Replies

1. GBA Editor
   Martin Holladay | Dec 03, 2015 01:18pm | #1

   Edward,
   I had never heard of the siding. I think it is actually spelled "Celect Cellular." It seems to be made of so-called cellular PVC (a type of vinyl).

   You proposed wall assembly will meet code requirements. It could be improved.

   In general, exterior rigid foam does a better job than interior rigid foam, for three reasons:
   1. It does a better job of insulating rim joists and partition intersections.
   2. It has fewer penetrations.
   3. It keeps the sheathing warmer (and therefore dryer).

   So one way to improve the performance of your proposed wall is to move the rigid foam to the exterior.

   The second way you could improve the performance of your proposed wall assembly is to create a deeper rainscreen gap between the siding and the WRB. For more information on rainscreens, see All About Rainscreens.

2. Expert Member
   Dana Dorsett | Dec 03, 2015 01:30pm | #2

   +1 on the exterior foam rather than interior. A 1" you'll have ~R4.2 outside the sheathing, which is more than sufficient dew point control for a zone 4A climate. The IRC doesn't require ANY insulating sheathing to be able to use standard latex paint on wallboard as the interior vapor retarder in chapter 7, and R4.2 would be adequate even for much wetter zone 4C:

   http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm

   It's also easier to install the foam on the exterior than on the interior(!). I takes a few percent more foam (but there will probably bee less scrap), since it covers the rim joists and foundation sill plate, unlike an interior foam approach.

3. 
   iLikeDirt | Dec 03, 2015 01:51pm | #3

   If this is new construction, why not put the EPS on the outside? Easier and more effective.

4. Expert Member
   Dana Dorsett | Dec 03, 2015 02:33pm | #4

   Even easier would be to install 1.5" ZIP-R sheathing, where an inch of polyiso foam is bonded to the interior of the sheathing OSB. It will perform as-well or better than an inch of EPS in a zone 4 climate, and it's fewer steps.

   ZIP has the advantage of the WRB being integral to the sheathing as well- no housewrap required. With ZIP-R it IS worth building out at least a 6mm (1/4" ) rainscreen, especially since the OSB is on the cold side of the assembly.

5. 
   EdSe | Dec 04, 2015 04:10pm | #5

   I have looked up Zip R and though I'm not a fan of OSB it looks like a good solution. I want to install a rain screen using Core A Vent then Celect Cellular siding. Do I need any type of vapor barrier over interior insulation and studs before installing drywall?

   Thanks

   Ed Serafin

6. Expert Member
   Dana Dorsett | Dec 04, 2015 04:20pm | #6

   You don't want a vapor retarder of any sort over the interior, and definitely not a true vapor barrier.

   The R-value of 1.5" ZIP-R is MORE than sufficient for preventing wintertime moisture accumulation in the wall cavities in your climate, and a vapor retarder would just impede the ability of the assembly to dry toward the interior. Latex paint on wallboard is good, but avoid vapor barrier finishes such as foil or vinyl wallpaper.

7. 
   EdSe | Dec 04, 2015 04:46pm | #7

   Thanks Much!

   Ed Serafin