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We are trying to maximize wall assembly R-value: Very cold climate in CO

Arapahoe | Posted in Energy Efficiency and Durability on

New home construction: 2×6 ext wall with 2″ of closed cell spray foam with the additional 3.5″ of cellulose or fiberglass batt. I was thinking of going with Dow’s SIS panel (1″ R5.5), but I am reading from one of Robert Riversong’s comments that the impermeability of the SIS panel (Polyiso) is a bad idea for our climate.

I admit, 2″ of XPS allows better permeability but I thought we could save time with the use of the SIS panel and taping. We are in Frisco, CO and almost 10,000 ft. I am fairly certain we can keep moisture from condensing on the interior side of the SIS panel because of the flash and batt technique. Let me know your thoughts!!!!

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  1. user-723121 | | #1

    I would forget the flash and batt and consider a double wall or a 2x6 wall with several inches of foam wall sheathing.

  2. Foamer | | #2

    With 2 inches of closed cell foam and the SIS panel you will have close to R-20 outside of the potential condensing surface (inside foam boundary). Adding cellulose will still keep 2/3 of the overall R on the outside which should be fine. If you want to work the numbers, I believe Building Science Corp recently had an article about the topic. Check their website.

  3. jklingel | | #3

    I would suggest you not even consider fiberglass batts, whatever you do.

  4. GBA Editor
    Martin Holladay | | #4

    1. If you really want "to maximize wall assembly R-value," you have a long ways to go. Your proposed wall has an R-value of only about R-25 (actually less, considering the thermal bridging through the studs).

    2. SIS panels are available in two thicknesses: 1/2 inch (R-3) and 1 inch (R-5.5).

    3. I don't know your climate zone, but at 10,000 feet you may be in climate zone 7. A 2x6 wall needs at least R-15 of exterior foam to avoid condensation or moisture accumulation problems in the wall assembly. (For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.) Two inches of closed-cell spray foam comes close, and 2 in. of spray foam plus SIS comes even closer.

    4. Don't worry about the vapor permeance of your sheathing. Both types of wall you are describing -- either a flash-and-batt wall or a wall with SIS -- is designed to dry to the interior. You can't expect either type of wall to dry to the exterior, but that's OK. As long as the foam layer is thick enough, either wall will perform well, as long as it can dry to the interior.

  5. Lo3H5B3Ck7 | | #5

    If you consider using polyurethane SIPs then you really don't have a vapor issue at all.

  6. Arapahoe | | #6

    Being at 10,000 ft we are still in a dry or high alpine desert we call it. We have built with Tyvek, OSB, 2x6 framing w/ fiberglass for years and we don't really see rotting OSB. Why is this the case? Granted improper flashing is bad, but bad in all climates.

  7. GBA Editor
    Martin Holladay | | #7

    As long as you don't install any foam sheathing, the OSB dries readily to the exterior. That's why you don't see any rotting OSB.

    Once you install foam sheathing, however, your stop outward drying. Such walls can still perform very well, as long as:
    1. There is no interior polyethylene, and
    2. The exterior foam is thick enough to keep the OSB above the dew point during the winter.

  8. iy7yBuHns9 | | #8

    There are several companies in Colorado that manufacture Polyurethane(PUR) core SIPS. Some of the advantages of PUR SIPS.
    1. High R-Values over 7 per inch. 6-1/2" panel will give you a R38-R40 wall.
    2. No wall cavity, so no condensation in wall.
    3. Can provide nearly air tight wall.
    4. Eliminates almost all thermal bridging
    5. It will be a whole lot easier and faster to build and build it right.

  9. Arapahoe | | #9

    Great ideas and advice, and cost is an issue. My original thought was to stay away from SIP's because of cost (and admittedly I have never done them before). I then thought of the DOW 1" Structural sheathing panel (prevent thermal bridging), with 2" closed cell at R13, and maybe blown cellulose for the remaining 3.5" of the wall cavity at R12.6. Total R31. Another driver for this was the ability to sheet and insulate the exterior all at once. At this point, with the SIS panels costing $32/sheet with closed cell spray.....maybe I am better looking at SIP's and it sounds like I will sleep better knowing that the dew point will never exceed the inside temp of the sheathing. I have no idea what a good price per sq ft is for SIP's really is? Is it based on sq ft of living space or exterior wall area? Kent...I would be interested in hearing of a reputable SIP's contractor here in CO and close to Summit County. I would be more than happy to take that response offline to [email protected]. Thanks for everyones input!! Great forum!!

  10. iy7yBuHns9 | | #10

    I am NOT associated with either of these companies, so I hope it OK to list them.
    Earthcore SIPs
    Insulated Component Structures
    I do work for a PUR SIP manufacturer in North Carolina.
    Considering the level of performance you are looking for, the cost for SIPS might look pretty good.

  11. PLnQUWAkNS | | #11

    I first considered SIP and then exterior foam. Both were quite expensive (SIP more so). I finally settled on 10" double stud walls with dense pack cellulose and plywood sheathing (better permeability than OSB) for my 1000 sq. ft. single story. Unfortunately I haven't started construction yet (soon!) so I can't tell you how it worked out.

    On paper though it has higher actual R value (R-30+), is pretty cheap (only a couple hundred $$$ in lumber to frame the second wall), and is no more complicated than framing a 2nd, non load bearing, wall. Plus easier to run wiring through the walls (no drilling through studs).

    And no foam (good for the environment)!


  12. Arapahoe | | #12

    I looked at the idea of a double stud wall but that will reduce our interior space and that is already quite small <1900sqft. Does the architect care if I made this move? What kind of a space does one need between double stud walls? Right now it is a 2x6 ext wall. It is simply another interior non load bearing wall. What kind of design implications do I need to heed? I know windows are one of them.

  13. Arapahoe | | #13

    Alan, we are building this summer as well. I would assume you are in a cold climate zone 7 as well. Where are you located? Email me and we can chat off line. [email protected].

  14. JOHNX2EH | | #14

    I am preparing to break ground on a 1300sf home. I cost analyzed PUR SIPS R28walls (4.5") and R42 roof (6.5"). Cost initially was concerning up front. However the walls in 2x6 with spray in insulation, 2x6 framing and all dimensional lumber with ridge boards, sheathing (osb), miscellaneous (nails, adhesives, etc) and Labor for framing only comes in at $15,800. This is to get only an insulated shell. The cost of the PUR SIPS for the sa$16,500. Though the cost seams high initially, there will be lees than 5% difference in cost, and I thing a better producr.

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