Condensation on window glass is one reason homeowners turn to replacement units. But it can also be one of the reasons for a callback after the windows have been replaced. What starts out as a selling opportunity — visible evidence of energy-wasting windows —can turn around and bite you when the condensation persists after new high-performance units have been installed.
On a hot, humid summer day when the air conditioning is cranked up high, condensation can form on the outside of windows. Since there's no way to regulate outdoor humidity, the best defense against this is installing high-performance windows that include warm-edge technology and gas-filled insulated glass.
It's important from the very start to educate homeowners about the causes of condensation, and not to oversell what the windows can do. Be clear with them that, although cold glass exacerbates the problem, windows do not cause condensation; high humidity does.
CAUSES OF CONDENSATIONMoisture 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.
Typically, the coldest surfaces in a home are on the windows — most often at the edges where conduction is greatest. In extreme cases, when indoor humidity is very high, chronic condensation at the edges of the glass can create a significant moisture problem that leads first to peeling paint, then to mildew (a type of mold), and eventually to rot.
Keep in mind also that if condensation frequently forms on the glass, it's likely also to be forming inside walls. This can happen where warm, moist air leaks into the wall at a point where a pocket of poor insulation creates cold surfaces.
Condensation can form in both very hot and very cold weather. On a cold winter night when the indoor temperature may be more than 50º F higher than outside, condensation on the inside edges of an insulated glass window is possible, depending on indoor humidity.
Similarly, on a hot Florida summer day, condensation may form on the outside surfaces of the windows of a heavily air conditioned home.
In winter, you can do something about the humidity — ventilate. In the hot summer, the solution may only be found in using high-performance windows.
The window on the left has a standard edge spacer — a highly conductive piece of extruded aluminum that quickly lowers the temperature at the edge of the glass and allows the indoor humidity to reach its dew point. The window on the right has “warm-edge technology” — edge spacers that are not as conductive and, therefore, have a lower potential for edge condensation.
The first line of attack should always be to examine the humidity conditions in the home. If the relative humidity is greater than 50%, condensation may be inevitable. Keeping humidity levels as low as possible in very cold weather is the first best strategy, but selecting the right window can help alleviate the problem. Window options include:
The colored lines on the chart above show when condensation will appear under different environmental conditions (indoor relative humidity and outdoor temperature) on the window glass of four types of windows.
The chart at left helps to explain when condensation will occur, depending upon the type of window. For example, look at the left axis labeled “Relative Humidity (%).” Relative humidity is simply the amount of moisture in the air relative to its temperature. The scale starts out from zero at the bottom, and goes all the way up to 100% indoor relative humidity at the top. If the indoor relative humidity is above 65%, the very best window available is still at risk. (A humidity level this high indoors will likely cause other problems besides dripping windows.) However, if we look at the single-pane glass (represented by the red curve on the bottom of the graph) and an average winter outdoor temperature of 30º F, we'll get condensation on the window when the indoor relative humidity is just 30%, which is exceptionally low in many areas. To solve this condensation problem, simply switch to double-pane.
Contractors should carry a digital hygrometer to measure and record indoor relative humidity while they are in customers' homes. These are relatively low-priced tools that can go a long way toward communicating clearly with clients about the indoor environment.
Prices for a digital hygrometer range from about $30 to $300, but a low-cost model such as this one is sufficient for most remodelers. It's primarily an aid for talking to clients about temperature and humidity and explaining the causes of condensation on glass even when it's not visible.
Of course, some callbacks associated with condensation can be blamed on the window.
The most obvious failure is condensation between the panes of glass in an insulated glass unit (IGU) caused by a broken seal. When the seal breaks, moisture-laden air leaks in and condenses on the coldest surface inside the IGU. The only cure is to replace the IGU (or, more commonly, the whole sash unit).
Even if a broken seal does not cause condensation inside the unit, the low-E coating, which is typically put on one of the inside surfaces, will slowly oxidize. This appears as a permanent smudge or fog that can't be wiped off. This, too, warrants replacement of the unit.