Indoor air is heated but not humidified. Relative humidity outside is high, but the temperature is very cold, so the absolute amount of water in the air is very low. Bring that same air indoors, heat it up by ~150 degrees Fahrenheit, and the resulting relative humidity is very low.

Hard to measure at such low numbers. I have a handful of cheap consumer-grade sensors with me, and they all report "0%" or "lower limit" or equivalent. I can't share readings from official NSF-owned equipment.

Static buildup is a real issue. It's muscle memory now for me to ground myself before touching any electronics. There's makeshift grounding points (metal tape, metal frames, etc) everywhere. I'm still not used to the little shocks every time.

I humidify my bedroom at night using portable humidifiers. This takes ~1.5 gallons of water per night, which is deemed an acceptable use of water. Most people do this. I can get my room up to ~15% humidity with a reasonable amount of effort. The difference in sleep quality is noticeable.

Frost buildup is a real issue in humidified spaces, given the temperature gradient to the outside, even with triple-pane windows. The greenhouse has no exterior windows, so this isn't a problem.

The greenhouse is kept at ~25% relative humidity. It's the only place on station with infrastructure in place to provide humidity directly through the HVAC system.