The author posted a question in Family
Plastic windows They put a plastic window. And they didn't put anything under the window sill. There's a lot of empty space. What can be caulked with? and got a better answer
Response from Антон Владимирович[+++++]
With foam and scraps of whatever tile there is. You don't fill it all in, of course, but in pieces.
You have to fill it with foam. But this should do installers and plus the warranty on the installation should be.
They should have sealed everything. We did it, the only thing left is to wallpaper. I don't know what was used to seal it with.
If the gap is large, then put wooden spacers, blow out the foam and put something heavy on the window sill so it won't be lifted by the foam
with foam. you can put bricks on the mortar if the gap is big and if the window sill will have flowers or someone to sit on it, use angles
put some plaster on top of the foam