JJ Ben-JosephCondensation on windows occurs when the temperature of the interior glass surface drops below the dew point of the indoor air. Water vapor then changes to liquid, creating foggy panes, dripping water, and potential damage to frames. While many guides mention dew point, few give homeowners an easy way to evaluate actual condensation risk for their specific windows. This calculator fills that gap by estimating the interior surface temperature based on your window's insulation value and the temperature difference between indoors and outdoors. It then compares that surface temperature with the dew point computed from your indoor temperature and relative humidity.
The dew point is calculated using the Magnus formula, which is accurate for typical indoor conditions. The surface temperature calculation uses a simple one-dimensional heat flow approximation with an interior film coefficient of 8 W/m²K. This approach is sufficient for assessing whether your window is likely to fall below dew point. If the surface is warmer than the dew point, condensation should not form under steady conditions. If it is cooler, moisture will accumulate.
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. These equations run entirely in your browser without sending data anywhere.
Consider a winter day with indoor temperature at 21°C and relative humidity of 45%. The outdoor temperature drops to -5°C. If your double-pane window has a U-value of 1.8 W/m²K, the calculator computes a dew point of roughly 8.5°C. The surface temperature estimate is 13.5°C. Since the surface temperature is above the dew point, condensation should not occur. However, if the humidity rises to 65% with the same conditions, the dew point increases to about 14.3°C, exceeding the surface temperature and indicating a risk of condensation. The calculator clearly reports this condition and provides a table showing how different window types would perform.
The following table shows the estimated surface temperature for a fixed indoor environment of 20°C and 50% RH with an outdoor temperature of -10°C for common window types.
| Window Type | U-value | Surface Temp (°C) |
|---|---|---|
| Single Pane | 5.0 | 0.0 |
| Double Pane | 2.7 | 6.3 |
| Triple Pane | 1.0 | 12.6 |
Comparing these surface temperatures with the dew point helps you evaluate which upgrade might solve condensation issues. For example, if the dew point is 10°C, single and double panes would condense while triple panes would remain dry.
The model uses steady-state conditions and assumes uniform interior surface temperature. In reality, air movement from vents or curtains can create cold spots. The interior film coefficient of 8 W/m²K is a typical value but can vary depending on airflow; faster air movement raises the surface temperature. The calculator also does not account for thermal bridging through window frames. Despite these simplifications, it provides a quick risk screening tool. For detailed analysis, consult a building professional or use advanced simulation software.
Condensation can lead to mold growth and structural damage. If you already see moisture forming, increase ventilation, reduce indoor humidity, or improve insulation. Combining this calculator with the Mold Growth Risk Calculator and the R-value to U-value Converter offers a more complete approach to building moisture management.
Understanding dew point and surface temperature is also valuable for energy efficiency. Keeping indoor humidity in check allows you to set the thermostat lower without discomfort, saving energy. Conversely, overly dry air can cause health issues. Use this tool as part of a broader strategy for comfortable and efficient homes.
Finally, remember that weather fluctuates and so does condensation risk. Revisit the calculator during seasonal changes or after making upgrades. Over time you'll develop a sense of how your home responds to humidity and temperature variations, enabling more proactive maintenance.
Historical buildings present a special case. Single-pane windows in older homes are notorious for condensation, yet owners often hesitate to replace them to preserve character. In such situations, interior storm windows or seasonal plastic film can raise surface temperatures enough to stay above the dew point. The calculator can help evaluate whether these temporary measures are sufficient or if a full window upgrade is warranted.
Another consideration is occupant behavior. Activities like cooking, showering, and drying clothes indoors can spike humidity levels dramatically. By entering different humidity values before and after such activities, you can see how quickly condensation risk changes. This highlights the value of ventilation fans and dehumidifiers as practical mitigation tools.
Manufacturers sometimes advertise low-emissivity coatings and inert gas fills as solutions. While these features improve insulation (lowering U-value), they also raise cost. Running their specs through the calculator allows you to estimate whether the investment will actually prevent condensation in your climate.
On the flip side, extremely low humidity may prevent condensation but cause other issues such as dry skin, static electricity, and damage to wooden instruments or furniture. The calculator reinforces that the goal is balance: maintain humidity high enough for comfort but low enough to keep surfaces above dew point.
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