On 5 Oct 2004 17:14:22 -0700, a particular chimpanzee named
(Alex) randomly hit the keyboard and produced:

The walls of my terrace house are made of two leaves of standard
bricks and the cavity is no more than 30mm.
Is there any way to install cavity insulation into such narrow gap?

Are you sure the cavity is meant to be 30mm? I notice further down in
this thread you mention that the inner leaf is being rebuilt because
of bowing. Cavities were normally 50mm, or none at all. It could be
either that your inner or outer leaf has bowed inwards reducing the
cavity, or it was meant to be a solid wall, and the headers have
snapped (or non-existent, which I've seen happen). Either way, you
should be looking to rebuild the wall plumb and maintain the original
width.

Most cavity insulation requires a clear cavity. In sheltered
locations Celotex or polystyrene requires at least a 25mm clear
cavity, and in more exposed locations, the full 50mm should be
maintained. There are some insulations such as Dritherm mineral fibre
batts, which can be used to fully fill the cavity, but these don't
come in anything less than 50mm thicknesses. I remain to be convinced
about the effectiveness of foil-faced bubble wrap; nevertheless, from
memory it requires at least 25mm clear space both sides in a cavity.

If for some strange reason your cavity is 30mm, I'd keep it clear and
insulate on the inside.
--
Hugo Nebula
'What you have to ask yourself is, "if no-one on the internet wants
a piece of this, just how far from the pack have you strayed?"'