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Community and Q&A

Zip vs. Thermo-Ply in retrofit wall assembly

nhbean | Posted in Energy Efficiency and Durability on

I am a homeowner renovating a 100-year old foursquare in Manhattan KS (mixed-humid, zone 4). We need to remove our wall sheathing to address some dry rot issues, and are taking the opportunity to improve the envelope details and work closer to a pretty-good house.

The walls are 2×4 balloon framing with interior lathe-and-plaster and 1×12 exterior board sheathing. The cavities are insulated with blown cellulose. After repairing the rot, our plan is to add the following layers on top of the existing board sheathing:

1 – ½ Zip system panels with taped seams
2 – Two staggered layers of reclaimed 1” Polyiso (foil-faced) with taped seams
3 – A ¾” vented rainscreen gap created from 1×4 furring strips screwed to the studs
4 – Boral TruExterior beveled siding

However, on this forum I recently saw the Ox Thermo-Ply product, and am curious if it would make sense to swap the Zip panels for Thermo-Ply Green panels. This would reduce the overall weight and cost of our assembly considerably.

It looked like the primary complaint leveled at Thermo-Ply is related to using it in a structural manner, but as we will retain our existing board sheathing any shear loads should already be addressed. Also, it has a lower permeability than Zip, but with two layers of foil-faced polyiso, we’re already limited to drying to the interior.

Or is there some additional benefit from using Zip on top of the existing board sheathing that I am missing? Would it handle high winds better (we are in tornado country)?

My thanks for any advice and thoughts you can offer!


Nathan Bean

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  1. GBA Editor
    Martin Holladay | | #1

    There is no doubt that Zip sheathing (or any other brand of OSB you care to mention) can handle high winds better than Thermo-Ply. (Thermo-Ply is foil-faced cardboard.)

    That said, there may be no reason for you to worry about high wind loads -- especially if your existing board sheathing is well nailed. If you are worried about tornadoes, you should talk to an engineer.

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