Cavity wall cock-up

This morning, I was woken up by a very loud noise outside. It turned out to be construction plant equipment:

You'll note from the photo that the van is emblazoned with "insulation extraction".

The street where I live is a mixture of private and council properties. Originally it was built as local authority housing, but over three-quarters of the houses have been sold and are now privately owned (including mine). The local council arranged to have cavity wall insulation installed in its remaining properties.

In theory this is a good idea as it theoretically reduces heating bills; however all of the houses here have lightweight thermal blocks for the inner leaf, which provides a similar (if not greater) insulation value to concrete block inner with insulation in the cavity.

So, why is this a problem?

Because Thermalite blocks are porous. Meaning that if there isn't an air gap between the two leaves of the wall, damp can (and will) permeate through to the inside walls.

My neighbour next door (privately owned) had cavity wall insulation fitted some years ago. Within a few months they had damp problems, whereas before none existed.

It turns out that the local council are now, at great expense, having to go around all their properties that use this type of material for the inner walls and remove all the cavity wall insulation.

I did investigate having it installed some years back. My parents' house had it retro-fitted, however the inner leaf there is made of concrete blocks (which are impermeable) rather than thermal blocks. In that instance it did make a difference and cut down the heating costs. However, I was hesitant, and when next door had the damp problem I decided against having it installed.

Turned out to be the right decision. It would have made a negligible difference to my heating costs (given the relatively high insulation of the thermal blocks that are already there) and would almost certainly have caused similar issues with damp.

Quite simply, the thermal blocks require an air gap to be present between them and the outer brick leaf. If cavity wall insulation is retro-fitted, it will bridge this air gap and allow moisture to cross to the side of the thermal block that faces the cavity. Because the blocks are permeable, the moisture will then find its way inside the house and cause all sorts of problems.

Which is why I was advised by a friend in the building trade that it would not be a good idea to install it with this type of construction.

Surely someone at the council ought to have known this?