Replies

1. 
   dcdunn | Jul 29, 2016 04:02pm | #1

   I need a total R 27 R 23 cavity, R 4 continuous exterior to meet the requirements of my res-check , so a wall structure as follows meets this criteria and seems to give me protection from the dew point in my dry Northern Utah Zone 6 climate.

   2"x6" wall
   Roxul ComfortBatt R 23 in cavity
   MemBrain, over interior cavity
   OSB Exterior with Tyvek homewrap over (maybe?)
   Roxul ComfortBoard 80 R 4
   Stucco

   This gives me the overall R 27 I need with no dew point problems because the wall readily dries each direction, the Roxul ComfortBoard performs as insulation only, not trapping moisture. In fact the Roxul ComfortBoard will actually act as a rain screen.

   Maybe I'm looking at it wrong, but all the foam plastic rigid insulation seems to cause more problems than their worth.

2. GBA Editor
   Martin Holladay | Jul 29, 2016 04:17pm | #2

   David,
   To answer the original question: If you want to add rigid foam to the exterior of a 2x6 wall in Climate Zone 6, the rigid foam needs to have a minimum R-value of R-11.25 -- so your two layers of rigid foam (R-1.9 + R-10 = R-11.9) works just fine.

   This type of wall assembly does not require an interior vapor retarder, so skip the MemBrain and skip the interior polyethylene. The rigid foam layers already constitute a vapor barrier.

   For answers to all of your questions about this type of wall assembly, you should read this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

3. GBA Editor
   Martin Holladay | Jul 29, 2016 04:33pm | #3

   David,
   To answer the comments you posted in Comment #1:

   Q. "All the foam plastic rigid insulation seems to cause more problems than they're worth."

   A. What problems are you worried about? If you are worried about the global warming potential of the blowing agents used to make XPS, you could switch from XPS to EPS or polyiso.

   If you want to install a continuous layer of mineral wool insulation on the exterior side of your sheathing, you can. Your proposed details will work. You should know, however, that (1) Roxul ComfortBoard may be difficult to purchase or expensive in some locations, and (2) Roxul ComfortBoard is trickier to install than rigid foam because it is squishier than rigid foam.

   For more information on wall design, including a discussion of using of a continuous layer of exterior mineral wool, see How to Design a Wall.

4. Expert Member
   Dana Dorsett | Jul 29, 2016 04:34pm | #4

   From a dew point control perspective with R23 batts you need R11.25-R12 on the exterior.

   At 25F average temp through the Type-I EPS R-tech (mid winter that would be about right) it would deliver about 4.3/inch, so the exterior half-inch would give you R2.15

   Fully depleted of it's HFC blowing agent the2" of Type-II XPS only delivers R8.4 @ 75F average temp, but would be good for about R9 with an average temp through the layer of 40F, or R9.4 at 25F (it's not that cold in UT- the outdoor temps would have to average in negative digits to be that cold.)

   Add it up and you're at about R11.15 - R11.5 wintertime performance over the long haul with that approach which is probably good enough, but MemBrain would be cheap insurance. It's a safer bet under stucco than the rock wool approach, and higher performance too.

   With 1" of rigid rock wool and interior MemBrain you'd be fine in winter, but you might run into issues with the high shoulder season & summer moisture drives of stucco cladding with an exterior that permeable. Rather than Tyvek you may need to go with double-layered asphalted roofing felt, or use a different type of siding. You might get away with it in high-dry Utah, but you need something to limit the peak vapor drives from sun on dew-wetted stucco.