Now, anyone who has any rudimentary knowledge of science knows if the air's coming in, it's also going out of the house somewhere. I couldn't figure out where--I thought maybe the other chimney, maybe the windows, basement, doors, but nothing is as drafty out as the fireplace is drafty in. And anything that can create as much airflow in as we were getting must be a pretty big leak.
Finally, last month I was reading This Old House magazine, and it's full of green ideas, one of which is replacing your recessed can lights with a compact fluorescent retrofit kit that not only saves energy with more efficient lighting, but seals the recessed fixture, and makes it so you can put insulation right against it.
In our family room/breakfast nook we have 6 recessed lights. One is underneath the office floor, but the other 5 are underneath the attic space over the family room (the house is only one story there.) The two breakfast nook lights say they're sealed, and they have insulation coming out of them, so I'm assuming they're okay.
I take the bulbs out of the other cans, and sure enough, there's an open slot through which I can see light from the gable vent in the attic area. Not a big hole, but there's three of these lights and the total volume would be significant. Also, in the winter you'd get a great convection flow as warm air rises out of the house, and cold air gets sucked in.
I finally looked into this attic area (the access panel is in our office closet upstairs, and we had never been in there) and surveyed the situation. Sure enough, the cans are unprotected and have no insulation above them (which they're probably not supposed to due to fire considerations). The insulation fills most of the joists (probably 2x6s) but we can lay another batt crosswise on top. Once I get the retrofit kits I can insulate around the cans too, and probably tape shut any other spaces I see as a precaution. We also noticed a gap by the fascia boards where warm, moist air is probably leaking out, and it's damp there, so we need to address that to avoid some rot issues eventually. It's also on the north corner of the house and gets no sunlight, so there's a bit of moss we have to deal with. Sealing the cans should fix those moisture problems. Also, once I'm ready to crawl back there for real, I can see the situation about how 3 roofs come together, which is a chronically bad area for leakage, both heat and water, and I might be able to address some of that even more. There's at least one spot that looks completely uninsulated (which again would be contributing to our air infiltration problems.)
So I've ordered the kits, will buy some insulation at a home improvement store, and next weekend hopefully make it so our family room is not bitterly cold all winter. Also, we should save buckets of money not heating the world. The best part is I'll feel like we can use the gas fireplace again. The other side effect of the airflow through the fireplace was that it would be so cold that you couldn't start a fire in it--the gas logs would light, the moisture of combustion would immediately condense on the logs, and would put the fire out, resulting in unburned gas pouring into the fireplace. Nice. I could only get the fire started by running a space heater by the fireplace for 15 minutes to heat it up enough so the condensation didn't happen. Without the airflow that shouldn't be a problem anymore.