My walls were insulated 30 years ago with UF foam insulation. I've the certificate to prove it. However, they neglected to fill the tops of the wall, probably about 18" short of the wall plate. This part is measurably colder than the rest of the walls.
I'm currently topping up the loft insulation, and could pour some forms of insulation in over the wall plate. I've filled up one spot with the polystyrene extrusions used to pack boxes. I'd like to put some more insulation in before I complete the loft insulation. Any suggestions on what to use that can be got in easily. PU foam would be much too expensive in aerosols.