Replies

1. GBA Editor
   Martin Holladay | Sep 15, 2010 12:29pm | #1

   Chris,
   If you go with your proposed R-7.5 exterior foam, your OSB is at risk of accumulating moisture.

   According to Table N1102.5.1 in the 2007 Supplement to the International Residential Code, the minimum R-value of exterior wall foam in Climate Zone 6 is R-11.25 for 2x6 walls. If you plan to use XPS, that means you need foam that is at least 2.25 inches thick, not 1.5 inch.

   The question of whether to use fiberglass batts is up to you. If the batts are very carefully installed, and if you have a six-sided air barrier in each stud bay, convection currents are unlikely. However, most studies confirm that dense-packed cellulose insulation performs better than fiberglass batts.

2. 
   J Chesnut | Sep 15, 2010 01:51pm | #2

   I'm a designer in Minnesota. I have experience going through a hygro-thermal analysis of an assembly with insulation in the stud bay combined with outboard rigid insulation. I liked how the building scientist we worked with framed the discussion in terms of high and low risk. Your proposed assembly would be less risky the more you add to the outboard insulation (as Martin describes) and if you empty the stud bays of insulation.
   In a hygrothermal analysis you assume some rain water infiltration as well as interior vapor getting into the assembly through air exfiltration (vapor diffusion is now considered less an issue). Your rigid insulation restricts drying potential to the outside. The batt fill in the studs to a lesser extend restricts drying potential also because it restricts convection. If the stud cavities are empty there is greater drying potential to the inside (but you loose the R-value).
   Using OSB is higher risk because it is more suspectable to mold growth. Using plywood would be less risky.
   I agree with Martin that dense packed cellulose is a better alternative to fiberglass batts to restrict air exfiltration (among other reasons) although this is no substitute for a well defined continuous air barrier layer.
   I forget, is the Kraft-facing meant to serve as a vapor retarder? If so you don't want a vapor retarder to sandwich your assembly between the vapor retarding effect of the outboard rigid insulation (don't know the perm rating of XPS per inch off the top of my head).
   I would continue to use the vapor barrier in your ceiling assembly but would add that you should again define what the air barrier is in this condition and make sure it is continuous with the air barrier of your wall assembly. (often a properly detailed drywall installation serves as the air barrier).

3. 
   qcCTLJrGgz | Sep 15, 2010 02:50pm | #3

   Thanks to both of you for your comments, much appreciated.

   Just need a clarification on what Martin said about code minimums for exterior insulated sheathing. The way I interpret the code as it's written is that if you have a 2X6 stud wall with exterior insulated sheathing with an R-value of at least 11.25, then you are allowed to eliminate the class I or II vapor retarder required at the warm in winter side of the wall, and instead are allowed to use a class III vapor retarder on the warm in winter side (which would be latex or enamel based paint on the drywall). This would then imply, in my situation, where the R-value of the insulating sheathing is less than 11.25, I need a Class I or Class II vapor retarder on the warm side of the wall. Kraft-faced batts qualify as Class II vapor retarders (perm rating between 0.1 <= 1.0), and this is why I chose them. While this might not be the best idea (due to what Chesnut said above), it doesn't mean that it's not allowed by code, but it's, in fact, required by the code for my situation. This might be wrong, but that's how I read it. Please clarify if this is incorrect.

   There is also some confusion here about vapor retarders and vapor barriers. A vapor retarder, according to ASTM E 1745, is a plastic material with a permanence rating of 1 perm or less, and allows for some vapor transport. A vapor barrier on the other hand is a plastic material having a permanence of 0.01 perms or less (per ASTM). Vapor barriers are typically only installed below slabs, and are rarely installed in wall and ceiling assemblies due to their very limited potential for drying.

   Exterior XPS has a permanence rating of 1.1 for 1" thick, and decreases with increase thickness. None the less, 1.5" XPS would qualify as a vapor barrier.

   All this being said, I don't want to increase the thickness of the XPS because it would make the installation of siding and trim difficult and would probably require adding furring strips for nailing (thus breaking the continuous blanket of insulation at the exterior of the house and eliminating some of the effectiveness of the insulation, not to mention add more cost to the project).

   Would I be better served to forgo the exterior insulation and batts and use a spray foam in just the 2X6 cavities that would give my an equivalent R-value?

