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It sounds to me like there are significant differences in the construction methods. You guys live a lot further north than I do. Insulation on houses here is not taken as seriously, I suppose. Most of my houses have used roll insulation with an attached vapor barrier. But there are big gaps in the coverage.
For example in my former California residence, the recessed lights required an 6" airgap according to the installation instructions. With only about 8" of insulation in the attic in the first place, that amounts to a big hole in the insulation that is a square about the same size as the joist spacing. That section not only leaked air at the recessed fixture, but the only separation between the living space and the attic air was a 5/8" gypsum board. That same house had no insulation in the exterior walls.
In NM it gets a bit colder. More attention is paid to insulation, but there is still little finish or closure work done around recessed lighting fixtures. Ken is certainly right that using gaskets to seal up the airflow is a benefit. I have used caulking before, but that is a problem if you have to remove the fixture for some reason.
Another thing that I have done in the past to narrow the airgap around recessed fixtures is to cut a piece of 10" round steel duct and place that around the fixture. That provides about a 2" airgap and will keep insulation from closing onto the fixtures and overheating them. This also allowed me to have a much smaller uninsulated patch. Fixtures that are rated for direct insulation contact would solve all of these problems. But, it occurs to me that places like Lowe's and Home Depot may stock different types of fixtures for different areas and codes.
Recessed fixtures in my old house in NM had holes directly through the metal. Heat loss wasn't such a big problem but yellow jackets were. With holes about 5/16 through the metal and into the attic space, the bugs would come right through these fixtures and into the house. You could put gaskets on the rims, but the holes were for ventilation and were a real PITA.
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Theres a local company up here that has a neat slant on home building, the houses are built in a factory in pieces small enough to be trucked down the road then bolted together on site.
The really different part about it though is that they build the house inside-out sort of, after the wall is framed the first thing they put on is the drywall, then they put in all the windows and electrical, from the outside, they they use a sprayed on vapour barrier, then they put in the insulation, then they put on the housewrap and finally the outer skin.
Apparently by doing it this way the structure is VERY well sealed since there are no perforations in the vapour barrier. Also, since the wood framing is on the inside of the vapour barrier it tends to swell and shrink less seasonally then if it was not sealed in.
Best of luck.
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The link below goes to an article (.5M Acrobat) that describes some problems in Northern Climates. The problem is as much moisture from condensation as energy efficiency. Modern building techniques here go to a lot of trouble to ensure an intact barrier.
What can happen is that moisture still goes through some types of ceiling material or seams, and the air is warmer than the outside. It is trapped under vapour barrier but will migrate around and exit through any opening that isn't lower than the barrier. One problem is if it percolates through insulation it cools, moisture condenses in the insulation, rots wood structural members etc. I don't imagine there is this type problem in your area but that's what I was thinking of. What I am familiar with is intact barrier with wide overlap of the seams with insulation on top, and the edges are tucked around structures at the ends so the barrier goes down. Murf mentioned spray on barrier, which I think is the modern preferred method. But I'm just blabbing and it's easy for a person to sound like they know more than they do.
It's curious to note that the same government agency that provides the article had an energy efficiency grant program for older homes during the late 70's. The program subsidized contractors who did things like blow cellulose insulation into flat roof structures and put UF foam into wall cavities--on structures that lacked vapour barriers. Well, the victims of that program gassed themselves inside their houses; had their rafters rot and their property values crash to below 'handymans' specials.' Some program!
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10-30-2003, 15:00 
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