Insulation of radiator niche

Hello, I bought a terraced house from 1969, it still has the thick radiators installed and I want to replace them with 21 cm flat radiators! I now want to insulate the niche about 18 cm deep but I’m not quite sure how! On the outside, I have red clinker bricks without insulation in between! I plan to glue Styrodur boards with flexible adhesive and then close it with an MDF board + drywall and then put the flat radiator on top! Is that okay or are Styrodur boards unsuitable? Above the radiators, under the window sill, there are windows where 3-pane glass will replace the current double glazing! Please help or suggest improvements! Best regards, Waldi

Internal insulation is something to seriously consider due to the dew point shift. Possibly (or probably?) moisture condenses nicely behind your drywall and causes everything to mold. You should definitely consult a professional.

I agree with the previous writer!

Our neighbor has successfully cultivated mold cultures in this way. I also recommend consulting a professional for triple glazing. Windows and walls should match, otherwise moisture can condense on the walls.