Good Humidity for a House: The Range, the Reading and What to Check

What the public sources say the number should be

Several public sources give a number, and each one answers a slightly different question.

• EPA, general advice: below 60 percent, ideally between 30 and 50 percent.
• ENERGY STAR, general: 30 to 50 percent is the optimum, and anything above that range may promote mold growth.
• ENERGY STAR, heating season in colder climates: 30 to 40 percent, to prevent window condensation.
• EPA, for a home that runs a humidifier: do not go above 50 percent.
• UF/IFAS Extension, in a county office post: around 50 percent is a perfect indoor humidity.
• Florida Solar Energy Center (FSEC), for empty Florida homes: below 65 percent most of the time and below 70 percent nearly all of the time.

FSEC called 70 percent a critical level, above which mold growth is likely or probable. Our reading: the figures answer different questions. The 30 to 50 percent band is the target, 60 percent is the line EPA says to stay under, and FSEC set 65 and 70 percent as limits for vacant homes.

ENERGY STAR's dehumidifier sizing chart also describes what a room can look like at higher readings when nothing is drying the air:

• 50 to 75 percent: the space feels damp and has a musty odor that may come and go.
• 75 to 90 percent: the space consistently feels and smells damp, and damp spots appear on walls and floors.
• 90 to 100 percent: walls or floor sweat, or seepage is present.

Why the same moisture can read differently from room to room

FSEC says each degree Fahrenheit of temperature rise lowers RH by about 2 percentage points. Our reading: a cool room and a warm room can hold the same moisture and still show different RH readings, because RH depends on how much the air can hold at that temperature.

The National Weather Service defines the dew point as the temperature to which air must be cooled to reach an RH of 100 percent. It gives an example: an 80-degree day with a 60-degree dew point produces an RH of 50 percent.

The National Weather Service Tampa Bay Area office says the rainy season is typically accompanied by persistent dew point values above 70°F. Our reading: when outdoor dew points sit above 70°F, a house that lets outdoor air in can find 30 to 50 percent hard to hold.

How to measure it, and why a cheap gauge can mislead

ENERGY STAR defines a hygrometer as a gauge that measures relative humidity. EPA says a moisture or humidity meter can measure relative humidity, and that many hardware stores sell them.

A UF/IFAS Extension county office post, quoting guidance on hygrometers, says mechanical ones are frequently in error by more than 20 percent. It says electronic meters tend to be more accurate but should also be calibrated. The post describes a salt test: a meter sealed in a plastic bag with a cup holding water and table salt should read 75 percent after at least 12 hours at room temperature.

Our reading: take readings in more than one room and at more than one time of day, and write down the room temperature each time.

When the number is too high: what the sources say to check first

EPA says condensation on windows, walls or pipes can be a sign of high humidity. It says to dry the wet surface quickly and to reduce the moisture source.

EPA lists humidifiers, steam radiators, dryers and stoves as things that can raise humidity, and says cooking and showering add to it too. Among its general tips, EPA says to run the bathroom fan or open the window when showering. UF/IFAS Extension says Florida nights can reach 80 to 100 percent humidity, and that open windows let all that humidity into the house. Our reading: EPA's tips are written for the whole country. In a Tampa Bay summer the outdoor air can be the wetter air, so an open window may add moisture instead of removing it.

EPA also lists clean air conditioning drip pans and unobstructed drain lines among its moisture-control tips. Drain problems are covered in air conditioner leaking water, and mold inside the system in mold in HVAC systems. If the reading stays high while the AC runs, why a house feels humid with the AC running covers the cooling-system causes, so this page does not repeat them.

Florida cases: humid nights, power outages and an empty house

UF/IFAS Extension gives the average relative humidity in Florida as 74.5 percent. It says to check indoor humidity when power comes back after an outage, because without electricity nothing has been removing moisture from the air. Storm preparation is covered in hurricane season HVAC prep.

FSEC's vacant-home study, reported in 2005, monitored three homes in Brevard County, on Florida's east coast. With no air conditioning, dehumidifier or heating running, indoor RH averaged 62, 69 and 73 percent in the three houses, while outdoor RH averages about 77 percent from June through October. FSEC found that setting the thermostat to 85°F did not control humidity, mainly because the air conditioner did not run long enough to remove moisture.

