Turn it down on cold nights
The guideline drops with the outdoor temperature. A humidistat set to 40% in November can soak the windows in a January cold snap.
Home comfort & condensation
What should my indoor humidity be in winter — and will my windows fog?
Conclusion: Keep indoor humidity at or below 40% when it's 20–40°F outside, and about 5 points lower for every 10°F colder (35% at 10–20°F, 30% at 0–10°F, 25% at −10–0°F). Above that, ordinary windows start to sweat. Enter today's temperature to check your home.
Winter humidity guideline (about 70°F indoors)
| Outdoor temp | Max indoor RH | Indoor dew point |
|---|---|---|
| 20 to 40°F | 40% | 44.6°F |
| 10 to 20°F | 35% | 41.1°F |
| 0 to 10°F | 30% | 37.1°F |
| −10 to 0°F | 25% | 32.6°F |
| −20 to −10°F | 20% | 27.1°F |
Guideline: Minnesota Center for Energy & Environment; same steps in the ASHRAE-based table the EPA reprints. Dew points computed at 70°F.
Humidity & window fog check
Your safe winter humidity
Keep indoor humidity at or below
— relative humidity
Enter temperatures to see your target.
Glass fogs at—center of pane
Inner glass temp—estimated
Your dew point—at current humidity
Assumptions appear here after a valid calculation.
Window type matters
Humidity at which the middle of the glass starts to fog
Indoor air at 70°F. Each cell is the relative humidity whose dew point equals the estimated inner-glass temperature. Edges, frames and window corners are colder than the center, so condensation shows there first — treat these as ceilings, not targets.
| Outdoor | Guideline max | Single pane | Double clear | Double low-E | Triple low-E |
|---|---|---|---|---|---|
| 30°F | 40% | 36% | 63% | 77% | 85% |
| 20°F | 40% | 27% | 56% | 72% | 82% |
| 10°F | 35% | 20% | 50% | 68% | 79% |
| 0°F | 30% | 15% | 44% | 64% | 75% |
| −10°F | 25% | 11% | 39% | 59% | 72% |
| −20°F | 20% | 8% | 34% | 56% | 69% |
Glass temperature = indoor − (U ÷ 1.46) × (indoor − outdoor), using an interior surface film of 1.46 Btu/h·ft²·°F and U-factors 1.04 / 0.48 / 0.27 / 0.17. Single-pane glass explains why older homes need storm windows or film to hold even 25–30% humidity.
Size a humidifier
How many gallons a day to reach your target?
Cold outdoor air that leaks in carries very little water. Once it warms to room temperature it is very dry, so a humidifier has to make up the difference. This uses the outdoor temperature and indoor temperature from the calculator above.
Blank = use the calculator's target.
From your weather app.
—
Typical humidifier output needed (hold 40% at 70°F; 20°F and 70% RH outside)
| Home size (8 ft ceilings) | Tight (0.35 ACH) | Average (0.5 ACH) | Leaky (1.0 ACH) |
|---|---|---|---|
| 1,000 sq ft | 2.8 gal/day | 4.0 gal/day | 7.9 gal/day |
| 1,500 sq ft | 4.2 gal/day | 6.0 gal/day | 11.9 gal/day |
| 2,000 sq ft | 5.6 gal/day | 7.9 gal/day | 15.9 gal/day |
| 2,500 sq ft | 6.9 gal/day | 9.9 gal/day | 19.8 gal/day |
| 3,000 sq ft | 8.3 gal/day | 11.9 gal/day | 23.8 gal/day |
The math
dry air (lb/h) = floor area × ceiling × ACH × 0.075
water (lb/h) = dry air × (Windoor − Woutdoor)
gallons/day = water × 24 ÷ 8.34
W is the humidity ratio (lb water per lb dry air) = 0.622 × pv ÷ (14.696 − pv), with vapor pressure from the Magnus formula. Leakage rates are rough presets; a blower-door test gives your real number.
