Water forms where a surface is colder than the dew point of the room air
Whether a window drips does not depend on the humidity number alone. Any surface colder than the dew point of the indoor air collects water. Air at 70°F and 45% humidity has a dew point of 47.7°F, so a window pane that cools below 47.7°F starts to bead up.
This calculator estimates the surface temperature from the type of glass, so no thermometer is needed to get started. On a 0°F night, center-of-glass temperatures in the National Glass Association's NFRC 100 table are 16°F for a single pane, 56°F for double glazing with low-E and argon, and 63°F for triple glazing. At 70°F and 45%, the single pane is far below freezing and frosts over, while the double low-E glass stays dry until the humidity passes 61%.
Why work it out before the glass fogs
Condensation tends to show up only on the coldest nights, so the same room can be dry for weeks and then drip overnight. Working it out ahead of time shows how far a humidifier can safely run and which night will be the first to fog the glass.
- Running a humidifier in winter: check the highest humidity the window glass tolerates before choosing a target.
- Water on the sill every morning: see what humidity stops it and how cold the night has to get before it comes back.
- Mold on a wall corner or behind a closet: no droplets are needed. Measure the spot and see whether its surface humidity passes 80%.
- Choosing replacement windows: compare single, double low-E and triple glass for the same room and the same weather.
- Running the air conditioner hard in summer: see when dew forms on the outside of the glass and how warm to keep the room to avoid it.
How it is calculated
- Dew point: the indoor vapor pressure is e = humidity ÷ 100 × 6.112·exp(17.62T ÷ (243.12 + T)) hPa, and the dew point is the temperature at which that pressure saturates the air. The formula is the one in the WMO guide to meteorological instruments.
- Surface temperature: surface = indoor temperature − r × (indoor temperature − outdoor temperature). The ratio r says how far the surface falls toward the outdoor temperature. A measured surface temperature replaces the estimate.
- Verdict: water forms when the surface temperature is at or below the temperature at which the air saturates. That is the dew point for a surface above freezing and the frost point for a surface below freezing. The two values are compared as shown, rounded to one decimal, and equal values count as wet.
- Mold conditions: for a measured spot, which is treated as a wall material that absorbs moisture, the surface humidity (indoor vapor pressure ÷ saturation pressure at the surface temperature) is flagged at 80% or more.
- Humidity limit: the highest whole-number humidity at which the surface stays dry, found for each surface at its current temperature.
- Outdoor temperature limit: the warmest outdoor temperature at which the surface cools to the point where water forms at the current humidity. If the overnight low is at or below it, water forms by morning.
- Cooling season: when it is warmer outside, the outer face of the glass is estimated as outdoor temperature − r × (0.04 ÷ 0.13) × (outdoor temperature − indoor temperature) and compared with the dew point of the outdoor air. ISO 13788 uses 0.13 m²·K/W for the indoor surface of a window with horizontal heat flow and 0.04 for the outdoor surface. The outer ratio is capped at 1 − r so that the inner and outer ratios never add up to more than 1.
Formulas dew point = 243.12·ln(e ÷ 6.112) ÷ (17.62 − ln(e ÷ 6.112)) · frost point = 272.62·ln(e ÷ 6.112) ÷ (22.46 − ln(e ÷ 6.112)) · surface = Tin − r × (Tin − Tout) · r from a reading = (Tin − Tsurface) ÷ (Tin − Tout) · T in °C, e in hPa; the page shows °F
Three kinds of glass and how cold they get
The National Glass Association (NGA) lists the temperature of the interior glass surface, measured at the center of the glass, under standard NFRC 100 rating conditions of 0°F outdoors, 70°F indoors and a 12.3 mph wind. The ratio r used here is how far the surface falls from the indoor temperature toward the outdoor temperature: r = (70 − surface) ÷ 70.
| Glass | Temperature drop ratio r | Surface at 70°F indoors, 0°F outdoors |
|---|---|---|
| Single pane, 1/4 in | 0.771 | 16.0°F |
| Double pane, low-E, argon | 0.200 | 56.0°F |
| Triple pane, 2 low-E coatings, argon | 0.100 | 63.0°F |
NGA values for a 1/4 in single pane (U-factor 1.02), a double 1/4 in – 1/2 in – 1/4 in unit with one low-E coating and 95% argon (0.24), and a triple unit with low-E coatings on two surfaces and 95% argon (0.12). They describe the center of the glass only.
The same 70°F room needs a very different humidity limit depending on the glass. Because the NGA figures are for the center of the glass, the edge of the glass and the frame are not covered. Measure those separately.
