Humidity and Condensation
People are generally comfortable in homes when relative humidity ranges between 30 and 60 percent. Below 30 percent, some people experience dryness in their nose and throat; over 60 percent, the air begins to feel uncomfortably sticky. Human comfort is one consideration for indoor humidity levels, the other major consideration is keeping condensation from occurring on interior surfaces and within structural cavities like exterior walls. Excess moisture in these areas can cause problems from peeling paint, cracking of siding, deterioration of building materials and insulation. On the home's interior, moisture can promote mildew formation and contribute to health problems.
Other disadvantages of high humidity include the growth of mold, odors becoming more noticeable, and staining when condensation occurs on windows and around nails or screws in walls and ceilings. In addition, high humidity can worsen respiratory problems for people with allergies.
An early indication of high humidity levels in your home is condensation on windows. Because they are usually the coldest surface exposed to room air, they fog up first. By taking action to reduce condensation on windows, you should be able to avoid condensation problems from occurring inside the walls.
Occasional condensation or frost on windows is normal. Frequent occurrences, or periods of prolonged duration, are warnings that inside humidity conditions may be causing condensation inside wall cavities.
Inexpensive color-change relative humidity indicators can also reveal high moisture levels. These should be installed near the thermostat.
On a positive note, a certain amount of humidity in the home can help prevent dry throats and make people generally feel more comfortable because less moisture evaporates from the body thereby reducing the cooling effect. Also, higher humidity levels results in less static electricity and improved furniture maintenance as wood moisture is maintained reducing cracking and shrinkage.
To understand condensation, one must first understand a couple of other concepts. Humidity refers to the amount of water vapor in the air. Relative humidity is a measure of the amount of water vapor in the air compared to the maximum amount possible at a given temperature. Air with a relative humidity of 50% is holding half the total amount of water vapor it is capable of holding at that temperature.
The amount of water vapor that air can hold depends on the temperature of the air. If the air temperature decreases, the maximum amount of water vapor the air can hold is reduced. If air at 70°F and 50% relative humidity is cooled to 52°F, the relative humidity will reach 100% and condensation will begin. The "dew point" is the temperature at which air saturation occurs, and condensation begins.
If air at 100% humidity is cooled, condensation will form as fog in the air or on surfaces at or below this temperature. This phenomenon may be observed on a cold winter day when you "see your breath" in the air; your warm breath is cooled enough to condense part of its water vapor, producing the tiny water droplets as fog.
A similar process occurs when an air-water vapor mixture flows through walls and ceilings of a home. The air is cooled as it moves through the thickness of the building envelope.
When moisture laden air reaches its dew point, condensation will occur if it is cooled to a lower temperature. The dew point for a given temperature of air from the home varies according to the amount of humidity in that household air.
If the dew point is above 32°F, condensation will form as a liquid. If the dew point is colder than 32°F, the water vapor will condense and immediately form frost or snow.