Replies

1. 
   user-958765 | Jul 26, 2011 12:06am | #1

   I should add, the home is located in NW Iowa.

2. 
   davidmeiland | Jul 26, 2011 02:54am | #2

   I think the recommendation is going to be to use thicker exterior foam. Other options would be to use plywood sheathing instead of OSB, felt instead of housewrap, and possibly a larger rainscreen gap than will be provided by DrainWrap (I have used and like Obdyke Homeslicker).

   Notice how easy it is for me to spend your money....

3. 
   davidmeiland | Jul 26, 2011 02:57am | #3

   Here is Martin's article on foam thickness... https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing
   ...and there are several other pieces relevant to foam in his collection that you should check out.

4. GBA Editor
   Martin Holladay | Jul 26, 2011 07:37am | #4

   Brad,
   Assuming you are in Climate Zone 5, you need at least R-7.5 foam. Your detail shows R-3.5 foam.

   Polyiso is more environmentally friendly than XPS. I would suggest using 1.5-in. polyiso (about R-9). Of course, 2-inch polyiso would be better.

   I think you need a proper rainscreen air gap between your siding and the foam. Use 1x4 vertical furring strips to create the rainscreen gap.

   What's that "vapor barrier" shown on the plan under the drywall? Not polyethylene, I hope. I guess you could use MemBrain, but nothing would be better.

   And I'm always suspicious of any architect who draws plans that don't show the location of the air barrier. Where is it located? How will you detail it?

5. 
   user-958765 | Jul 26, 2011 09:34am | #5

   Martin-

   The air barrier is the TYvek drain wrap applied to the insulation board behind the cement board siding. James Hardie recommends and calls out for Tyvek drain wrap when a "weather barrier" is used on the exterior surface. Why not polyrthylene?

6. GBA Editor
   Martin Holladay | Jul 26, 2011 09:45am | #6

   Brad,
   I don't recommend the use of housewrap as an air barrier. Because of rips and fastener penetrations, it's hard to make such an air barrier work. For your house, you would be better off creating the air barrier at the sheathing layer (I assume you have OSB or plywood under the foam). This can be done by sealing the plywood seams with high-quality tape.

   Another alternative is to use the Airtight Drywall Approach.

   For an explanation of why polyethylene vapor barriers are rarely used anymore except in the colder regions of Canada, I suggest you read these articles:

   Vapor Retarders and Vapor Barriers

   Forget Vapor Diffusion — Stop the Air Leaks!

7. 
   user-958765 | Jul 27, 2011 12:15am | #7

   Martin- thanks for the quick response I greatly appreciate it. I've spent a few hours researching the articles mentioned to include others , pretty educational! I’m located on the edge of zone 5 &6. Below I believe are my options. If you could please take a look at them and let me know your thoughts; you can refer to my previous details attachment. Is there a huge concern on the top 8” of the foundation wall not being insulated on the exterior? See attached picture)

   BASEMENT OPTION 1:
   1” xps (Already in place running from footing up to 8” below top of foundation), 8” concrete wall with moisture barrier spray applied (already in place), 1” xps insulation board with all seams taped (thinking of getting the boards that are channeled and gluing them to the foundation wall to allow any moisture to drain down to footing drain tile), 3 ½” fg bib insulation, ½” gwb (no vapor barrier/poly). R value R-20.

   BASEMENT OPTION 2:
   1” xps (Already in place running from footing up to 8” below top of foundation), 8” concrete wall with moisture barrier spray applied (already in place), 3” closed cell spray foam, ½” gwb (no vapor barrier/poly). R value R-20.

   ABOVE GRADE WOOD FRAME WALL OPTION 1: (wall cavity internal dry)
   Cement board siding, tyvek drain wrap, R-11.25 rigid insulation board (all seams taped), plywood or osb sheathing, 5 ½ fg bib insulation, ½” gwb (no vapor barrier/poly). R value R-34.25.

   ABOVE GRADE WOOD FRAME WALL OPTION 2: (wall cavity internal dry)
   Cement board siding, tyvek drain wrap, plywood or osb sheathing, Flash and batt insulation-2” closed cell spray foam and 3 ½ fg bib insulation, ½” gwb (no vapor barrier/poly). R value R-28.

   ABOVE GRADE WOOD FRAME WALL OPTION 3: (wall cavity external dry)
   Cement board siding, tyvek drain wrap, plywood or osb sheathing, 5 ½ fg bib insulation, vapor barrier/poly, ½” gwb. R value R-23.

   
8. GBA Editor
   Martin Holladay | Jul 27, 2011 05:23am | #8

   Brad,
   1. You wasted your money on the 1-inch exterior XPS on your foundation wall. Since you forgot to insulate the part of the foundation where it matters most, it is virtually worthless. Of course, you could still extend it to cover the exposed above-grade part of your foundation, and then cover it with stucco, but that's a lot of work for just R-5 insulation. The stucco would be easier to justify if you had insulated the exterior of the foundation with R-15 or R-20 insulation -- but you chose to use the thin stuff.

   2. For your basement wall insulation, option 2 is definitely better than option 1. You don't ever want to install fiberglass batts to insulate a basement wall.

   3. For your above-grade walls, option 1 is clearly the best, because it has the highest R-value. Be sure to use plywood sheathing, not OSB. The wall would be greatly improved if you included a true rainscreen drainage gap by installing vertical furring strips over the foam. Otherwise you'll have to use very long nails to install your fiber-cement siding.

9. 
   user-958765 | Jul 27, 2011 10:35am | #9

   Martin-
   1. Wish I could redo this. Might just add the top 8" and cover it anyway.
   2. Wouldn't the 1" xps work the same as the sprayed closed cell foam or vapor and the fg insul would remain dry by drying to the interior?
   3. Do you see any problems with option 2?
   thanks

10. 
    user-651098 | Jul 29, 2011 06:25pm | #10

    Brad,
    Diagrams for insulating a cantilevered floor assembly
    Taken from the Canada Mortgage and Housing Corporation (CMHC) website – http://www.cmhc.ca

    http://www.cmhc-schl.gc.ca/en/inpr/bude/himu/codemo/codemo_063.cfm

    http://www.cmhc-schl.gc.ca/en/inpr/bude/himu/codemo/codemo_043.cfm

11. GBA Editor
    Martin Holladay | Jul 30, 2011 04:28am | #11

    Brad,
    Q. "Do you see any problems with [above-grade] option 2?"

    A. No problems, but it could be improved by having a rainscreen drainage gap. The R-value is not particularly high. The wall design does not address thermal bridging through the studs.