Humidity limits by outdoor temperature
Each cell is the highest indoor humidity at which no water forms on that glass when it is 70°F indoors. The collapsible table in the calculator recalculates for your indoor temperature.
| Glass | −10°F | 0°F | 10°F | 20°F | 30°F | 40°F |
|---|---|---|---|---|---|---|
| Single pane | 7% | 11% | 16% | 23% | 32% | 43% |
| Double pane, low-E, argon | 56% | 61% | 65% | 70% | 75% | 81% |
| Triple pane, 2 low-E coatings, argon | 75% | 78% | 81% | 84% | 87% | 90% |
The EPA advises keeping indoor humidity below 60%, ideally between 30% and 50%. A single pane cannot meet that on a cold night: at 30°F outdoors it stays dry only up to 32%, and at 70°F and 45% it starts to bead once the outdoor temperature is 41.1°F or lower. The double low-E glass would not fog at 45% until the outdoor temperature is −41.3°F or lower, and the triple glass stays dry across the whole range the calculator accepts.
Measure the surface to find the ratio for your own window
Older windows, a leaky frame or a cold corner behave differently from the table. One measurement gives the ratio r for that spot, which the calculator then uses for colder nights.
- Pick a day when it is much colder outside than inside. The bigger the difference, the more accurate the ratio.
- Wait until the heating is steady, then write down the indoor temperature, indoor humidity and outdoor temperature at the same time.
- Measure the surface where condensation shows first: the bottom corner of the glass, the frame, a wall corner or the wall behind furniture.
- Select the surface card in the calculator and enter the reading under "surface temperature". For anything not in the list, use the "Measured spot" card.
For example, at 70°F indoors and 30°F outdoors, a window frame that reads 55°F gives r = (70 − 55) ÷ (70 − 30) = 0.375. On a 0°F night that frame falls to 43.8°F, below the 47.7°F dew point at 45% humidity, so it beads up. It stays dry only at 38% or lower, and at 45% humidity water forms once the outdoor temperature drops to 10.6°F.
Infrared thermometers can misread shiny or reflective surfaces such as glass. Follow the instrument's manual for the material, and check against a contact probe when possible.
Mold needs no droplets
ISO 13788 sets a critical surface relative humidity of 0.8 (80%) for mold growth unless national regulations say otherwise, and defines it as a monthly mean. The calculator shows the surface humidity for a single moment, so treat a mold flag as a warning for spots that stay that way for days. The EPA also lists the surface of walls behind furniture as a place where condensation forms and hidden mold can grow.
Take a wall behind a wardrobe that reads 55°F with the room at 70°F. At 45% humidity the surface humidity is 76% and the spot is dry. At 50% it is 84%, above the mold line, and the wall stays below 80% only at 47% or lower. No water appears in either case, since the 55°F wall is warmer than the 50.5°F dew point.
Below freezing, frost forms instead of water
When a surface is colder than 32°F, water vapor deposits as ice. The calculator then compares the surface with the frost point, computed from the saturation pressure over ice, 6.112·exp(22.46t ÷ (272.62 + t)) hPa. In very dry air the frost point is slightly higher than the dew point. At 70°F and 15% humidity the dew point is 20.2°F and the frost point is 21.5°F. A surface below 32°F is judged against the frost point and labeled frost.
Dew on the outside of the glass in summer
With the air conditioner running, the glass is cooled from the inside and its outer face falls below the outdoor air temperature. If the outdoor dew point is higher than that, water forms on the outside. Enter an outdoor temperature higher than the indoor one and an outdoor humidity box appears for this check.
The outer face cools much less than the inner face because of the small outdoor surface resistance. For a single pane (r = 0.771), 0.771 × 0.04 ÷ 0.13 would be 0.237, but the cap 1 − 0.771 = 0.229 applies, so the outer ratio is 0.229. With the room at 68°F and 88°F outside at 90% humidity, the outdoor dew point is 84.7°F and the outer face of a single pane is 83.4°F, so dew forms. It stays dry only if the room is held at 73.9°F or warmer. The outer face of double low-E glass is 86.8°F, and dew stays away as long as the room is at 35.2°F or warmer.
Common mistakes
- Judging by humidity alone: at 70°F and 45%, double low-E glass is dry at 0°F, while a single pane is already beading with 41.1°F or lower outside.
- Entering the outdoor temperature as the surface temperature: the surface sits between the indoor and outdoor temperatures, and the ratio r says where. Entering the outdoor value greatly exaggerates the condensation.
- Measuring only the center of the glass: the NGA figures cover the center of the glass. The edge and the frame are usually where water appears first, so measure them too.
- Treating the limit as a target: the limit is the humidity at which water is about to form. Allow a few points of margin for hygrometer error and for a colder night than forecast.
- Assuming airing out always dries the room: if the outdoor air holds more water vapor than the indoor air, the humidity goes up. The indoor humidity calculator compares both.
Limits
- The glass temperatures are center-of-glass values from the NGA table at NFRC 100 conditions. Wind, sun, curtains, a heater under the window or furniture in front of the glass change the real surface temperature, so use a measured value for an exact answer.