   Thanks,
   Chris

4. GBA Editor
   Martin Holladay | Sep 15, 2010 03:49pm | #4

   Chris,
   1. You're off by a factor of 10 on your definition of a vapor barrier. A vapor barrier (also known as a Class I vapor retarder) has a permeance rating of 0.1 perm or less, not 0.01 perm or less.

   2. Concerning the code question: you're right. As Joe Lstiburek says, the code doesn't prevent people from designing bad wall assemblies. However, Lstiburek has been lobbying to change the code so that it at least allows good wall assemblies.

   It's legal to build a wall with foam sheathing on the outside and a low-permeance vapor retarder on the interior. Legal, but not wise.

   If you choose foam sheathing, you want your interior layers to be high permeance -- something that (fortunately) Table N1102.5.1 allows. But as you point out, Table N1102.5.1 does outlaw stupidity or bad design.

5. 
   J Chesnut | Sep 15, 2010 04:30pm | #5

   Chris,
   Sounds like you've done your research. I can understand the difficulties of increasing the thickness of exterior rigid insulation. I decided not to do it while retrofitting my own home because of cost and detailing complications although in new construction there is more opportunity to properly work out the details cost effectively. You can layer rigid insulation so that any nailing strips you may need are installed over the first continuous layer of rigid insulation.
   Just wanted to point out again that much of the risk surrounds your sheathing choice OSB. It takes more to start mold growth or damage to the solid wood studs. Upgrading to plywood or using a Densglass structurally rated gypsum sheathing should do much to reduce risk.
   If you go with spray foam the lower density open cell only provides about as much R-value as cellulose. Close cell spray foam (~ R 5.5 per inch?) will get you close to your proposed assembly but then you have thermal bridging through your framing reducing the effective overall R-value. Closed cell spray foam cures hard so the installer won't completely fill the stud cavity because it would make installing the drywall very difficult so don't assume 5.5" when calculating R-value. Its also expensive, is a product whose blowing agent has global warming implications (as does XPS), and when applied in thicknesses over 2" you need to be sure to hire a competent installer to avoid curing problems.

6. 
   qcCTLJrGgz | Sep 15, 2010 05:12pm | #6

   Chesnut,

   Thanks for your comments, I think I'll take your advice and use plywood instead of OSB. That way I still have the strong pullout strength of wood to fasten all my siding and exterior finishes to.

   Martin,
   As I said, this is ASTM (the American Society for Testing and Materials, ever heard of them) definition of a vapor barrier, not a "vapor barrier/Class I vapor retarder" you refer to.

7. 
   farfigngn | Dec 10, 2010 12:54am | #7

   I dont know if it is too late to continue this discussion, but I am in a similar situation doing a renovation currently and I think the contractor might have created a potential problem. The exterior of the house was just finished. The contractor first put tyvek over the old existing plywood sheathing, then installed 1" xps foam board, then put vinyl siding over the foam. Am I going to have moisture problems now becase of the tyvek? We still havent put insulation in the walls yet, but had it was to be R-13 fiberglass faced in the 2x4 bays. Is that also a mistake? Finally, the contractor proposed as an alternative to the fiberglass to then spray foam the interior side of the sheathing with closed cell foam half inch and fill the rest of the cavity with blown in insulation. Does that sound even worse with the closed cell foam creating a barrier between itself and the xps with the sheathing between it, oh and I forgot the tyvek too? Any ideas before I close up the walls? Thanks.

8. GBA Editor
   Martin Holladay | Dec 10, 2010 04:18am | #8

   Far Fig NGN,
   1. As usual, I have to remind questioners that it is often hard to answer your question if you don't provide a location or climate.

   2. Whenever you install exterior foam insulation, you have to be sure it is thick enough. To find out whether your contractor chose thick enough insulation, we need to know your climate. Find out more here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

   3. There's nothing wrong with the Tyvek; your contractor did the right thing by installing Tyvek over plywood. Tyvek is vapor-permeable, so it doesn't trap moisture.

   4. Installing spay foam on the interior of your plywood is probably a bad idea; you don't want to create a foam sandwich with plywood in between, or the plywood won't be able to dry out if it ever gets wet. The best insulation to use in your stud bays is probably dense-packed cellulose, although carefully installed fiberglass batts will also work.