Ok then how tight in air leakage numbers is tight? How much air leakage should a new house be allowed?

Would you accept a window open all the time? a door? both halves of a patio door not there all winter?

10 is crap but I think 5 is ok and 3 possible.

Guidance for achieving one would be much appreciated?

I know have a conversion factor as I went to a very good AECB seminar today given by John Willoughby and Peter Warm. The building regs requirement is 10m3/h/m2 @50pa This equates to roughly the same number of ach at 50pa Divide by 20 to get normal air pressure [I think?]. What this boils down to is that as Nigel says 10 is crap, 5 is okay and 3 is possible.

Great to see you Paul here but how did you get in?

Finally I'm registered. One thing that struck me about the m^3/h/m^2 figures for air leakage is that smaller buildings will naturally be worse since they have proportionately a larger surface area for their volume.

From what I can tell 1.5ACH@50Pa is about the same as 3m^3/h/m^2 - if that's the case, then a figure of 10 as a target is dismal indeed. Over here, mechanical ventilation is mandated in the building codes so there's no excuse for leaky construction. If I recall correctly, R2000 certified construction requires a blower door test to be performed post-construction to verify the air tightness detailing. It's certainly not rocket science, though, as an estimated 70% of builders here can and do build to R2000 standards, whether or not the properties they build are R2000 certified. The new house we built was not R2000 certified - in fact, based on energy usage, we exceed R2000 standards by quite a bit (mainly due to the GSHP). Even the 109 year old house, post-renovation, exceeds R2000 energy usage standards, but is certainly not leak free (it measures 6.7ACH@50Pa - which I believe is still better than the target 10m^3/h/m^2 specified in the UK!)

Paul in Montreal