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Community and Q&A

Is this a risky wall assembly?

jcoenen | Posted in Energy Efficiency and Durability on

I am in the process of finalizing plans for a 2 story 2385 square foot home in Zone 6 WI 44.3353° N, 88.6389° W. I have been trying to get in the pretty goodish house category as far as insulation levels and while not where I’d like them to be am currently sitting with the following setup due to budget.

R10 EPS below slab insulation
R10 EPS exterior Foundation walls (will finish off basement later and can add interior insluation)
2×6 R20 damp spray cellulose with R7.5 Atlas Thermalstar LCI-SS EPS insulated sheathing
R55 blown cellulose attic insulation
Single Hung Alliance U .29 Windows

It just seemed anytime I tried to bump any of these components up any higher the price tags went up exponentially.

The only real concern I have right now is the wall assembly as I know with the exterior foam is below the minimum threshold for my zone and there is the potential for some wintertime condensation in the wall.

My question is – is this wall assembly inherently risky. I know with exterior foam its go thick or go home, but being only R4 away from the threshold where there would be absolutely no winter time moisture accumulation and by planning on adding Certainteed Membrain smart vapor retarder on the interior side of the wall and having cellulose in the wall cavities with some moisture absorbing capabilities am I mitigating this risk?

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  1. GBA Editor
    Martin Holladay | | #1

    The building code says that this type of wall needs a minimum of R-11.25 exterior insulation, and you want to install R-7.5 exterior insulation. So the first person you need to talk with is your local code inspector -- who may or may not approve of your plan.

    You've chosen to purchase a type of foam that is 2 inches thick and rated at R-7.5. You might consider switching to 2-inch-thick polyiso -- nominally about R-11.4 or so, and able to perform at cold temperatures at about R-9 or R-10. Better than what you are choosing, and no thicker.

  2. user-1072251 | | #2

    You might also look for recycled foam insulation which is often 1/3 or 1/2 of retail cost.

  3. jcoenen | | #3

    Thanks for the answers. The insulation is integrated to the sheathing and that is the thickest foam Atlas sells as part of the LCI-SS sheathing product. This product saved us tremendously on labor and was over $8k cheaper than 2" of XPS due to combined sheathing and insulation package. Obviously poly ISO will be more.

    The other option is to drop down to 2x4 construction to get back within safe limits or just go 2x6 without any exterior foam. I know the 2x4 will perform better, but then my wall insulation is fairly weak. Just seems hard to get above r20 without breaking the bank based on all quotes I've seen. Exterior foam always seems to add $15-20k+ in costs which is hard to justify given the cheap cost of heating and cooling which I think could be here to stay.

  4. rocket190 | | #4

    I feel your pain about the thick foam required in a Wisconsin climate. I ended up using a 2x8 wall with 2"x4s cross strapped on the flat to reduce thermal bridging.

    I tried some interior polyiso I don't recommend it. I had a difficult time keeping the wall plane flat. It was easy to squish the foam into the studs.

  5. GBA Editor
    Martin Holladay | | #5

    If I were you, I would choose a 2x4 wall with continuous exterior foam over a 2x6 wall with no exterior foam.

  6. charlie_sullivan | | #6

    The LCI-SS sheathing product says:
    "Unlike other nailbase products, the OSB of LCi-SS faces inward, with the insulation
    on the exterior side"

    That's too bad. The other way around mitigates some of the condensation risk, but this isn't any different in that regard from separately applied exterior foam.

    The smart membrane would help, but consider Rick's suggestion (comment 4)--you could end up with a less risk and lower cost assembly.

  7. jcoenen | | #7

    Yeah unlike Zip-R and others with the insulation inward - it's on the exterior side. Which at first I was excited about as I could never get my mind around the condensation risk with ZIP-R since the polyiso was on the inside of the sheathing.

    LCI-SS is less than half the cost of ZIP-R, but unfortunately maxes out at R7.5. Which I'm sure is so that you can still easily apply siding without having furring strips. I emailed them and they said of course its fine to use with 2x6 in Zone 6, but as with anyone selling a product they tell you what you want to hear.

    I've yet to run across a builder or even energy star rater who is aware of the minimum thickness for exterior rigid foam. Everyone just blows it off and says that the risk is overblown and it won't be an issue. Every builder I ever talked to gave me proposals for exterior foam below the requirements and thought I was nuts for asking for 3" of exterior foam and well when they gave me the cost I thought so too, but just doesn't seem to be much awareness around the code and I also wonder if the local code inspectors are aware as obviously these builders are building houses and must be getting away with these assemblies.

