Indoor Humidity

Why is indoor humidity so low in the winter?

People say there air is drier but it tends not to be so much of a problem outdoors.

It turns out air going from outdoors in during the winter gets dramatically dryer due to two effects, the expansion of the air spreading out the same amount of water over more air and the warmer air having more capacity for holding humidity making the relative humidity lower.

• At 20°F and 100% RH, you have about 1.2 grams of water vapor per cubic meter.
• At 72°F, the air could hold up to 19.4 grams of water vapor per cubic meter.

How much circulation is measured in air changes per hour (ACH) Typical ACH Ranges - Tightly sealed homes: Might have ACH as low as 0.5 or even lower. - Average homes: Typically fall in the range of 1-3 ACH. - Older, drafty homes: Can have ACH of 5 or more.

Based on Charles's Law, a cubic meter of air at 20°F will expand to approximately 1.10 cubic meters when heated to 70°F.

Cost to Heat the Air

This calculation assumes the air is completely dry and is heated from 20°F to 70°F. - Energy Required: 20.94 MJ (or 5.82 kWh / 0.20 therms) - Cost using Natural Gas: $0.30 - Cost using Electricity: $0.87

Cost to Humidify the Air

This breaks down the cost of adding enough water vapor to raise the humidity from 0% to 50% at 70°F. It also addresses your point about heating the water, which in this case is the water that is added to the air. The mass of water added is approximately 5.24 kilograms (1.2 gallons).

Vaporizing the Water (turning it from liquid to gas)

This is the most energy-intensive part of humidification. - Energy Required: 11.84 MJ - Cost using Natural Gas: $0.17 - Cost using Electricity: $0.49

Assuming you want 50% humidity and 72F you need to add all the humidity to the air via humidification about a third of the energy costs.

So you take normal air and you make it dry by heating it not by doing anything to the humidity in the air but simply by changing what the baseline of what humid vs. dry is for that air.