When condensation appears on a window, it can be either a positive or a negative sign depending on which part of the window is attracting condensation, and on indoor and outdoor climate conditions. Below, we explore the most common situations and provide some guidance for addressing attendant problems.
If a sealed dual-pane or triple-pane window is fogging up on account of condensation in between its glass panes, the insulated glass unit has failed and cannot be repaired. The only solution is to replace the glass unit. Moreover, if the condition of the window frame or sashes is especially poor, complete window replacement is strongly recommended.
The gas between the glass panes is not air. It is a heavy isotope of argon (40Ar) which has been desiccated (all moisture has been removed). The gas maximizes the efficiency of the window, and once it has escaped, there is no reliable process for replacing it, or for resealing the glass unit.1
Water condenses on the interior surface (“roomside”) of a window for the same reason that it collects on a cold soda can—the temperature of the surface is below the dew point temperature of the air the room.
Air can contain only so much moisture, and how much exactly is a function of air pressure and temperature. In general, as temperature falls, the less moisture the air can hold.2 If the room’s humidity is at 50%, and the temperature is 70°F, the window will attract condensation just in case the roomside glass temperature is 50°F or less. Explore Figure 1 to see how condensation interacts with temperature and humidity.
If condensation appears on the interior surface of a window when the room is at a temperature of 65–75°F and the air outside is near or above 32°F, it would indicate that the humidity of the indoor air is well above the recommended range of 15–40% for the winter season (see Table A).
|Outdoor Temp. (F)||Indoor Humidity|
|20 – 40°||≤ 40%|
|10 → 20°||≤ 35%|
|0 → 10°||≤ 30%|
|-10 → 0°||≤ 25%|
|-20 → -10°||≤ 20%|
|< -20°||≤ 15%|
Excessive humidity can cause paint to peel, will encourage mold and mildew growth, and may accelerate the deterioration of sensitive materials (EPA 2012). The underlying problem is that home ventilation systems are inadequately offsetting indoor sources of humidity, which contribute several gallons of water to the air each day (Corrin 1992). Most likely, the HVAC system is not replacing enough indoor air with dry outdoor air. Additionally, exhaust fans in the kitchen or bathrooms may not be working properly. Adjusting the HVAC system and repairing exhaust fans and dryer vents will usually bring the interior humidity to an appropriate level. Adding a dehumidifier can help significantly, as well. If the problem persists in spite of these measures, attempt to reduce the water vapor produced by bathing and housekeeping tasks (particularly laundry, cooking, and dishwashing) by limiting their frequency and duration where practical, and by running exhuast fans for several minutes after bathing or cooking has finished. Table B offers further guidance:
|Humidity Source||Direct Cause(s)||Remedy|
|Forced evaporation||Dryer, dishwasher||Use the kitchen exhaust fan(s); ensure dryer vent is clear.|
|Steam||Shower/bath, cooking, dishwashing, ironing||Minimize steam-producing activity; run exhaust fans; cover cooking pots.|
|Standing water||House plants, groundwater in basement||Run fans; add a dehumidifier.|
|Water-retaining material||Firewood, house plant soil||Store outside; run fans; add a dehumidifier.|
If the old windows were airtight and used uninsulated glass, condensation will lessen. However, if the new windows are tighter—as is usually the case—condensation may increase. This is because high-performance windows improve energy efficiency by stopping air leaks. This increases humidity by retaining air that has absorbed moisture produced within the home. The increase in humidity and roomside condensation therefore only indicate that the windows are not leaking.
Condensation on the outside surface of a window is not commonly noticed by homeowners, and does not signal a problem. Generally, this happens when the AC system brings the air temperature inside the home quite low (e.g., 65°F) on a humid summer night with high barometric pressure (a cloudless sky indicates high pressure) and minimal breeze. The window glass, cooled by the AC system inside the house, drops below the dew point temperature of the stagnant outdoor air, causing the water in the air to condense on the glass.
If the condensation is bothersome (e.g., it may create visible water spots after drying), increase the indoor air temperature by several degrees, and remove outdoor obstructions that may inhibit airflow around the house.
 Argon is non-flammable and non-toxic, so there is no need to worry about it getting into your house.
 That is, granting a ceteris paribus clause as regards air pressure.