Understanding condensation

Moisture is present in all homes, but when it becomes excessive, homeowners need to take an active role in protecting their investment.

Humidity and condensation

  • Humidity is a measure of the amount of water vapor in the air.
  • Condensation is water that forms when warm, moist air hits a cooler surface. In homes, it might occur on surfaces such as windows, bathroom walls and cold-water pipes.
  • When interior humidity levels are too high relative to cooler outdoor temperatures, condensation can form on the coldest surface in a room — often the glass in a window or door.
  • While windows and doors do not cause condensation, they may be one of the first places it shows up.

What causes condensation?

  • Excess humidity is typically the cause of condensation. There are many sources for moisture in a home: showers, dishwashers, clothes washers and dryers, house plants, humans and pets, among others. In newly built homes, additional moisture may escape from building materials such as lumber, plaster and masonry for up to three heating seasons, even with proper air-flow and temperature management.

Older vs. newer homes

  • Condensation may be less of an issue in older homes if conditions allow for more air exchange between indoors and out, often from around aging, loose or poorly installed windows and doors. However, the tradeoff is higher energy bills.
  • Newer homes, on the other hand, are more airtight and energy efficient. Many have vapor barriers — plastic within the wall cavity that blocks moisture passage in either direction. With tighter fitting doors and windows, vapor barriers and increased insulation, energy costs are lower, but humidity levels must be monitored more closely.
  • No matter when your home was built, the key is to strike the right balance when it comes to humidity levels. Air that’s too dry can cause furniture to dry out and crack, joints and studs to shrink and twist, and paint and plaster to crack. Excessive moisture in the home can cause paint to peel and insulation to deteriorate, and condensation on windows and doors can damage sills and trim.

Measuring and controlling humidity

  • To determine how much humidity is present in your home, you can purchase a humidity gauge from a hardware store or home center. Invest in a quality unit for greater potential accuracy in assessing humidity levels or consult an expert. Another way to monitor the interior humidity level is to watch your windows. When interior condensation begins to form, reduce the humidity by opening windows, running exhaust fans or dehumidifiers, or minimizing sources of moisture.
  • For more detailed information visit this booklet: http://web.pella.com/SiteCollectionDocuments/PellaCondensationManual.pdf

Exterior condensation

  • Exterior condensation on windows occurs primarily in the morning when days are warm and humid, but nights are cool. Typically, it clears as the day warms. Exterior condensation can occur at any time, especially in warm, humid climates where interior temperatures are cooler than outdoor conditions. Exterior condensation means that windows are doing their job properly. However, if you spot excessive condensation on the inside of your windows, check your inside humidity – it may be a signal of potential problems if not addressed.

Tips for winter-time moisture management

  • Closely monitor the furnace humidifier and any other humidifying devices.
  • 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 crawlspaces.
  • 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 since as wood dries, it gives off moisture.
  • Open a door or window for several minutes each day to refresh the inside air.
  • Open window coverings — such as blinds, shades, drapes, and curtains — during daylight hours to increase airflow over the glass. 
  • Install energy-efficient windows, such as those with the ENERGY STAR® designation.

For more information on condensation in the home, consult the following:


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