An energy-efficient home in a hot, humid climate should have a tight envelope, thick insulation, energy-efficient appliances, and low-solar-gain windows. If you include these features in a new home, your air conditioner won’t run as often as your neighbor’s. That’s good.
But there is a downside to the fact that your air conditioner runs rarely: during the hours that your house has no active cooling, it also has no active dehumidification. As a result, your indoor relative humidity is going to rise.
This problem is fairly well understood. In hot, humid climates, a run-of-the-mill house has better control of indoor humidity levels than an energy-efficient house. If you live in an energy-efficient house in a humid location like the Southeastern U.S., you may need a dehumidifier.
Why do some homes need supplemental dehumidification?
To figure out whether you need a dehumidifier, it’s worth considering the factors that raise indoor relative humidity levels. Some of these factors apply to all climates: indoor humidity levels rise when there are many people or pets living in the house; when residents take many showers; when there are lots of houseplants; when residents cook frequently; and when someone mops the floor.
Other factors — some of which are counterintuitive — apply particularly to houses in hot, humid climates:
A study of homes in Houston
One of the landmark studies looking at humidity problems in energy-efficient homes was conducted by three researchers from the Building Science Corporation (Armin Rudd, Joseph Lstiburek, and Kohta Ueno).
The researchers monitored humidity levels in Building America homes with excellent thermal envelopes — high levels of insulation, low levels of air leakage, and above-average windows — and compared the homes with a control group consisting of ordinary new homes. Somewhat surprisingly, “The houses without energy efficiency improvements and without mechanical ventilation had much fewer hours of high relative humidity than those built to the Building America metrics.” In other words, the energy-efficient homes were more humid and less comfortable than the run-of-the-mill homes.
The researchers monitored different types of dehumidification equipment installed to address these issues, and discovered that the least expensive solution was the most effective.
The researchers reported, “The stand-alone dehumidifier in an interior hall closet system, with central-fan-integrated supply ventilation and fan cycling, had the lowest initial cost and operating cost while providing good humidity control. This system is recommended. … The stand-alone dehumidifier system involved installation of an off-the-shelf 50-pint-per-day dehumidifier in an interior closet with a louvered door near the central air return. The dehumidistat built into the dehumidifier energized the dehumidifier whenever the humidity level rose above the user setting. The fan cycling control [for the supply ventilation system] was set to 33% duty cycle (on for 10 minutes if it had not been on for 20 minutes), to intermittently average air conditions throughout the house and distribute ventilation air.”
The solution is fairly simple
A stand-alone dehumidifier costs between $200 and $250. There are two basic types: units that are hooked up to a drain, and units that come with a plastic tub that needs to be emptied once or twice a day. Needless to say, most people prefer the convenience of a unit that is hooked up to a drain.
The capacity of a residential dehumidifier is reported in “pints per day.” Typical residential dehumidifiers have capacities ranging from 25 to 75 pints per day. If you choose to buy a unit with a plastic tub that is emptied by hand, you should be aware that most dehumidifiers have tub volumes that are less than half the rated capacity of the unit. That means that if you expect the dehumidifier to achieve its rated capacity, you would need to empty the tub two or three times each day.
Most stand-alone dehumidifiers have controls that allow the user to choose a relative humidity (RH) setpoint and a fan speed. Of course, if you choose a low RH and a high fan speed, the appliance will use more energy than if you choose a higher RH and lower fan speed.
The efficiency of a dehumidifier (its energy factor or EF) is reported in liters per kWh. The federal government sets minimum efficiency standards for dehumidifiers:
- 1.35 liters/kWh for units rated at 35 pints/day or less
- 1.5 liters/kWh for units rated at 35 to 45 pints/day
- 1.6 liters/kWh for units rated at 45 to 54 pints/day
- 1.7 liters/kWh for units rated at 54 to 75 pints/day
The minimum requirement for an Energy-Star-labeled dehumidifier is more stringent, of course:
- Any Energy Star dehumidifier rated at 75 pints per day or less must have a minimum EF of 1.85 liters/kWh.
- For Energy Star dehumidifiers with a capacity of more than 75 pints per day, the minimum EF rises to 2.8 liters/kWh.
Dry air is good, but wringing moisture from the air is energy-intensive
Keeping your indoor humidity low is generally a good idea. During the winter, lowering the indoor humidity is easy (and relatively cheap): all you have to do is operate a ventilation fan. If you bring in cold outdoor air, the outdoor air will be dry. (Cold air can’t hold much moisture.)
The longer you run a ventilation fan, the lower your indoor relative humidity. To achieve this benefit, either an exhaust fan (for example, a bath exhaust fan) or a supply fan — or both — will work.
Operating your ventilation fan at a high rate during the winter carries an energy penalty, of course, because all of that outdoor air has to be heated. But the energy penalty associated with winter ventilation is much less than the energy penalty associated with running a dehumidifier. And fortunately, it’s rare to have high indoor humidity during the winter. Generally, the problem only occurs if the house is small and occupied by a large family, or if the house has a wet basement. Most homes have the opposite problem: indoor air is too dry, not too humid, during the winter.
