Best Humidifier for a Bedroom or Large Room, Sized by Square Feet

Output is listed in milliliters per hour; multiply by 24 and divide by 3,785 for gallons a day. The makers' own coverage ratings line up with output: 270 mL/h (1.7 gallons a day) for 432 sq ft, 350 to 400 mL/h (2.2 to 2.5 gallons a day) for 600 sq ft, 550 mL/h (3.5 gallons a day) for 753 sq ft, and 1,500 mL/h (9.5 gallons a day) for 3,000 sq ft.

Size up for a room with high ceilings, an open doorway into a hall, or a drafty older house; all of those lose moisture faster than the rating assumes. An oversized unit is not a problem when it has a humidistat, because it just reaches the set humidity sooner and idles.

Tank size decides how often you refill. At full output a 4.2 liter tank empties in about 15 hours, so the long run times on the box assume low or sleep settings. A 6 liter tank gets most bedrooms through a night and a day; a 10 liter tank or the 6 gallon Superior 6000S goes several days.

Evaporative humidifiers blow air through a wet wick. There is no visible mist and no white dust, they cannot over-wet a room easily because evaporation slows as the air gets more humid, and they use little power. The trade-offs are a higher price, fan noise on high and a wick that needs washing or replacing.

Ultrasonic humidifiers shake water into a fine cool mist with a vibrating plate. They are the cheapest and quietest, and they use little power; the Classic 300S is rated at 26 W. Everything dissolved in the water goes into the air with the mist, which is where white dust comes from.

Warm mist units boil water before releasing it, which leaves minerals in the tank and feels pleasant in a cold room. The heater is the catch: the Levoit warm mist models here are rated at 280 W, roughly ten times an ultrasonic, and the hot water is a burn risk around small children.

If your kettle or shower head scales up, your tap water is hard, and an ultrasonic humidifier will coat nearby furniture and electronics with a fine white film. The EPA recommends water with low mineral content, such as distilled water, in ultrasonic and impeller humidifiers for exactly this reason.

Buying distilled water by the gallon is the hidden running cost of an ultrasonic unit in a hard-water home; a bedroom unit at full output uses more than a gallon a day. An evaporative unit avoids it: minerals stay behind in the wick, which is why the wick discolors and needs washing or replacing.

Whatever type you buy, clean it. The EPA advises emptying, drying and refilling portable humidifiers daily and cleaning them every third day, because standing water grows bacteria and mold that an ultrasonic unit can then spray into the room.

Electricity is small for evaporative and ultrasonic units. Running 24 hours a day at the 17.3 cent US average, the 12 W Superior Studio costs about $0.05 a day ($1.50 a month), the 26 W Classic 300S about $0.11 a day ($3.24 a month), and the 30 W Superior 6000S about $0.12 a day ($3.74 a month).

Warm mist is the expensive one. At its 280 W rating, an OasisMist 450S or LV600S on warm costs about $0.39 for an 8 hour night, or $1.16 a day running nonstop, about $35 a month. Switch it to cool mist and it costs about what an ultrasonic does.

Filters are the other line item. The Superior Studio's wick is washable and rated for up to 2 years with weekly washing; the Superior 6000S wick lasts 3 to 6 months per Levoit. Ultrasonic units have no wick, but in a hard-water home the distilled water costs more than either.

Past about 750 sq ft, one tabletop unit cannot keep up. Either buy a floor unit like the Superior 6000S, rated for 3,000 sq ft, or put a smaller unit in each room you care about, usually the bedrooms. If the whole house is dry every winter and you have forced-air heat, a furnace-mounted whole-house humidifier is the other route, with no tanks to fill.