I always thought it was ice-damming. Like many of the homeowners in our area, we've got huge ridges of ice along the bottom of the roof of our 1.5 storey. Given the thin (possibly non-existent) insulation in our roof, there's certainly the potential for leakage due to ice dams.
Last spring, however, a home inspector told me that attic condensation is the more likely culprit. My understanding of attic condensation is that it's caused by moist warm air leaking from the insulated area of the house, condensing on a cold surface in the uninsulated attic, and then freezing. When the outside temperature warms up, the ice and frost inside the attic melts and the water finds its way into the house.
Unfortunately, we didn't get to test his theory because it was spring already - too warm to check for the telltale signs of attic condensation.
Earlier this afternoon, I finally got around to doing some poking around. Below are a few photos of what I found. I'm pretty sure there's warm air leaking into the attic - we've got icicles in there that are bigger than Michael Clark Duncan's arms.
Apparently there are no easy solutions. My wife phoned several insulation contractors who said they don't do 1.5 storey houses because their design makes it difficult to eliminate problems like this.
This may be a job that I try to tackle on my own. I couldn't make it much worse . . .
Here's some pretty good information about ice-damming and attic condensation from CMHC.
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