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Exterior spray foam insulation over OSB?

toology | Posted in Energy Efficiency and Durability on

Getting ready to building a home and couldn’t find much info on this except in commercial applications. Had a meeting with the insulation contractor (in business since 1976) and explained to him that I wanted an airtight home with good insulation, I mentioned ZIP-R with blown in cellulose or loose fill fiberglass.

He recommended instead I use OSB sheathing and he could spray the exterior with 2″ closed cell spray foam after the brick ties were on for a completely air tight and water tight home, then use blow-in spider plus fiberglass or blown-in mineral wool (owens corning insul-fill I believe) in the cavities since he has seen cellulose settle first hand years after application. He said I could also skip house wrap by doing this.

He showed me pics of the exterior closed foam on commercial applications they’ve done. Would this be okay since the inside wall should be able to dry in case of a water getting in the cavity? I want to make sure there are no issues with the wall not being able to dry and causing wood rot issues down the road.

Thanks in advance.

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Replies

  1. toology | | #1

    BTW - I'm in Atlanta, which is climate zone 3

  2. user-2310254 | | #2

    Raj,

    There is no requirement for exterior foam in Zone 3, and I would imagine that much spray foam would be quite expensive. What is your objective? Passive Haus? Earthcraft Platinum? Pretty Good House?

  3. toology | | #3

    I'm not looking for any certs, but I and trying to achieve the best price / performance when it comes to having both an airtight, water tight home and very comfortable living space. The reason I was looking at zip-r or closed cell on outside is I wouldn't have to worry about all the thermal bridging with corners, studs and headers. I don't particularly like spray foam on the inside walls due to flammability. Why I was looking at rockwool or cellulose.

    What would you recommend instead in zone 3?

    Thank you

  4. user-2310254 | | #4

    Raj,

    I live in Atlanta and can tell you that exterior foam on a two-story house is a PITA. You will be much better off aiming for a Pretty Good House level of construction. Search GBA using this phrase and you will find lots of information. With good air sealing and advanced framing, you can limit thermal bridging without getting too exotic.

    On the exterior spray foam... That sounds like a risky idea for our region since it’s likely the OSB will have accumulated a lot of moisture during a typical nine to 12 month build process.

  5. GBA Editor
    Martin Holladay | | #5

    Roger,
    I disagree with Steve Knapp. From a moisture performance perspective, as well as an R-value perspective and an airtightness perspective, the exterior spray foam is an excellent way to proceed.

    There really are no disadvantages other than (a) cost and (b) the high global warming potential of the blowing agents used for the spray foam (a problem that can be avoided by using the new Lapolla spray foam or one of the similar competitors).

  6. toology | | #6

    Thank you both for your feedback.

    Steve, per your recommendation I searched Pretty Good House I came across a few articles by Allison Bailes at Energy Vanguard whom I'll probably use now for HVAC design.

    Martin, I appreciate the guidance and have enjoyed reading GBA for the past 5+ years!

  7. JC72 | | #7

    Roger,

    Both Steve and Martin are correct. Atlanta is located in Koppen climate zone humid-subtropical. As you're aware humidity levels are high 9 months of the year so your build will absorb a lot of moisture which you're going to have to deal with via dehumidification regardless of whether you use ZIP-R or exterior ccSPF.

    May I suggest that you reach out to Carl Seville. IIRC he built a LEED house in Decatur using ZIP-R and just installed a dehumidifier.

    https://www.greenbuildingadvisor.com/blogs/dept/green-building-curmudgeon/topping-out

  8. user-2310254 | | #8

    Raj,

    Energy Vanguard and Carl Seville are two great local resources. You also might want to talk to Luis Imery. He is a PHIUS certified builder and very detail oriented.

    Last, I think your "pretty good house" search came up a little short. (Google seems to do a better job indexing the site and returning results.) See this link and additional links in the sidebar: https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/pretty-good-house

  9. toology | | #9

    Yes, I plan on having a dehumidifier. I'll reach out to Carl for sure.

    I've heard of Luis. Also did read the GBA article as well :-)

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