ᐅ Roof insulation made of 100mm thick rigid polyurethane (PU) foam

@Nordlys Of course, insulation between and under the rafters is sufficient. It’s a tried and tested method that works well. Combined with insulation over the rafters, it’s even better—especially when the beam thickness limits the insulation thickness.

From inside to outside: wallpaper, filler, drywall, OSB board, mineral wool with horizontal battens perpendicular to the rafters, vapor-permeable membrane, insulation between the rafters, and optionally insulation over the rafters. Make sure the underlay membrane is vapor-permeable.

More insulation is always better, but at some point, it becomes economically inefficient.

@Ankeffm (Ffm like Frankfurt am Main? I’m from Neu-Isenburg)
I understand your uncertainty and have been there myself. In the end, I am just an amateur who has read up on the topic. One of the contributors said: mass is everything for thermal and sound insulation, and he’s right. And mass doesn’t mean a disadvantage in cold protection—you just need to know the right amount.

Regarding the options:
Insulation below the rafters is not advisable in your case because it reduces your living space, and you will be opening the roof completely for the work anyway. Instead of gypsum board for wall cladding, you can also use wood fiber or clay plasterboard. That depends on your budget. It also contributes to thermal and sound insulation. A renovation company I contacted suggested exactly this and referred to a list of references in the airport area...

Insulation between the rafters is sensible and reduces the overall thickness compared to insulation above the rafters. However, it is more labor-intensive because the insulation has to be installed in two steps: first a layer between the rafters, then a layer on top of them. Using only insulation between the rafters will not achieve the thermal transmittance value (=U-value) required by the energy saving regulations. I believe this value is around 0.14 for roof renovations.
Also, some sort of battens or sheathing must be fixed to the inside of the rafters as a base for the insulation, and the insulation mats must be precisely cut to fit the varying rafter spacing. This means more working time and material waste. In my house from the 1950s, almost every rafter spacing is different.

Insulation above the rafters is good because it involves only one step: laying thick panels on the rafters or on mounted sheathing. Roofers prefer this method. Another advantage is that the roof is completely enclosed by a continuous insulation layer.

In new builds, a combination of insulation between and above the rafters is usually used to avoid an overly bulky roof structure. Adding 25cm (10 inches) or so on an existing roof can significantly change the appearance of the house.

You could do this combination as well. You could do the insulation between the rafters and the drywall or cladding yourself. But the position of the vapor barrier in the entire assembly must be considered carefully... and you shouldn’t compromise on this. An interior vapor barrier is sometimes difficult to install because studs or purlins get in the way. I have often read that this part is recommended as DIY work because homeowners tend to be more careful than contractors... which, unfortunately, matches my experience too. Building cheaply means building twice! To verify this, a blower door test can be done if you want.

I recommend trying out the options using a U-value calculator. All building materials are included there. If you are completely unsure, I would ask an energy consultant to design a roof assembly for you, which a roofer can then implement.

Alternatively: just call a field representative from a wood fiber insulation manufacturer and get advice. I did this informally myself. Whether you choose one manufacturer or another is usually decided by the roofer. Most insulation materials, especially PUR, come from the same factories and are just sold under different brand names...

So pick up the phone, read online (Steico and Gutex have very good brochures for your purpose), calculate with the U-value calculator, and if in doubt, contact an energy consultant or architect and clearly state what you want!

My advice: I would rather consult an energy advisor than an architect.

Don't forget about the available subsidies. So, hire an energy consultant who is listed with the KFW.

Hello
@roof_builder
Thank you for your detailed report... very, very interesting.
Yes, we are from Frankfurt... hello, neighbor
I have now tried out this U-value calculator... and as I understand it, a continuous insulation above the rafters with wood fiber boards plus insulation between the rafters is the best solution for sound and heat protection, and of course also for cold protection.
I am currently looking for a good manufacturer of wood fiber boards for the continuous insulation above the rafters.
Yes, I have also considered consulting an energy advisor or architect... but I guess they wouldn’t recommend anything different anyway.
It will remain with the continuous insulation above the rafters (since the entire roof is being redone). I imagine that such a consultation may not be cheap either.
The fact is that these PU boards are not particularly effective for heat and sound insulation... and that is exactly what I want to avoid.
If anything, these days everything should be effective.
We will do the insulation between the rafters ourselves. I think we can manage that. The important thing is to install the vapor barrier properly and, above all, airtight.
@ Many thanks to all other contributors
And always happy to receive more opinions, tips, help, etc., etc.
It’s great that people help each other here like this.
Thanks and best regards, Anke

The KfW consultant is also subsidized. Please at least check the KfW website. I assume that with on-roof and between-rafter insulation you will reach the required value. In that case, it’s worth it anyway because you want to do it regardless.