Humidity, Condensation and Your Home

During winter cold snaps, you might experience condensation or sweating on the inside surface of your windows.

Regardless of the window manufacturer or whether the window is made of wood, vinyl or aluminum, humidity will condense on any window if conditions are right. The situation is usually temporary and can be handled by making some adjustments to control interior moisture. Others may require additional effort and are more difficult to control.

Interior water vapor can be annoying to a home owner and destructive to a home. It can damage sheetrock, paneling, and window sills. It can also penetrate the walls, deteriorating wood framing and reducing the effectiveness of insulation.

Frequently Asked Questions

What is humidity?
Humidity is water vapor, or moisture, in the air. Usually it is visible, but sometimes, such as with steam or ground fog, it's concentrated enough to be seen. But see it or not, all air contains a certain amount of moisture.

Where does the moisture come from?
There are many things that generate indoor moisture. The normal perspiration and breathing of the occupants of a home adds moisture to the air.  So does cooking, baths and showers, doing the laundry, etc.  In fact, every activity that uses water adds moisture to the air. The normal daily activities of a family of four can add more than 18 gallons of water a week into the air in their home. And more water vapor in the air means a higher indoor relative humidity.

What is relative humidity?
Air can hold only a limited amount of water vapor, and that amount depends on the air temperature. When air at a certain temperature contains all the vapor it can hold, it's said to be "saturated", which means a relative humidity of 100%. When it holds only half the water vapor it can hold, the relative humidity is 50%. Cooler air cannot hold as much water vapor as warmer air.

What is condensation?
Fog on windows is a form of condensation. So is the water that forms on the outside of a glass of iced tea in the summer. It all comes from water vapor in the air.

What causes it?
Excess moisture in the air. When warm, moist air comes into contact with cooler surfaces, the moisture condenses. That's because the cooler air surrounding cooler surfaces cant hold as much moisture as warmer air.

What does condensation on windows mean?
Condensation usually appears in cold weather on the glass and/or frames of windows and sliding glass doors. It can block out the view, drip on the floor, and freeze on the glass. It's annoying. While it is natural to blame the windows, you shouldn't. Window condensation is the result of excess humidity in your home. And the glass only provides a visible cold surface on which humidity can condense. The important thing is your foggy windows and sliding glass doors are trying to tell you to reduce indoor humidity before it causes hidden, costly problems elsewhere in your home.  Foggy windows and sliding glass doors are warning signs that humidity could be damaging your home.

What are some other symptoms of excess humidity?
Problems like peeling paint, rotting wood, buckling floors, insulation deterioration, mildew, and even moisture spots on ceilings and walls.

How do I know if I have excess indoor humidity?
Check for damp spots on ceilings and room-side surfaces of exterior walls, particularly closets. Look for water and ice on windows. Even water-filled blisters on outside paint surfaces indicate excessive indoor humidity.

What does excess humidity do to my home?
Excess humidity contributes to the deterioration of a home. Excessive humidity can pass through walls and freeze in the insulation. In spring it melts, damaging your ceiling and walls. Or, humidity can force its way out through siding to form blisters under your exterior paint.

Can moisture actually go through walls?
Yes! It's because of a force called "vapor pressure". Moisture in wet air tries to flow toward drier air to equalize itself. This flow acts independently of air currents. In winter, inside air is much more humid than colder outside air. So, the vapor pressure, or equalization process, actually forces the inside moisture through cement, wood, plaster, and brick, toward the outside.

What happens then?
Because certain varnishes and paints block the flow of moisture, condensation can occur between the inside and outside walls, or under exterior paint surfaces. It can rot a homes wood frame and blister the paint.

Is condensation more prevalent in any geographical region?
Yes. Condensation is more apt to occur in climates where the average January temperature is 35 F or colder.

Does condensation occur only in winter?
Usually, but it can occur during cold weather anytime, and occasionally it will form on the outside of windows on hot, humid summer days, when your air conditioner has cooled the glass.

