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

Yes, thanks for the tips, Nordlys...
Our attic will be used as a living space after the roof work is completed. That’s why thermal insulation and soundproofing are so important.
Are there other users here with experience in roof insulation???

Have you discussed with an architect or similar professional what is feasible and practical for your situation? Is everything being renovated including the interior, so that you will also have access from the inside?

I don’t see any reason to skip cost-effective insulation between the rafters. What is planned in that regard? If that is not sufficient, an additional layer of insulation above the rafters is typically installed when the roof is replaced. For protection against heat, materials with high density, such as wood fiber, are more suitable.

Only 10cm (4 inches) of insulation in total provides very little protection against heat. I would also avoid expensive insulation with a thermal conductivity of 0.023 W/mK; it is only used when there is very limited construction height available.

Hello Boxandroof, thanks in advance.
No, we have not contacted architects or similar professionals, only roofing contractors, including getting quotes.
The plan is:
1. To reroof the house. That’s why we were told it makes sense to install an exterior insulation layer on top of the rafters (roof battens insulation).
2. Insulation against cold and heat / The attic is intended to be used as living space.
3. We were told that if you install exterior insulation on top of the rafters, no insulation between the rafters is needed anymore.
Therefore, we wanted to remove the "old" insulation from the inside and cover it with drywall panels.
The products planned are this type: puren Protect N 023
So, should you rather use wood fiber boards for exterior insulation? Or would it be better to skip exterior insulation entirely and work with insulation between the rafters only?

There are many options. I would recommend reviewing and planning everything with an architect. If the roof space up to the ridge is going to be used as living space, effective heat protection is important, which is not achieved with your current approach.

Insulation between rafters: it depends on how much thickness can fit between the rafters, do you know that? This can be sufficient against cold, but heat protection won’t be very effective. In new construction, 20cm (8 inches) of 035 mineral wool insulation between the rafters is cost-effective and considered the minimum; in older buildings, less may fit.

It is better with additional insulation above the rafters. For heat protection, dense interior materials (not drywall) can also be installed.

Five years ago, I installed exterior roof insulation according to the energy saving regulations using PUR foam and a supposedly special insulation material that has a mineral wool facing of several centimeters (inches). Overall, I have a U-value of 0.14. I can’t tell you the exact thickness, but it’s definitely around 20cm (8 inches). One thing I can say is that the heat protection is poor, as is the sound insulation. I regret that neither the energy consultant, the architect, nor the roofer pointed out the benefits of wood fiber insulation given the airplane traffic around us. Due to its high bulk density, wood fiber insulation provides better protection against heat and noise. However, it is somewhat less effective for cold protection, meaning that a greater build-up height is needed. Nowadays, the common approach is a combination of insulation between the rafters (since that space cannot be used anyway) and exterior roof insulation. Most roofers won’t suggest this because it’s simpler to just lay a layer of PUR boards on the outside of the rafters. Many currently work with unqualified helpers to keep costs low. But that doesn’t really solve the problem.

If you want certainty, check a U-value calculator online—unfortunately I can’t share a link here (but you can search for Ubakus). Try inputting a hypothetical roof assembly and pay attention to the phase shift values. This should be more than 10 hours. Phase shift refers to the time it takes for heat to penetrate through the material. Everyone focuses only on cold protection, which is excellent in our roof. But in summer, after three hours of sun it becomes unbearable, not to mention after three days of sun.

Take a look at companies like Steico, Gutex, or Pavatex for information about wood fiber insulation. It is also more environmentally sustainable and does not create hazardous waste. So I wholeheartedly recommend wood fiber insulation. Of course, it is a bit more expensive than common polystyrene foam, but if you try to save money, you might end up rebuilding. I am now installing an air conditioning unit and adding additional insulation between the rafters with Steicoflex mats, counter battens, and Steico universal boards instead of drywall.

I also have a PUR insulation with a layer of mineral wool (about 5cm (2 inches)). I think heat protection is good; the apartment only gets warm after a few days and rarely goes above 26°C (79°F). In my opinion, using wood fiber insulation doesn’t make much of a difference. It simply heats up during the peak of summer, and there’s not much you can do about that.

The point about sound insulation is correct, it’s not particularly good. However, it probably depends on the angle. It’s louder under my dormers than under the sloped ceiling. By louder, I mean you can hear some low-flying planes, but under the slope, it’s not like that. If sound insulation is important to you, I’d recommend choosing something else, like rigid foam.