On 18/02/2020 19:36, John wrote:
My house was built in 1988. The cavity walls are filled with slabs of
rockwool stuff - possibly not as well as I would have liked.

Occassionally I get salesmen trying to sell me the sort of injected cavity
wall insulation. They usually claim there is a grant toward it.

I don't like to be pressured - I like to decide if it is a good thing.


Any views?


You may do more harm than good by adding foam insulation to rockwool
slab insulation as there is a danger of bridging the cavity which could
cause damp problems.

In any event you would are unlikely to significantly improve the u-value
enough to justify the cost of filling with foam.