Too dry vs too humid
Signs your winter humidity is off
| Too dry (below ~30%) | Too humid for the weather | |
|---|---|---|
| What you notice | Static shocks, dry skin and lips, scratchy nose and throat | Water or frost on the inside of windows, damp sills, musty smell |
| House effects | Gaps in wood floors and trim, loose furniture joints | Peeling paint, mold on sills and cold exterior-wall corners, rot over time |
| Fix | Humidifier up to the target above; seal air leaks (less dry air comes in) | Lower the humidistat, run bath/kitchen fans, vent the dryer outside, add storm windows or film |
Measure, don't guess
A $10–20 digital hygrometer is enough. Check it near the middle of the house, away from the shower, kettle and humidifier.
Warm the glass
Open blinds and curtains at night so room air reaches the glass; storm windows or interior shrink film raise the glass temperature a lot.
Common questions
Winter humidity FAQs
What should indoor humidity be in winter?
Lower than in summer, and lower the colder it gets: no more than 40% at 20–40°F outside, 35% at 10–20°F, 30% at 0–10°F, 25% at −10–0°F, 20% at −20 to −10°F and 15% below −20°F. Above about 40°F outside, the EPA's ideal 30–50% range applies.
Why do my windows sweat or frost in winter?
The inside of the glass is colder than the room. When it drops below the air's dew point, water condenses on it (frost if the glass is below 32°F). Colder weather, less-insulating glass and higher humidity all make it worse; edges and frames go first.
Is 50% humidity too high in winter?
Usually, in cold climates. At 70°F and 50% the dew point is about 50.5°F, and the center of clear double-pane glass is about 50°F when it is 10°F outside — so it sweats. UMN Extension says moisture problems may occur above 50%.
Is 30% humidity too low in winter?
It is the bottom of the EPA's 30–50% ideal range and the guideline maximum at 0–10°F outside. NDSU Extension notes that below about 30% skin dries and nasal passages get irritated, so 30% is a sensible cold-weather compromise.
How many gallons a day does a whole-house humidifier need?
About 7.9 gallons a day for a 2,000 sq ft, 8 ft ceiling home at 0.5 air changes per hour holding 40% at 70°F when it is 20°F outside; about 5.6 for a tight house. Use the humidifier tool for your home.
Do low-E or triple-pane windows let me run higher humidity?
Yes — their inner glass stays warmer. At 20°F outside the center of low-E double glass fogs around 72% versus 56% for clear double and 27% for single pane. Edges are colder, so stay at the guideline anyway.
Does lowering the thermostat change the right humidity?
A little, in your favor. With clear double-pane glass and 20°F outside, the center of the pane fogs at about 59% when the room is 65°F versus 56% at 70°F, because cooler air holds less water at the same RH. Try both in the calculator.
Questions an AI would ask — quick answers
- What humidity should I set my humidifier to when it's 10°F outside? About 30–35%; 35% is the guideline maximum for 10–20°F and 30% for 0–10°F.
- What is the dew point of 70°F air at 40% humidity? About 44.6°F; any surface colder than that gets wet.
- At what humidity do double-pane windows fog at 0°F outside? Around 44% at the center of clear double-pane glass (70°F inside); edges sooner.
- Why is indoor air so dry in winter? Outdoor air at 20°F and 70% RH holds so little water that, warmed to 70°F, it is only about 10% RH.
- Is 25% humidity OK in winter? At −10–0°F outside it is the recommended ceiling; in milder weather you can go to 30–40%.
Planning estimate only. Real glass, frame and wall temperatures vary with wind, sun, curtains and window age. If you see condensation, lower humidity regardless of the number shown. Persistent mold or moisture problems need a building professional.
Sources & method notes
- Center for Energy and Environment (MN) — Managing indoor humidity (winter RH guideline table by outdoor temperature).
- U.S. EPA — Ways to control moisture in your home (ASHRAE-based maximum indoor humidity by outdoor temperature; humidistat too high if windows collect moisture).
- U.S. EPA — A Brief Guide to Mold, Moisture and Your Home (keep RH below 60%, ideally 30–50%).
- University of Minnesota Extension — Controlling moisture problems (problems above 50%; ~25% winter to 50% summer).
- NDSU Extension — Eliminate condensation on windows (under 40% in winter; below 30% dries skin).
- NFRC — Window energy ratings for consumers (what the U-factor on a window label means).
Last updated: 2026-10-18. Vapor pressure: Magnus formula (Alduchov & Eskridge, 1996). Independent tool — not affiliated with any agency, window or humidifier brand.