Individuals with a finished basement may have no choice. If they don't use a dehumidifier, mold might grow on their furniture and other belongings.
However, there are a few downsides to dehumidifiers:
- They use a fair amount of electricity. If you absolutely need a dehumidifier, make sure it is Energy Star rated.
- They vary in effectiveness when the temperature drops. Check the manufacturer's specifications for a given brand or model, or search for "low temperature + dehumidifier," if your basement area chills down in the winter. Perhaps you think your dehumidifier is working when it isn't.
- They don't cure a mold issue. Mold will still be present even though a dehumidifier has been run for a long time.
- They don't prevent mold growth due to moisture wicking up through the foundation onto wood sill plates or studs or other cellulosic surfaces.
- The main concern is one that I was tipped off to by a water restoration expert, Edward "Dutch" Eaton, waterproofing engineer from Eaton's Waterproofing and Basement Restoration Service on Long Island. Dutch was my waterproofing mentor and told me stories of how he saw foundations weakened through years of dehumidifier use. Use of a dehumidifier speeds up the action of lime leaching out of the foundation.
Dutch's dehumidifier stories:
- At one basement, Dutch noticed the large amount of efflorescence. "Efflorescence" is the white mineral deposit on some interior foundation walls. Efflorescence typically is made up of mostly lime deposits. Where does the lime come from? From the cement of the foundation. Lime is like the "glue" that holds the foundation together, and lime is water-soluble.
As moisture passes through the foundation, it can dissolve the lime and bring it to the interior surface. When the moisture evaporates, lime is left behind as efflorescence.
Dutch said to the homeowner, "You've been running a dehumidifier for a long time, haven't you?" The homeowner, surprised (since the dehumidifier wasn't in evidence at the time), replied, "Yes. How did you know?"
Dutch responded that a dehumidifier speeds up the process of deliming a foundation. Since moisture migrates from higher concentrations to lower concentrations, when the dehumidifier reduces the relative humidity in the basement, moisture migrates in from the outside soil, carrying lime with it and increasing the amount of efflorescence.
One of my clients had about 3/4" of efflorescence on his basement walls and thought it was mold. He had plenty of mold, too, but I recommended that he call in a foundation specialist.
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- Dutch told me that particularly with block foundations, he occasionally saw them eaten away from the outside, contributed to by dehumidifier use.
- If the basement is waterproofed, i.e., if there are moisture barriers (such as polyethylene wrap over wet-dry tar) on exterior foundation walls and under the slab, then a dehumidifier can be used without risk because the foundation is protected.
What is a basement?
- Historically the basement was the buffer zone between the earth and the living area of the house. We can get in trouble when we finish the basement, because concrete is not waterproof, and moisture or moisture vapor can seep through concrete into wall cavities, fostering mold growth.
- A waterproofed basement can be finished. Waterproofing should ideally be done at the time of construction when it costs less. A vapor barrier is put down before the slab is poured. Then, before backfilling the foundation walls, apply a heavy coat of wet-dry tar to the exterior wall, followed by wrapping in 6-mil polyethylene plastic. Add in a perimeter French drain system with drainage tiles. Inside, apply silicone to the crack where the wall meets the floor. A dehumidifier could be used in such a waterproofed basement without the risk of leaching lime from concrete.
- Dutch did basement demolition work. I asked him once to estimate the percentage of finished walls where he found mold behind them. He said, "100%." But then, he was dealing with the worst of the basement situations.
You can reduce the need for a dehumidifier in an unfinished basement:
- Make nutrient material unavailable to mold, i.e., encapsulate every square inch of wood, including ceiling joists, subflooring, steps, workbench, wood backing of electric panel.
- Store items carefully so as not to feed lunch to mold. Keep books upstairs. Place clothing in plastic lawn bags or in cardboard bags inside plastic lawn bags. Or, store porous material in plastic bins. Do not store upholstered furniture or mattresses in the basement. Do not store wicker or baskets in the basement.
- Improve air movement, perhaps with a fan. HEPA vacuum a couple of times a year.
- Reduce moisture intrusion from outside.
A tribute to Dutch:
I was introduced to Dutch on an inspection job. When he learned about my environmental inspections, he said to me: "I believe in what you're doing and will do anything I can to help you." And so he became my waterproofing mentor. I frequently called him with questions relating to waterproofing, soil, and foundations, until his passing some years ago. His concern was always for the children, that they shouldn't live in a moldy house or a house with other environmental issues.
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