Sealed Unvented Attic Moisture/Temperature Issues
New self-built home with unvented, conditioned attic space started becoming very humid. I added a dehumidifer and now it is becoming very hot in the space, therefore defeating the purpose of the conditioned unvented attic idea.
Specs:
Coastal North Carolina Zone 3 Warm Humid
Roof: Asphalt shingle gable roof, 6:12 pitch, zip sheathing with taped seams, 2X12 rafters
Vapour Diffusion Port: 1″ slit on each side if ridge entire length, covered by Tyvek housewrap and Cobra Ridge Vent 3
Insulation: R-38 fiberglass batts between roof rafters, sealed foam board at eve/wall line, no insulation in attic floor
Area: 16′ wide x 26′ long = 416 sf
Supply Air: 4″ takeoff directly from main trunk roughly 20 cfm, no return air
Problem:
House is very air tight and energy efficient, not requiring the HVAC to run much. Installed 2 stage HVAC with variable speed air-handler to help dehumidify the space and run with extra cool/dehumidify setting. Added Alorair 70 PPD stand alone crawl space dehumidifier in attic. This is solving the humidity issue (now 43% RH @ 85.6 F) but causing the space to get over 90F on hot days. Does this defeat the purpose of having the HVAC system in the “conditioned” attic space? Considering increased supply to 6″ but also wondering if I should add a room transfer vent. I can hear the attic air pushing through the access doors since it has no where to go. Would that be a fire hazard to create a transfer vent? Also worry about open fiberglass batts blowing loose fibers through the vent. Could I add a furnace filter to the vent or attempt to add mesh netting over the fiberglass batts?
Thank you for your insights
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Replies
Unvented dark colored shingles applied directly to sheathing without an air gap can transfer a lot of heat. And you don’t have exterior insulation to buffer it. In addition to bumping up the supply rate, can you also add a return grill? Might promote more airflow especially if you can place it in an opposite corner of the attic. Have you measured the supply airflow to ensure it is really supplying 20 CFM?
To my ear your attic does not sound like a conditioned space as a conditioned space would be more or less the same temp and humidity as the rest of your home.
Note a dehumidifier is a super-efficient space heater in disguise. In fact, can make 2-4 times more heat than a space heater.
At best a conditioned attic is only marginally better than the dumb idea of a vented attic full of HVAC equipment. This is because of the fact that the sloped roof has a much larger area losing heat.
It seems you are committed to the conditioned attic part of that commitment is buying and operating the equipment necessary to keep the attic at more or less the same temp and humidity as the rest of your home.
Walta
Do you really need to run the dehumidifier to bring the attic RH down to 43%?
What temperature will the attic stay at if you only bring the humidity down to 50%?
Where in the attic are you measuring your humidity?