Quick answer
If your attic is sealed, with spray foam or insulation at the roofline, and its humidity stays above 60%, buy a dehumidifier with a built-in pump, and for most sealed attics that is the ALORAIR 120 PPD ENERGY STAR Crawl Space Dehumidifier with Pump, rated 4.1 stars across 278 Amazon ratings: 55 pints a day at AHAM conditions and a stated 19.6 ft pump lift. If the attic is vented to the outdoors, do not buy one, because it would be drying outdoor air. Find the moisture source instead: a bathroom fan ending in the attic, air leaks around lights and the hatch, or a roof leak.
Buyer's Guide
What to know before you buy.
Vented or Sealed: The Question That Decides Everything
Most American attics are vented. Air comes in at the soffits and leaves at the ridge or gable vents, and the insulation lies on the attic floor. In that kind of attic, the air is outdoor air, so a dehumidifier up there would spend its effort drying the weather, around the clock, without end. The fix for a damp vented attic is to stop moisture coming up from the house and to make sure the vents actually move air. A sealed attic is different. When spray foam or rigid insulation is applied under the roof deck and the vents are closed, the attic becomes part of the house's conditioned space, and its humidity can build up like a basement's. That is the attic where a dehumidifier, or a supply duct from the HVAC system, belongs. Look up before you buy: insulation on the floor means vented, foam on the rafters usually means sealed.
Find the Moisture Source Before Buying Anything
Attic moisture almost always comes from below or above, not from the attic itself. The usual culprit is a bathroom, kitchen or dryer exhaust duct that ends in the attic instead of outdoors. Residential building codes require exhaust air to discharge outside, and EPA advises venting moisture-producing appliances to the outdoors. Next come air leaks: warm, damp house air rising around recessed lights, the attic hatch, plumbing vent pipes, wiring holes and the tops of interior walls, then condensing on cold roof sheathing in winter. Signs include frost on nail tips, dark staining on the sheathing, damp insulation and a bathroom fan that drips. A roof leak shows up as a stain that grows after rain. Fix the duct, seal the leaks with foam and caulk, and repair the roof. In a vented attic, that is the whole job; no dehumidifier is needed.
Heat and Cold: Read the Operating Range Before You Buy
None of the four listings on this page states an operating temperature range, and that is the biggest gap for attic use. At the cold end, ENERGY STAR notes that frost can form on dehumidifier coils below 65°F, which is why auto defrost matters in a sealed attic on cool nights; both AlorAir units and the Airzentti list it. At the hot end, a compressor has an upper limit too, and a vented attic on a summer afternoon can run far hotter than the outdoor air. A sealed attic usually stays much closer to the temperature of the house, which is one more reason dehumidifiers belong only in sealed attics. Before buying, place a logging thermometer and hygrometer in the attic for a week of hot weather. Then ask the maker for the unit's maximum operating temperature and compare. If the attic exceeds it, the answer is an HVAC supply duct, not a dehumidifier.
Getting the Water Out of an Attic
The 55-pint AlorAir is rated to pull almost 7 gallons a day at AHAM conditions, and in an attic every drop of that sits above a ceiling. Choose a unit with a built-in pump: the attic floor is a grid of joists, and a gravity hose needs continuous fall to its outlet, which is rarely possible. The AlorAir picks list up to 19.6 ft of lift, and Airzentti lists 16.4 ft. Common destinations are a nearby plumbing drain with the proper fitting, or a line run down through a wall to the outdoors, away from the foundation. If the attic has an air handler, ask an HVAC technician before sharing its condensate line, since one clog would then back up both. Set the dehumidifier in a drain pan with a float switch that cuts power if water rises, the same protection attic air handlers get. Check the pump and pan each spring and fall.
Sizing and Setpoint for a Sealed Attic
Size from the attic's floor area and how damp it runs. ENERGY STAR's chart calls for 20 to 30 pints a day for a slightly to moderately damp space under 2,000 sq ft, rising to 30 to 50 pints when the space is wet, and it advises oversizing rather than undersizing. Use AHAM figures when comparing, since saturation numbers run about twice as high. A sealed attic over a 1,500 sq ft house that holds 65% RH in summer sits comfortably within the range of the 55-pint AlorAir; the 35-pint Wi-Fi unit suits a smaller section. Set the target at 50% RH. EPA's guidance is below 60% indoors, ideally 30% to 50%, and a sealed attic is indoors. Read the result with a hygrometer at the far end of the attic, away from the machine, and watch it through the first summer.
Who Should Not Put a Dehumidifier in the Attic
Skip the dehumidifier if your attic is vented; fix the moisture source, and if the vents are weak, see the attic fan and soffit advice linked below. Skip it while the roof is leaking, because the machine would only hide the leak. Skip it if the attic has no outlet and no safe place for a 24/7 appliance; an electrician should add a dedicated outlet first, and a long extension cord to a compressor is a fire risk. And if mold already covers the sheathing, more than about 10 square feet, which EPA treats as beyond a routine homeowner cleanup, the job is remediation first: cleaning the wood, removing wet insulation and correcting the ventilation. A dehumidifier holds a dry attic dry. It does not clean mold off rafters, and it cannot keep up with a bathroom fan pouring shower steam into the attic.
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