Questions about roof insulation

Hello everyone I am new here in the forum. I recently bought an old wooden house (built 1920). The roof was renovated about 5 years ago but not insulated. I now want to properly insulate the roof. It will not become a living space, because after insulation it is too small. Now I have questions about this: - The first layer of Homatherm Holzflex 100mm will be clamped between the existing rafters. The second layer (exactly the same product) I want to simply attach on top (double-sided tape on the lower layer or on the rafters), but without doubling the rafters (lengthwise/crosswise) because then I create fewer thermal bridges. However, this causes the problem that I cannot properly attach the finishing OSB-3 board 18mm. I could now screw wooden cubes 100x100x100mm onto the existing rafters at certain points and attach the boards to these or drill through the 100mm Homatherm mats with long screws (probably extreme thermal bridge). Has anyone faced this problem before? I am very grateful for all tips and tricks. Many thanks, Nick

Hi Nick,

So you are already on the right track with the lengthwise and crosswise arrangement because of thermal bridges.

A professional would actually screw a 100 mm squared timber crosswise onto the rafters because the wood fiber insulation is a soft mat that needs a support structure. This makes the whole thing really stable and also provides a proper connection for your interior cladding.

Alternatively, there are also rigid, dimensionally stable wood fiber boards with the same insulation quality (040), I don’t know about Homatherm, but from the big three (Gutex, Pavatex, Steico), which you can also use as 100 mm under-rafter insulation. They are 2.50 m long, with tongue and groove or stepped joint, installed crosswise to the rafter direction (you place one onto the other, they lock together), get advice at the timber wholesale, they have the right screws for that (guess: 160 or 180 mm), not toy screws like those in the hardware store.

Then comes the vapor barrier directly (don’t forget!) or you can use OSB.

Happy working

Susanne Junghans

Perfect! Thank you very much Susanne!
Nick