Understanding Condensation

The Role of a Window

Tighter modern windows help reduce air leaks, which limit ventilation and trap moisture. But tight windows also help reduce heating costs and limit drafts. Modern windows with special glass may even
reduce condensation problems because the glass temperature remains higher.
If your old windows were drafty, those cracks did more than just let in the wind – they allowed excess moisture to escape outdoors.
Your new windows are better insulated, so indoor humidity can’t escape.

The moisture that suddenly appears

in cold weather on the interior or exterior of window and patio door glass can block the view, drip, or freeze on the glass. It can be an annoyance.
While it may seem natural to blame the windows or doors, interior condensation is really an indication of excess humidity in the home.
Exterior condensation, on the other hand, is a form of dew; the glass simply provides a surface on which moisture can condense.

What Is Condensation?

Condensation appears when warm, moist air comes into contact with cooler surfaces, like your window glass on a cold, winter day. That’s true with even the best-performing windows. Although condensation is most common during winter, it can occur whenever warm, moist air encounters a cold surface. That’s why your iced tea glass sweats during the summer. The glass is cold and condensation forms when warmer, moist air comes into contact with it.

Indoor Humidity’s Effect on Condensation

Excessive humidity is the cause of most window condensation. As the outside temperature drops, the window glass temperature also drops. When moist air comes into contact with the cold glass pane, the moisture condenses and forms water droplets. Determining when the condensation will occur and preventing it depends on the energy efficiency of the window, the relative indoor humidity of the home, and the exterior and interior temperature.

Sources of Indoor Moisture

Everyday living generates a surprising amount of indoor moisture. Cooking three meals a day adds eight or ten cups of water into the air. Each shower contributes another cup. Every activity that uses water adds moisture. Regardless of its cause, the moisture’s source doesn’t have to be where the condensation occurs. Vapor pressure can actually force moisture through plaster, wood, brick, and cement.

Managing Exterior Moisture

Condensation on the outside of a window is the same as dew on your lawn. It occurs mostly during the spring and fall, when there are cooler nights and warmer days. It is usually an indicator of energy-efficient windows, since it means the outside pane is thoroughly insulated from the heat indoors.

Fibrex Material

While wood and fiberglass windows may require regular maintenance to combat the effects of condensation, Renewal by Andersen® replacement windows are made with Fibrex® material, a revolutionary composite that provides exceptional resistance to rot and fungal growth.¹

Tips for Reducing Humidity in the Home

Vent all gas appliances, such as clothes dryers and exhaust fans, to the outside. Cover the earth in the crawl space with a good vapor barrier. Run an over-the-range exhaust fan when you cook. Avoid storing firewood indoors. Ventilate your attic and crawl space. While bathing, run a bathroom exhaust fan until the mirror is clear. If you have a forced-air furnace, contact your heating and air-conditioning specialist for guidance in ensuring your home is properly ventilated.

Measuring Indoor Humidity

You can use humidity-measuring instruments called hygrometers, inexpensive tools that can be purchased at most hardware stores. Many of today’s new programmable thermostats also include controls for humidifiers and dehumidifiers. Remember that humidity levels quoted in weather reports indicate outdoor humidity—they have little bearing on your home’s humidity level.