Volume 8, Issue 8 - September 2007
eye on energy
When Good Windows Look Bad
I shouldn’t have said anything, but I did. “Buy Energy Star™ windows,” I recommended when my mother-in-law asked about replacing the single-glazed ones with storm panels after her husband had died. She had had concerns that the windows were really worth the investment since they would be more efficient than the walls in her 1940s house. I assured her that any improvement in the building envelope was a good thing. Time passed, the windows were installed … and then the dreaded call came. “If these are so good, why do I have water on the window now?”
A Real Delimma
Surprisingly, it was inside the house in the fall. It might have made more sense in winter, but according to John Carmody’s Residential Windows book (now in its third edition), the vinyl-framed, low-E, argon windows should have alleviated the problem. Instead, we had the opposite. “My mother-in-law had condensation on an efficient window, where the problem had never existed before even with the worst window on the market.”
That situation was occurring in a temperate month made it even more puzzling. I went back to thermodynamics 101—hot goes to cold, wet goes to dry. Somehow, she had humid air in the house that was warmer and wetter than the inside surface of the window. I would have had an answer if she lived in the South. The air conditioner was now oversized, so the unit was short cycling and not pulling out humidity. But in upstate New York? Where she didn’t have an air conditioner? At a time when she was leaving the windows open during the day because it was so nice out?
Ah, maybe that was the problem. Maybe the air outside was too humid and there was enough humidity trapped in the house during the day to condensate at night when the temperature dropped. Except, these were efficient windows—there shouldn’t have been much difference in temperature between the air and the window surface. I checked the Weather Channel and the data—the temperature difference between night and day, the dew point and the humidity levels –all indicated that the moisture source originated inside the house.
I probably should have. Several nights later, she called to tell me the other family expert, her contractor nephew, provided the answer. Silly me, I live in Florida, so how was I supposed to know years of weather patterns? Apparently, heavy rains that spring and summer brought had saturated the earth. As drier and cooler fall came, physics took over. Hot goes to cold, wet goes to dry, wicking the moisture right up the empty wooden walls of the 1930s house that had never been air sealed.
Moral of the story? Remember physics is your friend when you give advice to relatives.
Arlene Zavocki Stewart is a member of the Efficient Windows Collaborative and an energy code advocate. She can be reached at firstname.lastname@example.org. Ms. Stewart’s opinions are solely her own and not necessarily those of this magazine.