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Community and Q&A

Wall assembly help

jrpritchard | Posted in Energy Efficiency and Durability on


I wanted to get some advice on two wall assemblies I am considering. I am in Iowa- climate zone 5. Cost is a factor but would like to have an efficient assembly as we are going to live in this house for a long time.

Option 1 from outside in
LP Smart side
1/2” furring strips/rain screen
1 1/2” XPs rigid foam
TyVek commercial D drain wrap
2×6 standard framing with BIBS and my insulation contractors basic air sealing package
1/2 Sheetrock latex paint

Option 2
LP smartside
Homeslicker drain mat
ZIP R sheathing 1.5” r vale 6.6
2×6 wall BIBs and air sealing
1/2 sheet rock latex paint

I like that I can surface mount and flash my windows in the zip system but am afraid of getting my structural OSB to far away from the framing with the thicker ZIP panels. I would much rather use jamb extensions inside than exterior window bucks. The attic will be sprayed with a butter coat (as they call it around here) of 2” of foam topped off with blown cellulose for an R50. Rims sprayed as well. Foundation will have 2” exterior foam with interior framed 2×4 walls and Roxul bats. Same 2” of foam placed below foundation slab as well.

What do you guys think of this overall and what would be the better option for my wall assembly? Are there any areas that need to be beefed up or areas I can cut back on?

Thank you! GBA is a great resource

Jeremy from Iowa

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  1. GBA Editor
    Martin Holladay | | #1

    Green builders try to avoid the use of XPS, which is manufactured with a blowing agent that has a high global warming potential. (For more information on this issue, see "Choosing Rigid Foam.")

    That's why I would advise you to use Option 2 (which uses Zip R sheathing, a product that incorporates polyiso rigid foam) instead of Option 1 (which uses XPS).

    Experienced builders find that the so-called "butter coat" of spray foam that your builder wants to use to air seal the attic is unnecessary. Most builders use air sealing techniques for attics that don't require 2 inches of spray foam. For more information on this issue, see "Air Sealing an Attic."

    That said, if your builder is unfamiliar with conventional techniques of air sealing, the "butter coat" approach will work, if conscientiously applied. As usual, it's best to confirm air sealing work with a blower door test.

  2. jrpritchard | | #2

    Martin - thank you very much for the response. Would using a different exterior foam change your opinion?. I have read that the ZIP R system which uses interior foam may not be as good as exterior foam because it would still allow the OSB to condense on its surface I have also read that it doesnt really matter because it still increases the R vlaue overall and decrease bridging thru the studs. I am going to use 2x6 walls and am interested in an efficient-green-above minimum code wall. What would be the best 2x6 wall assembly for climate zone 5? Thanks again - I greatly appreciate the advice Jeremy

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Would using a different exterior foam change your opinion?"

    A. Yes. Either EPS or polyiso is a better choice (from an environmental perspective) than XPS. If Option 3 is similar to Option 1, but with EPS or polyiso instead of XPS, then either Option 2 or Option 3 would work. You should choose whichever of these options your builder is most comfortable with.

    Q. "I have read that the Zip R system (which uses interior foam) may not be as good as exterior foam because it would still allow the OSB to condense on its surface."

    A. That's not quite true. As long as you select the right thickness of Zip R -- with enough R-value to meet the rules spelled out in my article on the topic, "Calculating the Minimum Thickness of Rigid Foam Sheathing" -- then the interior surface of the Zip R will never get cold enough to permit condensation or moisture accumulation. The polyio keeps everything warm (and therefore immune from condensation).

    Q. "I have also read that it doesn't really matter because it still increases the R-value overall and decrease bridging through the studs."

    A. That's not the reason "it doesn't really matter," but you're correct: Zip R sheathing decreases thermal bridging through the studs.

