Replies

1. GBA Editor
   Martin Holladay | Dec 21, 2015 09:35am | #1

   Greg,
   Unfortunately, you should have asked your question a little earlier in the process -- before you installed your siding.

   Rigid foam can help improve the performance of your wall, but the rigid foam is best installed on the exterior side of the wall sheathing. Now it's too late to do it the right way, because (if I understand correctly) you have already installed your siding.

   The method being suggested by your friend is called the "cut-and-cobble" method. It is definitely an inferior method when compared to installing rigid foam on the exterior side of the wall sheathing. It's also somewhat unconventional.

   If you want to learn more about the cut-and-cobble method, here is a link to an article on the topic: Cut-and-Cobble Insulation.

2. 
   gregwharrey | Dec 21, 2015 10:14am | #2

   Martin,

   Thank you for your response. Yes I have read about installing the rigid board on the exterior of the wall and realize that I missed an opporunity. With the cut and cobble method should you use craft faced insulation or should it be paperless?

   To reiterate my layers would be:

   1.) House wrap
   2.) Sheathing
   3.) 1/2 inch rigid foam with spray foam sealing the edges
   4.)R-13 Insulation
   5.) Drywall

   Thanks!

   Greg

3. GBA Editor
   Martin Holladay | Dec 21, 2015 10:23am | #3

   Greg,
   Kraft-faced insulation would be preferred, but unfaced insulation will work. If you choose to install unfaced insulation, you probably want to use a vapor-retarder primer when you paint the drywall.

4. 
   gregwharrey | Dec 21, 2015 10:41am | #4

   Thanks Martin.One final question. Can I use a insulating sheathing extruded polystyrene (XPS) insulation board?

5. GBA Editor
   Martin Holladay | Dec 21, 2015 11:34am | #5

   Greg,
   I forgot to mention another problem with your plan: In Climate Zone 5, the rigid foam layer needs to meet a minimum R-value requirement for this type of "rigid foam plus fiberglass batt" approach. For a 2x4 wall, the minimum R-value of the foam layer in your climate zone is R-5. For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

   So 1/2 inch of rigid foam won't work. You will need 1 inch of polyiso, 1 inch of XPS, or 1.5 inch of EPS.

   Green builders try to avoid using XPS, because XPS is manufactured with a blowing agent that has a very high global warming potential.

6. Expert Member
   Dana Dorsett | Dec 21, 2015 04:46pm | #6

   What sort of siding do you have on the exterior?

   In zone 5 if the siding is back-ventilated, such as the air behind vinyl siding or a uniform gap of at least 1/4" between the sheathing and the siding, you don't need or want an interior side vapor barrier. Half-inch foam would not be vapor barrier unless it has a facer on it (even half-inch XPS is about 2 perms without facers, half-inch EPS is over 5 perms sans facer) but it doesn't really buy you anything in thermal performance either.

   Read the details & exceptions in IRC chapter 7 for zone 5 carefully:

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

   Vented cladding (like vinyl siding or any rainscreen type of siding assembly ) buys you plenty of protection in zone 5. It's only a problem if the siding is tight to the housewrap, with no air gap.

   Fiber insulation in contact with the wall sheathing will wick moisture away rather than potentially trapping it there. It's far better than cut'n'cobbled interior side foam. The "right" thing to do with half-inch foam would be to add it to the stud edges only, and compress some mid-density R20s into that now 4" deep space. Or skip the foam entirely and install R15 rock wool or fiberglass batts in there not R13s. To meet current code minimums adding 2" thick foam stud-edge strips and using R20-R23 batts designed for 2x6 framing can work, but that may be awkward for dealing with the window sills and finish trim, etc.

   If you DON'T have vented siding, you'll need an interior side vapor retarder that's tigher than 1-perm. Half-perm paint gets you there, so do kraft facers, but unfaced R15s and a layer of a membrane type smart vapor retarder such as Intello Plus or Certainteed MemBrain would be worth it.

   Some of the box stores are starting to carry the latter in my area, and would likely ship to your local store if ordered online. eg:

   http://www.homedepot.com/p/CertainTeed-MemBrain-100-in-x-50-ft-Air-Barrier-with-Smart-Vapor-Retarder-902018/205920791

   http://www.lowes.com/pd_750463-27-902018_0__?productId=999958593

   Smart vapor retarders are variable permeance, varying by the humidity levels of the air in the stud bays. They slow the wintertime moisture accumulation in the sheathing without slowing the drying rate when the moisture levels in the stud bays reach levels high enough to support mold growth.

   Before installing any insulation, take the time to air-seal the framing to the sheathing inside EVERY stud bay, and any penetrations of the framing for electrical & plumbing with can-foam, including lateral runs between stud bays, not just the top/bottom plates.