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Community and Q&A

Perfect Wall assembly

rodrob15 | Posted in Green Building Techniques on

Hello All,

I’m building a small cabin in the mountains of North Carolina later this year (Beech Mountain). Elevation is at about 5,000 feet and the climate can actually be pretty brutal for the southeast in the winter (Zone 5 Climate). Average January high is around 35 and low around 15, but temps can get 10 to 20 below zero on occasion.

I’m planning on building an envelope based on Joe Lstiburek’s “Perfect Wall” theory.

My wall will be constructed as follows (from exterior to interior):
– HardiPlank or Galvalume Siding (haven’t decided yet) for exterior cladding
– 4 inches of exterior rigid insulation (R20)
– Air/Water/Vapor peel & stick membrane. I’m thinking Carlisle CCW 705 or similar. This is a Class I Vapor Retarder
– 2×6 stud walls with R30 batt insulation. This will be compressed into the cavity, which will basically equal R21.
– Gypsum board & T&G Panels for interior wall covering. Any paint/stain I’ll use will have maximum permeability.

The goal here is put the vapor barrier in the middle of the wall and allow any moisture a pathway to dry, either to the inside or to the outside. The exterior rigid foam should also be plenty thick enough to keep moisture from accumulating in the wall.

Does anyone have any experience with this type of wall? Am I missing anything in my approach?


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  1. GBA Editor
    Martin Holladay | | #1

    Plenty of people build walls with exterior rigid foam, so lots of people here at GBA have experience with this type of wall.

    For more information on this type of wall, see these articles:

    Combining Exterior Rigid Foam With Fluffy Insulation

    How to Install Rigid Foam Sheathing

    From your description of your wall -- one with 4 inches of rigid foam rated at R-20 -- I'm guessing that you are either (a) planning to use XPS or (b) conservatively de-rating the R-value of your polyiso to R-5 per inch.

    I hope it's (b), not (a). Green builders avoid using XPS, because XPS is manufactured with a blowing agent that has a high global warming potential. For more information, see Choosing Rigid Foam.

  2. rodrob15 | | #2

    I'm looking at a number of insulation options, but my original post was in reference to XPS. I'll likely use a combination of products. I'll be insulating around my foundation as well. I'm currently looking at mineral wool for exterior insulation and hope I can find some products that will fit my needs. For XPS, there is technology that vastly reduces the blowing agents (European companies are using it), but I don't think american companies have made that transition yet or plan to anytime soon. Owen Corning's formular claims its better, which may be true, but its still probably not that great.

  3. JC72 | | #3

    IMO, having spent the first half of my life in WNC I think your largest hurdle will be finding a builder who has experience with exterior rigid insulation and won't charge 3x the labor rate. My brother who lives in Asheville received what he calls "deterrent pricing" for a similar project. Everything in WNC is expensive anyways with all the wealthy retirees so look around for design companies which have done passive houses (I think there's at least one in WNC) and perhaps they can point you to a builder who has experience building better than code.

  4. rodrob15 | | #4

    I'll be doing the build myself. My father will help me occasionally and I'll hire some local labor when I need it. The benefit here is I will be able to afford high quality building materials. I'm keeping the design very simple and small (around 950 sqft). I have some building experience but this will be my largest project yet.

    Re Western NC prices: you are not joking there!!!! The prices are ridiculous, it's purely price gouging. Many common products like CMUs, insulation, drywall can be 1.5x - 2x the price. I will do a lot of price searching and might even have to buy a lot of materials from Charlotte, Hickory, or Johnson City. The good news is lumber is reasonably priced in western NC since that's where a lot of its grown and there's a lot of lumber mills that keep prices competitive.

  5. Robert Opaluch | | #5

    Good to see an owner-builder upgrading the thermal performance of their home. Would be interested in hearing your experiences as an owner-builder going forward.

    Wondering about your insulation choices. Why compress R-30 batts into 2x6 stud cavities? Resulting R-21 is not much better than cheaper R-19 batts. Have you considered 5.5" thick Roxul ComfortBatt R-23 or denser grades of fiberglass batts with higher R-value than R-19? Or build 2x8 walls to take advantage of R-30 batts?

    Polyiso has higher R-value per inch, depending on manufacturer, R-6 to R-7 range, at least for the inner layers that will be warmer where polyiso R-value is higher. Likely cheaper than XPS too. But cannot be used at or below grade (hydroscopic, can absorb moisture). Dana and others will suggest you find used or seconds foam board insulation at about a third of the price of new product sold through lumberyards. Big savings even after delivery charges.

  6. rodrob15 | | #6

    For the interior batts, I'll be completely focused on cost (i.e. because i'll be putting more cost/effort into the exterior insulation). I've found some great deals on R30 batts, which are only slightly more expensive than R19, which is why I'd do that. If the price difference was greater, I'd absolutely just use R19. I would also consider using R23 Roxul batts so long as the price was competitive.

    I will likely use Roxul Comfort Board 80 for my exterior wall insulation. A little less R Value, but I love the fire resistance, pest resistance, and vapor permeability of the product. I'm not sure what I'll use for my roof and below grade, that will likely be between XPS and EPS. Used boards are a great idea, I'll definitely look into that.

    I'm really not a fan of Polyiso. I think the R 6.5 per inch rating is false advertising since it weakens in colder climates. And while it's a much greener product than XPS, the price is just too high for it to make sense for most people. I'm on a tight budget, so will do everything environmentally friendly I can, but some things will be out of reach for me and I'd put Poly in that category. I also believe the green aspects of Poly are probably way overstated. Since this is a specialized product, I wouldn't be surprised if a lot more energy is used making it versus XPS, which greatly reduces its "green" qualities.

    This is a complicated topic, but I also believe that having some bad products in your home to help get you to net zero is probably better than using all green products, but continuing to use local electricity which is likely created by burning coal.

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