By Jason Hammond
Have you ever seen condensation on your windows in the summer time? I’ve recently noticed that the windows on the south side of our house have developed condensation on them, early in the morning. Since we made such a large investment in windows, and in particular low-E, triple pane glass, I was concerned that there was something drastically wrong. I know that interior condensation occurs in the winter time and usually is a sign that the cold air is making it’s way into your home, through your windows. When the cold air meets with the warm interior air it results in a condensation on the interior surface of the glass. Besides being a sign of poorly performing windows this condensation can also result in rot and mold issue inside your home. However, the condensation that I’m seeing on my windows is happening on the exterior not the interior, so it got me to wondering why?
After a few web searches, I came across a series of similar explanations to my window condensation issue. The explanation I found to be most clearly stated, was on the website of RLC Engineering, LLC. from Craig DeWitt, PhD, PE, who writes:
“…condensation occurs when a surface falls below the dew point temperature of the air. The outside glass in an energy efficient window will be closer to the outside temperature, and the inside glass will be closer to the inside temperature. The glass in an inefficient window will be more heavily influenced by both inside and outside temperatures. A Low-E coating (that reduces radiant heat transfer) will tend to warm the inside surface in the winter, and the outside surface in summer sunshine. A Low-E coating will also lower the outside surface temperature at night in the summer. Therefore, a low-E coating will reduce the potential for winter condensation, while creating more potential for summer condensation situations (especially if the inside thermostat is set near or below the outside dew point temperature.)”
Based on the information stated above, I’m sure that the condensation issue in our house is a combination of our highly efficient, low-E coated windows in conjunction with the fact that our house envelope is incredibly energy efficient maintaining a more consistent temperature, therefore magnifying the difference between the changes in inside air and outside air temperature. Although the issue was mildly concerning when I first notice it, I guess that it’s a reassuring sign that my windows are doing their job, and apparently doing it quite well.
You can see the slightly frosted appearance of our widows, as condensation has gathered on the outside of the glass.
In this shot, you can actually see some of the dew running down the face of the window exterior. The condensation usually disappears shortly after the sun rises and the exterior temperature begins to rise.