With the new addition to the 2009 IRC, section 315.1, requiring carbon monoxide alarms outside of sleeping areas in the immediate vicinity of the bedrooms, where fuel fired appliances and attached garages exist, raises an interesting question. I've had many electrical contractors ask me where to put the CO detectors, up high, or down low? We've been allowing the combo CO/smoke detector units to be installed on the ceiling in the hallway, or up high on the wall in the hallway, where smoke detectors are already required. Is carbon monoxide heavier than air, and if so, what good is it to put a combo CO/smoke detector up on the ceiling? I've read that CO is slightly lighter than air, but have also had many people tell me it's heavier than air. Any thoughts??...