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Best insulation method for LP Smartside panels as sheathing & siding?

Eric Mikkelsen | Posted in General Questions on

Building woodshop in cold climate…zone 6-7. Using LP Smart side panels as sheathing and the siding. 190 series, product is .53″ thick.

Concerned about condensation on interior side of this sheathing/siding in cold weather. Will dense pack cellulose be fine, or should this siding/sheathing be first sprayed with 2lb closed cell foam?

2×6 wall construction. I’m thinking I’ll rip strips of 2″ foam board to apply over interior side of studs to increase wall depth to 7.5″ & to decouple thermal transfer from the studs. With wet sprayed cellulose, the R-value of the wall would be about R-28.5

Cost is an issue, but…don’t want condensation issues either. To spray or not to spray a layer of the 2lb foam is the question. Thoughts?

If I should spray the foam, would 2″ suffice? That would boost the wall cavity to about R-34 so I would get that benefit, but again, trying to hit that good cost/benefit balance.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Eric,
    You don't have to install spray foam if you don't want to. Although the exterior wall panels will get damp in February, they will dry in April. For more information, see How Risky Is Cold OSB Wall Sheathing?

    In Climate Zone 7, if you decide to follow the "flash-and-fill" approach -- a variation of "flash-and-batt" -- then the closed-cell spray foam needs to have a minimum R-value of about R-15 for a 2x6 wall, according to most building codes, although you could cheat a little. To acheive R-15 with closed-cell spray foam, you need about 2.5 inches of closed-cell spray foam. For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. Eric Mikkelsen | | #2

    Thanks Martin;
    - since this sheathing is also the siding and will be painted, does that present any unusual situations? Should it be back primed before installing cellulose?

  3. GBA Editor
    Martin Holladay | | #3

    Eric,
    I don't think there are any special concerns arising from the fact that the panels are used as both sheathing and siding (other than the fact that air sealing is compromised). Of course, you should follow the installation instructions published by LP for their SmartSide panels.

  4. Steve Knapp CZ 3A Georgia | | #4

    Eric. You might want to install the spray foam against a felt or house wrap backer. See this article for more info: https://www.greenbuildingadvisor.com/articles/dept/musings/insulating-walls-old-house-no-sheathing

    If you install the foam against the LP sheathing/siding, it will be difficult to replace any panels that may become damaged.

  5. Eric Mikkelsen | | #5

    Does this wall assembly need a vapor barrier such as tyvek? I'm planning on calking the seams of the 4x8 panels to help air seal the walls. I don't see any reason for the tyvek but am I missing something?

  6. Jon R | | #6

    A water-resistive barrier is required. Maybe DrainWrap for better moisture performance.

  7. GBA Editor
    Martin Holladay | | #7

    Eric,
    Q. "Does this wall assembly need a vapor barrier such as Tyvek?"

    A. Building codes require a water-resistive barrier (like Tyvek) behind the siding. In your case, the barrier would go between the studs and the Smartside panels. But this WRB is not a vapor barrier. WRBs (including Tyvek) are vapor-permeable. You don't want a vapor barrier in this location.

    If you live in Climates Zone 5, 6, 7, or 8, most building codes require a vapor retarder (a less stringent barrier than a vapor barrier) on the interior side of the wall assembly. This requirement can be satisfied with a layer of vapor retarder paint. For more information on this issue, see Vapor Retarders and Vapor Barriers.

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