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Dry 18°C air _feels_ significantly colder than humid 18°C air. Doesn't humid air also retain the heat longer than dry air?

As for the air cooling down when it hits the ceiling, this is true.. for a period of time. In an ideal situation you have a significant amount of insulation on the other side of the ceiling which inhibits thermal loss.

In my case, I have a heat pump and electric resistance backup. A few years ago I added a whole-house humidifier, which made my home feel significantly warmer at the same set-point and reduced the amount of electricity that I used each month under similar weather conditions.