It seems almost everyone has basement dampness problems. Where does all that water come from?

Humidity can enter your house naturally in the air, in warm weather through open doors and windows, and through air leakage when the heat or AC is on, such as poorly sealed windows and doors, cracks in exterior wall plaster in older houses, cracks in upstairs ceilings, or attic hatches that lack proper weatherstripping. Anything that draws air out of your house, such as a woodstove, exhaust fan, or a low- or mid-efficiency furnace, will also pull outdoor air in.

So one way you can reduce moisture is to restrict the flow of fresh air, especially during humid weather.

Water can enter through your basement walls from outside. You need to direct surface water away, seal the outside of the foundation, improve drainage below the foundation, and you may even need to seal the inside of the masonry wall as well.

Aspects of your own daily life such as breathing, sweating, showering and cooking, can also create moisture when the house is closed up. The daily activities of inhabitatns can add as much as 20 gallons of water a day to your house.

Rain collecting in your driveway, or pouring out of an eaves trough, makes its way downhill. If the soil is porous around your foundation, or if there is a gap between the pavement and the house, the water will percolate through the ground or the crack until it meets the water table or is otherwise halted. Once it can’t flow down, it flows across, which is when it starts seeping through your foundation walls.

So keep surface water away from the walls of your house. Keep eaves troughs cleaned out and properly angled, and downspouts in working order with their outflow running away from the house. Your driveway should slope away from your house, or you can run a line of mortar along the gap between the driveway and the house to keep water that accumulates in the driveway from working its way down along your foundation walls.

Gardens, lawns, and other surfaces should also draw water away from your home, and if you have a sump pump it should drain far from the house as well.

Protecting your basement from groundwater is a bit more challenging and can be expensive. A basement contractor can dig a trench to your foundation walls from outside, and apply a sealant to the exterior below-ground walls. They should also backfill the trench with gravel and sand to improve drainage, and they can put in drainage tile at the bottom to draw groundwater down below the floor of your basement.

A more affordable option, if you have exposed masonry in your basement, is to scrub down the masonry to a hard, clean surface free of grit or mineral sweat, and use a parging compound to add waterproofing on the inside of your walls. This works for minor humidity problems but won’t solve serious basement moisture problems.

Any moisture you can avoid producing indoors will help cut down on humidity problems. You can’t stop breathing, but you can cook with lids on, and turn on the range hood when boiling water; you can cut back on showers or run the bathroom fan when showering; and be mindful of aquariums, decorative fountains, and other possible humidity sources.

If winter humidity is a problemIf you have humidity problems in the winter and your home is recently built and well insulated, it is possible that your home is sealed too well. A well sealed home cuts your heating costs, but if it’s too well sealed moisture will build up, as will off-gassing from synthetics, woods, carpets, and other materials that may harm your health. You might consider buying a heat exchanger rather than a dehumidifier if this is your problem. Heat exchangers let air flow in and out of your house, while keeping the heat in as the exhaust air is blown out.

Before go out and buy a dehumidifier to solve moisture problems that can’t be solved by the tips above, you should understand how humidifiers work, their rankings, how ENERGY STAR dehumidifiers are ranked, and how to choose the best model for your situation.

Dehumidifiers remove moisture from the air using a compressor, condenser coils, and fan. They work the same way a refrigerator works, except that their cooling power is used to condense water out of the air. Dehumidifiers have a humidistat that keeps them on until the moisture reaches a set lower level, or until the tank fills up, whichever happens first.

Dehumidifiers are grouped based on extraction capacity — how much humidity they extract from the air per day — and by tank capacity — how much water they hold. In the United States, extraction capacity is rated in pints per day and capacity is measured in quarts. In most other countries, extraction capacity is rated in liters per day and capacity in liters.

Once your tank fills up, the dehumidifier will not extract any water from the air until you empty the tank. If you place your dehumidifier near a basement floor drain, you can run a hose from the tank to the drain, so that you never need to empty the tank. (Most dehumidifiers come with an opening for a hose.) If you can’t do this in the room where you’re putting the dehumidifier, make sure you buy a unit with adequate tank capacity.

The energy factor for dehumidifiers is the number of liters or pints of water removed from the air per kilowatt hour (kWh) of electricity used. The higher the energy factor value, the more efficient the dehumidifier is.  So bigger is better — unlike the efficiency rankings for most other types of appliances.

ENERGY STAR dehumidifiers have energy factors ranging from 3.56 (excellent) to 1.2 (marginally efficient) with a median rating of 1.62. Only two companies, Therma-Stor Products and Munters Corporation, make dehumidifiers with an energy factor better than 2.2.

But bear in mind that an ENERGY STAR rating does not mean the dehumidifier is all that efficient — just more efficient than some others in its class. For example, ENERGY STAR rated dehumidifiers can have an energy factor of as little as 1.2, while the best score is 2.02 and the worst for a non ENERGY STAR dehumidifier is 1.0.

Always buy a dehumidifier that can handle the amount of humidity in your house. You probably do not require a pints-per-day rating above 25 unless your basement is sopping wet and at least 1,200 square feet in area, wet and at least 1,500 square feet, or usually damp and mildewy (but not wet) with at least 1,800 square feet.

On the other hand, ENERGY STAR units that can withdraw more moisture are typically more energy efficient because of the different capacity thresholds used to rate dehumidifierS. So don’t skimp and buy too low a capacity unit — overestimate rather than underestimate.

If your basement is very cool, buy a dehumidifier built to handle lower temperatures. Otherwise, the cold basement will cause frost buildup on the coils, which reduces their efficiency and may cause the motor to short cycle. If notice this short cycling, turn the dehumidifier off until the ice has melted and fallen into the tank. If the behavior persists you may need to replace the unit with a unit built for cooler temperatures.

Assuming you do own or buy a new dehumidifier, you’ll want to reduce your energy costs with whatever dehumidifier you own.

If your basement is humid, and the dehumidifier is on, keep the doors to the basement closed so damp air doesn’t creep in from upstairs.

Keep your dehumidifier coils and fan clean and free of dust. Some dehumidifiers include a washable air filter you can clean to get rid of dust build-up; if you have one, keep it clean.

If you replace an old dehumidifier with a new, properly sized energy efficient dehumidifier, and you tackle your moisture sources as best you can, you could reduce your electricity use by half or even two thirds over what you were paying before for the same level of comfort. But more likely, you’ll use a little less energy with a new and improved dehumidifier, while controlling your humidity more effectively, or you’ll use more energy (if you didn’t have a dehumidifier) but you’ll be far more comfortable and won’t be spending a fortune on electricity.

With all the news surfacing about the health effects of indoor dampness and the resulting mold and mildew, you’ll definitely benefit from addressing your humidity problems and buying an energy efficient dehumidifier. You’ll save money and be healthier.