Improve cavity insulation in old buildings

Hello everyone,

I hope someone can give me a quick advice here...

We bought a house from the 60s. The roof was renewed in 2001 (roof tiles, underlay membrane, glass wool).
Unfortunately, the rafters have a depth of only 110mm. "Sauerkraut boards" were attached under the rafters. We have removed these. What is the best approach now?
1. Leave the old glass wool (still looks good), double the rafters and clamp another layer of glass wool underneath
2. Double the rafters and use completely new, thicker glass wool
3. Attach new beams perpendicular to the existing rafters and clamp new glass wool between them (no cold/thermal bridge due to offset)

With all options, I do not want to lose too much space. What minimum thickness should the new rafter have?
A sub-rafter insulation of 30mm (lathing 50x30) is also being considered.

Thank you very much in advance!

If the old glass wool still looks good, I wouldn't see any reason to replace it. Perpendicular to the rafters seems to me the most sensible, then the thermal bridges are gone. If you're already doing the work, I would insulate properly right away, an absolute minimum of 50mm, clearly better would be nearly 100mm insulation. The additional cost for the insulation is hardly noticeable, so that overall you achieve 180 to 200mm of insulation. I think that is worth it in your case. Is the loss of space only on the slope? Do the few extra centimeters really make such a difference that you want to do without them?

Hello Garfunkel,

thank you very much for the reply!

No, you are right. The additional costs compared to the rest are rather low and at the moment everything is disclosed anyway. I am also concerned about the static structure. 110mm rafters and then screwing others of the same thickness underneath? A roofer is coming for another matter. I can ask him, but that will take a while.

Rafters 110mm
crosswise: battens 40x60 (are still available) possibly screwed upright (= +60mm)
vapor barrier
construction for drywall (50x30) with under-rafter insulation (= +30mm)
drywall

This would give me 190mm. Or is that rather uncommon?

Thanks in advance!

Sounds not bad at all. Let's wait and see what the roofer says and then get to the cordless screwdriver :)