Window Condensation: Causes, Dew Point and Why Ventilation Matters

Window condensation is water vapor from room air turning to liquid on a window surface that is colder than the air's dew point. Humidity supplies the water, and the cold surface collects it. A window is where condensation shows first because the glass and frame are usually the coldest surfaces in the room. Fixing it means lowering humidity, raising the surface temperature, or both; the window can help with the second but not the first.

What the dew point means

The dew point is the temperature to which air must be cooled to become saturated, according to the National Weather Service. Once a surface is colder than that temperature, liquid water forms on it. The warmer the room air and the more moisture it carries, the higher the dew point.

Building Science Corporation gives a worked example. If the air in the middle of a room is 70 F at 40 percent relative humidity, any surface below 45 F can condense water, and any surface below 54 F has air next to it at 70 percent relative humidity, the level at which mold growth becomes possible. At 25 percent relative humidity in the same 70 F room, the condensing temperature falls from 45 F to 32 F, and the mold line falls from 54 F to 40 F.

Relative humidity measured in the middle of a room is different from the relative humidity at a cold surface. Air next to a cold surface is cooler, so its relative humidity is higher.

Where condensation appears

Natural Resources Canada lists the same places: the glass, the frames and sashes near the weather seals, the walls around a skylight, and between the panes if the seal is broken.

Why it forms: four causes

1. Indoor moisture. Showers, cooking, dishwashing, clothes drying, humidifiers, houseplants, breathing and unvented combustion appliances add water vapor. EPA says to vent clothes dryers, stoves and similar appliances outdoors where possible, to run bathroom fans or open a window when showering, and to use exhaust fans when cooking and dishwashing.
2. Cold surfaces. The colder the weather, the colder the glass. Natural Resources Canada states that the risk increases as the weather gets colder or the inside humidity rises.
3. Poor air movement at the glass. Air flowing across the glass keeps it warmer. Deep window sills, closed drapes and blinds, and nearby furniture block air movement and let cool air pool at the window (Natural Resources Canada).
4. Weak points in the window. Green Building Advisor's analysis stresses that window condensation is a thermal bridging problem, not an average U-factor problem. The coldest part of most modern windows is the bottom half-inch of the glazing, and a metal spacer can create a low condensation resistance even when the window's overall U-factor is low.

Interior condensation and humidity targets

EPA advises keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent, and treats condensation on windows, walls or pipes as a sign of high humidity. EPA also recommends drying wet surfaces quickly and addressing the moisture source.

A single target does not fit every condition. Building Science Corporation points out that the surface temperature matters: in winter in cold climates, indoor humidity should be kept as low as possible within the comfort and health range, because colder glass needs lower room humidity to stay dry. It also calls the commonly quoted 40 to 60 percent range an incorrect recommendation, particularly in very cold and cold climates. German environment-agency guidance gives 40 to 60 percent as its target for indoor humidity. These sources differ in emphasis, which is why the practical approach is to measure humidity with an inexpensive hygrometer and adjust until the glass stays dry in the coldest weather of the year.

The climate matters in the US. A house in a very cold climate zone has colder glass for more of the year than a house in a mild zone, so the humidity that is safe in October may be too high in January.

Exterior condensation

Exterior condensation is the reverse problem. On high-performance glass, little heat escapes from the room, so the outer pane stays close to the outdoor temperature or colder. On cool, calm and humid nights, water can condense on the outside surface around sunrise (Natural Resources Canada). Green Building Advisor explains that night-sky radiational cooling can make glass colder than the surrounding air, the same effect that forms dew on grass, and that it is most likely on a window facing open sky. It clears as the sun warms the glass and does not indicate a problem with the window.

Natural Resources Canada also notes a higher condensation risk where the low-e coating is on the room-side surface of the glass.

What windows can do, and what they cannot

A window cannot remove moisture from the house. It can only change how cold its inner surface gets.

What a better window does:

• Natural Resources Canada says ENERGY STAR certified windows have less condensation than ordinary models, and lists the features that contribute: triple glazing, low-e coatings, inert gas fills, thermally improved spacer bars, insulated frames and sashes, and greater air tightness.
• A lower U-factor raises the interior glass temperature. See U-factor for windows.
• A warm-edge spacer and a thermally broken frame reduce cold spots at the perimeter, where condensation starts.
• The NFRC condensation resistance (CR) rating, when provided, measures how well a product resists condensation on a scale from 0 to 100, where a higher number means less build-up (ENERGY STAR). DOE adds that CR ratings are listed for some products in the NFRC Certified Products Directory, and recommends specifying high CR ratings in extremely cold regions. ENERGY STAR qualification is based on U-factor and SHGC only, so CR is a separate line to ask for. See how to read an NFRC label.

What it cannot do:

• It cannot fix a house with a wet basement, unvented appliances or no mechanical ventilation.
• It cannot stop condensation if humidity stays high enough for a warm glass surface to reach the dew point.
• It cannot compensate for blinds, drapes or furniture that block airflow at the glass.

Ventilation, including tilt and turn windows

Ventilation lowers indoor humidity by replacing humid indoor air with drier outdoor air in winter. Natural Resources Canada also advises making sure a heat recovery ventilator (HRV) is running if the house has one.

The German Environment Agency (Umweltbundesamt) gives precise guidance on opening windows. It says leaving windows tilted for long periods raises energy use and heating costs drastically, and the wall beside the window can cool quickly, which can lead to condensation and mold. It recommends short, wide openings instead: in the cold season about 5 minutes, two to three times a day, per room with one or more windows wide open; 5 to 10 minutes in the bedroom after getting up; and 5 to 10 minutes in the kitchen and bathroom during and after cooking or showering. Air exchange is much greater when windows on opposite facades are open at once.

For a tilt and turn window that means:

• Turn position is the airing mode: it opens the whole sash and gives the fast air exchange this guidance describes.
• Tilt position gives a small, controlled opening and is useful for brief or sheltered airing. It is not a substitute for wide airing, and leaving it tilted for hours in cold weather works against the glass temperature.
• Sashes swinging into the room need clear space; see inward-opening windows for drapery and furniture clearance.

In tight modern construction, bath and kitchen exhaust fans, a balanced ventilation system or an HRV do the continuous work; window airing is the supplement.

A short checklist

1. Measure indoor relative humidity with a hygrometer.
2. Keep it below 60 percent, and aim lower in cold weather until the glass stays dry.
3. Vent moisture sources outdoors.
4. Keep air moving across the glass: open blinds partly, avoid furniture against the window.
5. Air rooms briefly and widely; avoid long tilted ventilation in winter.
6. If condensation persists between panes, replace the glass unit.
7. For new windows, compare U-factor, spacer, frame and the CR rating when available. Terms are in the glossary.

If mold appears, EPA says the moisture problem must be fixed before cleanup can be considered finished.