October, 2006

Condensation on Windows

Condensation on windows spells trouble—and not just for the windows.

By Steve Easley

 “I have customers who complain about condensation forming on double-pane windows, and they claim the problem is with the windows. Is the window at fault or is there another reason?”

Steve responds:

Windows do not cause condensation; high humidity does. Understanding how condensation forms on windows will inform how we correct the problem.

Causes of condensation.

Moisture vapor in the air tends to condense on cold surfaces in a home where the air reaches its dew point. This is common when there is great difference between inside and outside temperatures and when the relative humidity of the warm side is high. Glass is generally colder in winter due to the fact that windows typically have lower R-values than the rest of the building envelope.

Chronic condensation can create a significant moisture problem that leads first to peeling paint, then to mildew and mold, and eventually to rot. Keep in mind that condensation on windows is not just an inconvenience to the person who is looking through the glass. If condensation is frequently forming on the glass, it’s likely to be forming inside walls where there are pockets of poor insulation (and where air leaks are bringing warm moist air into contact with cold surfaces).

Solutions to the problem.

In winter, don’t over-humidify homes. The first sign of over-humidification is condensation on double-pane windows. Second, ventilate. We always control moisture at the source so make sure customers properly use bathroom and kitchen exhaust fans to remove excess moisture. If the relative humidity is greater than 50% under cold conditions, condensation may be inevitable. The first line of attack should always be to examine the humidity conditions in the home. Keeping humidity levels as low as possible in very cold weather is the first, best strategy, but it is not the only cure. It is also important to select high performance windows. These windows are better insulators and have warmer surfaces that are less likely to create conditions for condensation.

Window options.

Advising customers to switch from a double-pane to a double-pane low-e window will often solve the problem. It’s not uncommon to see condensation on single-pane windows, even in normal humidity conditions.

Investing in a window that uses warm-edge technologies to reduce conduction at the edges of insulated glass units also makes sense. These technologies keep the window edges warm and reduce the chances for condensation to form. Windows with an argon or krypton gas fill provide a little better insulation value and reduce convection between the panels. These gasses further reduce the potential for condensation.

I always recommend windows with a minimum NFRC label U factor of .4 or lower (see www.nfrc.org). U factor is a rating of how much heat a window conducts. Some manufacturers are including a condensation resistance factor. This is a rating between 1 and 100. The higher the number the better the window resists condensation.