Have you looked into those cavities? Cavity wall insulation

Insulating wall cavities can help you cut energy use and save money by reducing heat lost through walls in colder months, and keeping it out during warmer months, which cuts back on cooling needs.

Cavity wall insulation can be used in any double-walled or double-skinned building. The cavity between the walls needs to be at least 50 mm for the insulation to be effective.

Tip

There are different types of cavity wall insulation:

Fibre: Mineral wool is a proven technology comprised of granulated spun glass or rock wool treated with water repellent. While cellulose loose fill is a fibrous material created from recycled waste products, which has to be kept free of water/moisture to remain effective.

Foam: Urea formaldehyde is water repellent and resistant to rot, fungi and vermin. It is also suitable for buildings with non-standard cavities. Polyurethane is particularly suited to narrow cavities and high-rise buildings. Cost is a limiting factor, but performance gains can compensate for that.

Bead: Expanded polystyrene beads are suitable for most buildings with standard cavities and no obstructions. It can also be recovered.

Perlite/vermiculite: This is a free-flowing injectable product which makes it a good void/gap filler. It can be reclaimed as well.