Not long ago, Erik Henson an energy auditor with HD Engineering and Design in the Kansas City area, posted the photo below on LinkedIn. As you can see, frost has formed on the walls and ceiling inside the house. It’s not uncommon to see frost on windows in cold climates. But walls? That’s a rarity. What’s going on here?
When I shared the photo and asked how many people had seen something like this before, almost no one had. Here are the numbers that help describe what was happening:
Outdoor temperature: -5° F
Indoor temperature: 66° F
Indoor relative humidity: 49%
At those conditions, the dew point was 46° F. As you can tell from the photo, that part of the walls and ceiling were below 32° F, well below the dew point for the indoor air.
Two things are going on here:
- The building enclosure wasn’t doing its job.
- The relative humidity was too high.
First, let’s look at the building enclosure problem. The owners took the frost photo, and Erik went to the house a week later to check things out.
The photo below shows the attic right above where the frosty corner was. The insulation wasn’t missing when he arrived. He pulled it back so he could see what was going underneath. As you can see, the framing and the electrical wire prevented the insulation from making good contact with the air barrier. Also, it’s close to a soffit vent, and it was windy when the frost formed.
The other factor is the high humidity in the home. In the winter time, it’s better to keep it lower, closer to 30% than 50%, to prevent moisture problems. 49% RH isn’t horrible…unless it’s really cold outside and your insulation doesn’t work. The reason their humidity was so high was that they have 8 kids and kept the humidifier going 24/7.
Moisture problems more often happen inside walls in cold climates because the interior drywall is typically too warm to pull the water vapor out of the air. The humid, indoor air has to find the exterior sheathing to find a cold enough surface to get into the materials.
If you ever doubted what the building science crowd has been saying about proper installation of insulation, let this home be a warning for you.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.
Photos used with permission from Erik Henson.
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