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Advice on insulating sheathing in climate zone 5?

jwyman | Posted in GBA Pro Help on

Climate zone 5, western Massachusetts.

I am looking into various exterior wall systems for a two family residence and came upon the Dow Styrofoam SIS insulating sheathing:

In my climate zone with 2 x 6 framing, it is recommended that I use a minimum of 1 1/2″ rigid insulation on the building exterior for a thermal break. SIS sheathing only comes in 1/2″ and 1″ panels but satisfies both sheathing and insulation installation in one application. Is it a bad idea to use this type of sheathing in my climate zone?

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  1. GBA Editor
    Martin Holladay | | #1

    Here's the article with the chart showing minimum R-values for foam sheathing: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    In your climate zone, you need foam sheathing with a minimum R-value of R-7.5 for a 2x6 wall.

    Here's more information on structural insulated sheathing: Styrofoam SIS.
    The one-inch-thick product has an R-value of R-5.5. That's not thick enough for your climate, so you shouldn't use SIS.

    You can use either 1.5 inch of XPS, 1.5 inch of polyiso, or 2 inches of EPS.

  2. Foamer | | #2


    If you are sold on SIS, you can apply a layer of closed cell spray foam on the back of it to reach your required R-value. Do not use open cell foam for this purpose.

  3. GBA Editor
    Martin Holladay | | #3

    If you are willing to put up with the disadvantages of vinyl siding -- both environmental and aesthetic -- you could install R-2 insulated vinyl siding over 1-inch-thick SIS to achieve R-7.5.

  4. jwyman | | #4

    Thanks for the suggestions. What I really need is a solution that will allow the house to be constructed with most work being done in the indoors during winter. Also with standard detailing for the carpentry crew. That being said, the double wall may be my solution.

    On the other hand, what about applying 1" rigid polyiso over the SIS? The material numbers for SIS and 1" polyiso are approaching $60 per 4' x 8' sheet. I'm going to lose a lot of real estate with the double wall.

  5. GBA Editor
    Martin Holladay | | #5

    If you are installing two layers, why not just install ordinary OSB (or better yet, plywood) followed by 2 in. of polyiso? That will give you R-13 -- it's a better wall.

  6. jwyman | | #6


    I do agree that it may be a better wall, but the house is not a simple shape and has several trim details. We are starting construction in late November and keeping much of the work indoors will be an advantage. I am still weighing the differences, but I do feel that plywood and 1 1/2" to 2" polyiso will be more affordable than the SIS and polyiso system.

  7. wjrobinson | | #7

    Excuse me for a little aside to follow... but... it is somewhat part of the discussion above (though I second Martin's suggestion and say skip OSB. Plywood or Zip wall.)

    OK... my aside... is... Contractor pep peaves... number X... starting construction on the worst date....

    I am missing days of yore... when we dug foundations after the spring thaw, then framed during black fly season, roofed before the dog days of August, sided, and went on to another project to do the same saving all the homes and the next inside work steps... for winter.

    Shingles this way do not seal down if the work is done here much later than September.
    Hardwood floors layed in August here... well, I have seen floors open up over a 1/2" the following winter.

    Forcing work to be done when the season is the worst... is...

    definitely no joy for contractors...
    just sayin

  8. jwyman | | #8

    I understand what you are saying AJ but in this is a fire/replacement project with job conditions that require it to be done by a certain time frame. In this economy, I find we must take on any work that comes along, and all work is not indoors during winter months.

    That being said, we have made the decision to frame typical 2 x 6 walls with 2" polyisocyanurate over plywood sheathing. This is saving almost 150 square feet of floor space and will be our first project using this wall system.

    Using the details and recommendations from this site, we are excited to begin this project. Thanks for the advice!

  9. user-723121 | | #9

    This sounds like a good wall system for this climate, try and get energy heel trusses into the building specifications. I like a minimum 12" energy heel, this allows room for vent baffles and adequate insulation over the exterior wall plates. Ice dams are a real problem in a cold climate and the detail I have described will eliminate this.

  10. homedesign | | #10

    Jon, you say "the house is not a simple shape"
    Can you post an image of the house or of a similar house?

    Which GBA window detail did you decide to use?

    What is your air barrier strategy?

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