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High Humidity in Unvented Attic

schulerz | Posted in GBA Pro Help on

Last year after a severe hailstorm the attic insulation and the entire roof deck of my 1600 sqft 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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  1. GBA Editor
    Martin Holladay | | #1

    You need to extend a supply-air duct from your forced-air system to the attic, and to install one or more supply-air registers up there so that your attic is air-conditioned. You may also need a return-air grille. For more information on this issue, see Open-Cell Spray Foam and Damp Roof Sheathing.

    Six inches of open-cell spray foam provides only about R-22, which isn't much. Most building codes require roof insulation to be at least R-38 or R-49. For more information on this issue, see It’s OK to Skimp On Insulation, Icynene Says.

    1. Carhus | | #4

      Would vapor diffusion ports at the roof ridge work in this scenario? Dr. Joe Lstiburek created the concept. Wondering if that would work here with specifically with open cell foam?

      I am in a similar bind. I have open cell foam. A small unvented attic (super humid) with the HVAC (leaky) and the rest of the house is a cathedral ceiling. I am seeing condensation creeping out of the side of the cathedral ceiling near the ridge where timbers meet a small gap in the sheetrock. I am working on installing the supply register and a fan at the interior ridge which is vented to the outside in the small attic. My fear is that it isn't enough. Just bought this house and the open cell foam is giving me grief.

      Maybe Joe Lstiburek's vapor diffusion vents would be a good solution?!?


      1. walta100 | | #5

        Carhus I think you need to condition the attic air so it is more or less the same temp and humidity as the rest of the air in your house and you are unlikely to have moisture problem.

        Did they remove all the old insulation from the attic floor?

        What is the R value of your roof insulation?


  2. charlie_sullivan | | #2

    I suspect that what is going on is that the hot, humid air leads to condensation in the foam at night, and then during the heat of the day, the heat drives that moisture out into the attic again. If you take the humidity out with A/C as Martin suggests, or with a dehumidifier, that would help the immediate problem. The questions become 1) is there a source of moisture that makes that dehumidification effort expensive, and 2) will the wet roof problem get worse in the winter when you have the roof cold all the time, rather than just a night.

  3. schulerz | | #3

    Thanks for the advice I reached out to Icynene and I'll keep these solutions handy during the discussion. On Wednesday night it was real good at 75 degees and 45% humidity. I checked it today and couldn' believe it. Outside here in Fort Calhoun, NE it's currently sunny, 76 degrees, and 48% relative humidity. I had the windows open and on the main floor of my house it's 78 degrees and 51% humidity. In my attic right now it's 83 degrees and 89% humidity. It feels like a rainforest up there and there are visible water droplets on the underside of the Icynene sprayfoam. I shut the windows and turned the air conditioning back on. I have the commercial exhaust fan in the attic that is pushing the humid air from attic outside into the open garage. It's dropping the humidity quickly, but I was stunned at how high it was.

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