ᐅ Topping layer in screed insulation: yes or no?

I’m not sure how things are on your end, but usually the widths in question can no longer be simply overlapped according to today’s standards. It’s not just about the gaps next to the pipes, but also about the width of the pipe duct—altogether that quickly adds up to 20 or 30 cm (8 or 12 inches), which is definitely too much for cavities.
At that time, I just asked the screed installer how he preferred it and then followed his approach. Personally, I wouldn’t start cutting corners in such “sensitive” (and especially later inaccessible) areas without a very good reason.

But it’s your house, so you can do whatever you want 🙂

That’s correct.
However, in the mentioned room, there are at most two empty conduits with a diameter of 25mm (1 inch) side by side, so 5cm (2 inches) in total. Let’s see, maybe I’ll redo that part to make it perfect.

However, I have the impression—regardless of the trade—that I tend to be more precise than the builders.
In this one room, I also wanted to be “like them” for once ;-)

denz. schrieb:
However, I get the impression—regardless of the trade—that I tend to be more thorough than the construction workers.

You’re not alone in that ;-)

We installed the insulation beneath the screed ourselves in almost the entire house. Using a styrofoam cutter and bonded loose fill—perfect!
However, in one room, the drywall installers weren’t finished yet, so the heating contractors had to install the insulation there. They simply broke the boards and “kind of” placed them around the pipes, and despite gaps of up to 25cm (10 inches) in some spots, they didn’t add any loose fill. After much grumbling, they finally did it on my instructions, and following even more complaints, they vacuumed up the styrofoam.
Since they probably didn’t take the vacuuming seriously, we now have numerous inclusions of small styrofoam beads in the screed—only in that room! :-(

Dr Hix schrieb:
You’re not alone with that ;-)

We installed the insulation under the screed ourselves in nearly the entire house. Used a proper polystyrene cutter and bonded loose fill – worked perfectly!
However, in one room, the drywall contractor wasn’t finished yet, so the heating installers had to lay the insulation there. They just broke the boards and somehow placed them around the pipes and didn’t add any loose fill, even where there were gaps of up to 25cm (10 inches). After a lot of complaining, they did it on my instruction, and after even more fuss, they vacuumed up the polystyrene.
Since they probably didn’t take vacuuming seriously, we now have lots of small polystyrene beads trapped in the screed… and only in that area! :-(

We’re planning to install the insulation ourselves in the next 2–3 weeks. Do I really need a polystyrene cutter, or would a utility knife be enough?

A utility knife or cutter is usually sufficient (depending on the thickness of the panels, however, the sawing motion can get quite annoying). You will, however, end up spreading a lot of polystyrene beads around the room, which should then be carefully vacuumed up for the reasons mentioned.

For this reason, I recommend using a polystyrene cutter.

Dr Hix schrieb:
A utility knife/cutter is usually sufficient (depending on the thickness of the panels, the "sawing" can get quite annoying). However, you will spread a lot of expanded polystyrene beads around the room, which should then be carefully vacuumed up for the reasons mentioned.
Therefore, I recommend a polystyrene cutter.

Or a reciprocating saw with a blade for polystyrene. I also find that very clean. It doesn’t produce as much odor either.

A utility knife just makes way too much mess.