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Community and Q&A

Open-cell or closed-cell SPF in 2×4 walls?

sxs3S4bLty | Posted in Energy Efficiency and Durability on

I am building a house (or rather my builder is, I am a semi-well read novice, not an expert at all) in Climate zone 4 in the western piedmont (near the foothills) of NC. We have relatively hot periods in the summer (several weeks of 90 deg +), and we have several weeks of below freezing temps in the winter. This is definitely a “Mixed” climate.

I have looked at all sorts of wall configurations, but due to time, money, and the fact that things progressed construction-wise while I was learning, I am now stuck with working within a 2×4 wall. I am familiar with dense pack cellulose, and like it (I am the Mkt. Mgr. for Insulweb(R), fabric used to blow insulation into walls), but I want more than the cavity R-value cellulose can reach in a 2×4 wall. I also want a super-tight house.

I’ve been looking at spray foam and read many articles for or against closed cell or open cell in my climate region. I would prefer closed cell for the high r-value per inch in my skinny walls. The “con” articles say the OSB sheathing could rot during hot humid summers b/c it won’t be able to dry to the inside. It just doesn’t seem that our summers here in this part of NC are humid enough long enough to not allow the OSB to periodically dry to the outside.

Right now, our plan is to have a inside-to-outside wall configuration of 1/2″ drywall+1/2″ air gap+3″ closed cell foam+7/16″ osb+Tyvek(R) house wrap+3/4″ air gap+4″ brick. Where there are studs, obviously they would replace the foam and the interior air gap.

I feel like the osb should be able to dry to the outside if damp (admittedly slower and more periodic occasionally in the summer), and the studs will dry to the inside.

Does anyone see a potential miosture problem? There should be no condensation insude the wall due the the dew point being inside the ccSPF, right? Would open cell foam not stay wet for a period of tim if wetted and have to dry out itself?

OSB is not very permiable itself, and it seems that if it soaks through and water gets deep in the wall, it will have a hard time getting out regardless of the insulation. With closed cell, you only have to worry about the osb, right?

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  1. GBA Editor
    Martin Holladay | | #1

    Relax. Your wall will be fine. The air gap between the Tyvek and the brick veneer will provide a way for the OSB to dry if it ever gets wet.

    If it's not too late, you might consider providing bracing with an alternative means (for example, steel L-profile diagonal bracing) and sheathing your house with rigid foam instead of OSB. Rigid foam is a great sheathing to use behind brick veneer.

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