How to close/insulate radiator niches in KSS-MW, Styrofoam, and gypsum?

Hello everyone,

I am renovating a house from the 70s, it has 24 cm calcium silicate brick masonry (construction 11 cm calcium silicate - 2 cm air - 11 cm calcium silicate). From the inside there are then 2 cm of Styrofoam and everywhere covered with drywall.

The radiators are being replaced by underfloor heating, the niches are to be insulated. Now there is a row of calcium silicate bricks and then Styrofoam/drywall installed.

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



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



Which materials are best to use for insulating and closing?
Build Ytong (aerated concrete) slightly staggered into the niche and then later attach a cut-to-size gypsum board. Then fill it all and it will look seamless?

Thanks a lot and best regards

Which materials are best for insulating and sealing?

Insulation is actually done with insulation materials - EPS, PUR, mineral wool, etc. You don’t insulate with masonry or so little that you hardly notice it. You can simply use Ytong, build out the niche with it, and then plaster it. If you absolutely prefer drywall, that is also possible - just be careful not to create any offsets.

Thank you for your feedback, what is better thermally? Fill everything with Ytong or in the niche Ytong -> Styropur & then GK

Thank you for your feedback, what is better thermally? Fill everything with Ytong or Ytong -> Styropur in the niche and then GK

Insulation is always better. But then please seal everything airtight so that no warm (moist) air can reach the inside of the exterior masonry. Otherwise, it will condense there. I would only use Ytong and eventually equip the exterior walls with an ETICS or similar. 1/3 sqm with a few cm of Styropur doesn't really help.

Hello, what is your assessment of gypsum wallboard panels? Can I also use them well for that? I have to order a larger quantity of them anyway..

I am renovating a house from the 1970s, it has 24 cm sand-lime brick masonry (structure 11 cm sand-lime brick - 2 cm air - 11 cm sand-lime brick). On the inside there is then 2 cm styrofoam as well as everywhere covered with drywall. The radiators are being replaced by underfloor heating, the niches are to be insulated. Right now there is a row of sand-lime bricks and then styrofoam/drywall installed.

Which materials are best to use for insulating and closing? Ytong (aerated concrete) somewhat offset in the niche and then later place a cut-to-size gypsum board on top. Then putty everything and it will be all seamless?

What is your assessment of gypsum wallboards? Can I also use them well for this?

Gypsum boards are not the worst when viewed on their own. However, the description of the wall structure reads to me as a very heterogeneous Inslata Mista sandwich; I am also more of a fan of clean code in thermal contexts. How certain are your findings about the "old" wall structure?