Replies

1. 
   Jon_R | Aug 28, 2021 03:53pm | #1

   > want to choose an exterior insulation

   Note that Zip-R isn't exterior insulation - - the OSB portion of the Zip will be cold and so more subject to moisture problems than if it were warm.

   Higher exterior perms are helpful - so consider taped plywood (if sheathing is being replaced), WRB, unfaced EPS or GPS. Furring strips can be attached through to studs.

   In any case, make sure the final result is compliant with the IRC here:

   https://www.continuousinsulation.org/content/2021-ibc-and-irc-adopt-improved-vapor-retarder-requirements

1. 
   JC72 | Sep 15, 2021 07:55am | #14

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2. 
   walta100 | Aug 28, 2021 04:46pm | #2

   Jon in most homes I would agree Zip+R is interior insulation but not if like in this case the poster will have a layer of sheeting inboard of the Zip+R makes it exterior insulation.

   “I’m Intrigued by Zip R12.6, because the existing sheathing is mostly 1/2″ fiberboard.”

   Walta

1. Expert Member
   MALCOLM TAYLOR | Aug 28, 2021 11:13pm | #3

   Walta,

   I'm not following you. if you install Zip-R you have a layer of sheathing outside the rigid insulation. What difference does it make that there is another layer somewhere else in the wall?

1. 
   walta100 | Aug 29, 2021 03:17pm | #7

   The way I see it if the original poster keeps the existing sheathing and adds Zip+R on the outside of the existing sheathing he will need to do a dew point calculation to prevent rot.

   To me his wall looks more like this drawing with the strapping being replaced with the Zips OSB.

   https://d4c5gb8slvq7w.cloudfront.net/eyJlZGl0cyI6eyJyZXNpemUiOnsid2lkdGgiOjg1MH19LCJidWNrZXQiOiJncmVlbmJ1aWxkaW5nYWR2aXNvci5zMy50YXVudG9uY2xvdWQuY29tIiwia2V5IjoiYXBwXC91cGxvYWRzXC8yMDE4XC8wOFwvMDgwNDM0NTBcL1JpZ2lkIGZvYW0gd2FsbCBzaGVhdGhpbmcgLSBGSEIgLSBjcm9wcGVkLW1haW4tNzAweDg1OC5qcGcifQ==

   There is a risk that the inner sheathing becomes a condensing surface and become wet over time.

   Walta

1. Expert Member
   MALCOLM TAYLOR | Aug 29, 2021 03:24pm | #8

   Walta,

   Ah - got it. So the protection of both layes of sheathing need to be thought through.

2. 
   Jon_R | Aug 29, 2021 04:10pm | #9

   > strapping being replaced with the Zips OSB

   There is a fundamental difference in wetting and drying between intermittent and high perm exterior strapping and continuous, low perm exterior sheathing (like the OSB portion of the Zip in this case).

3. 
   jwolfe1 | Aug 28, 2021 11:31pm | #4

   Before going too far make sure obtaining Zip R12 is even remotely possible in your area with the timeline you have in mind. I’d also check the price.

4. 
   Green_Geek | Aug 29, 2021 11:46am | #5

   To clarify, I plan to add a layer of insulation over the exiting sheathing. I was looking into GPS as exterior insulation, but wonder if Zip R12.6 might offer a better solution if it is not significantly impacted by the cold. My main question is whether or not Zip R12.6 would perform well in winter climates.

   I figured that Zip might cost less in labor to install. The current sheathing doesn't have any WRB.

   Thanks again for sharing your thoughts.

1. Expert Member
   MALCOLM TAYLOR | Aug 29, 2021 12:08pm | #6

   Green Geek,

   Jon has given you good advice. The concerns you should address before deciding to go ahead are:
   - Making sure the sheathing on your Zip-R has a path to dry.
   - Making sure the wall assembly is more permeable on the outside than the interior so that drying exceeds wetting.
   - Ensuring it conforms to the code requirements he linked to.

5. 
   johngfc | Aug 31, 2021 12:21am | #10

   To get to the question of insulation - yes, the polyiso will still insulate at cold temperatures, but likely not at the full R6 value. See: https://www.greenbuildingadvisor.com/article/cold-weather-performance-of-polyisocyanurate
   The second graph in that review shows that at 0 deg F, the R varied from ~4.5 t0 6.1 per inch for polyiso from a variety of sources. And keep in mind that only the outside 1/2"? or 1"? will be at the coldest temperatures - there will be a temperature gradient. So yes, using insulated Zip should dramatically increase insulation from what you have.

6. 
   Green_Geek | Aug 31, 2021 04:52pm | #11

   Thanks everyone.

   I called Huber and asked what zones Zip R12.6 could be used in, and they said it can be installed in climates as cold as Zone 7.

   I also asked about polyiso thermal drift, and they said it was neutral compared to other foams; given that polyiso offers the highest R valuer per half inch.

   Thanks again,

1. Expert Member
   NICK KEENAN | Aug 31, 2021 07:20pm | #12

   The key things to keep in mind are: 1. While polyiso loses insulating power at colder temperatures, it still keeps some. 2. Only the outside of the polyiso is at the outdoor temperature, the rest is somewhere between the outdoor and indoor temperatures and doesn't suffer as much loss in insulating power.

   You do have to heed what the other posters are saying about vapor management. Usually houses in cold climates are built with a vapor barrier on the interior side of the exterior walls, and are vapor-open on the exterior side. Any vapor that gets into the wall can dry to the exterior side, and the heat flow from warm to cold through the wall will tend to drive moisture in that direction.

   Polyiso foam has very low vapor permeance, so you'll be creating a wall that has vapor barriers on both sides (assuming your existing walls have vapor barriers on the interior). This can lead to mold and rot.

1. 
   Green_Geek | Sep 14, 2021 08:56pm | #13

   Thanks again everyone.

   At this point I'm planning to go with 2" of GPS foam. Should have Net Perms of 0.72 with factoring for cavity insulation, framing, sheathing, WRB, 2" GPS foam board, and siding.

   Best,

1. 
   charlie_sullivan | Sep 15, 2021 08:40am | #16

   That sounds great. Good decision.

2. 
   876543 | Jul 15, 2022 04:15pm | #17

   I'm curious to know which direction you ultimately took, and if you used a local contractor? I also live in the Des Moines metro, and am considering an exterior insulation upgrade in the near future.

   John

7. 
   JC72 | Sep 15, 2021 08:03am | #15

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