It’s been muggy in Atlanta this week. We’ve had dew point temperatures in the low to mid 70s Fahrenheit. I live in a 60 year old house, so the combination of high humidity and air conditioning can mean condensation. Currently we have the original windows plus the storm windows that someone added in the decades after the house was built. With the humidity and heat we’ve had this week, I’ve seen condensation on storm windows and on the main windows behind them.
Condensation on storm windows
I took the lead photo above first thing in the morning earlier this week. The outdoor temperature was in the low 70s Fahrenheit. The relative humidity was about 100%, which means the dew point and the dry bulb* temperatures were about the same. The condensation was on the outside of the storm window because that was the condensing surface nearest to the saturated air.
We keep our thermostat at 74° F, and the indoor temperature in the living room, where those windows are, was probably a bit lower in the morning, maybe 73° F. That made the temperature inside about the same as the outdoor dry bulb temperature and the dew point. And that gave the storm windows the extra little push they needed to condense water vapor out of the air.
Condensation on main windows
So, in the morning I had condensation on the outer surface of the storm window. When I came home from the office later that day, I took the photo below. There’s still condensation on the window, but now it’s in a different place. It’s on the outer surface of the main window, not the storm window now.
Let’s make this easier by numbering the surfaces 1 through 4, with 1 being the surface I can touch standing inside the house and 4 being the surface I can touch standing outside the house. The morning condensation occurred on surface 4. The evening condensation occurred on surface 2.
Why did the condensing surface move? In the morning, everything was at about the same temperature and the water vapor was outside. In the evening, the outdoor temperature was about 90° F. That would keep surface 4 too warm to be a condensing surface when the dew point is in the 70s Fahrenheit. But the storm windows aren’t airtight so there’s plenty of water vapor between the panes. With our thermostat at 74° F, and outdoor dew points about the same, now surface 2 becomes the condensing surface.
Condensation on storm windows, and behind them on the inner panes of glass, can happen in summer time in humid climates. It’s a regular occurrence in really humid places. When I checked the dew point map of the US yesterday, the highest dew point I saw was in Iowa. It was 84° F. With a dew point that high, even double pane, low-e windows would be susceptible to condensation.
How’re your windows doing?
Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog and is writing a book. You can follow him on Twitter at @EnergyVanguard.
* Dry bulb temperature is what we usually just call temperature.
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