  8. GBA Editor
    Martin Holladay | | #8

    For more on code stupidity, see The 2012 Code Encourages Risky Wall Strategies.

  9. Expert Member
    Dana Dorsett | | #9

    This product would not quite meet the IRC prescriptives for exterior R in zone 5, since the R7.5 is the total R includes the 7/16" OSB included. There is only 1-9/16" of Type-I EPS, which is typically at best R4/inch, so you're looking at only 1-9/16 x R4= R6.25 for the foam at 75F. At a mean temp of 25F it's about R4.35/inch, so you're getting maybe R6.8 out of the foam when it counts.

    IRC minimums for Zone 5 are R7.5 on 2x6 framing. Zone 6 needs R7.5 over 2x4 framing to drop to class-III vapor retarders. So up there in the hinterland between Oshkosh & Green Bay it's not even quite enough EVEN if you dropped back to 2x4 framing (!).

    See the description in the short spec:

    But with air-tight MemBrain and cellulose in the cavity it should be just fine, even with 2x6 framing if you limit the interior RH under 40% max in winter.

    A flash-inch of closed cell foam on the OSB would work too.

  10. rocket190 | | #10

    Jeremy, if you are in the Fox Cities area and looking for a builder experienced in insulated exterior sheathing, contact John Hofferber of Berhoff Homes. He's built 30+ homes with either Dow SIS insulated siding (2" foam) or zip-r. His last home tested under .40 ach and it was only a modest budget. All his homes are insulated with damp spray cellulose, and his subs are familiar with green techniques that are uncommon to our area.

  11. GBA Editor
    Martin Holladay | | #11

    Chris Kreple from Atlas EPS just sent me an email with comments that he wants to post on this thread. His comments follow.

    I noticed our product, ThermalStar LCi-SS, was mentioned on Green Building Advisor. There were a couple of mistakes regarding the product that I thought I would explain.

    1. The R-7.5 (2") version of the product includes a layer of 7/16" OSB. It is a insulated structural sheathing panel, unlike the polyiso panels that you mentioned would have a higher R-value.

    2. We are making this product with an enhanced EPS product made with graphite. It has an R-value of almost R-5 per inch. (The product tests at about 4.9 per inch so we will be selling our product slightly over thickness to get to R-5, R-10 ,etc). So, the R-value of the foam only is about 7.6, + the R-value of the OSB which is about R-0.6. (Dana Dorsett had the R-values wrong).

    3. Our opinion is that the insulation on the exterior of the sheathing in a cold climate should reduce condensation potential inside a wall cavity by raising the temperature in the wall cavity. If you have a different opinion, we would be interested to hear it. (Charlie Sullivan suggests that the opposite would be better).

    4. Based on experience, the wet applied cellulose should be allowed to dry before being closed up on the inside with a vapor retarder. I did not see that mentioned.

    5. Anecdotally, homes build with LCi-SS have been able to perform very well in blower door tests, assuming that joints are well taped.

    -- Chris Kreple

  12. Expert Member
    Dana Dorsett | | #12

    It's curious that the short sheet description does not mention using graphite loaded EPS, which is why I presumed otherwise:

    This is an important clarification, and it would be even better if the product literature were more specific. Thanks!

  13. charlie_sullivan | | #13

    Re-reading my previous comment that Chris Kreple refers to, I see that I wasn't very clear. If the foam R-value is high enough for the climate it's used in, I think the Atlas product is great, and it's true that the foam keeps the sheathing warm, and thus there's no problem with condensation on it.

    My comment was intended to refer only to the situation that the foam is not thick enough, which I think we can all agree is bad regardless of which side of the foam the OSB is on. In either case, there can be condensation on the first surface of that laminated product. But if you do have that situation, I think it's a little safer to have that condensation occur on foam than on OSB. If the surface of the foam gets wet, it can drip down and get parts of the wood structure wet, and there might be mold and rot as a result, but at least the foam itself won't rot and is unlikely to support mold growth.

    In summary, applied right, the Atlas product is great, and I'm actually pretty excited about having it available as an option, especially with the graphite. Applied wrong, it might be worse than some other products applied wrong, but that's not particularly relevant, because applying any of the products wrong is a bad idea.

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