ᐅ What factors should be considered when installing a new roof?

Hello,
I want to have my roof re-covered and insulated.
a) What should I pay attention to when getting a cost estimate?
b) What insulation is standard today (is WL40 sufficient, or is WL35 recommended)?
c) How thick does this thermal insulation need to be (100–140mm (4–5.5 inches))?
Thank you very much for your help.

The insulation thickness is inversely proportional to quality. For new constructions, typically 18cm to 22cm (7 to 9 inches) of insulation with a thermal conductivity of 0.035 W/(m·K) is installed. If a thinner layer is desired, a higher-quality material must be used. What is feasible in your case should be checked by a professional on site. Often, old, sparsely sized rafters cannot support the additional loads that result from modern standard renovations or expansions. A structural engineer should be consulted for this. Additionally, the exact required insulation can be calculated.

Proper planning of the individual layers is also important to avoid thermal bridges, moisture damage, or mold growth.

During installation, it is essential to ensure that the insulation is sealed airtight on the interior side and remains undamaged.

Speaking of insulation: Two years ago, I installed 16cm (6.3 inches) of full-rafter insulation in our attic, which is now going to be converted. Back then, I only placed it between the rafters. Now I want to continue working on it (adding another 5cm (2 inches) across the rafters first and then installing an airtight membrane).

Could the insulation have lost some of its thermal performance due to the absence of the membrane for over two years?

Subjectively, I don't notice any loss in insulation.

so long :-?

If you want to be very precise, you should check the airtight sealing of the insulation with a blower door test. This test can also detect other potential leaks in your house. It’s not very expensive and can save you a lot of trouble with mold growth and drafts!

We were offered Rockwool insulation with a thermal conductivity of 0.035 W/(m·K) and 12cm (5 inches) thickness. Is that sufficient for a roof renovation? According to the roofer, it should be very good.

to the last anonymous poster:
12 cm in 035 is a joke; no offense intended, but the state of the art is different now, as are the current regulations.
Everything else has already been mentioned further above.