What Humidity Avoids Condensation? Upper Limits by Glass, Frame and Outdoor Temperature

Condensation happens when window surface temperature < dew point

Whether condensation forms comes down to comparing the dew point of the room air with the temperature of the window's indoor surface. Air at 20°C and 50% humidity has a dew point of 9.3°C, so droplets form when the glass surface gets colder than 9.3°C. How to find the dew point is explained in detail in How to Calculate Dew Point.

So there are only two ways to prevent condensation: lower the room humidity to lower the dew point, or raise the window's surface temperature. And surface temperature depends almost entirely on the window's insulation and the outdoor temperature. That's why, at the same 50% humidity, one home gets condensation and another doesn't.

How cold does the window surface get?

The indoor surface temperature of a window can be calculated from its thermal transmittance (U-value), which shows how easily heat passes through. It's the number in window catalogs with the unit W/(m²·K), and the smaller it is, the better the insulation. The window type determines how much of the difference between room and outdoor temperature is lost by the indoor surface, and surface temperature = room temperature − (room temperature − outdoor temperature) × that share.

Single glazing lets two-thirds of the room-to-outdoor difference reach the surface, so with 0°C outside the glass cools to nearly 7°C. An aluminum frame gets colder still, which is why droplets form on the frame before the glass. Low-E double glazing and vinyl frames, on the other hand, keep the surface at 15–16°C even at 0°C outside.

Chart of upper humidity limits (20°C indoors, center of window)

Once you know the surface temperature, the humidity at which that temperature becomes the dew point is the upper limit that avoids condensation. These are the limits at 20°C indoors, at the center of a window with the curtains open, by outdoor temperature and window type. Use the lowest temperature around dawn, when condensation is most likely.

• Homes with single glazing and aluminum frames: the limit is around 50% at 5°C outside and around 40% at 0°C. Hard to reconcile with comfortable winter humidity
• Ordinary double glazing (12 mm): 67% at 0°C outside and 60% at −5°C. Keep to 40–60% and the glass hardly gets any condensation
• Low-E double glazing or better: 67% or more even at −10°C outside. Unless you overhumidify, condensation on the glass rarely occurs
• Even with double glazing, an aluminum frame sets the limit (39% at 0°C outside)

The limit drops further at window corners and behind curtains

The table above shows values at the center of a window, where warm room air reaches well. Warm air struggles to reach the bottom edge and corners of the glass, and closed curtains or shoji screens block the room's heat, so surface temperatures drop further. Moisture still slips through gaps, though, so condensation is much more likely than at the center.

The left value is the surface temperature and the right is the upper humidity limit (at 20°C indoors). Even with double glazing, on a morning with closed curtains, condensation forms above 51% humidity. If your window is fine by day but soaked when you open the curtains in the morning, judge by the behind-curtains conditions. Water collects at the bottom edge because corners get the coldest.

Raising room temperature lowers the upper humidity limit

It may be surprising, but raising the room temperature while holding the hygrometer's percentage the same makes windows more prone to condensation. At the same percentage, warmer air holds more water vapor, so the dew point rises. Here are the limits by room temperature at 0°C outside, at the center of a window.

For example, heat a room at 20°C and 50% to 22°C and the hygrometer drops to 44%, but the water vapor and dew point are unchanged, so condensation is no more or less likely. Humidify at 22°C back up to 50%, however, and condensation becomes more likely than in the 20°C, 50% room. Turning up the heating and humidifying as well is the hardest combination on windows.

Below what outdoor temperature do windows get condensation?

Conversely, given your current room temperature and humidity, you can calculate how low the outdoor temperature can go before condensation starts. Compare it with the forecast low to predict condensation tomorrow morning. These values are for 20°C indoors, at the center of a window.

With single glazing at 50% humidity, condensation starts once it's below 3.7°C outside. Even in Tokyo and Osaka, winter dawns are often 0–5°C, so condensation every morning is the natural result. With 12 mm double glazing, on the other hand, the center stays clear down to −5°C even at 60% humidity. At that same 60%, however, condensation behind curtains starts at 4.5°C outside, so if your double-glazed windows still get condensation, suspect the curtains and corners.

Striking a balance between dryness and condensation

40–60% humidity is seen as a comfortable range indoors in winter. Below 40%, your throat and skin tend to feel dry; above 60%, dust mites multiply, and above 70%, mold does. Laying this range over your own window's limit shows what to aim for.

1. 1

   Check your glass and frame type

   Look at the edge of the glass to see whether there are one or two panes, and whether the frame is metal or vinyl. The corner of the glass may also carry a product name or a Low-E mark or sticker.

2. 2

   Find the limit using the dawn low

   Find the upper humidity limit using the forecast low and, if you sleep with curtains closed, the behind-curtains conditions.

3. 3

   If the limit is 40% or above, aim slightly below it

   In a double-glazed home, aiming for about 45–55% avoids both dryness and condensation. Humidifier output can be calculated in Guide to Humidifier Output.

4. 4

   If the limit is below 40%, put the windows first

   With single glazing and aluminum frames in areas that drop below 0°C outside, lowering humidity alone can't also avoid dryness. Measures that raise the window surface temperature come first, such as inner windows and how you use curtains. See the comparison in Preventing Window Condensation.

FAQ

What humidity avoids condensation in winter?

It depends on the window type and outdoor temperature. At 20°C indoors and 0°C outdoors, the guide is up to 42% for single glazing, 67% for ordinary double glazing, 77% for Low-E double glazing and 39% for aluminum frames. Behind closed curtains, the limit drops another 7–16 points.

Why do my windows get condensation even at 40% humidity?

Because single glazing and aluminum frames get very cold from the outside air. At 20°C and 40% humidity, the dew point is 6.0°C, and below about −1°C outside, the surface of single glazing gets colder than that. Aluminum frames start getting condensation at 0.4°C outside.

Why do I get condensation despite double glazing?

Because behind closed curtains and at the bottom edge of the window it's 2–4°C colder than at the center. Even at 0°C outside and 20°C indoors, condensation forms behind curtains above 51% humidity. With aluminum frames, the frame gets condensation before the glass.

Does turning up the heating reduce condensation?

With the same amount of water vapor, window condensation hardly decreases. The hygrometer's percentage appears to fall, but the dew point doesn't change. Raising the room temperature with a humidifier running makes condensation more likely instead.

At what temperature do windows start getting condensation?

At 20°C and 50% humidity, at the center of a window, condensation starts below 3.7°C outside for single glazing and 5.0°C for aluminum frames. Double glazing (12 mm air gap) is calculated to stay clear down to −13.7°C.