Condensation
Windows & Water: Condensation Facts 

What is Condensation?
Condensation is the result of high humidity that produces a "fog" once it hits a colder surface. The humidity is caused
by excess water vapor in the home. This is commonly seen in a foggy mirror after a hot shower. Condensation
usually occurs first on windows because glass surfaces have the lowest temperature of any of the interior surfaces in
the home.
What Causes High Humidity?
Humidity comes from excess water vapor in the air. All homes have a certain level of moisture due to household
activities. Activities such as cooking, laundry and dish washing can add up to five pounds of water vapor into the air
daily! Other moisture-producing agents include plants, heating systems and humidifiers.
Seasonal changes, quick changes in temperature, reconstruction and remodeling can also add excess humidity
temporarily into the air. During construction, large quantities of water are displaced into the air from building materials
such as concrete, plaster, taping, etc. After one season of heating it will commonly dry out and your condensation
problems will disappear.
Does Condensation Depend on My Window Type?
Sometimes. Recessed windows like bow and bay windows usually experience more condensation than other window
styles. This is because air circulated around those window types is usually more restricted, and since they hang away
from the insulated house wall, bays and bows could be a few degrees cooler in temperature. Placing a common
electric fan near the window to produce air circulation may also be helpful. 

Maximum Recommended Humidity Levels 
(based on engineering studies at 70 F° conducted at the University of Minnesota Laboratories) 
Outside Temperature F° Inside Humidity
20 F° to 40 F° Not over 40%
10 F° to 20 F° Not over 35%
0 F° to 10 F° Not over 30%
-10 F° to 0 F° Not over 25%
-20 F° to -10 F° Not over 20%
-20 F° or below Not over 15%

Tips for winter-time moisture management.
  • Closely monitor the furnace humidifier and any other humidifying devices.
  • Open window coverings — such as blinds, shades, drapes and curtains — during daylight hours to increase airflow over the glass.
  • Be sure louvers and vents for the attic, basement and/or crawl space are open, adequately sized and cross-ventilated.
  • Run exhaust fans for kitchen, bathroom and laundry rooms for longer periods.
  • Make sure exhaust fans vent directly outside, not into attics or crawl spaces.
  • Be sure chimneys are free and clear so moisture in combustion gases can escape.
  •  Follow the manufacturer’s instructions for venting gas appliances. In most cases, that will mean directing vents to the outside of the home.
  • Make sure your furnace is in proper working order and is serviced regularly.
  • Store firewood outside or in the garage — as wood dries, it gives off moisture.
  • Open a door or window for several minutes each day to refresh the inside air. 
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