    Q. "What would be the best 2x6 wall assembly for Climate Zone 5?"

    A. Read this article: "How to Design a Wall."

  4. Stockwell | | #4

    You can also use Zip-R along with closed cell spray foam in your walls if you are concerned about condensation. You can calculate exact thicknesses of both with the info in the article Martin linked. You could then fill the rest of the wall with BIBS as planned.

  5. Expert Member
    Dana Dorsett | | #5

    The R6.6 ZIP-R isn't sufficient for dew point control for a 2x6 wall in zone 5. Per the IRC prescriptives it needs to be at least 7.5:

    At 1.5" XPS is labeled R7.5, but even the warranties don't suggest it it will perform at better than R6.75 after 20 years.

    If you're limited to 1.5", go with foil faced rigid polyisocyanurate. The labeled R-value is R9-R10 (depending on manufacturer, but the foil facer gives it another R1 or so where it counts. While some polyiso needs to be derated for temperature in this stackup & location, the average performance of the foam in mid-winter will always be at least R6.5, and the foil facer on one side of the 3/4" air space bumps it to R7.5.

    Dow alleges that they have the "magic forumula" that avoids the derating issue altogether, and would always exceed it's labeled performance in winter. See the derating curve for 2lb density roofing polyiso (labeled at R5.7/inch @ 75F average temp through the foam) in Figure 2 here:

    Any 1lb density foil faced polyiso will exceed that performance even without the air gap & foil facer. But compare that to Dow Thermax (the black line) in the plot on this page:

    So 1.5" of Thermax would beat even fresh, undepleted XPS in performance, and ANY vendor's 1.5" foil faced polyiso would be adequate for dew point control in your stackup & climate zone.

    Polyiso is blown primarily with hydrocarbons, the major component of which is a usually a variant of pentane (7x CO2 @ 100 years.)

  6. GBA Editor
    Martin Holladay | | #6

    Thanks for pointing out the issues concerning the R-value of the foam layer. Good points.

  7. Jon_R | | #7

    > The polyio keeps everything warm (and therefore immune from condensation).

    The "wrong side" polyiso in Zip R doesn't keep the OSB warm and the OSB will get cold enough to cause condensation in ex-filtrating (or circulating interior that makes it that far) air.

  8. GBA Editor
    Martin Holladay | | #8

    In a Zip R wall (which according to the manufacturer must have seams sealed with Zip System tape), exfiltrating air will never reach the OSB.

  9. Jon_R | | #9

    If perfect air sealing existed, it would solve many moisture problems. IMO, walls that count on it to avoid ex-filtration induced condensation are "not as good".

  10. Jon_R | | #10

    Jeremy, I'd take a look at other options too - for example plywood sheathing and external side EPS.

  11. Expert Member
    Dana Dorsett | | #11

    If EPS, it takes at least 2" of exterior side foam for dew point control on R23 cavity fill such as 1.8lbs fiberglass in 2x6 framing. The R7.5 prescriptive is (apparently) presumes R20 cavity fill. Making the ratio proportional for R23 it would take R7.5 x (R23/R20) = R8.6.

    While the labeled R-value of 1.5lbs density EPS is R4.2/inch (and thus only R8.4 @ 2"), in this stackup it's legitimate to use the 40F mean temperature R value for EPS, which is typical R4.5/inch, which would deliver R9-ish performance during the winter season that matters. But at 1.5" it would only deliver an up-rated R6.8, well shy of the needed R8.6.

    Brand-X foil faced polyiso might only deliver R5/inch at a mean temp of 40F through the foam, but the boost from the foil facer pushes it into the right range, and since the performance rises rapidly with temperature the drying season begins much sooner, even during the coldest handful of weeks in the winter it underperforms. (Dow Thermax performs at R6.6/inch at a mean temp of 40F, and higher at lower temps according to the marketing literature, so 1.5" should make it with some margin.)

