ᐅ Insulation Between Rafters – How to Install a Vapor Retarder When Using Spruce Boards?

Hello,
we are originally from Germany and now live in Bern in a 100-year-old house.
We are about to renovate the mansard floor (attic). A complete reroofing would cost in the six-figure range, so we have to postpone that for now. Therefore, the roof insulation will be done somewhat differently.

Continuous 3cm (1 inch) thick fir boards are installed on the rafters, and on top of these are the battens and the interlocking roof tiles.
We plan to install insulation between the rafters using mineral wool, a breathable membrane, and then gypsum board on the inside.
The question is how to handle the exterior side. Should a vapor retarder be installed between the mineral wool and the fir boards and sealed to the rafters, or should the rafters be completely wrapped?

Best regards from Bern
HausBern

Hello and greetings! The order of the insulation and the position of the vapor barrier are correct. Please attach the sheets only to the rafters. Completely “wrapping” the structure is possible but very challenging and does not improve the result. Make sure that the “top side” of the sheet faces outward; this is essential for the vapor diffusion to work properly. Also, ensure overlap between sheets. The end of the sheet should not rest on the masonry but on the eaves board, to prevent condensation from entering the masonry. Use the appropriate adhesive for the membrane (blue or green), preferably from the same manufacturer as the sheets. Have fun with your project!