Building a wall under windows and insulation behind

[kimf]

Hi, I've installed new, smaller windows in the new kitchen, and the mason has helped me fix the facade side. The exterior walls consist of
- Lättbetong
- Insulation
- Brick

I was thinking of using foam to fill the last bit (there's about 20cm of insulation missing down the hole, here)

Then masonry towards the foam. But it's not so easy when there's nothing to "foam against."
Is it overkill to do this, would you just foam up to the level with the brick, and then run the mortar all the way in towards the lättbetong on the inside?