Humidity Showing Up on Windows

What window condensation is really telling you

Condensation forms when humid air touches a surface that is cooler than its dew point. On a window, the glass is often the coldest surface in a cooled room, most of all when the air conditioner has been running and the outdoors is hot and wet. Moisture in the air turns to droplets on the glass, and in bad cases it runs onto the sill, the frame and the wall below.

The window itself is not always the culprit. It is the place where the problem shows up first because glass is thin and cools quickly. If the glass is wet, other cool surfaces in the house may be wet too, such as the inside of exterior walls, the backs of closets, the attic side of the ceiling and ducts. You just cannot see those.

In this region the pattern is reversed from what you may read in northern articles. Up north, condensation comes from warm indoor air meeting cold glass in winter. Here it comes from hot humid outdoor air meeting a cool surface that the air conditioning has chilled. The cause is outside moisture getting in rather than inside moisture getting out, although both can matter.

Where the moisture comes from

Part of the moisture is made inside. Cooking, showers, laundry, drying clothes indoors, aquariums, houseplants and even the breathing of the people in the home all add water to the air. An exhaust fan that vents into the attic or does not run long enough leaves that moisture in the home.

Most of it, though, comes from outside. South Florida air carries a large amount of water vapor for much of the year. Every time a door opens, and every time air leaks through a gap in the wall, ceiling or floor, that moisture comes in. Air conditioning removes some of it, but a leaky house can bring in more than the system can handle, most of all in mild, rainy weather when the unit does not run long enough to dry the air.

That last point is important. An air conditioner cools and dries air at the same time, but it only dries while it is running. An oversized system that cools the house quickly and shuts off leaves humid air behind. A system with leaky ducts can pull wet attic air into the supply side. Both situations raise indoor humidity and increase condensation.

• Daily activity such as showers, cooking and laundry
• Bath and kitchen fans that vent into the attic or do not run long enough
• Humid outdoor air entering through gaps and penetrations
• An air conditioner that cycles too briefly to remove moisture
• Duct leaks that draw humid attic air into the system

How air leaks drive indoor moisture

A house is full of small gaps. They are around windows and doors, at the tops of walls, around plumbing and electrical penetrations, at recessed lights, around the attic hatch, along the sill plate and at the edge of the floor. Each is small, but together they can add up to the equivalent of an open window.

Air moves through those gaps because of pressure differences. A house that is cooled and has exhaust fans running or leaky return ducts may be slightly under pressure, which pulls outdoor air in through every opening. That air is hot and wet and it carries water vapor straight into wall cavities and attics, where it can condense on cool surfaces.

The window is where you see it, but the leaks are elsewhere. Sealing them often reduces condensation more than changing the windows. The page on air sealing explains how those gaps are found and closed, usually before any insulation is added.

How thin or missing insulation adds to the problem

Insulation does two things related to moisture. It keeps surfaces closer to room temperature, so they are less likely to fall below the dew point. And, when it is paired with an air barrier, it limits the humid air that reaches cool surfaces.

Walls with little or no insulation, such as some older concrete-block homes, have inside surfaces that are cooler in air conditioning. Condensation can show up on the wall, behind furniture and in closets as well as on windows. Thin attic insulation leaves the ceiling cool and can lead to dampness on the ceiling surface or stains in corners where air is trapped.

Adding insulation raises the temperature of the interior surface and cuts the chance that it will fall below the dew point. It does not remove the moisture, but it can stop the moisture from turning into water on the wall. This is why insulation and air sealing together are better than either alone. The page on attic insulation explains how the ceiling layer is improved.

Windows, frames and flashing: when the window is part of the cause

Sometimes the window really is a factor. Single-pane glass is much colder than double-pane glass with a low-emissivity coating, and in a cooled room it will sweat more. Aluminum frames conduct heat and cold, so they can be wet at the edges even when the glass is only lightly misted. Seals that fail between panes can leave fog between the layers, which is a sign that the unit has lost its seal and no amount of wiping will clear it.

Water that appears on the inside of a window frame after rain may not be condensation at all. It may be a leak around the frame or through the flashing, which is the material that sheds water around the opening. Moisture that runs down the wall below the window, stains the drywall or swells the baseboard after a storm points to a leak. This is a different repair, and it needs a window or roofing professional.

How can you tell the difference? Condensation appears on the room side of the glass and often shows in patterns that follow temperature, such as in the morning or on cooler nights. Leaks appear after rain, at joints and corners, and may carry dirt. A technician can sometimes tell by looking at the pattern, and a moisture meter helps confirm.

Signs that the problem goes beyond the glass

Look for other clues. Musty odor in closets, most of all exterior closets, is a sign that the air in the closet is damp. Mildew on the walls in corners, most of all behind furniture, shows that those surfaces are staying cool and damp. Peeling paint, bubbling wallpaper or a white powdery residue on masonry suggests that moisture is moving through the wall.

