airtight is key to insulation
hi
i want to suggest something here, and it may appear to be --warning-- glib flip impertinent irreverent and allround badass mischiefmaking. Still, there is a good principle at work that is scientifically proven.
Any way you can seal all the air is a good solution, provided you also take into consideration how the heat may build up in the space you have sealed. So, sealing around and behind your light fixture is a GOOD thing, and not prohibited, and not a bad thing. The consequences are that you will have light-bulb-warmed air being held in a bubble, a three quarters closed container, and in summer you may actually want the opposite, i.e. to let heated air escape in tot hte attic and not stay in the light fixture.
Back to square one: if create an airtight "container" around the fixture, and use a material that isn't going to degrade under the effect of the heated air that will be contained in the container you create, and if you are not creating a space so small that it makes the light bulb act as a stove - which would raise temperature too high for the wires themselves not to degrade -- then you have done the right thing.
I am not there on site in your house, I don't know what your attic looks like, etc etc. So you will act without relying on me for anything specific.
In any attic that I have been in, I would consider using any large glass or metal container, placed over the light fixture, and then sealed at the bottom edge too. Then I would have made a container holding the warm air in, that the light bulb creates. No more exchange of hot and cold air, summer or winter. Then, to insulate this new airtight container in the attic, I would put something around it, something that does not let air move, so it has to be a kind of foam to start with as the first layer. I would also cover any fiberglass insulation in any attic with plastic sheeting or old plastic bags, to hinder air exchange, since fiberglass open on one side lets air move in and out of it, and that prevents it from being effective.
Where I live the climate is so cold that airtightness is the big thing to design and build in, and almost nobody has leaky light cans letting air escape. Just for info, i'll add that your entire warm conditioned house air is seeping through the gaps, not just the air that the bulb heats. The pressure of lighter air forces warmed air up and out. The fact that you felt cold air coming in, was just dumb luck on that day:: the flow was reversed at that leak at that moment in time. There are plenty of other small leaks (windows, other fixtures, vents) so the flow can be reversed anywhere.
You can get roof valleys (9"x9") or flashing (aluminum or galvanized steel) and make a box/tube with a top, or you might find 6",7",8" ventilation pipe, and make the tube with that. If an old glass pickle jar, or a couple old Folger's or Maxwell House coffee tins do the trick, don't tell anyone. Let it be our secret.
If you buy the UL tested fixtures, you'll be spending an extra $30 or so per fixture, and this will have transferred funds to the capital structure of the firm to compensate for many things including development (thought process) and formal contractual insurance coverage, and it may give you peace of mind to buy an official product, but you may not gain much real-life insurance coverage.
If you use compact fluorescent bulbs you won't generate sufficient heat to have to worry about heat buildup, after you seal to stop airflow.
david