Room cold at the feet - Is basement ceiling insulation useful?

Still some questions about it: - If you use 2x50mm instead of 1x100mm, you logically need twice the amount of adhesive (and have double the work). All this "just" for the advantage of being able to stagger the joints – is it worth it or is this really common practice? - Does the second layer even adhere properly with the normal basement ceiling acrylic adhesive when you stick two layers of aluminum foil facing each other?

If you use 2x50mm instead of 1x100mm, you logically need twice the amount of adhesive (and have twice the amount of work). All this "just" for the advantage that you can stagger the joints – is it worth it or is this really common practice?

You generally don’t do that. For example, if a pipe runs along the ceiling, you can simply make a notch in the thicker board. 50mm could hold without supports. 100mm will fall off the ceiling again without support.

- Does the 2nd layer actually stick properly with the normal basement ceiling acrylic adhesive, when you glue two foil facings together?

Acrylic adhesive? Use PU foam adhesive, for example from Soudal. It really sticks like hell. You could then also glue drywall onto the insulation and make the ceiling "nice".

Making the ceiling look nice is actually not necessary. Thanks for the tip with the PU foam adhesive. I interpret your "Macht man eigentlich nicht." now as meaning that one does not use 2 layers of 50mm each? And until now, there was no mention of supporting - the thick panels do not remain directly above with the adhesive?

Your "Usually one doesn't do that." I interpret now to mean that one doesn't do 2 layers of 50mm?

Exactly. There is also the 100mm insulation with stepped joints.

the thick boards don’t stay directly on the adhesive on top?

No. Supporting can also mean that you (depending on the board size) just hold the board in place with your hands for 3-5 minutes. Or you quickly build a T from roof battens and clamp it underneath. I bought the cheapest extendable supports for about ten each. For a 120mm board 120x240cm, four are enough, two also for 120x120. It only takes a few minutes. While the supports are underneath, you can already apply the adhesive to the next board – you have to let it set for a few minutes so that it can develop its adhesive effect.

For how many sqm is a 500ml PU foam can usually sufficient?

How many square meters does a 500ml PU foam can usually cover?

I use Soudal Soudabond easy 750 ml for about 10-12 sqm. It costs around €12 per can. Hell of a stuff ;)