ᐅ How to close and insulate radiator niches with mineral wool, expanded polystyrene, and drywall?

Hello everyone,

I am renovating a house from the 1970s. It has 24cm (9.5 inches) calcium silicate masonry (construction: 11cm (4.3 inches) calcium silicate – 2cm (0.8 inches) air gap – 11cm (4.3 inches) calcium silicate). On the inside, there is an additional 2cm (0.8 inches) of polystyrene insulation, and the walls are covered everywhere with drywall.

The radiators are being replaced by underfloor heating, and the niches need to be insulated. Currently, there is one row of calcium silicate blocks, then polystyrene and drywall installed.

This is how it looks before removing the drywall (unfortunately no better picture available):

This is how it looks after removing the drywall (also no better picture available):

Which materials are best to use for insulating and sealing this area? Would it be advisable to build a slightly offset block of aerated concrete (Ytong) in the niche and then later fit a cut drywall panel on top? Then finish by filling and smoothing, so that everything looks uniform?

Many thanks and best regards

Besenkammer84 schrieb:

What materials are best for insulation and sealing?

Insulation is usually done with insulating materials such as EPS (expanded polystyrene), PUR (polyurethane), mineral wool, and so on.
You don’t really insulate with masonry; it provides so little insulation that it’s hardly noticeable.

You can simply use Ytong blocks to fill the niche and then plaster over it. If you really prefer drywall, that’s also possible—just be careful not to create any uneven surfaces during installation.

Thank you for your feedback. Which option is better in terms of thermal insulation?
Should everything be filled with Ytong, or should the niche be filled with Ytong -> Styrofoam and then drywall?

Besenkammer84 schrieb:

Thanks for your feedback, what is better thermally?
Filling everything with Ytong or Ytong in the niche -> Styrofoam & then drywall

Insulation is always better. But make sure to seal everything airtight so that warm (moist) air cannot reach the interior side of the external wall, as condensation would otherwise occur there. I would just use Ytong and eventually add an external thermal insulation composite system (ETICS) or similar to the outer walls. Filling about 1/3 sqm with a few centimeters of Styrofoam doesn’t really help.

Hello, what is your opinion on gypsum wallboards? Can I use them effectively for this purpose?
I need to order a larger quantity anyway..

Besenkammer84 schrieb:

I am renovating a house from the 1970s with 24cm (9.5 inches) calcium silicate masonry (construction: 11cm (4.3 inches) calcium silicate block – 2cm (0.8 inches) air gap – 11cm (4.3 inches) calcium silicate block). On the inside, there is an additional 2cm (0.8 inches) of polystyrene insulation covered everywhere with drywall.
The radiators are being replaced by underfloor heating, and the niches need to be insulated. Currently, there is one layer of calcium silicate block and then polystyrene/drywall installed.

Besenkammer84 schrieb:

What materials are best for insulating and closing off these areas?
Would it work to build into the niche with aerated concrete blocks (Ytong) staggered slightly, then later apply a cut drywall panel on top? Then finish with joint compound for a seamless appearance?

Besenkammer84 schrieb:

What are your thoughts on gypsum-based drywall panels? Are they suitable for this purpose?

Gypsum boards alone are not necessarily a bad choice. However, your description of the wall assembly sounds quite like a heterogeneous, mixed-material sandwich. In terms of thermal performance, I prefer a more straightforward and well-defined build-up. How certain are you about the details of the original wall structure?

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