Replies

1. GBA Editor
   Martin Holladay | Mar 08, 2019 06:00am | #1

   Ben Ross,
   Here is a link to the article you're looking for: "Calculating the Minimum Thickness of Rigid Foam Sheathing."

   If you're installing rigid foam or Zip R sheathing on the exterior side of a 2x6 wall in Zone 5, the rigid foam layer needs to have a minimum R-value of R-7.5. So your R-6 Zip R sheathing isn't enough. You need to upgrade to the Zip R sheathing that is 2 inches (nominal) thickness, with an R-value rating of R-9.6.

2. 
   BenRoss | Mar 09, 2019 02:58pm | #2

   Thanks Martin this is a great resource!

   Do the same rules apply for using ZIP-R on a vented roof? I saw the section about an unvented cathedral ceiling but wasn't sure about a vented one.

1. GBA Editor
   Martin Holladay | Mar 09, 2019 04:37pm | #4

   Ben Ross,
   You wouldn't generally use Zip-R for a vented ceiling assembly, because a vent channel under the sheathing would render the R-value in the Zip sheathing worthless. (Insulation installed on the exterior side of a ventilation channel is as useful as fiberglass batts hanging on a clothesline in your backyard.)

   Of course, maybe you want to install a ventilation channel on the exterior side of the Zip-R. That would be unusual but possible. You'd still need fluffy insulation between the rafters, and either a mild climate or lots of rigid foam to meet the requirements of the rigid-to-fluffy ratio rules.

3. 
   user-6185887 | Mar 09, 2019 04:16pm | #3

   I like Zip sheeting but I do not like the ZipR for structural reasons in seismic hurricane and tornado zones. When you separate the sheeting from the structure you have a weak wall no matter how many nails you use.

   I say put the sheeting where it belongs and put the foam on the outside.

   Walta

4. 
   Peter Yost | Mar 11, 2019 05:43am | #5

   Hi Ben -

   Last I talked to tech staff at Huber, their ZIP-R is NOT approved for roofs.

   Peter

5. Expert Member
   Peter Engle | Mar 11, 2019 08:03am | #6

   Walta,

   Huber has good engineering data on their Zip-R for use in shear walls. It can be used in seismic and wind braced wall assemblies, so long as you follow their recommendations. Considering their product-specific engineering testing and manufacturing controls, I would be somewhat more comfortable using Zip sheathing products than off-the-shelf OSB from an unknown source.

1. Expert Member
   MALCOLM TAYLOR | Mar 11, 2019 11:14am | #8

   Peter,

   There are specific limits on which seismic or wind zones Zip-R can be used in. I don't know if Oregon falls within them.
   http://www.huberwood.com/assets/user/library/14075-R_e-04-18.pdf

6. Expert Member
   Dana Dorsett | Mar 11, 2019 09:18am | #7

   >"...is that exterior insulation thickness enough to keep the dewpoint outside the exterior wall?"

   That's a meaningless question. The term "dew point" refers to a temperature (the temperature at which the air is 100% saturated, below which dew or fog can form). It is specifically NOT a location, inside, outside, under, or above a building assembly.

   When talking about "dew point control" within in a building assembly it's generally referring to controlling the AVERAGE temperature at a specific layer within the assembly above some presumptive dew point of the indoor air. Ideally the average temperature at the first low vapor permeance layer within the assembly (aka "first condensing surface") going from the interior to the exterior will have an average temperature above the indoor air's dew point.

   The interior wall finish is usually semi permeable, and fiber insulation is VERY permeable, foil faced polyiso IMpermeable. OSB & plywood are semi-impermeable, and susceptible to moisture damage, and when on the "cold in winter" side of the assembly have the potential to accumulate enough moisture to support mold/rot even from vapor diffusion through standard interior latex paint alone. (Air leaks are much worse than vapor diffusion.) With a ZIP-R assembly the polyiso side of the ZIP-R isn't damaged by accumulating condensation, but wetting the cold side of the fiber insulation can create problems beyond lowering it's effectiveness while wet.

   The 2" ZIP-R would provide sufficient dew point control, but usually can't meet the structural requirements. The building would need shear panels or let-in bracing, or interior side structural sheathing to meet those requirements. Putting 1.5" of polyiso on the exterior of standard plywood or OSB sheathing works though.