[MattRix]

“The reason why humidity isn’t included in cars, or most HVAC systems, is that it is not possible for most of us to attach a specific value to the humidity level.”

I would argue that humidity control is included in cars by the air-recirculation button. It’s certainly not exact control, but if you recirculate the air in winter everything will quickly get fogged up (and often vice versa in summer).

[andix]

I think a lot of cars do consider the humidity in their calculations. Also because they have a way to regulate it: with the AC. So even in the winter they use the AC to reduce humidity, and then immediately heat up the air again.