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Community and Q&A

Confused about continuous insulation in Climate Zone 5

ccutrer | Posted in Green Building Techniques on

I’m planning a home in zone 5 (northern Utah). I’m so confused by all the differing opinions on how the wall assembly and different barriers should be installed. And I’m definitely not getting much help from my general, or his insulation or exterior subs. He’s definitely for trying out exterior insulation, and understanding the thermal break it can create, but he doesn’t even know his EPS from his XPS from his PolyIso. The insulation guy was somewhat helpful, but his expertise mainly lies with inside-the-wall insulation. The exterior guy was basically pretty whiny, first asking “why would you even do that when you can just do better insulation on the inside?’, and then complaining that his fasteners for Hardie Board (the house is primarily Hardie, with a few minor sections of thin-brick veneer) aren’t even long enough. I don’t know if Utah is just a somewhat unique climate having somewhat extreme summers and winters (near-zero for several days and a few weeks of teens in the winter; a one to three weeks of 100+ in the summer, and very very dry around here). But I’m hard pressed to find anyone locally to do anything outside of the ordinary. The only thing that there was no argument about was 2×6 exterior walls. But I’ve never seen a home around here built with exterior foam board, and never any foundation insulation.

Anyhow, after doing lots and lots of reading, my current personal design idea is:

* 1″ EPS
* WRB (just regular Tyvek)
* 1/2″ OSB sheathing
* 2×6 walls, insulated with flash-and-batt of 1″ closed cell spray foam, followed by blow-in-blanket fiberglass ( filling out the rest.

My questions are:
* Some articles give a strict rule that the exterior foam thickness needs to be calculated so that the inside edge of the foam will never be below the dew point, even going so far as a strict ratio of interior insulation to exterior insulation. Is this really as big of a deal with EPS that’s more permeable than say PolyIso? I just want enough out there to break the studs’ thermal bridge.
* Is the location of the WRB in the correct spot? Do I need any other air/water/vapor barriers?
* Should I avoid the closed cell spray foam, as it adds an additional barrier?
* As for attaching the cladding, I found a Hardie Board note of using furring strips through the >1″ of sheathing + foam, and then attaching the Hardie Board to the furring strips. Which I’m sure the exterior guy will whine about, but tough.
* What about the thin-brick veneer. Can that attach directly to the EPS? Or do I need to do furring strips, and then an additional backer-board to give it a smooth surface?

I’m getting kind of frustrated, and almost to the point of giving up on continuous insulation and just doing what they want to do of BIBS insulation, and nothing else.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name?

    Q. "Some articles give a strict rule that the exterior foam thickness needs to be calculated so that the inside edge of the foam will never be below the dew point, even going so far as a strict ratio of interior insulation to exterior insulation. Is this really as big of a deal with EPS that's more permeable than say PolyIso?"

    A. Yes, you need to follow the rules if you hope to avoid damp OSB sheathing. Even 1 inch of EPS will reduce the rate of outward drying enough to put you in the danger zone. The solution is simple: follow the rules, and install a minimum of R-7.5 of exterior rigid foam. To learn the rules, as well as the logic behind the rules, read this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Q. "Is the location of the WRB in the correct spot?"

    A. Read this article: Where Does the Housewrap Go?

    Q. "Do I need any other air/water/vapor barriers?"

    A. Every wall needs an air barrier. You can't buy an air barrier -- you have to create it. More information here: Questions and Answers About Air Barriers.

    Q. "Should I avoid the closed cell spray foam?"

    A. Yes. If you are installing exterior rigid foam, the spray foam between the studs is unnecessary, expensive, and damaging to the environment. For more information on this issue, see these two articles:

    Installing Closed-Cell Spray Foam Between Studs is a Waste

    How to Design a Wall

    Q. "What about the thin-brick veneer? Can that attach directly to the EPS?"

    A. I'm not a fan of thin brick veneer. It's not a siding type that makes much sense, either from a green building perspective or from the perspective of a designer trying to choose a logical siding for a home with exterior rigid foam. If you insist on thin-brick veneer, you should follow the instructions of the veneer manufacturer.

  2. ccutrer | | #2


    Thanks for the reply. Sorry, I just signed up and couldn't see where to put my name. It's Cody Cutrer. I think I have it set in my profile now.

  3. Reid Baldwin | | #3


    I understand the frustration of trying to do something that differs from the usual way it is done in your region. I encountered many of the same issues building my house in zone 5 in Michigan. (See the series about Airport House.) I was fortunate that my general contractor and the framing sub-contractor were open to learning and trying something new. The framing sub installed the exterior foam and the siding. The insulation sub didn't need to do anything new (except get into the attic in a different way, which proved to be too much change for them).

    We used a combination of Hardie Board and brick veneer over 2" of graphite infused EPS. Furring strips were used behind the Hardie Board but not behind the brick. Attaching the furring strips required the contractor to procure a different type of fastener than they normally used, but it was readily available.

    Pushing a few people in your local contractor base to learn something new is a valuable service to your community. Providing that service will require you to do a lot of learning. You have found the right site to do that learning. People on this site will appreciate the service you are doing and give you encouragement. Don't expect much appreciation from the locals. It will also likely cost you some money. I paid a premium for the extras on my house which went toward climbing the learning curve.

  4. Sal_123 | | #4

    Hey Cody, on my project I found the contractors to be of little help. Once you stray away from the way they have been doing it for decades, your on your own. They follow the dogma they know and are VERY reluctant to adopt new ideas. Despite some great advice and new techniques, they plow ahead and are critical to anything outside their box. This site is very good, read all the links Martin gave and more. You need to educate yourself above and beyond your contractors or find new ones that understand the concepts. I recommend as well, some Youtube stuff is pretty good, search buildingscience, ask questions on this site. Read, read, read. Consider a local building science or energy efficiency consultant.

  5. KeithH | | #5

    Hi Cody,

    I'm not a pro, just a DIYer/homeowner. First, I've learned tons in the 4 years I've been reading Gba. It is an amazing resource.

    A few thoughts for you:
    - is the site in town or off in the woods/chaparral? If you are in the country, check out Roxul Comfortboard. More breathable than EPS, less flammable. More $. Tech guide for exterior insulation with quite a few details
    - if you are going to take risks with the insulation thickness, perhaps sheathing with plywood is a better choice? It is much more moisture durable and much more breathable under moist conditions. One blog about it:
    - I love Gba but you might look at as a complementary resource. The videos on there vary from this old house garbage to videos actually showing tradespeople dealing with the challenges that more insulation brings, etc. You might reach a contractor via video vs using a PDF.
    - I haven't used zip systems products (other then trying the tape indoors ... Too smelly for interior!) but it might be easier to sell your contractor on zip R (with or without straps) than straps on insulation. One PDF to look at:
    - You probably know that in your cold winter climate, polyiso is not a good solo exterior insulation due to blowing agent condensation (~-5F?). You can use it behind EPS or skip it.

    Hope that helps.

  6. chrisinutah | | #6

    I'm sorry to resurrect an old thread, but @ccutrer, did you have any luck with your general contractor or subcontractors for your project? I am also trying to build a house in Utah, and I'm finding it difficult to find contractors who are willing to consider these types of details. I would definitely appreciate any recommendations for contractors to talk to.

  7. utah_matt | | #7

    I am building in Tooele which was tough to find a contractor. I went with Clegg Contracting. I considered Green Tech out of Provo if they are close to you they seemed to be willing to try many of these methods.

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