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Is my wall design OK?

user-959871 | Posted in General Questions on

I am in zone 3A, Fort Smith, AR.
I understand that for this zone, I do not need any foam board insulation on the exterior of my wall sheathing to prevent condensation on the interior of the sheathing.

But I am fairly close to zone 4. Should I add some foam just to be sure ?
I am on a limited budget and would rather not if it’s definitely not necessary.

The wall will be (from the outside inward):
Vinyl siding, Advantech Zip panels for the sheathing (joints taped with Zip tape), 6″ Roxul batt, drywall, latex paint.

Also, Advantech says I don’t need felt over the Zip panels (neither for wall panels nor roofing panels), but should I add a layer of felt between the vinyl siding and the zip wall sheathing anyway, or will the looseness of the vinyl on the sheathing be enough for a drain plane.

Finally, I’m concerned that the denser resin in the Zip panels, along with the joints being taped, will make the sheathing nearly as much a vapor barrier as poly sheeting, thereby limiting any wall drying to the interior only. Will this be OK ?

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  1. Mike Eliason | | #1

    I don't understand the utilization of vinyl siding.

  2. user-869687 | | #2


    Your climate is pretty warm, and the use of exterior foam to prevent condensation is primarily a cold climate strategy. However, a layer of continuous exterior insulation would lower your operating cost. If you plan to run air conditioning, then your walls will dry toward the interior, especially when it's hot and humid outdoors (the indoor air will be much drier than outside). In this case the low perm rating of Zip panels would be helpful. Definitely avoid vinyl wallpaper or other impermeable interior wall finishes.

    I would recommend adding felt over the zip panels to ensure longevity, because there's no way to know how long the zip tape will hold. Maybe a decade of humid summers would lift it off the panels, but shingle lapped felt should shed water for the life of the wall.

  3. GBA Editor
    Martin Holladay | | #3

    Your wall design is OK. Not great, but OK.

    If you can scrape together the money for a layer of foam sheathing, your house will perform better and your energy bills will be lower. If energy prices double or triple in the future, you won't regret the investment.

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