Window Condensation Risk Calculator
Introduction: How window condensation risk is estimated
Window condensation starts when the interior glass surface cools below the dew point of the indoor air. At that point, water vapor leaves the air and collects on the pane, trim, or frame as fog, beads, or runoff. This calculator estimates that threshold for a specific window by combining indoor temperature, indoor relative humidity, outdoor temperature, and the window's U-value. It gives you a quick way to judge whether the glass surface is likely to stay above the dew point or slip into the condensation zone.
The dew point calculation uses the Magnus formula, which is reliable for the indoor temperatures most homes see. The surface-temperature estimate uses a simple steady-state heat-flow model with an interior film coefficient of 8 W/m²K, so the result reflects how the glass tends to warm up or cool down under stable conditions. If the estimated surface temperature is higher than the dew point, the pane should remain dry under those conditions. If it is lower, moisture is likely to appear on the colder parts of the window.
The formula for dew point (Tdp) is:
where T is indoor temperature in °C and RH is relative humidity in percent. The interior surface temperature (Ts) is approximated as:
where U is the window U-value and h is the interior surface heat transfer coefficient, taken as 8 W/m²K. Those two numbers are enough to show why a slightly higher humidity level or a weaker window can tip the result from safe to risky, and the calculation runs locally in your browser so the inputs never leave the page.
Worked example: a humid room against a cold pane
Consider a winter evening with the indoor temperature at 21°C and relative humidity at 45%, while the outdoor temperature sits at -5°C. If your double-pane window has a U-value of 1.8 W/m²K, the calculator estimates a dew point of about 8.6°C and an interior surface temperature of about 16.2°C. Because the glass stays well above the dew point, this setup should not produce condensation.
If the same room climbs to 75% relative humidity, the dew point rises to about 16.3°C while the surface temperature stays near 16.2°C. That tiny shift is enough to move the window into the risk zone, which is exactly why humidity control matters so much in cold weather. The calculator makes that crossover easy to see before moisture starts forming.
Window-type scenario comparison at the same indoor conditions
The table below keeps the indoor air at 20°C and 50% RH with an outdoor temperature of -10°C, so you can see how the window U-value changes the inside-surface temperature. Lower U-values keep the glass warmer and move the result farther from the dew point.
| Window Type | U-value | Surface Temp (°C) |
|---|---|---|
| Single Pane | 5.0 | 8.5 |
| Double Pane | 2.7 | 12.4 |
| Triple Pane | 1.0 | 16.7 |
At a dew point of 10°C, the single-pane case would be the most vulnerable because its interior surface is colder than the indoor air's dew point. The double-pane and triple-pane examples stay above that threshold, so they are less likely to fog under the same room conditions.
Assumptions and limitations for window condensation estimates
This window condensation calculator assumes steady indoor and outdoor temperatures and a fairly even glass surface. Real windows often have colder corners, spacer bars, and frame edges, so the first visible moisture may show up at the perimeter before the center of the pane reaches the dew point.
The 8 W/m²K interior film coefficient is a practical screening value, not a promise that every room behaves the same way. Air movement from supply registers, ceiling fans, radiator placement, or a curtain pressed against the glass can change how quickly warmth reaches the surface. Strong airflow tends to lift the interior surface temperature; still air can leave the glass cooler.
The model focuses on the window's U-value and does not try to simulate thermal bridging through the frame, sash, or installation details. That means a window with a decent center-of-glass value can still fog around the perimeter if the frame is much colder than the pane.
Use the result as an early warning rather than a final diagnosis. If a window repeatedly lands in the risk zone, the usual fixes are to reduce indoor humidity, improve ventilation, seal drafts, or upgrade the glazing before runoff, peeling paint, or mold become a problem.
Seasonal swings matter too. A room that looks safe on a mild afternoon can become prone to fogging once outdoor temperatures fall overnight. Rechecking the numbers after a weather change, after replacing a window, or after adding a humidifier gives you a clearer sense of the margin you actually have.
Older homes deserve extra attention because single-pane glass, loose sash joints, and leaky frames can create condensation even when the rest of the house feels comfortable. In those cases, interior storm panels or removable film can be enough to keep the surface above the dew point until a permanent upgrade is practical.
Occupant behavior changes the outcome as well. Cooking, showering, drying laundry indoors, or running a humidifier can drive humidity up fast, which is why this calculator is useful both before and after those activities. The visible glass may not react instantly, but the dew point does move quickly with moisture changes.
If you are trying to solve a recurring moisture problem, it can help to compare this result with the Mold Growth Risk Calculator and the R-value to U-value Converter. Condensation often sits at the intersection of comfort, insulation, and mold risk, so looking at those pieces together can make the next fix easier to choose.
The goal is balance, not absolute dryness. Very low humidity can avoid condensation, but it can also feel uncomfortable and damage wood, instruments, or finishes. This calculator helps you find the middle ground where the window stays warm enough while preserving a healthy indoor environment.
How to use this window condensation calculator
- Enter Indoor temperature (°C) in the temperature units shown by the form.
- Enter Indoor relative humidity (%) as a percentage from 0 to 100.
- Enter Outdoor temperature (°C) for the conditions outside the window.
- Enter Window U-value (W/m²K) for the glazing you want to check, then run the calculation and compare the dew point with the estimated glass temperature. If you are testing a borderline room, change the humidity or the U-value and run it again to see how quickly the result shifts.
Formula notes for window condensation risk
The calculator does not rely on a generic weighted sum. Instead, it compares the indoor air's dew point with the estimated interior glass temperature for your window. When humidity rises, the dew point climbs; when the outside air gets colder or the U-value gets worse, the glass temperature falls. A condensation warning appears when the dew point overtakes the surface temperature.
That relationship is the main thing to watch in a real home. Higher indoor humidity pushes the risk upward quickly, a colder night pushes it upward too, and a lower U-value pulls it back down by keeping the glass closer to room temperature. The result is best read as a practical risk check, not as a substitute for measuring the exact surface temperature of a specific pane.
Arcade Mini-Game: Window Condensation Risk Calculator Calibration Run
Use this quick arcade run to practice spotting the window conditions that really matter—humidity, indoor temperature, outdoor temperature, and the U-value—before you trust the condensation result.
Start the game, then use your pointer or arrow keys to catch the inputs that change window condensation risk and avoid the misleading ones.