On Saturday, 9 April 2016 18:09:57 UTC+1, polygonum wrote:
It seems to be pretty good but not totally air-tight. And there will be,
for example, an extractor fan in the utility, off the kitchen, through
which air can come -

Unless that fan is also running, in which case your house will implode :-)

If it's direct through the wall just use a 12" wall fan. A bit noisy while running but will clear the air in a typical kitchen in about 2 minutes.

Owain