Condensation on windows: the window's part and the building's.

Water forms on a window when the surface is colder than the dew point of the air that touches it. Only two things decide that: how cold the surface is, and how humid the room is. The window sets the first. The building sets the second. A better window raises the surface temperature. It cannot lower the humidity, and on its own it will not end a condensation complaint in a humid apartment.

Find out where the water is.

Where	What it means	What to do
On the room side of the glass	Room air is too humid for that surface temperature	Read the rest of this article
Between the panes	The edge seal of the glass unit has failed	Replace the glass unit. Cleaning and ventilation will not help
On the outside of the glass	The outer pane is cold, because little heat escapes through the glass	Nothing. It appears on clear, still mornings and burns off. It is a sign that the glass insulates well

The physics, in one table.

Air at a given temperature and humidity has a dew point. Any surface colder than that gets wet. For a room at 70°F:

Indoor relative humidity	Dew point, about
30%	37°F
40%	45°F
50%	50°F
60%	55°F

Read it from the bottom up. At 60% humidity, any surface below 55°F gets wet. On a cold night the edge of many glass units is colder than that. At 30%, the same surface must fall to 37°F before water forms. So the colder the weather, the drier the room has to be. The same apartment can be fine in October and wet in January.

Why the edge fogs first.

The center of a glass unit is its warmest part. The edge is the coldest. There the panes are joined by a spacer, and heat crosses the spacer more easily than it crosses the gas-filled cavity. Inside the cavity the coldest gas also sinks to the bottom. That is why condensation starts as a band along the bottom edge of the glass.

What the window can do.

Everything on this list makes the room-side surface warmer.

• A low U-factor. Low-e coatings, a gas fill and a third pane all keep the inner pane closer to room temperature. On a Vi6 casement, the standard insulated double-pane glass with an aluminum spacer is NFRC-certified at U 0.27. The triple-pane with krypton and a warm-edge spacer is certified at U 0.17.
• A warm-edge spacer. It replaces the aluminum spacer with a material that conducts less heat. The center of the glass hardly changes. The edge, where condensation starts, gets warmer.
• A frame that does not conduct. uPVC is a poor conductor, and the profile is divided into closed chambers, so the frame needs no separate thermal break. Aluminum conducts heat well, so an aluminum frame depends on a thermal break between its inner and outer halves. A well-designed thermally broken aluminum window performs well, and aluminum remains the better choice for very large spans and very slim frames. Wood insulates well and needs its finish kept up where moisture is present.
• Tight seals. Cold air leaking past a gasket chills the surface beside it.

The NFRC label can also carry a Condensation Resistance rating, from 1 to 100, where higher is better. It is optional, and it is only for comparing one product with another. It does not predict whether a given room will see condensation.

The tight-window surprise.

Old, leaky windows ventilate a building by accident. Replace them with tight ones and that accidental fresh air stops. Humidity that used to escape now stays indoors. Owners sometimes find condensation after a window replacement that they never saw before, on windows that are much better than the old ones. The windows are doing their job. The building now needs ventilation that is there on purpose.

A tight window also keeps out street dust, pollen and noise. That is good for the air indoors only if fresh air comes in another way.

What the building must do.

• Exhaust moisture at the source. Bathroom and kitchen fans must be ducted to the outside, and they must actually run. Dryers must vent outdoors.
• Supply fresh air on purpose. That can be mechanical ventilation with heat recovery, or a system that draws air through the windows. Where the design uses the window as the air path, the part that does it is a trickle vent. The mechanical engineer makes that decision, not the window maker.
• Control the sources. Humidifiers, drying clothes indoors, long showers, cooking without a hood, many plants and many people all add water to the air.
• Expect a wet first winter. New concrete, plaster and paint give off moisture for months. Condensation in the first heating season of a new building often fades in the second.
• Let room air reach the glass. Closed blinds, heavy curtains and deep reveals trap a layer of cold air against the window. Heat below the window helps. Furniture pushed against it does not.

The window sets the surface temperature. The building sets the humidity.

A checklist for a complaint.

1. Find where the water is: room side, between the panes, or outside.
2. Measure the humidity in the room with a hygrometer, on a cold morning. Compare it with the table above.
3. Check that the bathroom and kitchen fans run and move air.
4. If the windows have trickle vents, check that they are open.
5. Look at the blinds, the curtains and any furniture against the glass.
6. Check that the sash closes tight all the way round. A draft at one corner points to the gasket or the lock adjustment.
7. If the water is between the panes, the glass unit must be replaced. Go to service and parts.

If the water only appears in the coldest weeks, only along the bottom edge, and clears during the day, it is a humidity matter. It is not a defect. If it runs onto the sill every day, the room needs less moisture or more fresh air, whatever window is in the wall.

The certified U-factors, glass by glass, are on the energy page.