Should I Replace My ThermoPly Sheathing
My 1990’s house was damaged in the Marshall Fire in Louisville, Colorado (CZ5). As a result I will be replacing windows and re-siding my house. In the process I’m considering other changes to the wall assembly. I’d like to improve energy efficiency and also strengthen my structure as the winds are crazy here and racking is noticeable at times.
From outside to inside my existing wall assembly consists of wood fiberboard siding, ThermoPly Red sheathing, 2×4 with fiberglass batt, plastic film, then drywall. We are planning to re-side with James Hardie. I’m considering replacing the existing 1/8″ ThermoPly sheathing with 1.5″ thick R-6 Zip R-Sheathing panels to add strength, R-value and air tightness. I’ve read that testing of ThermoPly shows it does not hold up to the shear loads advertised by the manufacturer. I’m also considering having the sheathing installer reach in and cut the plastic film from behind the drywall to prevent water getting trapped inside the wall. Then to keep condensation outside the wall cavity, I think I will keep the existing R-13 batt (providing 13+6=19 R value and thermal bridging protection). If I were to blow in high-density cellulose or retrofit foam I’m afraid with the higher R value I would risk condensation in the wall cavity.
Is this a good plan? Anything I should do differently?
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Replies
Anybody? I'm meeting with my contractor tomorrow. Any advice?
Whether it is worth while to replace the sheathing is hard to say but now is definitely the time to do it. If you can get it done for a reasonable cost, the improvement in efficiency and comfort would be worth it.
Dense packing the walls works well if your batts are in very rough shape or there is no other way to air seal. Since you are looking at replacing the sheathing, the simplest is air seal at the sheathing layer by taping the seams. If the batts are in decent shape, I would keep it as is and only replace the sloppy installs. The R value increase between dense pack and R13 batts is not that much once you take into account the thermal bridging of the studs.
With R6 on the exterior, you could skip the interior poly, but I would not go too far out of my way to remove it. R5+2x6 walls with interior poly have been built around me (zone 5) for a long time and it works just fine. The poly was most likely also detailed as an air barrier, which is a good thing. I would focus your efforts instead on getting your window flashing details right and air sealing the sheathing. These two things will make a much bigger difference in the durability of the wall.
Thanks for explaining in such a reasoned way. Like you said, hopefully I can get the Zip R Sheathing for a somewhat reasonable price and find a contractor that knows how to install and tape properly. Seems like the sheathing and window installers will need to work together to remove and re-install my windows as the house is being re-sheathed. We are thinking of installing Gerkin vinyl windows with nailing fins. For the price they seem to have good U values and air infiltration.