Reduce window opening, aerated concrete or brick?

[kimf]

Hey,

I'm going to make this window opening smaller to accommodate base cabinets in the new kitchen. I've done a bit of casting, but never masonry work. However, this seems like a manageable project to start with

The standard wall construction is
<outside>
--------- 120mm facade brick
~~~~~ 100mm insulation
===== 180mm aerated concrete (including plaster)
<inside>

So, the question is, what's the easiest way to build up?
- I happen to have a pallet of regular bricks lying around, which are fairly easy to handle and cut (I think ), but I guess I'll lose some thermal efficiency right here under the windows with just bricks? Does it matter on two such small areas?

- Gluing new aerated concrete, like leca or H+H, feels easier to handle as long as you find blocks that only need to be cut on the short end, but no building supply seems to have them in convenient dimensions in stock. I guess it's not fun to cut them in all possible directions.

The openings I need to build up are 1300 * 180 * 280 mm & 1200 * 180 * 280 mm



On the outside, a mason will work on the facade brick to make it look nice. I didn’t want to take on the task of chipping away every other joint, setting in, and making the bricks uniform, while trying to mix a matching mortar (a rather unusual dark mortar with small white stones in it). But I don’t really know when he’s coming, so I want to do the inside now so I can proceed with the other surfaces.

 

[kimf]

Found H+H aerated concrete slabs on Optimera now, available in dimensions 75x200x600 and 200x200x600. With this, I can build almost exactly 280mm in height with one of each, including the adhesive seam of about 2mm. It's a bit too thick, so there's room for less insulation than intended. But it feels like a convenient option. Then I would only need to cut two stones lengthwise in the 1300mm wide gap. In the 1200mm gap, it might be possible to fit them without any cutting.

These don't need plastering either, just spackling, so this actually feels like a good option.

Something to consider for an amateur?
Is it penny-wise and pound-foolish to glue this little with PL400 instead of buying a 20kg bag (the smallest packaging) of their own thin adhesive mortar?

 

[kimf]

Ended up chasing down the accompanying tunnfogslim anyway, felt unnecessary to take a chance with PL400 in case it would lead to cracking later, considering I'm going to tile on top.



The height turned out perfectly, and I'm satisfied with the first bricklaying. It took a lot more time than I expected, but it was my first time. The blocks were very easy to cut with a regular handsaw, which made things easier.