Q & A: Does a dehumidifier actually kill mold, or is that a myth?

Dehumidifier information on proper dehumidification to stop mold growth

by Thad Godish, Ph.D

Editor's Note: You can link to FAQs and articles by Thad Godish and others in our Mold Center.

INFO LINKS Dehumidifiers Overview - view all Dehumidifiers sold by AllergyBuyersClub.com Dehumidifiers Comparison Chart - a comparison with pluses and minuses of all the best Dehumidifiers Mold Solution Guide - a quick buyers guide of the best recommended products to solve mold allergy problems Best Dehumidifier Articles - Editor's top picks on dehumidifier info

Q. Does a dehumidifier actually kill mold, or is that a myth?
A. A dehumidifier does not kill mold. However, when used properly dehumidifiers can be used to prevent mold infestation in a variety of building spaces such as basements and crawl spaces. Dehumidification is often essential to prevent re-infestation after a mold remediation.
Relative humidity is one of the major risk factors for mold growth in houses and other buildings. Buildings with relative humidity that doesn't exceed 60%, rarely become mold infested (there are exceptions of course).

Relative humidities in excess of 70% for extended periods (several weeks or months) provide optimum conditions for the germination of mold spores and colonization of suitable substrates such as paper, wood, dust, etc. That is why it is essential to operate dehumidifiers in basements (up to 6 months here in the Midwest) to keep humidity levels under control. Basements are at particular risk because they are poorly ventilated, serve as storage areas for materials that mold can easily grow on, and because of their cooler conditions (often less than 60-65°F), relative humidities will be higher even with the same amount of water vapor that is present in the air upstairs.

I have a fully finished basement under my house. If we did not have a dehumidifier, the paneling would warp and mold would be growing on the paneling, walls, paper, etc. As soon as it becomes warm in the spring, the humidity starts increasing significantly. This is the time to turn the dehumidifier on (and keep it on for another 6 months) so that the relative humidity is maintained below 65% (preferably below 60%). If one's basement is unheated or only partially so, one may have to use a small heater near the dehumidifier to bring the area temperature up to 65° so that the dehumidifier will work without freezing up. Purchase and use a good quality humidity indicator to monitor humidity levels.

Dehumidifiers work like refrigerators and air-conditioners. They have cooling coils. Water vapor is condensed and removed from the air going through the dehumidifier. It requires a lot of energy to condense water vapor and the higher the background humidity level, the more energy it takes. For a2000 square foot basement in the Midwest a 40-50 pint dehumidifier being operated to maintain humidity levels at 60% will cost about $30-40 per month in electricity costs. However, it is well worth the cost. It protects your belongings, and makes your basement and home more livable.

A dehumidifier must be properly sized to provide the dehumidification required. For most basements a 40-50 pint unit is appropriate. Always have a working backup dehumidifier in reserve. They tend to require a lot of maintenance and may not be operable when you need them the most.

Because of the relatively high electrical operating costs, it is better to purchase a more expensive, high-end dehumidifier than a less expensive, low-end one. You can make up the cost difference in a single summer.

The best way to operate a dehumidifier is to drain the condensate to a sump, or to your central air-conditioning drain line. When one has to constantly drain the dehumidifier by hand, you will tire of it and then tend to neglect it. As you do so an infested basement is not far away, and then there is that two-week vacation!

See all the dehumidifiers at Allergybuyersclub.com

First Published: early November 2002
Updated: April 2007

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