In article ,
Jeff Layman writes:
There was a problem with fibrous insulation blown into the cavity. When
this got wet for whatever reason (rising damp, falling damp, leaks,
condensation on the inner surface of the outer brick skin) it made the
fibres stick together and heavier, and they tended to sink. In the end,
a mass of soaking wet insulation ended up at the bottom of the cavity.
That caused the ties to rust quickly, and severe mould problems on the
inner wall.

I understand that it can be removed with powerful vacuum machines, and
there are companies which specialise in this.

Even polystyrene balls can be an issue if they aren't used properly.
Everyone always finds a few balls coming out when a hole is drilled into
the cavity, but when we had new patio doors fitted the stuff just poured
out when the old doors were removed. I'd always though that when blown
in a weak adhesive solution was used at the same time to make the balls
stick together. But not, apparently, in our place!

Someone in the road had polystyrene balls installed some 30 years ago.
A few months afterwards, the tail-end of a hurricane passed over, and
the whole street and all the gardens were full of polystyrene balls for
months. I presume they blew out the tops of the cavities.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]