    Given how SIP roofs fail at the ridge lines from exfiltration-deposited moisture I'm inclined to agree with JON R here, but only to a degree. With the additional rapid-drying capacity of the rainscreen construction siding the risk isn't as high as with SIP roofs. SIP walls seem to have much lower risk related to air leakage compared to SIP roofs.

  12. jrpritchard | | #12

    Thanks for all the input. I like the option of 1.5 Thermax on the exterior with 1/2” thick furring strips as a rain screen. My last question is - with exterior foam can I use a standard WRB like TYVEK or do I need something like Commerical D or stucco wrap that create more of a capillary break? Thanks again

  13. Expert Member
    RICHARD EVANS | | #13

    If it were me, and I were using Thermax with a rain screen gap, I would just tape the seams of the Thermax and let that be my WRB.

  14. GBA Editor
    Martin Holladay | | #14

    Assuming that you are planning to use furring strips to create a rainscreen gap, or Homeslicker, any kind of housewrap (like Tyvek) is fine.

    If you are trying to save money by skipping the rainscreen gap, then you might consider a product like StuccoWrap. But a real rainscreen gap is a better approach.

    For more information, see these two articles:

    "All About Water-Resistive Barriers"

    "All About Rainscreens"

  15. user-7114751 | | #15

    Thank you again - I have read your articles and they are incredibly helpful. Last question I promise. My concern is that if I have 'innie' windows (which is my preference) behind my foam, then my WRB should also go behind the foam - as noted in one of your articles. My concern is that a smooth face of a WRB like TyVek, secured tightly against the smooth face of the foam would not allow a capillary break thus not allow the water to drain out making my furring strip rain screen, which would be between the siding and the foam, useless. The assembly I am leaning towards is as follows -
    LP or Hardie Lap Siding
    1/2" plywood strips for rainscreen
    1.5" Thermax foam
    WRB of some kind
    OSB sheathing
    BIBS in a 2x6 wall
    drywall and paint
    My question is with this assmebly do I need a draininge plane between the foam and WRB or will the wall dry thru the Thermax?
    Thanks - Jeremy in Iowa

  16. GBA Editor
    Martin Holladay | | #16

    Q. "My concern is that if I have 'innie' windows (which is my preference) behind my foam, then my WRB should also go behind the foam - as noted in one of your articles."

    A. Yes. If you haven't seen this article, there are more details here: "Where Does the Housewrap Go?"

    Q. "My concern is that a smooth face of a WRB like Tyvek, secured tightly against the smooth face of the foam, would not allow a capillary break thus not allow the water to drain out making my furring strip rainscreen, which would be between the siding and the foam, useless."

    A. Your rainscreen gap won't be useless. It will allow bulk water that gets past the siding to drain away or evaporate harmlessly. Those are good things.

    The outer surface of the rigid foam will stop almost all water, which is a good thing, even though the outer surface of your rigid foam is not, technically speaking, the WRB.

    The WRB is your absolute last line of defense. In your assembly, it is highly unlikely -- almost impossible -- for liquid water to reach it. That's why a gap for drainage isn't really needed at the WRB in this configuration.

  17. Expert Member
    Dana Dorsett | | #17

    With an innie window using a crinkle type WRB such as Tyvek Drainwrap between the foam and structural sheathing gives micro-channels for bulk water to drain if your window flashing is mis-lapped or fails in some other way, and at least a modest capillary break. It may not be absolutely necessary if there is rainscreened siding, but it's a very small upcharge from the flat-stuff- very cheap insurance. (This is what the folks at Building Science Corp usually recommend.)

    With foil faced polyiso the rainscreen gap between foam & siding (rather than a mesh type rainscreen) does double-duty as a capillary break, but also adds about R1 to the performance of the foam, for greater dew point margin and an R1 improvement in "whole-wall R".

    With only half-inch plywood for the rainscreen furring you probably don't have enough fastener retention for fiber cement siding, and may need to use very long fasteners that reach into the structural siding. Consult the siding manufacturer if going with anything thinner than 3/4" furring.

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