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When it comes to taking care of your house and the health of the people living in it, there is one problem that may regularly arise, especially if you live by the Great Lakes or the Atlantic Coast: excessive moisture. High humidity, whether due to poor air circulation and ventilation, an uncontrolled leak or simply the climate where you live, can lead to mold infestation and structural damage, and it can even exacerbate health problems like allergies and asthma.

A dehumidifier is device that removes moisture from the air by condensing it into liquid water. They can be important tool to help reduce humidity in your house, prevent moisture-related problems and improve your air quality. Similar to how a humidifier helps to add moisture, a dehumidifier can be especially useful in the basement, crawl space or bathroom–areas of your home that tend to have excessive moisture. About 13% of US households use a dehumidifier, with the highest use in regions that experience very humid weather. This article will examine how dehumidifiers work and explain when, where and how to use them properly to keep your household humidity problems under control.

Where, when and why you may you need a dehumidifier

The areas in your house where high humidity problems are most common may have poor ventilation or do not get the benefit of air-conditioning. Typically, that means your basement and your attic or crawlspace are the prime spots for humidity problems. If you do not have a window or exhaust fan in your bathroom, this can cause humidity problems as well.

Often, the signs of a humidity problem can be as simple as walking into the room and feeling that it is muggy or clammy. A musty smell is another important sign of problematic humidity levels and could indicate a developing mold problem.

When humidity is especially high or lasts for a very long time, you will start to see more visible signs, such as:

  • Surfaces in the area can actually be wet to the touch.
  • Soft surfaces like carpets, wood or ceiling tiles might show damp spots or water stains.
  • Eventually, wood may become discolored and start to rot.

Rotting joists in your house’s structure are a serious problem that can cost thousands of dollars to repair.

You might need to use a dehumidifier even at moderate humidity levels if you store sensitive items in your attic or basement. Books and photographs are extremely sensitive to humidity and will deteriorate over the years if the humidity isn’t kept low. Many collectibles can be damaged by humidity as well, so it is probably a good idea to use a dehumidifier where you store your baseball cards or comic books.

What a dehumidifier can do for your health

Keeping household humidity low with a dehumidifier is good for your house’s structural integrity, but it is good for your family’s health as well. More importantly, humid conditions are ideal breeding grounds for a host of allergy triggers in the environment.

A dehumidifier may help reduce mold growth

Mold thrives in damp, humid conditions, and when mold thrives, it releases mold spores. The National Institutes of Health (NIH) reports that, “In people who have sensitive airways, allergy and asthma symptoms can be triggered by breathing in substances called allergens, or triggers…Mold is a common trigger.” The NIH also suggests using a dehumidifier as one method to control exposure to allergy triggers.

A dehumidifier can help with dust mite allergens

Humid air also supports a healthy population of dust mites. Dust mites and their by-products are among the most common allergens in the household air. A study (Arlian et al., 2001) in the Journal of Allergy and Clinical Immunology found that homes that maintained a relative humidity below 51 percent (by using air-conditioning and high-efficiency dehumidifiers) had significant reductions in the number of live dust mites and dust mite related allergens. After 17 months, the low-humidity homes saw live dust mite and dust mite allergen levels ten times lower than in the higher humidity homes

Dust mites are a common allergy and asthma trigger, though some clinical trials haven’t found much evidence that dehumidifiers reduce asthma symptoms (Singh & Jaiswal, 2013). For asthma sufferers, high humidity combined with high temperatures can create a cycle of difficulty breathing, excessive sweat and dehydration, according to the Asthma & Allergy Foundation of America.

How does a dehumidifier work?

Dehumidifiers work in much the same way as air conditioners. Both devices take advantage of the properties of a fluid (freon, or its more environmentally friendly modern alternatives) that readily compresses and expands. The cycle of expanding and compressing the refrigerant cools down coils inside the device while generating heat in another part of the unit. When air from the room is drawn in by a fan, it passes over the coils and is cooled down.

But how does a dehumidifier gets the moisture out of the air then? For that, we will need to talk about humidity itself. A given quantity of air can only hold a certain amount of water molecules, and that amount can change depending on the temperature of the air. Air expands as it gets warmer, so it can hold more moisture. That is why 90% humidity on an 85-degree day feels far worse than 90% humidity on a 50-degree day. This is known as relative humidity.

When air is cooled down, it can not hold as much moisture. The air contracts, like squeezing a sponge. The moisture is forced out of the air and will eventually condense onto a surface as liquid water.

That is what happens inside a dehumidifier or an air conditioner. The air passes over the cool condenser coils and is itself cooled down. Moisture is “wrung out” of the air, which collects onto the coils and drips down to a collection bucket or, in a window air conditioner, a drain pan that leads to outside. In a dehumidifier, the drain bucket usually has a float that will trigger a shut-off switch to prevent the bucket from overflowing. Some dehumidifiers use gravity-fed hoses or even pumps to automatically remove the water.

Either way, the water is removed from the air, and the less-humid air is returned to the room. Modern dehumidifiers have a humidistat that lets you set a specific humidity level (30 to 50 percent is typically suitable for most homes). The humidistat will automatically shut off the dehumidifier when the humidity reaches the set point, then reactivate it when the humidity rises.

