What should indoor humidity be?

30 to 50 percent relative humidity covers comfort and building health, holding nearer 50 in summer and stepping down toward 30 in deep winter cold so windows and wall cavities stay above their dew points.

Why do my windows fog if humidity is 'normal'?

Because the target moves with outdoor temperature: at 0°F outside, even 40 percent indoors condenses on ordinary glass. Fogging windows are the signal to step the setpoint down regardless of the number's reputation.

Is low humidity in winter actually harmful?

Below about 30 percent brings static, cracked skin and woodwork, aggravated airways, and gapping floors. The winter game is holding the highest humidity the coldest surfaces tolerate, not maximizing it.

Where should I put a hygrometer?

Main living space away from kitchens, baths, and supply registers, with a second unit in the basement, where the house's humidity problems usually live. Cheap digital units are accurate enough for the decisions involved.

Indoor Relative Humidity: Ideal Ranges by Season

Page last reviewed: August 2026 · Reviewed by Harold R. Bennett, retired home inspector

Indoor humidity is the most consequential number nobody measures: it decides window condensation, mold’s food supply, dust mite economics, static shocks, wood movement, and how warm a given thermostat setting feels, and it is managed well with a ten-dollar hygrometer and two setpoints. The ideal indoor humidity band is 30 to 50 percent relative humidity, held near the top in summer and stepped down with winter’s cold. This reference covers the band, the failure modes on both sides, and the tools.

The band, and why winter bends it

Relative humidity expresses moisture as a percentage of what air at its temperature could hold, which is why the same house reads differently room to room and season to season, and why the comfort-and-health consensus lands at 30 to 50 percent: high enough for airways, skin, and wood, low enough to starve mold, mites, and condensation. The winter adjustment is the part folklore misses: as outdoor cold deepens, interior surfaces, window glass first, then wall corners and closet backs, run colder, and humid air touching them crosses its dew point, per the condensation mechanics. The working winter ladder steps the setpoint down with the thermometer, roughly 40 percent in mild cold, 35 around 20°F, 30 near 10°F, and lower in severe cold, with the honest instrument being the windows themselves: persistent fogging says step down, whatever the dial claims. The stakes escalate invisibly, since the same dew-point crossing happens inside wall and attic assemblies where nobody wipes, which is why chronic winter over-humidification is a building problem wearing a comfort costume.

Above the band: the moisture economy

Sustained humidity above 60 percent runs the whole damp catalog: mold’s third ingredient supplied continuously, dust mites (whose populations track humidity closely) flourishing in bedding, condensation staining sills and growing the closet-corner mildew, musty basements broadcasting, and summer air that feels warmer and stickier than its temperature, taxing the AC’s dehumidification duty. The sources are the usual suspects, cooking, showering past the fan’s run time, drying laundry indoors, crawl spaces and basements exporting ground moisture, and humidifiers set optimistically, and the remedies run source-control first, exhaust ventilation second, dehumidification sized to the space third.

Below the band: the dry catalog

Winter’s furnace-driven dryness below 30 percent writes its own list: static discharge on every doorknob, cracked skin and irritated airways (with dry mucous membranes measurably less hospitable to comfort in cold season), nosebleeds in the susceptible, and the woodwork ledger, gapping floorboards, opening trim joints, sharpening piano and guitar seams, shrinking doors, that tracks the annual humidity cycle in every old house. Perceived warmth drops too, dry air evaporating skin moisture faster, so low humidity quietly raises thermostat settings. The remedies mirror the other side: humidification, portable or whole-house, run to the stepped winter targets rather than a fixed 45, and the envelope improvements, air sealing above all, that stop cold dry infiltration from replacing the house’s air several times a day, the leakage that makes winter dryness a symptom of the same holes the heating bill already reports.

Measuring and managing

The practice is two cheap digital hygrometers, one in the main living space away from moisture sources and registers, one in the basement where the problems live, glanced at like a thermostat, plus the window glass as the free winter auditor. The management tools then sort by direction and scale: exhaust fans and source habits for point moisture, dehumidifiers for damp seasons and below-grade spaces, humidifiers for winter dryness with the setpoint ladder respected, and the HVAC layer, AC runtime, ventilation strategy in tight houses, and humidifier interlocks, where whole-house equipment exists. The failure mode to avoid is fighting the instruments: a dehumidifier and humidifier dueling in one house, or a humidifier feeding windows that drip, is the dial arguing with the building, and the building keeps the receipts.

The bottom line

Thirty to fifty, near fifty in summer, stepped down with winter’s cold until the windows stay clear: that one moving target covers mold, mites, static, skin, wood, and walls. Spend ten dollars on the hygrometer, read the glass for free, and move the number with sources and machines instead of arguing with the season.