The release named three methods that could give good control in an empty house: a two-hour early-morning cooling run, a dehumidifier on a timer, and summer space heating. The report found that how long a dehumidifier needed to run depended partly on house size and more on the infiltration rate. Infiltration is the rate at which outside air enters the house. Our reading: that is one small study of three older east-coast homes, reported in 2005, so use it to frame questions for a contractor, not as a setting to copy.

Winter and dry air

ENERGY STAR ties its lower heating-season range of 30 to 40 percent to colder climates and to window condensation. EPA says humidifiers can ease nuisances brought on by winter heating, such as static electricity and cracks in paint and furniture. It also says that if water condenses on windows, walls or pictures, you should move the humidifier, lower its setting or use it less. Our reading: with outdoor air as humid as UF/IFAS describes, too much moisture is the more common worry in Florida, but a gauge reading settles which way your house leans.

Portable dehumidifiers: how they work and what they do to the room

ENERGY STAR says a portable room dehumidifier is typically used for a single room or space and can be moved to where it is needed. A GE owner's manual for one of its dehumidifiers says humid air is drawn over a cold refrigerated coil, and the moisture condenses on the coil and drains into a bucket. The manual says the dried air then passes through a condenser, where it is heated and discharged into the room.

The manual says it is normal for the surrounding air to become slightly warmer as the unit runs, and ENERGY STAR says the same. In FSEC's vacant-home tests, more than half of the drop in RH from running a dehumidifier came from the rise in room temperature. Our reading: a dehumidifier dries the air and warms it slightly, so it does not cool a room. Cooling is the air conditioner's job.

ENERGY STAR says the capacity you need depends on the size of the space and on its conditions without dehumidification. It also says to keep the doors and windows to the space closed while the unit runs. The GE manual adds that a unit in a basement will do little for a closed-off storage area nearby, such as a closet, unless air moves in and out of that area.

None of the sources cited on this page measured whether a portable dehumidifier shortens air conditioner run time. ENERGY STAR notes that a home with central air conditioning can take advantage of its dehumidifying capabilities. Our reading: the unit adds heat to the room, and the air conditioner already removes some moisture, so the two partly overlap. Your readings, not a rule of thumb, show whether a unit is worth adding.

ENERGY STAR describes whole-home dehumidifiers as typically installed to use a home's air ducts. Our reading: a search for a furnace dehumidifier is most likely after that ducted type, which whole-home dehumidifiers in Florida covers.

Looking after a portable dehumidifier

A GE owner's manual tells the owner to wash the air filter in warm, soapy water, rinse it, and let it dry before putting it back. It says not to run the unit without the filter, because dirt and lint will clog the unit and reduce performance. It says to clean the water bucket every few weeks to prevent growth of mold, mildew and bacteria. It also says to turn the unit off and unplug it before cleaning.

ENERGY STAR says to place the unit away from sources of dust and dirt, which can clog coils and grills. It adds that frost can form on the coils if the air temperature drops below 65 degrees. If that happens, it says, the unit should be switched off and allowed to defrost before it is turned back on. Our reading: follow the owner's manual for your own model. Work on the refrigerant or wiring inside the unit is for a licensed technician.

What to ask and check before you spend money

These questions turn the points above into things you can check or ask for in writing.

1. What does the gauge read in three rooms, at what room temperature, and at what time of day? One reading in one spot does not describe the house.
2. Has the gauge been checked against a second meter or with the salt test? A UF/IFAS Extension post says mechanical ones are frequently in error by more than 20 percent.
3. Is anything in the house adding moisture, such as a dryer, a stove, a shower with no working fan, or a drain that is not flowing? These all appear on EPA's lists of moisture sources and moisture-control tips.
4. Does the reading stay high while the AC is running? If so, ask for the cooling-system causes to be checked before a dehumidifier is added.
5. If a dehumidifier is proposed, is it a portable unit for one room or a ducted whole-home unit, and what reading is it meant to bring the house down to? A target and a gauge reading give you a way to judge the result.
6. If the house sits empty in summer, what is the plan for humidity while you are away? FSEC's report said that letting an empty house float cannot reliably keep RH below 65 percent most of the time.

UF/IFAS Extension recommends hiring a professional if mold covers more than 3 feet by 3 feet. Wiring, refrigerant and combustion equipment are work for licensed professionals, and this page gives no steps for them.