Calculator
Example Data Table
| Indoor Temp | Humidity | Glass Temp | Approx Dew Point | Window Result |
|---|---|---|---|---|
| 21 °C | 45% | 12 °C | 8.6 °C | Dry window likely |
| 20 °C | 60% | 10 °C | 12.0 °C | Condensation likely |
| 23 °C | 50% | 14 °C | 12.0 °C | Near condensation |
| 19 °C | 35% | 8 °C | 3.4 °C | Dry window likely |
Formula Used
The calculator uses the Magnus dew point approximation.
Alpha: α = ln(RH / 100) + (a × T) / (b + T)
Dew point: Td = (b × α) / (a − α)
Here, T is indoor temperature in Celsius. RH is relative humidity. Constants are a = 17.625 and b = 243.04.
Condensation margin: Window glass temperature − dew point temperature.
If the margin is zero or negative, condensation can form on the window.
Actual vapor pressure: e = RH × saturation vapor pressure / 100.
Absolute humidity: AH = 216.7 × e / (T + 273.15).
How to Use This Calculator
- Select Celsius or Fahrenheit.
- Enter indoor air temperature.
- Enter indoor relative humidity.
- Enter the measured window glass temperature if known.
- Leave glass temperature blank to estimate it.
- Enter outdoor temperature for the glass estimate.
- Select the closest window type.
- Press the calculate button.
- Review the condensation margin and risk message.
- Use CSV or PDF buttons to save the result.
Understanding Window Dew Point
A window dew point calculator helps compare indoor moisture with glass surface temperature. The dew point is the temperature where air becomes saturated. At that point, water vapor starts turning into liquid water. Windows often reach that point first. Their glass is usually colder than walls, furniture, and indoor air.
This tool is useful in winter. Warm rooms can still create wet panes. The cause is not rain entering the frame. It is indoor humidity touching a cold surface. Cooking, showers, plants, and drying clothes add moisture. Poor air movement can also raise local humidity near curtains or blinds.
Why Condensation Happens
Condensation appears when the window surface is at or below the dew point. A small positive margin is still risky. Glass readings change across corners, seals, and frames. Edge areas can be colder than the center. A calculator warning should be treated as a guide, not a building survey.
The calculator uses room temperature and relative humidity to estimate dew point. It also compares that value with measured or estimated glass temperature. If you do not know glass temperature, enter outdoor temperature and a window type. The tool then makes a rough surface estimate.
Better Moisture Decisions
The result helps you choose a practical response. Lower humidity by using exhaust fans. Open trickle vents when safe. Move furniture away from cold walls. Keep curtains slightly open for air movement. Raise surface temperature with better glazing or warmer room air.
Do not focus only on relative humidity. Cold surfaces change the risk. A room at forty-five percent humidity may still condense on very cold glass. Another room at fifty-five percent may stay dry if glass is warm. The margin number explains this difference.
Good Data Gives Better Results
Measure humidity near the window when possible. Use a clean hygrometer. Give sensors time to settle. Measure glass with an infrared thermometer, or use the estimate carefully. Sunlight, wind, blinds, and frame leaks can change surface temperature quickly.
Use this calculator for planning and daily checks. It can help protect paint, timber, drywall, and comfort. Persistent condensation should be investigated. Mold, staining, or damaged seals may need professional attention. Record readings each week during very cold seasons.
FAQs
What is dew point on a window?
It is the temperature where indoor moisture starts turning into water on the glass. If the glass is colder than the dew point, condensation can appear.
Why do my windows sweat at night?
Night air cools the glass. Curtains and blinds can trap cold air near the pane. If indoor moisture stays high, the glass may reach the dew point.
Is relative humidity enough to predict condensation?
No. Relative humidity must be compared with surface temperature. A room can have moderate humidity and still condense on very cold glass.
What is a safe condensation margin?
A larger positive margin is safer. Many homes should keep at least two Celsius degrees of margin. More margin helps during colder nights.
Can this calculator estimate glass temperature?
Yes. Leave the glass temperature blank. Enter outdoor temperature and choose a window type. The estimate is rough and depends on wind, frames, and curtains.
How can I reduce window condensation?
Lower indoor humidity, run exhaust fans, improve airflow, open curtains slightly, and warm the glass surface. Better glazing can also reduce the risk.
Does condensation mean my window is leaking?
Not always. Interior condensation usually means humid indoor air touched cold glass. Leaks, failed seals, and frame damage are separate issues.
Can dew point cause mold near windows?
Repeated moisture can support mold on trim, paint, drywall, or sealant. Clean wet areas, reduce humidity, and investigate persistent dampness.