Understanding Relative Humidity and Your AC

Humidity – the amount of moisture in the air – is of tremendous importance for your comfort and for your health. It also impacts the energy costs of operating your AC system, the biggest energy hog in your home. When you reduce the humidity in your home, you also reduce the risk of mold and mildew growth. When humidity is low, you also feel cooler in summer and warmer in winter.

Humidity is a complicated topic.  There is always some moisture in the air. Relative humidity is the amount of moisture relative to the amount of moisture the air can hold. If the relative humidity is 50%, then it is half “full” of water. If the relative humidity is 100%, it cannot “hold” more water. When rain clouds form, they are at 100 % humidity, but the ground-level humidity could be much less.  If the air around you is at 100% humidity, it is holding all the water it can. It does not mean it is raining, just that the air is saturated with water!

When relative humidity is low, we feel cooler because sweat evaporates easily. People feel most comfortable when the relative humidity is around 50%. Your AC system is designed to de-humidify the air, so it is important that it does this properly or you will not feel comfortable even if the temperature is cool. You will also find yourself sneezing and coughing, because humidity promotes the growth of allergens. If humidity is too low, you will have a dry throat and be trouble by static electricity as well as dry, itching skin.

As air cools, it become less able to hold water. At a certain temperature, it will condense into water (the dew point) or ice (the frost point). The dew point varies depending on the relative humidity. Currently, (October), the dew point in South Florida is 65 degrees… meaning condensation will form at this temperature. In Maine, on the other hand, the dew point is now 40 degrees.

Normally, during hot weather, condensation will form on the outside of your windows if you run the AC very cold. This is because hot air can hold more water than colder air. When the air touches your cold windows, it condenses. Indoor humidity that is too low is more common during winter, because cooler temperatures and heating cycles lower moisture in the air more rapidly. In the summer, relative humidity that is too high is a more common issue.

Condensation on the inside of your windows is a sign that something is imbalanced in your AC system. Your AC system includes your entire home, so if you have leaky windows that will affect the relative humidity inside your home and how hard your AC has to work to remove moisture. You buy humidity gauges (hygrometers) for around $15 apiece to measure the amount of relative humidity in the air. Buy several, leave them in place, check the relative humidity, and then swap the positions to check the accuracy of your readings.  If the temperature is at 76 degrees, the relative humidity should be around 50%.  In the winter, it should be no lower than 30%.

Stay Tuned for our next post “The causes of high relative humidity in air-conditioned buildings…

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