How much water is in the air?

My skin and sinuses don't like dry air, and nothing dries air like heating it: Heat freezing air at 40%RH to room temperature and the relative humidity drops below 10%, which is as dry as deserts at noon!

So when indoor heat comes on in the winter I break out the humidifiers. If my living space has a central air handler I install an automatic humidifier on that. If not I have to manually fill portable humidifiers. Which led me to wonder: How much water does it take to bring the humidity in dry air back up to comfortable levels?

It turns out that the moisture capacity of air is very non-linear with temperature: For example, air at 100°F holds 10 times as much water as freezing air!

Thanks to data here we can see that air at room temperature (20°C/68°F) holds up to 17g of water per cubic meter. So a 1000ft² living space with 9-foot ceilings holds just over 4kg, or 1 gallon of water, at 100%RH.

Humidity in living spaces should be kept below 50%RH because mold really thrives above that level. So when outdoor air is below freezing we need to add half a gallon of water to the heated indoor air of my hypothetical 1000ft² living space. But healthy living spaces also exchange fresh air – ASHRAE recommends eight air changes per day – so if that living space is properly ventilated then we will have to add four gallons of water per day!