Replies

1. 
   charlie_sullivan | Aug 11, 2015 09:55pm | #1

   My initial answer is that this would not be a good design for the reason you suggest. But you would get better answers with your climate zone or zip code included.

2. 
   decumanusgreen | Aug 11, 2015 10:41pm | #2

   Climate zone 5.

3. GBA Editor
   Martin Holladay | Aug 12, 2015 04:54am | #3

   Joseph,
   Your wall assembly will work -- but don't expect much inward drying through 2 inches of polyiso.

   For more information on this type of wall, see Walls With Interior Rigid Foam.

4. 
   decumanusgreen | Aug 12, 2015 06:51am | #4

   Thank you Charlie and Martin.
   I am not expecting any drying through the 2" of poly-iso. My thinking is that the back of the sheathing will be protected from outward vapor pressure and will dry to the exterior if the weather gets in. Any condensation from outward vapor pressure, it seems to me, would happen on the studs because of thermal bridging--so the interior edges of the studs could fall below the dew point and cause problems. But maybe that doesn't really happen. It seems to me that what I am doing is not all that different from a flash and batt technique, or a so-called hybrid wall assembly. I have enough impermeable and air tight insulation (I am taping and foaming all seams of the poly-iso) to the exterior to push the dew point out of the cavity at the sheathing and fill the rest with lower cost cellulose. Again, my concern is cold spots on the framing members that project into the cavity past the poly-iso.

5. 
   Reid Baldwin | Aug 12, 2015 06:56am | #5

   I don't think Joseph is talking about putting the polyiso on the interior side of the wall. Instead, he is talking about putting it on the interior side of the sheathing. If I understand correctly, it will not be continuous insulation. It will only be in the stud cavities, so each cavity has 2" of polyiso and 7" of cellulose.

6. GBA Editor
   Martin Holladay | Aug 12, 2015 08:31am | #6

   Joseph,
   Sorry -- I misunderstood your question. I now realize that you are talking about using the "cut-and-cobble" method. (I think.) For more information on this technique, see Cut-and-Cobble Insulation.

   In your climate zone (Zone 5), the method you are contemplating (a combination of exterior rigid foam and air-permeable insulation in walls) will only work if the R-value of the rigid foam makes up at least 27% of the total R-value of the wall. (If you're unaware of this, you might want to read Calculating the Minimum Thickness of Rigid Foam Sheathing.)

   Assuming that your stud depth is a true 9 inches, then your R-11 polyiso represents about 30% of the R-value of your R-37 (nominal) wall -- so you are OK on the issue of moisture accumulation.

   Of course, cut-and-cobble has its own disadvantages, which you can learn about when you read the article on the cut-and-cobble method. In all cases, a continuous layer of exterior rigid foam is preferable to thin strips of rigid foam inserted between your studs.

7. 
   charlie_sullivan | Aug 12, 2015 10:22am | #7

   What are your studs like? Just 1.5" x 9" solid wood, or engineered wood I beams? Or a double-stud wall?

   As Martin says, a continuous layer of insulation is better. That's both because it avoids the degradation of R-value, and because the thermal bridging will mean that the studs poking through the polyiso will be colder than the polyiso and could possibly have condensation problems. I don't think that's terribly risky, because with the membrane on the inside, you won't have much moisture going in, and any moisture on the studs can diffuse through them and out to the exterior. So it might be OK, but it's non-standard and not 100% bulletproof. Sometimes modeling with the "WUFI" simulation program can give insight on a non-standard configuration like this, but WUFI only does 1D, whereas what you want to understand is the 2D effect of having the studs bridging the polyiso.

   If you do a double-stud wall, an option is to put some kind of air barrier/vapor retarder in the middle. That could be your polyiso, or OSB, or a membrane of some kind. You then have to fill the inside and outside with cellulose, which sounds like a pain, but in my experience, non-expert insulation crews have trouble with deep cavities anyway, and filling two shallow cavities can be easier. And you could then skip the membrane on the inside.

   If you are doing engineered I-beams, you might consider putting the polyiso a little towards the middle, so that it fits in where the I-beam is thin, and it's not thermally bridged as much.

8. 
   decumanusgreen | Aug 12, 2015 11:04am | #8

   Yes, "cut and cobble" is what I had in mind. I apologize for my lack of clarity. I will check out the article by that title.

   I have an existing wall cavity with true 2x4 construction. The 2" poly is cut and cobbled (taped and foamed) within the bay. My plan was to build out with 2x4's from that. So really I only have a 7.5" cavity giving me even more margin for error on the percentage of insulation outside my moisture/air barrier (r-11 outside in a nominal total of r-32). My further question is: Am I better off running the 2x4 furring perpendicular with the existing studs and minimizing thermal bridging, or am I safer laminating them to the existing studs to allow the thermal bridging to keep the studs warm up to the poly-iso?

9. 
   charlie_sullivan | Aug 12, 2015 11:56am | #9

   So the cut and cobble is already done? That narrows the options a bit. But how about getting a layer of 1" polyiso (or EPS), putting that over the existing wall (interior side), and then adding the 2x4s for the additional thickness spaced out 1" from the existing studs by the extra continuous layer of polyiso? That layer could be your air barrier as well as solving the thermal bridging problem. If you don't want the extra thickness that gives you could go to 2x3s for the interior studs. And no need for the interior membrane.