As Allison Bailes writes in this recent Energy Vanguard article, many homes in hot, humid climates have encapsulated their attics with spray foam insulation to save on A/C costs by bringing the HVAC systems and ductwork inside the conditioned space. But if you don’t do anything to condition the air in the attic, the humidity can get very high. It also stratifies, with the highest relative humidity near the ridge. With closed-cell foam, the humidity gets trapped in the attic; with open-cell foam, the humidity ends up in the roof sheathing.

To find a solution to this problem, Bailes installed a small bathroom exhaust fan in his attic before having it insulated with open-cell foam, along with a HOBO data logger and remote sensor to read and record the attic's temperature and relative humidity over time. In this post, Bailes makes a few interesting observations about the results of his testing.

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