The Chart
Humidifier makers publish maximum indoor humidity by outdoor temperature. Aprilaire's numbers:(Aprilaire)(Aprilaire manual)
| Outdoor temperature | Indoor humidity, at most |
|---|---|
| 40°F | 45% |
| 30°F | 40% |
| 20°F | 35% |
| 10°F | 30% |
| 0°F | 25% |
| -10°F | 20% |
| -20°F | 15% |
- Honeywell's humidistat chart matches from 20°F down, but stops at 40% above 20°F.(Resideo (Honeywell))
- Aprilaire calls these settings a compromise between comfort and protecting the house, and says 50% in winter might feel ideal but would probably damage it.(Aprilaire manual)
- Turn it down ahead of a cold snap: Aprilaire's example is dropping from 35% to 25% the day 0°F is forecast.(Aprilaire manual) Honeywell suggests moving about 3 points a day.(Resideo (Honeywell)) Aprilaire also sells a control that reads the outdoor temperature and adjusts itself.(Aprilaire)
- The real test is the glass: University of Minnesota Extension says window condensation, not the gauge, shows how much moisture your house can take.(University of Minnesota Extension)
Wet or Icy Windows
- Condensation forms on any surface colder than the air's dew point, and in winter that's usually the windows.(National Research Council Canada) Dew point is the temperature air has to cool to before it's saturated.(National Weather Service)
- It's a humidity problem, not a window defect, according to both Andersen and Marvin.(Andersen)(Marvin) Frequent condensation on the glass suggests moisture is also building up where you can't see it, inside walls and roofs.(Marvin)
- It usually starts at the bottom of the glass.(National Research Council Canada) A strip around the edge usually comes from metal spacers carrying the cold in.(Andersen) At 40% indoors, double-pane windows start to condense once it's below zero outside.(NDSU Extension)
- Cutting it down: run kitchen fans while cooking and bath fans while showering, vent them and the dryer outdoors, open shades during the day, and don't store firewood inside.(Andersen) Make sure fans vent outside, not into the attic.(NDSU Extension) Andersen suggests 30 to 35% in winter, checked with a hygrometer.(Andersen)
- Between the panes means the seal has failed and the window needs replacing.(Andersen) Outside condensation on spring and fall mornings is normal.(Andersen)
- New or tight homes run more humid,(Marvin) and a new house often condenses in its first heating season while building materials dry out.(Andersen)
- Shopping for windows? The condensation resistance rating runs from 0 to 100, higher is better, and DOE advises a high rating in very cold places.(ENERGY STAR)(U.S. DOE)
Healthy Range
- EPA: keep indoor humidity below 60%, ideally 30 to 50%, and don't humidify above 50%, where dust mites and mold thrive.(EPA)(EPA) CPSC says the same 30 to 50%.(CPSC)
- Dry air causes static, dry skin and throat, and shrinking wood that loosens furniture joints and doors.(University of Minnesota Extension)(EPA) In the coldest weather the chart drops below 30%, trading some comfort to protect the house.
Humidifiers
- Whole-house units mount on the furnace ductwork. Bypass models use furnace airflow through a wet pad; fan-powered models have their own fan; steam models make steam and work even without forced-air heat.(Aprilaire)(Aprilaire)(Aprilaire)
- Care: replace the water panel once a year on most Aprilaire models (twice a season on the Model 400), and the steam canister yearly.(Aprilaire) Close the bypass damper for summer.(Aprilaire manual)
- Portable humidifiers: empty, dry and refill daily, clean every third day, and use distilled water.(EPA) Ultrasonic and impeller "cool mist" units spread the most minerals and germs; evaporative and steam units spread little mineral dust.(EPA) Dirty tanks grow bacteria and fungi that end up in the mist.(CPSC)
- If a portable unit fogs windows or walls, move it, turn it down or run it less.(EPA)
Attic Frost
- Warm, moist air leaking into a cold attic condenses as frost on the roof boards. The damage comes when it melts and the wood has to soak it up and dry out.(U.S. DOE Building America) Signs include wet ceiling spots and peeling outside paint.(University of Minnesota Extension)
- The fixes: seal air leaks into the attic, lower indoor humidity, and keep roof surfaces warmer. Air sealing the ceiling also helps prevent ice dams.(U.S. DOE Building America)
- A damp basement can supply up to half of a home's moisture.(U.S. DOE Building America)
Egress Window Wells
- A basement bedroom needs an egress window as its second way out.(Wis. Admin. Code SPS 321.03) The window must open from inside without tools.(Wis. Admin. Code SPS 321.03(6))
- If the opening is below grade, the window well must be at least as wide as the window and reach 36 inches out from the wall. Wells deeper than 46 inches need a ladder or steps. The well must keep water out of the house.(Wis. Admin. Code SPS 321.03(6))
- Wisconsin's dwelling code doesn't address well covers directly. A cover that someone inside couldn't push open would defeat the exit, so ask your building inspector before buying one. Egress windows need a permit in most towns; see the permit lookup.
Questions
What should my indoor humidity be in winter in Wisconsin?
Lower than you might think, and lower as it gets colder. Aprilaire's chart says 35% at 20°F outside, 25% at 0°F and 15% at -20°F, and Honeywell's is the same from 20°F down. Above that, EPA's healthy range is 30 to 50%. If windows fog or frost, turn it down.
Why are my windows wet or icy inside in winter?
The glass is colder than the dew point of your indoor air, usually because the humidity is too high for the outdoor temperature. Andersen and Marvin both say it isn't a window defect. Run bath and kitchen fans, open blinds, and lower the humidifier.
What does fog between the panes mean?
The window's seal has failed. You can't wipe it away, and Andersen says the window needs replacing. Condensation on the room side is a humidity issue; between the panes is a window issue.
How often should I change my whole-house humidifier pad?
Aprilaire says once a year for most models, or twice a heating season for its Model 400. Steam humidifiers need a new canister every year. Close the bypass damper on a bypass model for the summer.
Do window well covers have to open from inside in Wisconsin?
Wisconsin's dwelling code doesn't address covers directly. It requires egress windows to open from inside without tools, wells at least 36 inches deep from the wall, and a ladder in wells deeper than 46 inches. A cover that blocks escape would defeat that, so ask your building inspector before you buy one.