Whole-home humidifier settings for New York winters

Why a heated house runs so dry

Relative humidity (RH) is how much water vapor the air holds compared with the most it could hold at that temperature. Cold air holds very little, so outdoor air that feels damp in January carries only a small amount of moisture. When that air leaks into the house and is heated to 70 °F, its moisture content stays the same while its capacity rises sharply, and the relative humidity collapses.

Dew point is the companion measure that matters for buildings: the temperature at which the moisture in the air begins to condense. Any surface in the house colder than the indoor dew point will collect water.

Air leakage and ventilation replace indoor air with this dry outdoor air all winter. Leaky frame houses and brownstones dry out fastest. Homes that have just been air-sealed, re-windowed or re-insulated hold far more moisture, and many of them need little or no added humidity.

Dew point, cold glass and mold

Building Science Corporation (opens an external site), citing ASHRAE Standard 62-2001, places the lower end of the acceptable range at about 25 percent RH, and notes that complaints of dry nose, throat, eyes and skin appear when the indoor dew point falls below about 32 °F, which is roughly 25 percent RH at 68 to 70 °F. Very dry air also brings static shocks, gaps between floor boards and movement in millwork. A humidifier eases these effects. It is not a medical device, and we make no health claims for it.

At 70 °F, each step up in indoor RH raises the dew point: 25 percent RH corresponds to a dew point of about 32 °F, 30 percent to about 37 °F, 35 percent to about 41 °F, 40 percent to about 45 °F, 45 percent to about 48 °F and 50 percent to about 51 °F. Window glass is usually the coldest surface in the room. Mold can grow on a surface (opens an external site) well before visible condensation, once the air against it reaches about 70 percent RH; at 40 percent RH indoors, condensation forms below a surface temperature of about 45 °F, while mold risk begins below about 54 °F.

In practice, single-pane glass fogs or frosts at 30 percent RH on a 20 °F night. Older double-pane windows tolerate about 35 percent, although the bottom edge of the glass and aluminum frames condense first. Good low-e windows generally tolerate 40 percent at New York winter temperatures.

Damage inside walls, attics and masonry

Wet windows are the visible warning. The serious damage happens where moist indoor air leaks into the structure through gaps around ceiling fixtures, attic hatches, outlets and top plates.

• Attic frost: moisture freezes on roof sheathing and nail tips in January, then melts on the first thaw and drips onto the insulation and ceiling. Building Science Corporation recommends keeping indoor RH below about 45 percent (opens an external site) during the coldest part of the year for this reason.
• Wet insulation: damp fiberglass and cellulose lose much of their insulating value, and the resulting cold spots attract still more condensation.
• Rot and paint failure: repeated wetting of roof and wall sheathing leads to decay, and moisture moving outward lifts exterior paint on wood siding.
• Mold in cold corners: exterior closet walls, the wall behind a bed or dresser and the corners of north-facing rooms are the usual locations.
• Window sills and sashes: water running off the glass rots wood sills and stains plaster. It is often the first damage an owner notices.
• Interior-insulated masonry: when a brownstone or brick wall is insulated from the inside, the brick runs colder and wetter (opens an external site), which raises the risk of freeze-thaw spalling. These houses are run at a lower winter humidity.

Settings by outdoor temperature

The common 30 to 50 percent range is a comfort and health guideline, not a safe building limit when it is 10 °F outside. The table is our starting point for New York City and Long Island homes, based on Building Science Corporation guidance and manufacturer setting charts. The window test below takes precedence over it.

The window is the most reliable gauge. A light film of fog along the very bottom edge of the glass on the coldest morning marks the upper limit; water running onto the sill means the setting comes down. Set the system by the coldest window in the house, often a north-facing bedroom, not by the average.

A digital hygrometer in a central room reads better than the humidistat on the return duct, which sees mixed air from the whole system. After a renovation, record indoor RH for a few weeks before buying a humidifier.

Which humidifier fits the heating system

• Bypass evaporative humidifier: uses the furnace blower and a small duct to the return to pass warm air through a water panel. Lowest cost, and it fits gas or oil furnaces with warm supply air. Output depends on furnace run time, so an oversized furnace that cycles briefly delivers little moisture.
• Fan-powered evaporative humidifier: has its own fan and needs no bypass duct. Manufacturers rate these units at a set plenum air temperature (Aprilaire uses 120 °F (opens an external site), some competitors 140 °F), so capacities are compared on the same basis.
• Steam humidifier: produces its own vapor, so output does not depend on supply air temperature. It requires a dedicated 120 or 240 volt circuit, installed by our licensed electrical partner, and uses more electricity than evaporative units.
• Heat pump systems: heat pump supply air is noticeably cooler than furnace air, so bypass units underperform. A steam unit is usually the better choice, and Aprilaire recommends a hot water supply where an evaporative unit is used on a heat pump.
• Radiator, steam and ductless mini-split homes: without ducts there is no duct-mounted whole-home humidifier. The practical options are well-maintained room humidifiers, a steam humidifier with its own dispersion unit, or a ducted system planned as part of a larger ventilation project.

Water, drains, dampers and controls

• Drainage: evaporative and steam units send water to a drain, so the location needs a floor drain, standpipe or condensate pump, with freeze protection for any line in an unheated space.
• Seasonal damper: the bypass damper is open in winter and closed in summer. Left open in summer, it reduces air conditioning airflow; left closed in winter, the humidifier does almost nothing.
• Water quality: New York City water averages about 1.8 grains per gallon of hardness according to NYC DEP (opens an external site), harder where Croton water is served, so scale in steam canisters is mild in most of the city. Long Island and upstate supplies vary.
• Maintenance: replace the water panel at least once a season, and clean or replace the steam canister as the manufacturer directs.
• Outdoor temperature reset: an automatic control with an outdoor sensor lowers the setpoint as the weather turns colder, which follows the table above without daily adjustment.
• Safety interlocks: the humidifier runs only with the blower, and a high-limit humidistat on the supply side prevents wet ducts. A steam dispersion tube placed too close to an elbow is a common cause of water in ductwork.

We begin with the heating system, the windows, any history of condensation or mold, the attic air sealing, and the moisture sources already in the home, such as cooking without a ducted hood or a damp basement. A house that already holds enough moisture is usually better served by ventilation or source control. Where a humidifier is appropriate, we select the type and capacity for the heating system, plan the water, drain and electrical connections, and install the unit with an outdoor-reset control.