Old conservatory ripped out today, with a new one going back on top of the original dwarf wall with an insulated roof. The dwarf wall has a 70mm gap between the outer and inner layer of bricks which has been empty since it was originally built. So while I have the opportunity should I put insulation down the hole and if so what. Kingspan 50mm? There are ties holding the two walls together, just enough to make slotting something in a pain.

Polystyrene balls would fill up the cavity much easier than Kingspan and provide a reasonable amount of insulation. As its only a dwarf wall, heat loss will be minimal anyway.

70mm of Rockwool full-fill. Don’t use partial fill as you will need to leave a 50mm cavity, which will only allow 20mm of insulation.

Use something suitable for a fully filled cavity.

Is the conservatory heated?

Residual cavity required is 25mm.
You can get full fill rigid insulation that doesn’t require a residual cavity but don’t know how you’d install it without removing inner or outer leaf.

Have you submitted a Building Regulations application by the way?

Replacing a conservatory is a big enough modification that it falls in the scope of the Building Regulations.

If the conservatory is heated, or not separated from the house by an insulated wall/window, you may be legally required to meet the insulation requirements, see 3.15 of Part L2

You might be eligible for a grant for home cavity wall insulation, try your energy supplier
I don’t know whether a bag of Knauf Supafil could be tipped in; it’s a compressed bale and normally it’s blown in…

Residual cavity required is 25mm.

NO IT ISN’T. It is 50mm.

Thanks for all the replies. The “men” are going to sort it out. There will be building regs as its a solid roof jobby.