Updated: Jul 22
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.
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)