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“Thermoply” applications ?

user-1137156 | Posted in Plans Review on

“Thermo-ply” is an interesting material, less than 1/8″ thick and sold as a structural sheathing that can replace OSB as an air barrier etc. I’m considering it as my main air barrier in my thick walls, as a web in Larsen like trusses, under my roof trusses both to support the cellulose above and as the air barrier over a service cavity’ and as “tie” plates and around window & door openings for my double wall. My walls will have the primary air barrier on the outer face of an inner 2×4 wall with 2×4 top and bottom plates and an outer wall made of Larsen like trusses for studs. The inner wall is “structural” and sits on the foundation while the outer wall is “proud” of the foundation and is cantilevered over a conventional masonry brick exterior of the foundation. The “Larsen like trusses” consist of 2 ea. 2x2s spaced 3 1/2 ” apart and glued & stapled , on one side, to a strip of Thermoply that is 7″ wide the space between the 2×2’s is filled with a 1 1/2″ x 3 1/2″ strip of mineral wool. The two walls are tied to each other by strips of Thermoply at their tops and bottoms, for the inner wall this is an extra top and bottom plate, however for the outer wall it is the only top and bottom plate. I plan on using a 24″ OC staggered layout. The construction sequence is, one segment at a time: frame the inner wall, add the Thermoply sheathing layer, tape the seams, cut the openings add the top and bottom tie plates and “rims” around the openings, tape the sheathing to the tie plates and “rims”, construct the outer wall nailing the pre-built insulated trusses to the tie plates, add the outer 7″ of mineral wool between the truss-studs. The wall segments are erected sequentially with caulking and sealing to the previously erected segments. Once the wall, or any of it’s segments is vertical additional nailing/stapling of the outer wall trusses to the Thermoply can be done if needed. Once erected the exteror UN-SHEATHED mineral wool filled walls will be covered by a moisture permeable “housewrap” then DC14 drainage mat XPS, then either “Hardieboard” or Ambrico EZ wall panels and attached thin brick. What do YOU think?

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  1. Expert Member
    Dana Dorsett | | #1

    For structural applications it has to be nailed 3" o.c. to the framing, both perimeter& field, which is a LOT of nailing!


    I'm not convinced it's sufficiently structural for use as truss gussets or tying the tops of double studwalls together. It's pretty bendy stuff compared to half-inch OSB- take a peek at the vidi at about the 49 second mark. You'd probably need to install 2x framing every 16-24" between the top plates as nailers for the Thermo-Ply to have sufficient strength, in which case the Thermo-Ply would simply be redundant (or an air-barrier.)

    Also at <0.6 perms it's a Class-II vapor retarder, so it's important to pay attention to where it is in the wall stackup for dew point control.

  2. Expert Member
    ARMANDO COBO | | #2

    IMO, anyone using ThermoPly for structural sheathing is asking for trouble. See this report, ThermoPly plus crappie construction equals massive failures. See APA documents about TX building failures from their website. Document downloads are free if you register.

  3. user-1137156 | | #3

    Thank you for the quick responses! I really am not convinced for the tie plates,d window&door perimeters plywood is "right" for these. Also for the main wall sheathing, on the outer face of the inner 2x4 wall, the vapor control and added rigidity makes OSB a better choice. But for the ceiling air barrier I do believe the advantage goes to the Thermoply. Leaving the "webs" for modified Larsen trusses. The primary function of this web is to transfer vertical loading on the outer truss chord to the inner truss chord by "racking" the "panel". The panel is 3 1/2" wide and 9'6" tall and 0.113" thick. Such racking produces a shear stress in the vertical cross sectional area of the panel. That vertical area is 0.113" x 114"= 12.9 square inches! A 100 pound load transfer results in a stress of under 8 PSI.

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