In the attic, look for dark staining on the underside of the roof deck, damp insulation, rusted nails or a ceiling that feels cold and clammy from below. If ducts sweat, humid attic air is meeting cold surfaces. The page on sweating ducts in the attic explains that situation.

A hygrometer, a small and inexpensive meter, tells you the relative humidity in a room. Many people find that a reading that stays high through the day is a better guide than how the house feels. If you track the reading for a week, you can see patterns tied to weather, cooking and the air conditioner.

What you can do right away

There are practical steps you can try today. Use bath and kitchen fans during and after showers and cooking, and make sure they are vented outdoors. Keep interior doors open so air can circulate and conditioned air reaches every room. Avoid drying laundry indoors. Keep the thermostat at a steady setting rather than very low, which can chill surfaces below the dew point.

Wipe visible moisture from windows and sills so mildew does not take hold. Keep furniture a little way from exterior walls so air can move behind it. If a room is closed for long periods, such as a guest bedroom, leave the air conditioning supply open and the door ajar so the room does not become a damp pocket.

A dehumidifier can be useful in some homes, but it treats the symptom. If you need one running all the time, something is letting moisture in. Be cautious about portable units in closed rooms with no drain, since they can fill quickly and spill.

• Run exhaust fans during and after showers and cooking
• Keep interior doors open for airflow
• Wipe moisture from glass and sills each morning
• Leave a little space between furniture and exterior walls
• Track humidity with a small meter for a week

How different homes show the problem

In a concrete-block home with aluminum sliding windows, the first sign is often a damp track along the bottom of the glass and a film on the interior side of the block wall in a closet. The block is a thermal mass that stays cool when the air conditioner has been running, and humid air that reaches it forms water. These homes often have thin attic insulation and plenty of small gaps at the ceiling, and both feed the problem.

In a newer two-story house with large windows and a vaulted entry, condensation tends to appear on the tall windows where the glass is the coldest surface and air stratifies. Air leaks at the top of the walls and at recessed lights add warm humid air to the upper floor, which is also the hardest to cool. The solution there often starts at the ceiling.

In a condominium, the problem may be tied to the building's shared systems, a balcony slider that does not seal or a neighbor's activity. Some units sit on the leeward side of the building where air stays damp. Changes to windows and the exterior wall are typically controlled by the association, so interior steps such as sealing and humidity control may be the practical path.

In a waterfront home near the Intracoastal or the ocean, salt air and constant humidity make surfaces damp more of the year. Sliding doors and hurricane shutters that are closed for long stretches can trap humid air inside. A house left unoccupied for weeks can become a damp box with the air conditioner set too high, which is a point we return to in the page on vacant homes during snowbird season.

Thermostat settings, fans and system size

A thermostat set very low makes glass colder and condensation more likely. A thermostat set higher, with a steady cooling cycle, often brings humidity down more reliably than one that swings. If the fan setting is on rather than auto, the fan keeps blowing during off cycles and can re-evaporate water from the coil back into the home. Many people find the auto setting better in humid weather, but the right choice depends on the system and the home.

System size matters. A unit that is too large for the house cools quickly, satisfies the thermostat and stops before it has pulled much moisture out of the air. The house feels cool but clammy. This is a heating and air conditioning question, not an insulation one, but insulation changes the cooling load. After a house is sealed and insulated, the same system may run longer and dry the air better, or it may turn out to be larger than needed.

If your system has been serviced and the problem persists, a conversation with an air conditioning professional about humidity control is reasonable. An insulation technician can tell you what the building envelope contributes and refer you onward where that is the better path.

What a technician checks on the visit

The visit usually starts with the room that has the problem and then moves outward. The technician looks at windows, frames, trim and the wall below for signs of leaks or condensation. They check the attic for insulation depth, air leaks and moisture, and look at the air handler, ducts and returns. They check where exhaust fans terminate.

Measurements help. A moisture meter on drywall, a hygrometer in the room and attic, and sometimes a thermal camera show where surfaces are cool and where air is leaking. These tools turn a vague complaint into a map of where to work. The technician may also test whether the house is under pressure with the air handler running.

Typical recommendations include air sealing at the ceiling and wall penetrations, adding or correcting attic insulation, repairing or sealing ducts, correcting exhaust terminations and, where the window is clearly at fault, referring you to a window professional. Complicated jobs need an on-site visit before any quote, and quotes are given only by phone. A service call charge applies and is explained before dispatch.

Putting the pieces in order

The best results come from working in a sensible sequence. First fix any active leaks, because insulation cannot help a window that lets in rain. Then seal air leaks, which cuts the moisture coming in. Then add or correct insulation to keep interior surfaces warm enough to stay dry. Finally, check that the air conditioning is sized and set up to dry the air rather than just cool it.

The page on the order of energy-saving upgrades lays out the same logic for efficiency. If you have been wiping window sills for a long time, it may be the sign of a house that is asking for exactly this kind of attention. Call the number on this page to describe what you see and set up a visit.

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