"Peter" wrote in message
...

It was polystyrene?

Yup, had to clean the wiring where it was eating through, bits stuck all
the way along. Mind you it had taken 30 years to chew thro the outer cable
sheath. Rewiring was necessary of course. So far 2 houses, same street
same
builder same insulation.

The polystyrene should be bonded using pva glue and not be loose.
Then the plasticisers in the pvc cause the polystyrene to shrink which
remove the contact and stops the plasticiser being absorbed by the
polystyrene.
Polystyrene doesn't eat away pvc but it can make it brittle as it removes
the plasticisers.