Preventing Window Condensation: What Works First, with Inner Windows, Curtains, Ventilation and Dehumidifying Compared by Numbers

Only two remedies: lower the humidity or keep the window from getting cold

Window condensation occurs when the window's indoor surface temperature falls below the dew point of the room air. A room at 20°C and 50% humidity has a dew point of 9.3°C. On a 0°C morning, single glazing cools to about 6.8°C at the surface, which is 2.5°C on the condensing side.

There are two ways to close this gap. One is to lower the dew point by reducing the room's water vapor (ventilating, dehumidifying, producing less vapor). The other is to raise the window's surface temperature (inner windows, replacing the glass, directing warm air at the window). Absorbent tape and wiping only clean up droplets that have already formed; they don't reduce condensation itself. Keeping these three apart shows where to start.

Measures that lower humidity, and their limits

Lowering room humidity lowers the dew point and makes condensation less likely. At 20°C, the dew point is 9.3°C at 50% humidity and 6.0°C at 40%. Lowering humidity by just 10% lets a window get 3°C colder without condensation.

• Don't stop 24-hour ventilation: in an airtight home with an 8-tatami room and a 2.4 m ceiling, replacing half the room's air each hour removes about 70 g of water vapor per hour at 5°C and 60% humidity outside
• Dry laundry in another room or the bathroom: one load holds several liters of water, and much of it goes into the room air as it dries
• Cut back on open-flame heaters such as kerosene heaters and gas fan heaters: burning 1 L of kerosene gives off a little over 1 L of water vapor. Air conditioners and sealed-combustion (FF) heaters release no vapor indoors
• Lower your humidifier target: check the upper humidity limit for your windows and set it just below that
• Run the extractor fan longer after cooking and bathing: steam from pots and baths releases a lot of vapor at once
• Run a dehumidifier toward dawn: in winter, desiccant (zeolite) types, which handle low temperatures well, are the best choice

The effect of keeping windows from getting cold (inner windows, glass replacement)

Better window insulation raises the surface temperature and greatly widens the condensation-free humidity range. At 20°C indoors, 0°C outdoors, at the center of a window, the types compare as follows.

Adding an inner window (a vinyl-framed double window) to single glazing raises the indoor glass's surface temperature by 7.5°C, from 6.8°C to 14.3°C, and widens the upper humidity limit from 42% to 70%. By this calculation, condensation stops across the whole comfortable winter range of 40–60%, an effect no other remedy comes close to. Even in rentals, a simple tension-fit inner window is easy to remove when you move out and costs less than replacing the window.

The right-hand column is the humidity of air from a 50%-humidity room when it cools beside the window. Even without droplets, above 70% mold grows easily on rubber gaskets and curtain hems. If mold around the window bothers you even with double glazing, use this value as a guide.

Aluminum frames get condensation before the glass

Aluminum conducts heat very readily, so aluminum window frames get even colder than single glazing. At 20°C indoors and 0°C outdoors, surface temperature is 5.7°C for aluminum frames, 12.3°C for aluminum-vinyl composite frames, and 15.6°C for vinyl frames. This is why a home can switch to double glazing and still have soaked frames.

• Insulating tape or covers for frames: cover the aluminum surface and raise the temperature of the surface the room air touches
• Vinyl inner windows: they seal off the aluminum frame from the indoor side, so they also work best on frame condensation
• Condensation-absorbing tape: absorbs water collecting below the frame and keeps sills and wooden wall parts from getting wet (doesn't reduce condensation)
• Wipe up water in the tracks every morning: left alone, it leads to black mold on gaskets and corrosion of the tracks

Curtains and furniture placement increase condensation

Curtains and shoji screens block room heat from reaching the window, so the glass behind them gets even colder. Moisture still gets in through gaps in the curtains, though, so the space behind curtains becomes cold and damp, the place most prone to condensation.

1. 1

   Don't let curtains reach the floor, and leave a small gap from the window

   When the hem blocks the floor or sill, cold air collects and stops moving. Thick curtains help insulate, but the glass behind them gets cold, so on condensation-prone windows raise the hem slightly or leave only the sheer curtain closed for part of the time.

2. 2

   Aim a gentle breeze at the window with a circulator

   Moving air near the window helps room heat reach the glass and eases the cold at the corners. Placing a heater below the window follows the same idea.

3. 3

   Don't put large furniture in front of windows

   Furniture blocking a window has the same effect as closing the curtains. Walls behind furniture also get moldy more easily, so keep it at least 5 cm from the wall.

Remedies that tend to backfire

• Just turning up the heating: the hygrometer's percentage falls, but the water vapor is the same, so condensation hardly decreases. With a humidifier running, it actually increases
• Keeping windows closed and turning up the humidifier: everything above the window's limit collects on the window as droplets
• Leaving condensation and hiding it behind curtains: mold spreads to the curtains and window frame
• Heating the room with a kerosene heater: it warms the room but releases a lot of water vapor, so always pair it with ventilation

Checklist of what works first

1. 1

   Know your window's condensation-free humidity

   Find the limit from the window type, frame and dawn outdoor temperature. A chart is in What Humidity Avoids Condensation?

2. 2

   Reduce what produces water vapor

   Review indoor laundry drying, open-flame heaters and overhumidifying. This costs nothing and has a large effect.

3. 3

   Don't stop ventilation

   Many homes have closed the supply vents of their 24-hour ventilation, so keep them open. If the cold bothers you, angle the vent toward the ceiling.

4. 4

   Sort out curtains, furniture and airflow

   Adjust the curtain hems, furniture in front of windows and the direction of your circulator.

5. 5

   If the limit is still below 40%, consider inner windows

   With single glazing and aluminum frames, an inner window widens the upper humidity limit from around 40% to around 70%. Subsidies are often available, so also check local and national window renovation programs.

FAQ

What works best against window condensation?

In homes with single glazing or aluminum frames, an inner window (double window). At 20°C indoors and 0°C outdoors, the glass surface temperature rises from 6.8°C to 14.3°C, and the upper humidity limit widens from 42% to 70%. What you can do at no cost is reduce indoor laundry drying and open-flame heaters, and never stop 24-hour ventilation.

Why does closing the curtains make condensation worse?

Curtains block the room's heat, so the glass behind them gets even colder. With single glazing, against 6.8°C at the center, behind curtains falls to 3.7°C, and the upper humidity limit drops from 42% to 34%.

I switched to double glazing, but the frame still gets condensation.

If the frame is still aluminum, its surface gets colder than the glass. At 0°C outside and 20°C indoors, aluminum frames reach 5.7°C, and condensation forms above 39% humidity. Insulating tape for frames, or a vinyl inner window that covers the frame, is effective.

Do dehumidifiers help with window condensation?

Yes. At 20°C, lowering humidity from 50% to 40% lowers the dew point from 9.3°C to 6.0°C. But with single glazing at 0°C outside, the limit is 42%, so there's only so far you can go before it gets too dry. In winter, desiccant types that dehumidify at low temperatures are the best choice.

Are anti-condensation sprays and absorbent tape worth using?

They don't reduce condensation itself, but they stop droplets running down and wetting sills, walls and curtains. Think of them as cleanup that prevents mold and wood rot, and combine them with measures for humidity and window temperature.