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Relative Humidity and Why It Matters

January 31, 2013

You can control a key variable associated with proper air quality in your home by maintaining relative humidity levels.

Anyone who has watched the Weather Channel can grasp the concept that excess humidity is muggy and low humidity is dry. New Englander’s know that basements should have a dehumidifier to lower the humidity. Physicians recommend humidifiers to increase humidity in the winter when its’ dry or to relieve coughing and congestion. So, what’s the big deal with humidity in the home and what’s the ideal range? For indoor air quality purposes, the ideal range is between 35-45% relative humidity.

Let me try to clear the air regarding some of the basic terms which are not interchangeable.

Relative humidity is a ratio of the amount of water in the air compared with the amount of water the air can hold at a given temperature. Air’s capacity to hold water vapor increases with increasing temperature. Warmer air can hold more vapor than colder air. This is why 60% relative humidity at a temperature of 95F feels muggier than 60% humidity at a temperature of 40F.

Dew point is an absolute measure of the amount of water in the air. It is the temperature at which the air, at the current amount of water vapor, will be 100% saturated. Reduce the temperature any further and water will condense out and form dew or fog, hence the dew point. In fact, you’ll often see fog forming even before you reach that point.

Mold spores and dust mites will increase exponentially when the relative humidity exceeds 60% for a sustained period of time. A typical basement without a dehumidifier will read between 65-75% relative humidity during summer months and between 45-60% during winter months.

Most people will know when it’s dry or below 20% RH or muggy when above 70%. However while the average person will notice a five degree change in temperature they are not able to notice changes between 20-70% relative humidity. This is why monitoring the relative humidity in your home or basement is so important, yet few people do.

Relative humidity is measured using a hygrometer. You will want to set your dehumidifier to around 35% or continuous mode to ensure the relative humidity at the opposite end does not exceed 55%. Don’t rely soley on the relative humidity reading on your dehumidifier. These displays are notorious for being inaccurate. We recommend a quality digital hygrometer like the Extech brand sold through Professional Equipment for around $45.

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