Window Condensation Calculator
Enter indoor temperature, humidity, outdoor temperature and window type to see whether condensation will form on the inside of the glass, and the highest indoor humidity your windows can handle.
Calculator
37% highest indoor humidity before the glass edges fog
Likely along the bottom edges of the glass
- Dew point of room air
- 48°F
- Glass, center
- 52°F
- Glass, edges
- 43°F
- Limit at center
- 52%
Your room air condenses at 48°F. The glass edges are colder than that, so expect water along the bottom of the panes. Bring humidity below 37% to keep them dry.
How the calculator decides
Condensation forms when a surface is colder than the dew point of the air touching it. The calculator estimates two things and compares them:
- The dew point of your room air.
Calculated from indoor temperature and relative humidity with the Magnus formula, the standard approximation used in meteorology.
- How cold the inside of the glass gets.
Heat leaks through glass in proportion to its U-factor. Using typical center-of-glass U-factors and the standard indoor surface resistance from ISO 6946, the calculator estimates the glass temperature. Double and triple glazing is colder near the edges where the spacer bar sits, so an edge temperature is estimated too.
- The humidity limit.
The highest indoor humidity at which the coldest part of the glass stays above the dew point. Above it, expect water first along the bottom edges, then across the glass.
Real windows can be colder than the model. Heavy curtains, blinds and deep window wells trap cold air against the glass, and aluminum frames conduct more heat. If you see condensation below the limit shown, treat the number as an upper bound and aim lower.
Maximum indoor humidity by outdoor temperature
Edge-of-glass limits for a room kept at 70°F (21°C). Stay at or below these to keep the coldest part of the glass dry.
| Outdoor temperature | Single pane | Double pane, clear | Double pane, low-E with argon | Triple pane, low-E |
|---|---|---|---|---|
| 50°F (10°C) | 59% | 68% | 80% | 89% |
| 40°F (4°C) | 45% | 56% | 71% | 83% |
| 30°F (-1°C) | 33% | 46% | 63% | 78% |
| 20°F (-7°C) | 25% | 37% | 56% | 74% |
| 10°F (-12°C) | 18% | 30% | 49% | 69% |
| 0°F (-18°C) | 13% | 24% | 44% | 65% |
| -10°F (-23°C) | 9% | 19% | 38% | 61% |
| -20°F (-29°C) | 7% | 15% | 34% | 57% |
Frequently asked questions
Why do my windows sweat on the inside in winter?
Glass is the coldest surface in most rooms. When the glass is colder than the dew point of the indoor air, water vapor condenses on it, starting at the bottom edges where the glass is coldest. Lower indoor humidity or warmer glass stops it.
Is condensation between the panes the same problem?
No. Fogging between the two panes of a double-glazed window means the seal has failed and moist air has leaked into the gap. Lowering indoor humidity will not fix it; the glass unit needs to be replaced.
Does condensation on the outside of windows mean anything is wrong?
Usually not. Exterior condensation on cool, clear mornings is a sign that well-insulated glass is keeping heat inside, so the outer pane stays cold. It clears as the sun warms the glass.
What indoor humidity should I aim for in winter?
The EPA recommends keeping indoor humidity between 30 and 50 percent and below 60 percent. In cold weather, the lower end of that range, or the limit this calculator shows for your windows, keeps glass dry.
Sources and method
- A Brief Guide to Mold, Moisture and Your Home U.S. Environmental Protection Agency
- ISO 6946: Building components and building elements, thermal resistance and thermal transmittance (surface resistances) International Organization for Standardization
- Understanding Window Energy Performance Ratings National Fenestration Rating Council
- Update to Magnus-type equations for saturation vapor pressure (Alduchov and Eskridge, 1996) Journal of Applied Meteorology
Last reviewed . Results are estimates for planning; they do not replace an on-site assessment by a qualified professional.