1. Why Water Drops on Windows Come from Indoor Air, Not Outside
The water droplets on the inside of windows on winter mornings do not leak in from outside. They form when warm, moist indoor air touches cold glass or walls. The temperature drops, and the water vapor in the air turns into liquid droplets. The Tokyo Metropolitan Government guide explains this as "winter-type condensation," which occurs when indoor moisture meets cold surfaces. [1]
In a survey conducted by Tokyo in fiscal year 2016, 58.4% of households reported experiencing condensation in the past three years. Among those households, 83.8% saw it only on windows, while 14.7% saw it on both windows and walls. This means condensation is most visible on windows. [1] But where does the water vapor in the room air come from in the first place?
2. How People and Daily Activities Add Water to Room Air
Water enters the indoor air from human bodies and daily activities like laundry and bathing. A document from the Hokkaido Research Organization states that a family of four releases about 10L of moisture into the room each day. This moisture stays inside and condenses on colder surfaces like windows. [2]
10L is roughly the amount in one bucket. This is an estimate for a family of four and changes with the number of people and their habits. Water vapor spreads throughout the room. So why do windows get wet more than other surfaces?
3. Why Outside Temperature Makes Some Windows Colder
In winter, window temperatures are influenced by the outside air temperature. The Tokyo guide notes that condensation often happens on north-facing windows and walls. [1] According to the Flat Glass Association of Japan, the colder the outside air and the stronger the wind, the more heat escapes from the indoor glass surface to the outside. This lowers the glass surface temperature. [3] When is it coldest outside?
4. How Sleeping Bodies and Pre-Dawn Cooling Wet Windows
Two factors from the sources explain why windows are wet in the morning. First, moisture continues to be released from human bodies even while sleeping. [2] Second, on clear, calm nights, heat from the ground escapes into the sky. The outdoor air continues to cool until just before sunrise. As air temperature drops, window surface temperatures also drop, making condensation likely in the morning. [5]
This explanation is for clear, calm nights. There is no statistic in these sources saying "condensation is always highest in the morning." So why do some windows in the same room stay dry?
5. How Dew Point and Glass Temperature Decide When Drops Form
Air cannot hold water vapor beyond a certain point when cooled. That temperature is the dew point. For air at 20°C and 60% relative humidity, the dew point is 12°C. [3] If the indoor surface of a window is 12°C or lower, it gets wet.
In the calculation example where it is 0°C outside, the inside of clear single-pane glass is 6.0°C. This is 6.0°C below the dew point, so it condenses. Low-E double glazing is 15.5°C, which is 3.5°C above the dew point, so it does not condense. [3] These numbers are for specific conditions: 0°C outside, 20°C inside, 60% humidity. They are not actual measurements of every window.
The thermal transmittance (U-value) shows how easily heat passes through glass. Lower numbers mean less heat passes. Clear single-pane glass is 6.0 W/(m²K). Clear double glazing is 2.9. Low-E double glazing is 1.7. Vacuum glass is 1.0. Double vacuum glass is 0.8. [3] Windows that insulate better keep heat inside, so the indoor surface stays warmer.
Low-E double glazing does not guarantee no condensation. Manufacturer catalogs show that condensation can occur if the outside temperature drops further. These calculated values range from about -7°C to -22°C depending on the product. The catalog notes these are not guaranteed values. [4] What can we do to reduce condensation?
6. How Letting Water Vapor Out Helps: Opening Windows 5 Minutes Before Bed
The Tokyo health guide suggests ventilating during cooking and rice cooking, avoiding drying laundry indoors, and opening windows for about 5 minutes before bed. Opening windows for 5 minutes lets indoor humidity escape, reducing condensation. [1]
Even without changing window performance, reducing the amount of water vapor lowers the dew point. If the dew point becomes lower than the window surface temperature, no droplets form.
7. What You Can Check Yourself on a Morning with Window Drops
On a morning with condensation, you can check two things with your eyes and hands. With an adult’s help, gently compare the top and bottom of the window; do not touch damaged glass. Compare which part is colder. Look at where the droplets are. Wipe your hands after touching. [5]
Record mornings with many droplets and mornings with few in a notebook. Write whether the sky was clear or cloudy. Compare this with the previous night's conditions. [5] For reading, the Tokyo "Housing Health Guide" organizes condensation prevention and countermeasures with diagrams. [1]