Window Condensation – Moisture Problems & Energy Conservation

I like to go to Waffle House. Over the past few years, it seems, I’ve gone so often that I even order my breakfast in their special code: “Triple scramble wheat plate,” which translates to three scrambled eggs, whole wheat toast, and hash browns.

On our recent trip to the South Carolina coast to conduct certification field tests for some of our new home energy raters, Chris and I stopped at Waffle house for breakfast one day. As we walked up to the entrance, I noticed that the windows were dripping with condensation on the outside. It was a warm, humid morning so seeing condensation wasn’t too surprising.

When we opened the door and walked in, we discovered immediately what was going on. Their air conditioner thermostat was evidently not doing its job because the temperature inside felt like it was about 60 degrees F.

So, the air inside was cold. The single pane of glass in the metal framed windows got very cold. Outside, the air was full of moisture and had a dew point above 70 degrees F. Let’s say it was 72. When that outside air finds any surface with a temperate at or below 72 degrees F, moisture in the air condenses on that surface, which is exactly what you can see in this video we took. Notice how much water had already run down the windows and onto the concrete.

Normally, indoor temperatures should be maintained at 75 degrees F. If this Waffle House had been that warm, an oudoor dew point of 72 degrees would have resulted in no window condensation.

That wasn’t good enough for Charleston the next day, however. After lunch (not at Waffle House this time) we walked outside and saw condensation on the outside of the window at about 1 pm. The indoor temperature probably was close to 75 in that restaurant, but the dew point in Charleston that day was nearly 80 degrees!

From a building science perspective, there are a couple of problems with that Waffle House window condensation. First, all that condensation can get into the building materials and create moisture problems. In this Waffle House, it was just dripping down the aluminum and onto the concrete, so it wasn’t a problem here, but get enough of it onto siding or into the wood framing or fiberglass insulation, and you’ve got moisture problems in the materials.

The other problem here, of course, is one of energy conservation. They’re keeping the place too cold, so they’re increasing the heat gain. The rate of heat gain (measured in Btu per hour) depends on the temperature difference, so the higher that is, the more heat you gain – and then have to pump out with your air conditioner.