The air inside most houses is drier in the winter because homes are leaky. Cold air has very little capacity to absorb moisture. Take a situation where the outside air is at 40°F with a relative humidity of 50%. As that air moves into your house and is warmed to 70°F, its relative humidity changes to just 16%. How did that happen? The actual amount of moisture in the air did not change. This is reflected on the Y-axis of the graph labeled “Humidity Ratio: Grains of Moisture per Pound of Dry Air.”
The term relative humidity (RH) is a percentage of how much moisture the air could hold at saturation (for example, right before it rains). Dew point is the temperature to which the air must be cooled for condensation to happen, or in other words, 100% relative humidity. This is best illustrated by a psychrometric chart.
The type of heating system does not affect a home’s RH levels. Hydronic (hot water) or hot air systems will not “dry the air out.” With few exceptions, these are closed systems and do not add or subtract humidity from the building air.
On the other hand, a very tight house without outside air ventilation may end up with moisture issues, such as condensate on windows and doors. This is because in the absence of any significant outside air entry into the building, moisture levels increase in what is effectively a closed system. Condensation occurs when the temperature of a surface is below the dew point of the air. Typically, this happens on windows, since they have the lowest thermal resistance and therefore are the coldest surfaces.
Room condition No. 2 shows the air temperature at 70°F and 50% RH. To illustrate the dew point of this air, a horizontal line is drawn to the left until it intersects the 100% RH curve. This shows that the dew point is about 50°F. So a surface at or below 50°F will have condensation on it from moisture in the air.
Psychrometric charts show the properties of air and water mixtures for the conditions that we encounter in our environment. This chart shows two conditions: cold outside air entering a house and the effects of heating that air.