ᐅ Bricking Up a Window with Calcium Silicate Bricks and Insulation – Explanation of Cavity, U-Value, and Condensation

Good evening everyone,
I am currently renovating a house from 1973 – the exterior walls are built with 30 cm (12 inches) aerated concrete blocks, and 14 cm (5.5 inches) polystyrene insulation (WLG035) will be installed. I want to block up a few windows beforehand and still have plenty of clean 11.5 cm (4.5 inches) calcium silicate hollow bricks left from the demolition work.
Is there any reason not to fill the window openings with two rows of calcium silicate bricks and insulate the 7 cm (2.75 inches) gap in the middle? That should roughly equalize the U-values.

Could you also comment on my father-in-law’s statement: “just leave an air gap, that insulates too”? Wouldn’t that cause condensation problems in that cavity?

Thanks and best regards
Pascal

If you insulate from the outside, you can do whatever you want.

As long as the 14cm (5.5 inches) external insulation is not reduced in that area, there is no objection. I would avoid leaving any air gaps and prefer to seal everything within the masonry.