Does condensation depend on whether my home is new or old?
Generally, yes. Years ago, before the concern with energy efficiency, homes were built with less weather-tightness than homes today. Insulation concepts were not as advanced as today. Walls and ceilings were built with much more porous materials. Water vapor could easily flow in and out of walls.

Today's homes are much "tighter". Windows and doors are built to substantially reduce air leakage. Weather-stripping, modern insulation, vapor barriers, and construction techniques, which are intended to keep out cold air, lock moisture inside. As a result, moisture created by bathrooms, kitchens, laundries, and occupants no longer flows to the outside, unless provisions for mechanical ventilation have been made. So it's very easy to build up excessive, even harmful moisture levels in today's homes.

Controlling Indoor Humidity

How do I measure indoor relative humidity?
To get an accurate reading, you can buy humidity-measuring instruments, called hygrometers, or sling psychrometers. Otherwise, watch your windows for symptoms of excess humidity. When excessive moisture collects on the inside glass in a living room or bedroom, you're approaching the humidity danger level.

Isn't high indoor humidity healthy in winter?
That's a common belief, but there is little evidence to support it. Humidity in a heated house, whether it is high or low, is not an important health factor to a normally healthy person. And there is little scientific evidence that the health of most people will be adversely affected if humidity is kept at a level that prevents excessive condensation.

Then what are some recommended indoor relative humidity levels for winter?
The University of Minnesota Engineering Laboratories performed a series of long and careful experiments on that subject. The following table shows the maximum safe humidity for your home, paint, insulation, and structural members:

Recommended interior relative humidity for various outside air temperature ranges

-30 F or below -- not over 15%
-20 F to -10 F -- not over 20%
-10 F to 0 F -- not over 25%
0 F to 10 F -- not over 30%
10 F to 20 F -- not over 35%
20 F to 40 F -- not over 40%
(Assumes 70 F indoor air temperature)

When safe humidity levels are maintained, condensation is very unlikely, and a healthy interior environment exists for the home and its occupants.

How can I reduce inside humidity in winter?
There are at least two steps you can take to reduce indoor humidity in winter:

Control the sources of humidity. Vent all gas burners, clothes dryers, etc. to the outdoors. Use kitchen and bathroom exhaust fans.

Ventilate your home. Because outside air usually contains less water vapor, it will "dilute" humidity of inside air. This takes place automatically in older homes through constant infiltration of outside air. But again, in newer "tighter" homes, the only way outside air can get in is by ventilation.

How can I ventilate my home?
There are basically two types of ventilation: interior and structural ventilation.

As a temporary solution, open a window in each room for just a few minutes. Remember that inside air continually gains humidity through daily living activities. Opening windows allows the stale, humid air to escape, and fresh, dry air to enter.

After a shower, for example, open the bathroom window or turn on the exhaust fan, so steam can go outside instead of remaining in the home.

Structural ventilation is slightly more complex, but will save you costly repair bills in the long run. Miniature louvers in exterior walls can be installed to prevent moisture from condensing between the outside and inside walls. This will keep paint from peeling as a result of indoor vapor pressure.

Does structural ventilation include attics?
Yes. Many homeowners cover all attic louvers in winter in hopes of saving fuel. If the attic is properly insulated, this practice can cause harm. Because the indoor moisture penetrates ceilings, then condenses on the cool underside of the roof and can even form frost. If the attic were ventilated, moisture would be transferred to the outside air.

What harm can attic condensation do?
A lot. Moisture condensing in attics produces mildew, or rotting conditions. Or it drips down to the ceiling below to damage plaster or paint. Thermal insulation also becomes wet and provides less resistance to heat loss.

Are some kinds of attic ventilation better than others?
Yes. A combination of vents at the eaves and at the gable ends is better than gable vents alone. And, a combination of continuous eaves and ridge venting is best of all. However, regardless of the type you have, there should always be at least two vent openings, located so that air can flow in one and out the other.

How much attic ventilation should I have?
That's a difficult question to answer, because the size and number of vents depends on the location of the home, wind direction, physical size of the building, quality of workmanship and kinds of building materials used. A heating and ventilating contractor should be able to tell you how much ventilation your attic should have.

