ᐅ Rafter Insulation – Recommended Thickness and Costs?

Hello everyone,

We are doing the insulation between the rafters of the roof and the upper floor ourselves. I am now wondering what thickness is necessary. The attic will only be used as a storage and heating room. According to the energy calculation, we need 24cm (9.5 inches) of glass wool insulation with a thermal conductivity of 0.035 W/mK in the attic. However, the rafters have a thickness of 23cm (9 inches). I think that 24cm (9.5 inches) of glass wool should still fit here. Only a climate membrane foil will be applied afterward, with no additional cladding.

We also want to insulate the ceiling of the upper floor for soundproofing. The ceilings will be finished with drywall anyway. Behind it, we plan to install 24cm (9.5 inches) of glass wool insulation with a thermal conductivity of 0.035 W/mK plus a climate membrane. The wooden beams have a thickness of 24cm (9.5 inches).

Does this plan seem suitable as is, or would you suggest a different solution for the upper floor regarding costs?

Best regards

Mat91 schrieb:
For the energy calculation, we need 24cm WLG035 insulation in the attic. However, the wooden beams are only 23cm (9 inches) thick. I think 24cm (9.5 inches) glass wool should still fit. Only a climate membrane foil will be installed afterwards, no further cladding.

The terminology is a bit confusing. Just to be sure:
You want to use your attic / loft space only for storage purposes, and the heating system (gas?) will be installed there. You want to insulate the pitched roof slopes. The rafters are 23cm (9 inches) wide.
In the floor below this storage room, you also want to insulate the ceiling, which consists of 24cm (9.5 inches) beams, for soundproofing purposes.
Is that correct?

Attic / slopes:
Instead of compressing 24cm (9.5 inches) WLG035 insulation into 23cm (9 inches) rafters, you could also use 22cm (8.5 inches) WLG032 glass wool. It might cost about €3/m² more and will not be compressed (which is not allowed).

Upper floor ceiling:
What type of noise are you trying to reduce there? A gas boiler doesn’t produce that much noise, so it might not be critical. Additionally, with a full-rafter insulation of 24cm (9.5 inches) WLG035, the heating is no longer in the living area, so all losses are outside. With new heating systems, that’s not that significant, but it might not be fully accounted for in the heat demand calculation.

I would consider less thermally insulating soundproofing solutions. I can’t say exactly which ones—that’s not really my area—but surely someone here will be able to help.

Thank you for your reply. What you wrote was exactly what I meant. However, 23cm (9 inches) seemed a bit odd to me, so I measured again. It is actually 24cm (9.5 inches) as well. So, I will go with 24cm (9.5 inches) WLG035 for the sloped roof sections.

Regarding the upper floor, I already thought that adding glass wool insulation again would be excessive. My assumption is that sound can spread between rooms if we don’t use insulation between the rafters on the upper floor. The attic floor consists only of OSB panels.

Maybe this gives you a better idea: we are building a town villa with a hipped roof.

Mat91 schrieb:
that sound also spreads between rooms if we do not use insulation between the rafters on the upper floor.

I didn’t say you shouldn’t install any insulation there—just something different than 24cm (9.5 inches) of glass wool. For soundproofing, a combination of mass, air spaces, and insulation material is effective. So, OSB board – air gap – 10cm (4 inches) mineral wool – double layers of drywall or soundproofing panels.

But as I said: soundproofing is a detailed subject—better wait for someone who really knows about it.