One reason homeowners have replacement windows installed is because of condensation. Condensation is also the reason for callbacks after replacement windows have been installed. Windows are not the cause of condensation. It is caused by high humidity.

When the air reaches the dew point, moisture vapor condenses on cold window surfaces. The problem occurs most often when there is a substantial difference between the temperature inside and outside, and when the warm side relative humidity is high.

Windows are typically the coldest surface, especially at the edges. The conduction in this area is the greatest. High indoor humidity causes chronic condensation along the glass edges and leads to mildew, which is a type of mold.

If condensation forms on the glass it is likely forming inside the walls. Condensation is caused by warm, moist air leaking into the walls at points where poor pocket insulation creates a cold surface. Condensation forms in both very cold and very hot weather. Depending on the indoor humidity, when the indoor temperature is over 50ᵒ higher than the outside, condensation can form on the inside edges of insulated glass windows.

In much the same way, condensation can form on the outside surface of windows of a home that is heavily air conditioned. During the winter, ventilation can control humidity. The only solution for condensation problems that lead to mold in the summer is high-performance windows.

Problem Solutions

Examine the home’s humidity conditions. Condensation is nearly inevitable if the relative humidity is more than 50 percent. Keeping the humidity levels as low as possible is one of the best strategies. Selecting the right windows can aid in alleviating the problem.

Window options include switching from single-glazed to double-glazed window units. It is not unusual for condensation to form on single-glazed windows, even when humidity conditions are normal. Insulated glass units often solve the problem.

Conduction reduction at insulated glass unit edges is possible with the investment in windows using warm-edge technologies. The window edges are kept warm, and the chances of condensation forming are reduced. Condensation forming on insulated glass edges indicates excessive levels of indoor moisture.

An insulated glass window, with a krypton or argon fill, provides slightly better insulation and reduces the convection between panels. This option is suggested for kitchens and baths where exhausting humidity is insufficient in preventing condensation formation on glass edges.

Changing the dew point is an important means of condensation reduction. To accomplish the reduction, either the thermal performance of the window has to be increased, or the relative humidity has to be reduced.

Relative humidity is the amount of moisture in the air in relation to the temperature. An indoor relative humidity, which exceeds 65 percent, puts even the best windows at risk. Humidity levels this high have the potential of causing other problems in addition to dripping windows.

Single-glazed windows, exposed to the average outdoor temperature in winter of 30ᵒ, allow condensation when the relative humidity is 30 percent. Thirty percent is an exceptionally low relative humidity. The solution to the problem is to switch to double-glazed windows.

Purchasing a digital hygrometer that measures and records indoor relative humidity provides a low-priced tool that tells a great deal about the indoor environment. Contractors carry them when they want to communicate clearly with customers about the value of double-glazed windows.

When the Window Is the Problem

There are some condensation issues that are the fault of the window. The most obvious are condensation between insulated glass panes. It is caused by a broken seal. Broken seals allow moisture laden air to leak in and condense on the coldest surface inside the insulated glass unit.

Replacing the unit is the only cure. Even if condensation does not occur, the low-E coating, which is usually put on an inside surface, slowly oxidizes. A permanent fog or smudge that cannot be wiped off appears. This condition also warrants a unit replacement.

How Condensation Occurs

Most moisture generated in the home dissipates by moving moisture-laden air out of a home. With increased energy-efficiency, there is a reduction in the number of paths moisture can use to escape. Therefore, dealing with moisture has become more important.

During the winter, indoor humidity levels should be between 30 and 40 percent. When the indoor humidity exceeds 40 percent during the heating season, problems with moisture begin to appear. Condensation forming on surfaces that are cold is a symptom that the level of humidity is high.

The high humidity level is often the result of too much moisture being generated indoors. A modern family can generate 150 pounds, 18 gallons, of moisture per week. Cooking adds 4.5 pounds of moisture; showers contribute a half pound; weekly laundry is responsible for 30 pounds, human occupancy three pounds, and dishwashing 1.2 pounds. The moisture has to escape. Excessive humidity can be reduced by opening a window slightly somewhere in the home.

Nathan Bishop is the owner Go Green Glazing. They are an Australian business based in Geelong that specialises in the retrofit of double glazed windows. His team has over 100 years of combined experience in window repair, replacement & retrofitting.