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Community and Q&A

New unvented attic and high humidity

schulerz | Posted in General Questions on

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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  1. Expert Member
    Dana Dorsett | | #1

    The sun is driving accumulated moisture out of the roof decking into the attic during the day, then overnight when the radiational cooling takes the roof temp below the outdoor temp much of that moisture is re-adsorbed into the roof deck and the exterior levels in the foam.

    Exhausting attic air to the outdoors draws humid summertime air into the conditioned space- it's not really a solution.

    A better solution would be these two steps:

    1: Add a modest amount of air-conditioning to the attic space from a central air system which would replace the humid attic air with much drier air. Short of a new duct, actively ventilating / recirculating attic air with the fully conditioned space below would still do it.

    2: Add an air-tight layer of MemBrain smart vapor retarder to the inside of the open cell foam, which limits the amount of re-adsorption of the humidity back into the roof deck at night, and the humidity cycling would subside over days/weeks. (8' x 100' sheets are about $100 online at Menards, if you can't find it elsewhere.)

    One or the other will work on it's own, but both would work much better. To avoid high wintertime moisture accumulation in the roof deck the MemBrain part of the solution is highly advised no matter what. Code requires a class-II vapor retarder if you are using only open cell foam on the roof deck but that is often ignored.

    At only 6-8" of open cell foam it's woefully shy of the R49 code-min for US climate zone 5 (all of NE) under IRC 2012. Even a solid 8" is only R30-ish. If you have rafters rather than trusses there may be easier/cheaper ways to beef that up while fixing the humidity cycling problem.

  2. user-4053553 | | #2

    Interesting answer Dana, i would never have thought of that (it makes perfect sense though), but would the membrain work because the high humidity would allow it to reabsorb

  3. GBA Editor
    Martin Holladay | | #3

    This is the second time you posted this question. If you want to read the answer I wrote the first time you posted the question, you can find it here: Nebraska — unvented attic with high humidity.

  4. schulerz | | #4

    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.

  5. Expert Member
    Dana Dorsett | | #5

    You have a ton of moisture to purge, and you may even have a roof leak(?).

    The MemBrain would start letting the moisture out as soon as the air between the MemBrain & foam goes high-RH. If there is literally zero air exchange between the attic and conditioned space it will take a very long time to bring the cycling under control, but as long as you don't have a roof leak, it eventually would. But with the vapor retarder in place it would not accumulate a huge slug of moisture over the winter. Even a tiny amount of ventilation with conditioned space air would do quite a bit.

    Exhaust venting the space into the garage brings in some mix of conditioned space air and outdoor air into the attic, depending on where the leaks are. That represents a huge air conditioning load, and is not necessarily the best way to manage this. A large room dehumidifier would probably remove more humidity per kwh of power used.

  6. Brian Knight | | #6

    Great advice. Lots of sources for high interior humidity. Are you on a vented crawl or poorly detailed unvented crawl? Do you run your bath fans like you should? Does your vent hood exhaust outdoors? Have you ever had a blower door test done?

  7. schulerz | | #7

    I should have written that a bit better in that I ran the exhaust fan and opened the rest of the windows first until the humidity dropped to 60% RH in the attic. Then I closed the windows and turned on the air. I also measure attic temperature and humidity at the peak of the roof line. Which is about 8.5 feet from the floor of the attic. The attic is a very large space because it includes above the garage, so the it's probably 2,200 sq ft. The garage is insulated from the attic by 6 inches of spray foam in the ceiling. The other strange thing the more extreme the weather the better the attic RH. If it's 95 degrees and 70% RH outside, the attic will be 84 degrees at the peak and 62% RH. The attic currently has no ventilation to the rest of the house or anywhere else. We have bathroom exhaust vents that all vent outside.

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