Condensation on interior surfaces of windows and doors occurs because of high humidity and insufficient air exchange inside the home. In many older homes there were gaps in the windows where drafts could be felt and air would flow. This exchange of air, in many cases, was sufficient enough to prevent condensation from forming. The high-performance windows of today are designed to be air tight to reduce heat loss. This reduces air flow windows. Again, this is not a window defect.
Condensation on exterior surfaces of windows and doors occurs because of the increased efficiencies in today’s windows. On clear nights with still, humid air, condensation occurs when moisture-laden air comes in contact with a glass surface that is below the dew-point temperature (“dew-point” is the temperature at which the air will no longer hold its’ moisture vapor. Cold air holds less moisture vapor than warm air). In high-performance windows with LoE and argon gas, the outside glass surface will actually be colder than a similar, “regular” window without these features. This is because the high-performance window is reducing heat flow to the outside and preventing the warming of the exterior surface above dew-point. This is not a window defect; like dew forming on grass and car-hoods, it is a natural phenomena.
Other factors that influence condensation are:
• Window Size – Larger windows may have a higher tendency to show condensation.
• Window Location – Windows protected from the wind will have a higher tendency to show condensation.
• Screens – Windows protected by exterior screens may have different condensation than the same windows without screens under the same conditions.
• Air Circulation – Good air circulation, such as exposure to wind, reduces the occurrence of condensation. Building projections, foliage and other wind-breaks may contribute to condensation.
• Interior Shades – Opening interior shades or blinds may reduce condensation by allowing more heat to transfer to the outside.
• Minor differences in conditions can cause condensation to form on one window and not on another — even when they are side-by-side.
• Finally, condensation on windows can be a seasonal or a night-time event. When outside temperatures are warm, the glass temperature will usually be above dew-point.
• The same is true during cold, winter weather. Condensation will most often occur during transition months.