During the summer, ventilation won’t help you lower the indoor relative humidity — especially if you live in the humid Southeast. When the weather is hot, the outdoor air is usually very humid. The more you ventilate, the more humidity you bring into your house. So during the summer, the best approach is to keep ventilation to a minimum.
As long as the outdoor temperature is hot, your air conditioner will probably be running for a few hours every day — enough to keep the indoor humidity under control. But during the spring and fall, your air conditioner won’t be running. That’s when you may need to turn on your dehumidifier.
Many homeowners aim for 50% or 60% indoor relative humidity during the spring, summer, and fall. If you use a dehumidifier to reach this goal, your dehumidifier will add heat to the house. But that’s not a big problem; if the indoor temperature gets too hot, your central air conditioner will turn on, further lowering your indoor humidity level.
Dehumidifying your indoor air under these circumstances is a good thing to do. Dryer indoor air is better for you house and probably better for your health. The only downside is the energy required to dehumidify the air. Homeowners have to find a balance they are comfortable with — a balance between very dry air and affordable energy bills.
Operating a dehumidifier
If you have a dehumidifier in your house, here are a few points to remember:
- Keep windows and exterior doors closed. It’s very hard for a dehumidifier to work if it is trying to dehumidify the whole neighborhood.
- During hot, humid weather, you should minimize the run time of your ventilation fan. A tight house needs a little bit of fresh air, of course — but too much outdoor air will overload the ability of your dehumidifier to keep the indoor air dry.
- Experiment with different settings and run times of your dehumidifier to make sure that you aren’t running it for too many hours a day. If you are comfortable at 60% relative humidity, there’s no reason to try to keep your interior at 45% relative humidity. During hot, humid weather, it can be very expensive to try to make your indoor air dry.
Dehumidifiers use a lot of energy
About 19% of U.S. homes have at least one dehumidifier, and they use a lot of electricity.
Lauren Mattison and Dave Korn, two researchers employed by an environmental consulting company in Massachusetts called Cadmus, recently conducted a . The researchers monitored the energy consumption of 21 residential dehumidifiers in the Northeast and mid-Atlantic regions over a three-month period. According to the study, the average dehumidifier uses about 1,000 kWh per year — twice as much as an Energy Star refrigerator. In homes with dehumidifiers, the amount of electricity used by the dehumidifier equals 9% of the electricity consumption in an average home.
A summary of the researchers’ findings was published in . In that article, Mattison wrote, “The timing of this study” — the researchers’ monitoring extended from September 2011 to November 2011 — “did not allow us to measure the peak operation, which we expect would occur during the spring and summer months. Therefore the annual projections based on this study are conservative estimates of dehumidifier operation. … We would expect actual annual operating hours to be higher, because this study did not include metering during the time of year when dehumidifier operation is at its peak.”
- The average run time for the dehumidifiers in the study was 8.9 hours per day. Mattison wrote, “Because direct-drain units were not limited by tub size, they operated for longer hours, with average active run time of 11 hours per day for direct-drain units, compared to 8.1 hours per day for manually emptied units.”
- On average, the dehumidifiers used 459 watts when running. The range was from 375 watts to 964 watts.
- The average metered electricity consumption was 4.2 kWh per day (5.6 kWh per day for direct-drain units and 3.7 kWh per day for manually emptied units).
- On average, the homeowners enrolled in the study operated their dehumidifiers for eight months per year (2,160 hours per year).
- About half of the homeowners left their dehumidifiers on year round.
- The measured energy factor (EF) ranged from 0.2 to 2.1 liters/kWh, with an average of 0.8 liters/kWh. The measured EF was lower than the rated EF for all but two of the monitored dehumidifiers.
Dehumidifiers don’t save energy
Armin Rudd points out that “supplemental dehumidification, in and of itself, does not save energy. Rather, it is justified by enabling the energy savings from dramatically reduced sensible cooling loads in high-performance homes in hot-humid climates.”
In other words, running your dehumidifier adds to your energy bill. But running a dehumidifier in a hot, humid climate makes sense if it solves problems that accompany thermal envelope improvements like improved airtightness, improved insulation levels, and better windows. As long as the dehumidifier is run only when needed — not excessively — it will take less energy to run a dehumidifier and an air conditioner in such a house than it would to operate only an air conditioner in a house with a worse thermal envelope.
A weapon of last resort
Dehumidifiers use a lot of energy, and are a blunt and undesirable weapon in the war against high indoor humidity. Your first defense in this war is to build a tight building envelope to minimize the infiltration of humid exterior air. Your second defense is to install and use exhaust fans in humid locations (in bathrooms and above kitchen stoves). Your third defense is to operate your air conditioner during hot weather.
In almost all locations in the U.S., air conditioners do an adequate job of dehumidifying indoor air. If you live in a tight, well insulated house in a very humid climate, you may have a hard time keeping indoor humidity under control during the swing seasons (spring and fall). In these homes, a stand-alone dehumidifier may be the best solution. However, installing a dehumidifier is always a last resort.
Martin Holladay’s previous blog: “Do Cars Perform Better Than Houses?”