DIYbanter - View Single Post

wrote:
Hi again.

I realise it would only partially fill, leaving voids, but this is
still of value insulationwise. The voids would see less air movement
as a result of the cwi, so even there the heat loss would go down.

It wouldn't even be noticable, sort of putting small squares of loft
insulation down haphazzardly in the loft instead of covering the whole area.

Doing the job would not be hard, just a case of drilling many holes
and using an old fashioned double ended hoover with a box in the
tubeway to allow fibres to be put in the airstream. Certainly not as
easy as standard cwi, slow but doable. I wouldnt expect contractors
to do it.

An easier method would be to stud the wall and insulate behind the studding,
and no possibility of damp penetration.

Damp is the question. First I dont understand how whats in the cavity
could in any way cause water ingress. Ingress I would have thought
would be determined by the outer faces of the wall. Second, how does
the presence of a patch of fibreglass cause water to move across from
outer to inner leaf? Polystyrene would, as it soaks up water, but
surely not fibreglass, which afaik doesnt.

Polystyrene? it doesn't soak up water at all, fibreglass does, but only the
stuff you would see in your loft, not the treated stuff used for CWI.

Water runs down the inside of your exterior brick/stonework during heavy
rain...if you could take down the inner leaf and look at the stonework
during a rainstorm you would see a mini waterfall! - this is normal and the
cavity stops anything getting to the inner leaf, this is why brick ties have
a twist or a bump in the centre, to allow water to drain down the cavity.

I've done a diagram and put it up on tinypic:

http://tinypic.com/view/?pic=rw5uyq