- The calculation assumes that indoor temperature, humidity and outdoor temperature hold steady. A night that cools quickly, or humidity that spikes after cooking or a shower, can bring water earlier than shown.
- Condensation inside walls, moisture held in materials and leaks are not calculated. A spot that keeps getting wet may be a leak or missing insulation.
- The outer-face calculation uses the surface resistances from ISO 13788 as given. On a still day the outer face can fall further than calculated.
- This is an estimate for planning, not a building inspection or a rating of a window.
Sources
- 01National Glass Association, Glass Technical Paper FB49-17 (2022): Performance Improvements of Insulating Glass UnitsInterior center-of-glass surface temperature at NFRC 100 conditions of 0°F outdoors and 70°F indoors: single 1/4 in 16°F (U 1.02), double low-E with argon 56°F (0.24), triple with two low-E coatings and argon 63°F (0.12)
- 02U.S. EPA: A Brief Guide to Mold, Moisture and Your HomeKeep indoor humidity below 60 percent, ideally between 30 and 50 percent; act quickly on condensation on windows, walls or pipes
- 03ISO 13788:2012 public previewCritical surface humidity φsi,cr of 0.8 for mold growth unless national regulations say otherwise; Rse 0.04 m²·K/W; Rsi 0.13 m²·K/W for windows and doors with horizontal heat flow (Table 2)
- 04NCAR/EOL: Water Vapor Pressure FormulationsWMO (2008, CIMO Guide) ew = 6.112·exp(17.62t ÷ (243.12 + t)) hPa over water; ei = 6.112·exp(22.46t ÷ (272.62 + t)) hPa over ice
Frequently asked questions
Why does my window get wet when indoor humidity is only 45%?
Condensation depends on the surface temperature compared with the dew point, not on the humidity number alone. At 70°F and 45% the dew point is 47.7°F. A single pane with 30°F outdoors is about 39.2°F, so it collects water even though 45% is within the EPA's ideal 30 to 50% range. Double low-E glass at the same temperature stays well above the dew point.
What indoor humidity prevents window condensation?
It depends on the glass and on how cold it is outside. With 70°F indoors and 0°F outdoors, water stays off a single pane only up to about 11% humidity, off double low-E glass with argon up to 61% and off triple glass with two low-E coatings up to 78%. The EPA advises keeping indoor humidity below 60%, ideally between 30% and 50%.
What is the temperature drop ratio r?
It is how far a surface falls from the indoor temperature toward the outdoor temperature: r = (indoor temperature − surface temperature) ÷ (indoor temperature − outdoor temperature). A value near 0 means the surface stays close to room temperature. In the National Glass Association's NFRC 100 table a single pane has r = 0.771 (16°F), double low-E with argon 0.200 (56°F) and triple with two low-E coatings 0.100 (63°F) at 70°F indoors and 0°F outdoors.
How can I find the ratio for my own window?
On a day when it is much colder outside than inside, measure the surface where water shows first, such as a bottom corner of the glass or the frame, and enter the reading. At 70°F indoors and 30°F outdoors, a frame that reads 55°F gives r = (70 − 55) ÷ (70 − 30) = 0.375, and the calculator uses that value for colder nights. If the indoor and outdoor temperatures differ by less than 5.4°F, r is not calculated because the error is too large.
Do low-E or triple-pane windows really prevent condensation?
They lower the risk a great deal. At 70°F and 45% humidity, a single pane collects water once the outdoor temperature is 41.1°F or lower, double low-E glass with argon not until it is −41.3°F or lower, and triple glass stays dry across the whole range the calculator accepts. These are center-of-glass figures, so check the edge of the glass and the frame separately.
How is mold different from condensation?
Mold can start without visible droplets when the relative humidity at the surface stays high. ISO 13788 uses a critical surface humidity of 80% as a monthly mean unless national regulations say otherwise. For a measured spot the calculator flags mold conditions at 80% or more. A wall behind a wardrobe that reads 55°F in a 70°F room has a surface humidity of 76% at 45% indoor humidity and 84% at 50%.
Is it water or frost when the glass is below freezing?
Below 32°F water vapor deposits as ice, so the calculator compares the surface with the frost point, which comes from the saturation pressure over ice. In very dry air the frost point is slightly higher than the dew point. At 70°F and 15% humidity the dew point is 20.2°F and the frost point is 21.5°F. A surface below freezing is judged against the frost point and labeled frost.
Can dew form on the outside of the glass when the air conditioner runs?
Yes. When it is warmer outside, the outer face of the glass is cooled from inside, and dew forms if the outdoor dew point is higher than that face. With the room at 68°F and 88°F outside at 90% humidity, the dew point is 84.7°F. A single pane has an outer face of 83.4°F, so it gets wet, and the room would have to be 73.9°F or warmer to avoid it. Double low-E glass is at 86.8°F and stays dry.