Summer weather forecasts often include a heat index–a combination of temperature and humidity that predicts what the air actually feels like. It rises as the air becomes moister, which makes people feel uncomfortable and creates problems in houses too.
According to the National Weather Service, at 90 degrees and only 20 percent relative humidity, the Heat Index drops to 87 degrees. But at the same temperature and 80 percent humidity the index jumps to a blistering 113 degrees.
With houses, moist air and dripping condensation foster mold, bad odors and create a slew of cleaning and maintenance problems. On steamy summer days it’s not always easy to maintain, but 40 percent or lower relative humidity is a good target to shoot for indoors. That’s the level you need to control mold growth, says the American Lung Association.
Here are ways to make a humid home more comfortable:
– Basic air conditioner maintenance. Changing the filter and maintaining the system is a sensible first step. But the real culprit is dirt and dust that builds up on the coils where temperature is transferred. They act like an insulating blanket and get in the way of pickup and delivery. A study by Honeywell, a major manufacturer, found that typical buildup over several years reduced efficiency and increased electrical costs by 10 to 15 percent.
– Venting moisture. You’ll lose some cool air in the process of venting, but it’s a good tradeoff to reduce interior moisture–and there’s plenty of it. In most homes, cooking, cleaning and washing generate 7 to 10 gallons of moisture every day.
The most efficient approach is to install direct-exhaust vent fans where the moisture is produced. In bathrooms, for example, install a vent fan wired to a timer switch. You can set it to run for five or 10 minutes after a shower to exhaust excess moisture and then shut off automatically after you’ve left the room.
– Circulating air. Exhaust vents move air in limited areas for a while. Ceiling fans move air in much larger areas continuously. They also make people feel cooler. Fans also save money, particularly in warm climates, by allowing you to raise the air conditioner thermostat. In Florida, for example, where air conditioning accounts for about a third of the annual utility bill, fans allow most people to raise the thermostat 2 to 6 degrees without sacrificing comfort, according to the University of Florida Extension Service.
– Drying indoor air. Sometimes controlling moisture is especially important, for instance in a home shop or office where excessive moisture can corrode machinery and appliances. Dehumidifiers will do the job. But the machines don’t have an exterior outlet like an air conditioner and can’t exhaust heat from their compressors. The upshot: Your home office will be drier, but also warmer.
Among the 300 or so models from 30 manufacturers listed by the Association of Home Appliance Manufacturers, capacity ranges from about 10 to 50 pints per day. Unlike air conditioners, dehumidifiers are not evaluated by energy efficiency, only by capacity. That’s because there is not much variation in efficiency. Looking at models with a capacity of 20 pints per day running 1,300 hours per year (basically morning to night during the summer), a Department of Energy analysis of operating costs found only a $9 per year difference between the least and most efficient models.
If you only need to run a dehumidifier on the steamiest days, a small unit that removes 10 to 15 pints of water per day should do. Most can deal with rooms in the 500-square-foot range. Larger machines that collect up to 50 pints per day (that’s more than 6 gallons of water) can dry out basements three times as large.
After moisture is collected, some basic units keep pumping even if the pan overflows. It pays to buy a machine that cuts off the compressor when the collection pan is full.
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Trouble-shooting damp spots
Here’s a look at cures for some of the most moisture-prone parts of the house:
– Fixture condensation. Toilet tanks are prime candidates–regularly replenished with cold water that makes their surfaces cool enough to sweat puddles of condensation onto rugs, tile and grout. You could wipe up the water, in muggy weather sometimes several times a day. Otherwise the wetness fosters mildew growth and starts to erode tile grout. Eventually it can seep through the cracks and rot the wood subflooring underneath.
To control the sweating, try to catch at least some of the flow with a pan that mounts under the water tank. It collects condensation and funnels it through a small hose into the bowl. The ultimate fix is to install a mixing valve, which deals with condensation on the holding tank and the bowl. This small valve replaces the single inlet feeding cold water to the tank. It has two inlets, one for cold water and one for hot, but only one outlet that sends the tepid mix to the fixture. This keeps the normally cold fixture walls close to room temperature and prevents sweating.
– Dead-air zones. Almost all houses have some dead-air zones. The most obvious are closets. But cooling and air circulation can be limited in any small area closed off with a door. To combat moisture and mold growth you need to increase the airflow, increase the air conditioning input, or both. There are several options with closets. One is to cut a vent in the door, or a pair of vents–one high and one low–that increases through ventilation. Use wooden slat-type vents that blend in, or better yet, change a solid door for a fully louvered model.
– Air conditioning duct condensation. Match duct walls at 72 degrees with moisture-laden air in the attic at 100-plus degrees and you have a perfect recipe for condensation. The water can drip onto insulation and framing anywhere along the line.
Increasing attic ventilation will help. But whatever mix of inlet and outlet vents you use should create air flow throughout the attic without leaving dead areas of uncirculated air. To work effectively, you need about the same square footage of inlets, normally at the overhangs, and outlets, normally near the roof ridge.
Many new homes have the most effective outlet, a continuous ridge vent. To add one in an older home, the cap shingles along the ridge are removed. Then slots are cut in the plywood on each side of the ridge and covered with a vent that lets air out but keeps rain from getting in. Most vents blend well with shingles and are barely noticeable.
To eliminate condensation problems in attics or crawl spaces, it’s important to insulate the ducts. Where they run between joists, pack the spaces around them. Where they pass above joists, spiral wrap all exposed surfaces. This creates a buffer between temperature extremes that prevents condensation from forming on the duct walls.
— Mike McClintock
