I've come across some discussion of this on other forums (fora?) and am at a stage with my built where it would be possible to do this test now. I have installed all my internal PUR insulation and foamed up any gaps as best I can, so a test now will reveal any major leaks in advance of installing the vapour/airtightness layer. I would then run the test a second time to check that the barrier was doing its job.   It seems that the general idea is to use a large car fan running off