Inspection Insights High Humidity in the Home
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Inspection Insights: High Humidity in the Home

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A high relative humidity in your home encourages mold growth and dust mites, can make your house smell musty, and can potentially damage your home and your possessions. In simplest terms, “relative humidity” refers to the amount of dampness in the air. This Pillar To Post® Info Series offers suggestions for addressing high humidity.

Control Humidity at the Source
Whole House High Humidity
Lack of ventilation
- Newer homes are “tight”, meaning well-sealed, restricting ventilation. Without fresh air circulation, humidity builds up inside your home (only an issue during heating season when the windows are closed).

Oversized central air conditioner - Central air conditioning is an excellent dehumidifier. An oversized central air conditioner, however, has on-cycles that are too short to effectively remove humidity.
Caution - A gas-fired appliance not venting properly may cause high humidity. If you have any doubt, immediately contact a qualified heating contractor to investigate.

Localized High Humidity
- If an area, such as a basement, gets too much cold air supply, may create condensation and a high humidity problem. Adjusting the supply registers prior to the cooling season may help.
Clothes dryer discharging into house - This situation creates a huge source of moisture concentrated in a small area. Clothes dryers should discharge to the exterior even in winter. Verify that the discharge pipe is clear and connected properly at the back of the dryer.
Bathroom fans - Showers and baths add a great deal of moisture. Install an exhaust fan.
Basement dampness - Before you crank on a dehumidifier, find the moisture culprit and reduce or eliminate it at its source.
Crawlspace dampness - Put a sheet of plastic covered in sand or gravel over a dirt floor crawlspace to keep the dampness down.

A dehumidifier may be your only way to control moisture in a damp area. A dehumidifier removes moisture from the air and drains the liquid into a reservoir or drain. If the room is colder than 65 °F (18 °C) the dehumidifier may ice up. In this case, shut off the unit until it has defrosted then turn it back on. You may also have to increase the temperature of the space. Some units will operate down to about 42 °F (6 °C) and automatically defrost when they ice up.