Jason Advani thought he was on the right track when he insulated the bottom of the roof deck in his walk-up attic with closed-cell spray foam. As planned, the R-40 worth of insulation has drastically reduced his heating bill, but now he faces another, unexpected problem.
“Since then, the space has been plagued with very high humidity,” he writes in a Q&A post. “It’s at 60% to 70%, maybe higher. I’m forced to run a dehumidifier up there. I’m spending more money on running the dehumidifier year-round than I previously was to heat the house!”
With outside temperatures ranging between -5°F and 95°F, attic temperatures range between 48°F and 92°F. Only about 10% of the hip roof is shaded, so it gets plenty of direct sunlight.
Advani hopes to finish the attic and install a minisplit, although he’s not sure that will lower the humidity.
Most of the house is heated and cooled with a conventional gas-fired furnace and air conditioning system. But there is no effective way to circulate the air between the attic and the remainder of the house. Advani says he runs a dehumidifier in the basement during the summer and a whole-house humidifier in the winter.
“What do I do?” he asks. “Bath exhausts do go outside … all that typical stuff has been considered / examined.”
That’s where we start this Q&A Spotlight.
Sealing air leaks would help
Charlie Sullivan suggests that finding and sealing air leaks would improve conditions in the attic.
Air leaks in the lower part of the house — in the basement, for example — allow outside air with low humidity to get into the house. Advani is adding moisture to the air with a humidifier, and that hot air rises, finding ways of getting into…