New unvented attic and high humidity
Last year after a severe hailstorm the attic insulation and the entire roof deck of my 1600 sq. ft. ranch style home in Fort Calhoun, NE was replaced. With the replacement we upgraded the insulation to Open Cell Icynene spray foam on roof decking. The spray foam is at least 6-8 inches on the roof, and covers the sidewalls, soffits, and attached garage ceiling. This completely closes in the attic to the thermal envelope of the house.
I’ve been monitoring the relative humidity levels of the house for the last year. The attic continues to have relative humidity levels between 55-72% while the mainfloor and basement of the home is between 35-55%. Yesterday June 27th the outside temperature at 3:00pm was 90 and the outside humidity was 59%. The attic temperature at the peak of the attic gable was 85 and the relative humidity was 72%. I could also see tiny water droplets on the spray foam. On the floor of the attic the relative humidity was 64%. On the mainfloor of house the temperature was 78 and the relative humidity was 45%. The basement was 72 and the relative humidity was also 45%.
My concern is why the humidity is so high in the attic compared to the rest of the home? Also that moisture may be wicking from the attic to the outside or vice versa.
Other information that may be useful:
-The roof is completely exposed to the sun. No shade on the house
-The roof deck is 5/8″ plywood with ziptape on the seems of the plywood
-The shingles are 30 year Landmark shingles with standard underlayment
-When humidity is high in the attic, dark patches appear on the outside shingles
-I’ve run a commercial exhaust fan from the attic to the outside a couple times which drops to relative humidity to 55% in 1 – 2 hours, but with-in a day or two the relative humidity levels are back to their peak.
-There are no airducts in the attic, and the bathrooms of the home vent outside
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