Whether your windows drip
depends on humidity
and glass temperature.
The short version…
- At 20°C inside and 0°C outside, single-pane glass fogs up at about 41% humidity
- Double glazing holds out to about 65%, Low-E double glazing to about 79%
- Rather than drying the air, keep the glass warm and wipe it promptly
Will my windows fog up? Calculator
Choudo-kun is on it. Enter your room temperature and humidity (check a hygrometer), the outdoor temperature, and your glass type.
Same conditions, other glass
We estimate the inside surface temperature at the center of the glass as "room temp − (room temp − outdoor temp) × U-value × inside surface resistance (0.11)" and compare it with the dew point. U-values are typical figures from the Flat Glass Manufacturers Association of Japan (single 6.2, double 3.0, Low-E double 1.7 W/(m²·K)). With this method, at 20°C and 60% indoors, condensation starts at about 8°C outside for single pane and about −5°C for double glazing — close to the figures in Nippon Sheet Glass's catalog (about 8°C and −4°C). Real windows vary with frames, curtains and wind.
Humidity limits by glass type
At a room temperature of 20°C, the center of the glass starts to fog above these humidity levels (our calculation).
| Outdoors | Single pane | Double glazing | Low-E double |
|---|---|---|---|
| 5°C | ~51% | ~73% | ~84% |
| 0°C | ~41% | ~65% | ~79% |
| −5°C | ~32% | ~59% | ~74% |
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Our picks by type
- Water-applied, easy to remove
- Use rated film on wired or double glass
- Count the sheets you need
- Tape along the bottom soaks up drips
- Makes the morning wipe quicker
- Replace before it gets moldy
* Products and prices are loaded from Rakuten Ichiba when the page opens.
Want the details? (tap to open)
How condensation works (dew point and glass temperature)
The colder air is, the less water vapor it can hold. When room air touches a cold window and cools below a certain temperature — the dew point — the vapor it can no longer hold turns into droplets. That's condensation.
So it's a contest between the dew point of your room air and the temperature of the inside of the glass. The glass temperature depends on how cold it is outside and how well the glass insulates.
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Bar height is the temperature of the inside surface at the center of the glass. Aluminum frames get colder.
What humidity should I aim for?
| Guide | Source |
|---|---|
| 50–60% | Ministry of Health, Labour and Welfare, "Influenza Q&A FY2024": keeping dry rooms at this level with a humidifier can be effective |
| 40–60% | Tokyo Metropolitan Government guideline: above 60%, mold and dust mites thrive; keep it at 40% or more |
| 70%+ | Tokyo: the humidity mold likes (with 20–35°C and dirt or condensation water as food) |
Roughly 50–60% balances colds and mold. But as the table above shows, single-pane windows fog up at that level. The realistic split is: keep the humidity, but stop the window glass from getting cold and wipe any water away promptly.
5 ways to reduce condensation
-
Keep the glass from getting cold
Better-insulated glass is warmer on the inside and fogs less. Renters can use window insulation film; owners can add an inner window or switch to double glazing. Saitama Prefecture also lists double glazing and double sashes as effective. -
Don't add too much moisture
Run the extractor fan while cooking, avoid drying laundry indoors where you can, and don't overdo the humidifier. Saitama Prefecture advises against keeping a kettle on top of an open-flame (kerosene or gas) heater. -
Ventilate
Tokyo recommends running the bathroom fan for 30 minutes to an hour or more after bathing. Saitama says opening a window for about 5 minutes before bed reduces condensation. If you have 24-hour ventilation, leave it on. -
Keep air moving
Room corners and the space behind furniture are still, cold spots where condensation forms (Tokyo). Leave a little gap between furniture and walls. -
Wipe droplets the same day
Condensation water feeds mold. Wipe it off regularly and keep the drain channel at the bottom of the sash clear (Tokyo).
Good to know if you live in Japan
-
Search in Japanese
Condensation is 結露. Single pane is 単板ガラス or 一枚ガラス; double glazing is 複層ガラス or ペアガラス; Low-E double glazing is Low-E複層ガラス or エコガラス. Condensation absorbing tape is 結露吸水テープ. -
Many rentals have single-pane glass and aluminum frames
Check your windows: if you see two sheets of glass with a gap, it's double glazing. Ask your landlord before replacing glass or fitting an inner window. -
Mold at move-out
Black mold around windows can become a dispute when you move out. Regular wiping and ventilation is the cheapest insurance.
FAQ
If I lower the humidity, will condensation stop?
Why does only the window frame fog up?
Does air-con heating cause less condensation?
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