In dormer cottages, (new roof or renovations) there is a section of the roof that takes a little thought to insulate well because the inside (room) and the outside (roof) are in very close proximity where the pitch of the roof cuts through the room, some times there is as little as 100mm between the warmth of the room room and the cold outside air. In a Swiss chalet type building this area can be quite large, in a dormer cottage its usually smaller .
A solution is to insulate both between and under the rafters. Superfoil SF19, SF40 or SF60 can all be used under the rafter in a COMBI arrangement with HD foam board between the rafters. Which Superfoil and how much foam board depends on the U value to be achieved (current building regs require at least U 0.18 , in Scotland its 0.15 for a roof insulated on the slope of the rafters ).
If the rafters are small as is often the case with manufactured trusses the depth of the rafters may be very limited and in those cases the higher performance Superfoils are used under the rafter . If the space between rafters is deeper as in a cut timber roof then more foam board insulation between rafters and the standard Superfoil SF19 under the rafter should give the desired performance. The pictures shown here are from a customers project, they have used SF40 under the rafters combined with 60mm of Kingspan K5 PIR foam board between rafters, ( a 25mm air gap between them). a 38mm batten is in place for the fixing of the plasterboard. The result in this project is a better than building regulations 0.16 U value.
With SF60, which the highest performance multifoil avalabe in Europe under the rafters you can achieve U 0.18 even on rafters as small as 85mm deep by using 20mm of foam board eg PIR board. Combinations of Superfoil and foam board to meet U0.18 include SF60 with 20mm of foam board, SF40 with 45m of foam board, SF19 with 80mm of foam board. Assuming Kingspan K5 or similar and 25mm air gap. Specifications can vary from project to project, ask us for a U value calc for your particular construction.
Other solutions using Superfoil in DUET format (2 layers ) in under or over rafters combinations are also possible to achieve U values as low as 0.11.