After we finished insulating the walls and roof with sheep wool, we added vapor-open wood fibre boards. For the walls we used 6 cm of Gutex Thermowall. These panels serve a few functions: they act as an extra wind thight layer because the boards perfectly slide into each other; they add an extra thermal layer, continuous outer insulation that prevents thermal bridges where the wooden studs interrupt the wool insulation; they are also more vapor-open then the sheep wool, allowing moisture to easily travel from the inside to the outside.
Vapor permeability, from inside to outside:
ESB, 12mm | Schapenwol, 140mm | Gutex thermowall, 60mm |
0,48-0,96 µd | 0,14 - 0,28 µd | 0,18 m µd |
A good wall assembly is like a breathing membrane, more resistant inside, more open outside. That doesn’t mean that every layer has to be progressively more open in strict order. It just means moisture should never be trapped between two highly resistant layers.