What about the crawlspace? Should it be ventilated, too?
Yes. The crawlspace beneath a house is another place where ventilation is important. The crawlspace can evaporate gallons of water each day. When you seal the crawlspace, that water penetrates the floor above and causes more humidity problems in the home.

Providing foundation vents in the crawlspace reduces the humidity, and a vapor barrier (like polyethylene film) on the ground prevents moisture leakage into the house above.

Windows and Condensation

Do windows cause condensation?
Windows are not a cause, per se. They provide a ready medium on which the vapor can condense. But the primary cause of condensation is excessive moisture in the air. Windows are only indicators of excessive moisture.

Why do I see condensation on my windows and sliding glass doors first?
Condensation is generally seen first on windows and sliding glass doors because they tend to have the lowest temperature of any of the visible surfaces in the house.

Are windows the only place condensation is forming?
Possibly not. There is a point between the exterior and interior walls that is just as cold as the inside window surface. Chances are, if you can see condensation on your windows, it's also forming between the walls, too. Room-side surfaces of exterior walls are normally warmer, but occasionally condensation occurs on cold spots such as nail heads and in the corners of outside walls and closets. This is because insulation is weaker and circulation is restricted in these areas.

What causes condensation on windows and sliding glass doors?
Recall that cool air is able to hold less moisture than warm air. Therefore, when the warm, moist air of the room comes into contact with the cool glass surface, some water vapor that can no longer be held by the cooled air is deposited on the glass.

Do drapes and window shades cause window condensation?
Drapes and other window coverings don't cause window condensation, but they can contribute to the problem by restricting the flow of warm room air over the glass surface. Therefore, condensation is more apt to occur when drapes are closed and shades are pulled down.

What causes condensation on the inner surfaces of storm windows?
This indicates that air is leaking outward around the inner window, and is being trapped by a tight-fitting storm window. Moisture is then trapped between the panes, and condensation occurs on the outside pane. Tight storm windows should have some ventilation to the outside to relieve this problem.

What damage can excess window condensation do to windows?
It can cause the paint to peel from the sash or stain the inside. Water can run down into the window frame causing dampness around the frame. It can sometimes cause paint to peel on the outside of the window.

Are there any cases where window condensation is only temporary?
Yes, there are primarily three: new construction or remodeling; the beginning of each heating season; and quick changes in temperature.

At the beginning of the heating season there may be a certain amount of temporary condensation. During the humid summer your house absorbs some moisture. After the first few weeks of heating your house will dry out, and you should have fewer condensation troubles.

Sharp, quick drops in temperature can also create temporary condensation problems during the heating season.

Can windows help control moisture in my home?
Only in the sense that they can be opened for ventilation. Otherwise, windows are only indicators of excessive moisture in the air.

Is there anything I can do to my windows to eliminate condensation?
If you have windows with single-pane glass, you should seriously consider replacing them with windows that have double-paned glass with a low-e coating and argon gas filling. This is not guaranteed to eliminate condentation, but it should significantly reduce the visible signs of excessive humidity.

Is window condensation really reduced that much with double-pane insulating glass?
Yes. Double-pane insulating glass permits about 37% relative indoor humidity (at 70 F inside, 0 F outside) without condensation. Single-pane windows permit just 12% relative indoor humidity.

What other things can I do to prevent excessive condensation?
If you have a hot air furnace, install an outside fresh air intake.

Make sure your clothes dryer and all gas appliances have outside vents. Water vapor is one of the products of natural gas combustion.

If you have a basement, take the necessary steps to prevent leakage of soil moisture into the basement. These steps will vary with soil and drainage conditions on your lot.

Use exhaust fans in the kitchen, bathrooms, and laundry. They not only help reduce excessive moisture, but will clear away stale air as well.


The best way to avoid condensation is to reduce excess humidity inside your home.

Window condensation can certainly be a problem. But in the vast majority of cases, it can be controlled or eliminated. If you follow the steps and suggestions outlined here, you shouldn't have a serious condensation problem.

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