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Encapsulated attic – dehumidification – GreenBuildingAdvisor
My house (Climate zone 4A) has a small attic (200sf), which is encapsulated with 4″ of polyiso above the roof deck, mineral wool batts in the rafter space and an intello smart membrane below the rafters. The attic floor is uninsulated and air sealed with spray-applied foam. The attic is currently unvented and unconditioned. However, I have 2 bathrooms directly below the attic. They are properly vented through the roof. Despite the air sealing, I want to ensure I am controlling humidity in the attic space to prevent any mold issues. I have a hygrometer in the attic now. Humidity in the attic consistently bumps up 5% when the showers are being used (humidity currently hovers between 45-50% with a mean temperature of 73 degrees). Better air sealing may help eliminate vapor transfer to the attic, but that is never perfect.
I am considering installing a dehumidifier in the attic, similar to one of Allison’s recommendations (article linked below), but have no real way of draining the unit. I could direct the condensate through the exterior wall or could tie it into a vent stack (with trap and float switch). Accessing an existing sanitary line would be a challenging and arduous task. Any recommendations for draining dehumidifiers in an encapsulated attic?