupancy), a 1-in-10 design year and, on a roof, the surface taken 2 K below the air for night-sky cooling — so pick your nearest weather station for your own build-up: the result can change from one station to another.