An important means of reducing window condensation is changing the dew point. This is done by either reducing the relative humidity, or by increasing the thermal performance of the window.
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.
LEGITIMATE CALLBACKS
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.