said by LazMan:
Vapor barrier placement; if one is required, is dictated by temperature - it goes to the warm side of the wall - basically, if you spend more time heating, barrier goes on the inside; if you spend more time cooling, it goes to the outside...
Holmes on Homes was filmed around Toronto, Ontario, Canada... Hence vapor barrier on the inside.
It depends on where the dewpoint is inside the wall cavity, which is a function of the temperature differences inside to outside, and the type of insulation & thickness. Foam sprayed in a wall cavity up against the sheathing in sufficient thickness will change the dew point in the wall to the point where water can't condense. You then fill the remainder of the cavity with batt insulation and drywall right over the batts without a poly vapour barrier. This gives the wall assembly what it needs - the ability to 'dry' to one side - in this case the interior.
A former classmate of mine is the principal @ www.buildingscience.com You can read about insulation there. Joe is one of, if not THE leading expert on this in North America.