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Open-cell foam in unvented attic, Zone 1

Al0629 | Posted in Green Building Techniques on

I am a homeowner and have been doing a lot of reasearch on the topic,  but could use some assistance with regards to insulation options.  

We are building a new home in climate zone 1 (Dallas/ Ft Worth, TX) and my builder is using open cell (icynene) in the walls.  The walls are 2×4.   They are using OSB sheathing, which then gets wrapped in tyvek.  The tyvek will then have a rock or brick aplied for its exterior finish.   Are there any issues with this combination?  Im a little concerened about the use of OSB.   My builder told me that i shouldnt be concearned.  

The bigger issue is….. I enquired due to cost savings about foam in the attic and my builder offers an unvented attic option.   The process includes  spraying open cell icynene to the underside of the roofline (2×4 OSB sheathing to fill the cavity), then “shaving it flush.   When my builder does unvented attics they provide a fresh air supply system.    He said the purpose of the system is to provide fresh air into the home.  due to the tightness of the home the air can become stale.    The roof will have felt paper and asphalt shingles.   No use of venting/cooling for the shingles.  I believe that is called a hot roof if that is important to know.   

My big concern is the use of OSB for the roof sheathing, (i double checked this combination and he will be usimg OSB) from what i understand can absorb moisture/humidity faster than what the open cell foam can disperse it.  (Read about it on this website)   When i questioned him about the humidity that comes from the master bath, laundry room, kitchen, a simulated 180 gallon fish tank, etc… and then the moisture making its way to the OSB,  he said the HVAC system would be able to control that humidity.    I dont know about that.    Your thoughts?   I think I would prefer a dedicated humidty regulator/dehumidifier for that purpose.   But not sure if it is even required.   What is acceptable humidity in an attic with open cell?  

Lastly,  can anyone provide if a whole house fan/Attic fan would be okay in my situation.   I am considering installing a window at the opposite end of the attic from the attic fan.  Im thinking of opening that window in the cooler months and using the whole house celing fan to cool the home.    Again,  a little concerned about the humidity/open cell/OSB combination.  Maybe this is a horrible idea.     

If i choose not to go with the unvented attic option then an attic lid option is called for where open cell icynene is sprayed to the attic floor along with a bunch of blown in cellulose.  And then the attic is made vented by the use of open vents in the gables and soffits.  Radiant barrier roof sheathing is also provided.    My HVAC in either case of course will be located in the attic.   

Thank you for reading and i look forward to the responses.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Al S.

    First, there should be an air space between the sheathing or the brick or stone veneer. There are a number of ways to achieve that gap. You don't necessarily need a product for the brick. Some types of stone veneer are best installed similar to stucco, over a manufactured rainscreen product, like those shown in this article: Rainscreen Products for Stucco Installations.

    You are right to have concerns about the open-cell foam against the roof sheathing. There are is well documented research that point to it being problematic, more so in colder climates though. Some architects and builders regularly spec this detail in climates like yours without problems. Here are two articles I think you may find useful: Open Cell Spray Foam and Damp Roof Sheathing and Designing for a Hot, Humid City.

    1. Al0629 | | #2

      Thanks for your reply Brian. There will be an air gap for the exterior to breath and dry out. I was thinking again about vapor from the interior going thru the drywall, then the open cell and getting trapped in the OSB.

      I really want to pull the trigger on the envented attic but the investment doesnt seem like it is worth the risk. I am also looking at appliances that remove humidity and what optimal humidity should be in an attic.

  2. Expert Member
    Dana Dorsett | | #3

    >"I was thinking again about vapor from the interior going thru the drywall, then the open cell and getting trapped in the OSB."

    That's only an issue when the OSB's average temp over a season is below the dew point of the indoor air (a cold climate problem, not a Texas climate problem.)

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