I recently investigated an attic with spray foam insulation where we observed an interesting humidity pattern. We placed data loggers near the ridge and floor of the attic as well as in the living space and outdoors.
The graph at below shows dew point data for the four locations. The really interesting part is the big difference in dew point between the highest and lowest points in the attic, shown by the red and green curves in the graph.
Let’s take a look at the data for the ridge and floor in separate graphs now. The next graph shows the attic floor, and this time I’ve included the relative humidity. As you can see, the humidity, as measured both by dew point and relative humidity (RH), goes up during the day as it gets hotter outdoors and down at night when it cools off. But the humidity near the attic floor never gets out of control. The dew point and RH stay below 70° F and 70% at their peak (the peak of humidity, not the peak height in the attic).
In contrast, the humidity near the ridge in the attic gets very high. The dew point and RH get up close to 90°F and 90%. That’s way too high. On the bad days, when it’s sunny outdoors, the RH stays above 70% for about eight hours per day and above 80% for more than four hours. Not good!
Why is this happening?
This has been a topic of conversation in Joe Lstiburek’s backyard at Building Science Summer Camp for the past three years. In 2014, Foster Lyons, Joe, and I talked a lot about this. Foster, in fact, kept pressing for better explanations and is the one responsible for Joe’s latest article, Ping Pong Water and The Chemical…