On Oct 5, 10:30*am, harry wrote:
Further to above, if your house is very old it may have no insulation
at all in the cavity.
Cavity? What is this cavity of which you speak?

http://www.aecb.net/forum/index.php?topic=561.0 suggests cavity wall
construction moved east across the country between mid-19th and
mid-20th century. I'm from Cambridge, and would have guessed they
became common in the 50's.

Cavity wall insulation didn't become required until the 70's, which is
hardly "very old". In fact, it wouldn't surprise me if the majority
of our housing stock predates cavity wall insulation.