Replies

1. 
   Tiziano | Apr 25, 2025 12:15pm | #1

   Might have found my answer here: https://www.northernbuilt.pro/winnie-pretty-good-house-working-with-hubers-zip-r-sheathing/

2. 
   TracyS | Apr 25, 2025 05:03pm | #2

   In one of the first meetings with my structural engineer on a new home, they had a strong preference against zip-r. I think because of increased risk to shear resistance if the nailing isn't perfect. Likely, they would have designed in at least 2 steel moment frames if i insisted on zip-r. I am in climate zone 7, with design temperature of -13F, so i would have wanted zip-r12. I'm not an expert on zip-r, but it also seems like there may be challenges when attaching deck ledger boards or shed style roof rafters to walls. For reference, i listed below some of the design load considerations they used.
   It also seems like one of the big benefits to exterior continuous insulation is to keep the interior side of the sheathing warm enough in winter to prevent condensation. But maybe there is not much chance of this with the zip-r's polyiso being bonded to the sheathing.

   C. WIND:
   a. ULTIMATE WIND SPEED: 115 MPH
   b. EXPOSURE: C
   c. RISK CATEGORY: II
   d. INTERNAL PRESSURE: +0.18, -0.18 (ENCLOSED BUILDING)
   D. SEISMIC:
   a. DESIGN CATEGORY B
   b. RISK CATEGORY: II
   c. EARTHQUAKE IMPORTANCE FACTOR: 1.0
   d. SOIL SITE CLASS: D

1. 
   Malcolm_Taylor | Apr 25, 2025 05:33pm | #3

   Tracy,

   What matters with any wall assembly is where the First Condensing Surface of Interest (FCSI) is. That is a surface cold enough for moist interior air to condense on.

   In the most commonly used exterior walls with no foam that is the sheathing. But as long as there is good air-sealing, and an interior vapour-retarder to limit the amount of moisture taken up by the sheathing, the wall can dry to the outside and it isn't an issue.

   When you add exterior foam you limit the drying path to the outside, so you need to keep the sheathing warm enough that moisture doesn't condense on it.

   With Zip-R the foam becomes the FCSI. The sheathing has a good drying path to the exterior, has less moisture load from the interior, and doesn't need to be kept warm.

   The open question for both assemblies is whether you need to use thick enough foam to keep the FCSI warm enough to avoid condensation, or whether (as Tiziano is proposing) you can go with less foam by using appropriate interior vapour-retarders.

   That's the topic of these two blogs:
   https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
   https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness

3. 
   TracyS | Apr 25, 2025 06:42pm | #4

   Malcolm,
   I'm with you on all of this (i've read and reread those post a few times), and think your open question is really the crux of building a safe, efficient wall assembly. And i think the answer to that question is so dependent on your environment and how good is the air barrier and the interior vapor barrier. For where i am building, it's cold and relatively windy, so my priority is to get a tightly air sealed, high r value wall assembly. I probably have a larger margin of error regarding condensation - low humidity year round, minimal/no cooling load - and would be safe with less foam. That said, unless i change my mind (again) before i pull permits, my plan is to do smart vapor barrier, r21 wall cavities, taped zip sheathing, 2 layers of staggered seam 1.5" (recycled) polyiso, 3/4" furring/rainscreen.

1. 
   Malcolm_Taylor | Apr 25, 2025 07:19pm | #5

   Tracy,

   And I'm with you: What you are proposing sounds like a good wall assembly.

4. 
   kyle_r | Apr 25, 2025 09:02pm | #6

   Another option is to go with a 2x4 wall with Zip R9. Similar overall R value of 2x6 wall with Zip R6, but wouldn’t require a vapor retarder beyond painted drywall.

5. 
   Tiziano | Apr 26, 2025 09:05am | #7

   Zone 6 Minnesota code r-value minimums in 2x6 walls are R-21 or R20 + 5 CI.

   And I have a challenge. In my build the north wall has 160 square feet of surface area with 40 square feet of that being windows with U-factors of .46. I know that's terrible, but I need clear glass on that side and Pella's insulated clear air filled windows have that rating with a VLT at 73%. The other three walls have 6 square feet of windows each, with U-factors of .28 (VLT at 57%).

   Given all that, should I just go with simple 2x6 R-21 walls? I mean, am I losing so much insulating on the north wall that it negates whatever I might gain from either CI or Zip-R?

6. 
   TracyS | Apr 26, 2025 10:34am | #8

   I've used this spreadsheet to play around with different parameters. It's not a detailed manual -j and is specific to El Paso County, CO. But it gives a quick way to play with r values and u-factors to get a rough idea of relative heat loss. And how much changing the insulating values affects your overall heat loss.
   https://www.pprbd.org/File/ByAlias/HeatLossCalcXL

7. 
   Tiziano | Apr 26, 2025 07:51pm | #9

   Thanks Tracy!