I haven't tried the incense test yet to check for air egress yet, as boekeham (sp?) and others suggested (bought the incense sticks - man it's been a long time since I did that). I did aim the IR gun at both upstairs fan/light combos and they had showed cold temps there. Last time I checked on one of them, the crappy little integrated flapper that prevents air ingress got disconnected and there was nothing preventing air from coming in or out for that matter. Are these things typically a source of heat loss? I imagine they would benefit from a flap at the outdoors end of the duct like a dryer vent, but damn, that'll be a non-trivial project for me, involving ladders, heights, and/or a messy crawl in the attic. One guy at work said something to me a while ago about having a separate, quiet exhaust fan, mounted in the attic, similar in design to a radon removal fan. That seems intriguing. My current arrangement has the ducts exhausting through the venting under the roof underhang, which probably isn't super ideal since it could therorectially be sucked back in by the vents.