Choosing the right dehumidifier for your space

A dehumidifier essentially “works” the same, whether in a basement, bathroom, bedroom or crawl space, because how it removes the moisture from the air will be by the same method no matter the room it is in. However, the best choice can be affected by the type of room you are looking to reduce humidity in. In general, the square feet of the space, how damp the room gets and whether it will operate in temperatures below 65 degrees may impact what kind of dehumidifier is best for your needs.

Another major consideration when buying a dehumidifier is capacity (measured in how many pints of water the unit can extract from the air over 24 hours).

The capacity of a dehumidifier and the size you need

Energy star has a helpful guide that shows what capacity of dehumidifier you need, which is based on the 1) size of the space and 2) how damp or wet are the conditions of the room Obviously, a larger, wetter space will require a more heavy-duty dehumidifier. Below are some estimates for reference.

For a “moderately damp” room (feels damp and has a musty smell only when it is humid):

  • 500 sq. ft.: 10-pint dehumidifier
  • 1,500 sq. ft.: 18-pint dehumidifier
  • 2,500 sq. ft.: 26-pint dehumidifier

For a “very damp” room (space is always damp, smells “musty” and has damp spots showing on the floors and walls):

  • 500 sq. ft.: 12-pint dehumidifier
  • 1,500 sq. ft.: 22-pint dehumidifier
  • 2,500 sq. ft.: 32-pint dehumidifier

The temperature of the space and how it affects a dehumidifier

If the space you need to dehumidify has an air temperature below 65 degrees, you may need a dehumidifier with special features to prevent frost from forming on the condensing coils. These dehumidifiers have an anti-frost sensor that will shut the device down temporarily to prevent frost from building up.

What kind of dehumidifier do I need for my basement?

A basic mechanical/refrigerant (compressor) dehumidifier may be what you need to dehumidify your basement. If temperatures generally go below 65 degrees in your basement, it is a good idea to consider the anti-frost sensor mentioned above. Frost can impair the performance of the unit by causing the compressor to turn on and off without really removing the moisture from the air. You should also be considering a model with a hose attachment feature. This allows you to connect a hose and direct the other end to a floor drain or sump pump, allowing the dehumidifier to keep working without you having to empty the bucket – especially if the may need to be continuously dehumidified through a period of excessively humid months.

What dehumidifier should I use for an attic or crawlspace?

Attics and crawl spaces can present challenges for removing humidity, but it is crucial to dehumidify those spaces to prevent mold and wood rot. For a large attic, a regular compressor dehumidifier is a good solution. For smaller enclosed spaces, there are lower capacity dehumidifiers that can do the job as long as you can get an extension cord into the space. For really tight or tiny crawl spaces, using a desiccant might be your only option in lieu of using a dehumidifier.

What dehumidifier should I use for a bathroom?

For rooms with specific moisture problems, like kitchens and bathrooms, air conditioning will help, but those rooms really should have a vent with a fan to divert moisture outside the house. Moisture buildup in poorly ventilated bathrooms is a common source of household mold. However, if there is no fan that exhausts to the outside, or a window that can be opened, a dehumidifier may be a good option to remove excess moisture in the bathroom. In general, because of the small space you most likely don’t need a large capacity unit.

Use a desiccant for small, enclosed spaces

A desiccant “dehumidifier” has no moving parts and requires no electricity. It is essentially a bucket or dish filled with a substance that can absorb large amounts of moisture. When the desiccant substance is saturated, it stops working, so it needs to be replaced or refilled. Using a desiccant is not effective in large spaces with high humidity, like a basement, but they are excellent is small enclosed spaces where you need to control humidity, but do not have enough space to run a bulky dehumidifier. Cabinets, in camping trailers and crawl spaces are all places where desiccant could work well.

Controlling humidity in the rest of the house

If you have humidity problems throughout your house, one way to deal with it is to install central air conditioning. Since an air conditioning unit works like a dehumidifier, except that it dumps hot air outside instead of back into the house, it also dehumidifies the air that it cools. This can help keep overall humidity levels low throughout your house.

Ventilation can help with humidity in attics and crawl spaces as well. Ask a contractor to see if vents can be installed that will allow air to flow through these areas.

Keep in mind that excessively low humidity (below 30 percent) can also cause structural and health-related problems. This is typically a problem in the winter months in colder climates, when the furnace is operating frequently, but make sure you’re not creating overly low humidity by operating a dehumidifier.

Another solution you may often come across are adding “vapor barriers” in basements that attempts to “lock out” moistures. Unfortunately, these can often be highly problematic. It is effectively impossible to keep any moisture at all from entering a basement, so vapor barriers end up trapping the moisture and causing more mold and odor problems than they solve.

Dehumidifier to improve indoor air quality

The overall goal when using a dehumidifier is to improve the indoor air quality in your home. Another way to improve air quality, especially if you have an allergy to mold or dust mites, is to use an air purifier. One option to consider is the Molekule PECO technology, which can destroy mold and dust mite allergens, unlike other air purifiers that simply collect them on filters.

Controlling humidity levels is very important for air quality and may help with asthma or allergies. It may also help preserve the structural integrity of your home. If you live in a humid climate, or regularly deal with excessive moisture in your home, choosing the right capacity dehumidifier (and considering the temperature of the room) can help, whether it be in your basement, bathroom or bedroom.

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