I'm not saying you are wrong, I'm just saying it doesn't matter what the AC does.

When the dewpoint is around, say, 15F, as it probably was with a temp of 5F, the AC system can't get cold enough to produce fog without freezing up entirely. Now if the heater core is leaking, there could conceivably be enough moisture picked up to produce a fog. BUT, the defrost/fog scenario was presented as an alternative explanation to a heater core leak, not as a symptom of it. My point was simply that at that temp/relative humidity and without an internal source of moisture, the AC/defrost system could not produce fog.

My apologies to the board for sidetracking this topic with such minutiae...as a consultant for indoor air quality, I work with humidity issues quite a bit. <img src="/forums/images/graemlins/ignore.gif" alt="" />