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Vaseffa Fennick | Posted in Energy Efficiency and Durability on

My hubby and I are in Western PA (5A) and are in the works of building our home w a local builder. Their standard material is Blue Dow board or SIS. I found it a little curious at first using Styrofoam to hold a house up but hey what do I know about houses. From the inside out it goes: drywal, insulation, 2x4s with some metal reinforcement, then the SIS,some plastic wrap (for lack of better term) then the siding. This gives it a R-19 rating. I inquired later about plywood and they said I could swap the SIS for OSB? However doing so would negate the energy efficiency warranty that is offers for two years w the SIS.

Also it was recommened that if we were to choose the OSB, they recommend upgrading to 2×6 to increase R rating.
We do not know much, so I am.asking for guidance. It’s a two story 2200 sq ft house in rural w.PA. It’s situated on an open lot with no tree coverage “ontop” of home however the property is surrounded by them on all sides. The family room will have vaulted ceilings peaking at 12ft. In that room on the longest wall there will be a wall of Windows facing south.

I want to add the builder does use OSB in the corners of home

I feel like the SIS would be the better choice to keep the air in (energy efficient) however then the house won’t breath. On the flip side the OSB can rot and not dry if the R rating is to high or have thick wall according to an article found on here.

What would be better structurally?


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  1. GBA Editor
    Martin Holladay | | #1

    SIS comes in two thicknesses: either 1/2-inch thick (R-3) or 1-inch thick (R-5.5).

    The first question to ask your builder is, "What thickness do you plan to use?" In your climate zone (Climate Zone 5), the minimum R-value for rigid foam installed on the exterior of a 2x4 wall is R-5, so only the 1-inch SIS will work in your climate. For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    There are many Q&A threads on the GBA site covering this topic. Here is one: Advice on insulating sheathing in climate zone 5.

    If you use 1-inch thick SIS panels, your walls will have a higher R-value, and will perform better, than if you use ordinary OSB.

    There is no need for your walls to breathe.

    If you choose to install OSB plus a thick layer of exterior foam, the OSB will not be at risk of moisture accumulation, because it will stay warm and dry. If the wall is properly built, it will dry to the interior.

    Finally, there are lots of ways to improve the R-value of your walls. If your builders are planning to install fiberglass batts between the studs, you should know that fiberglass is a poor choice. If you want high-R walls, you should begin researching your choices here on the GBA web site.

  2. Vaseffa Fennick | | #2

    YThanks for your help. The SIS is an inch. Is it the better choice over the OSB? Structurely and efficienc

  3. GBA Editor
    Martin Holladay | | #3

    Structurally, the SIS will work fine. When it comes to thermal performance (energy efficiency), the SIS will work better than OSB.

    That doesn't necessarily mean that SIS plus 2x4 studs plus fiberglass insulation is a good way to build a wall. There are better walls. But the SIS is better than plain old OSB.

  4. Vaseffa Fennick | | #4

    Thanks again not to a be crazy person, how would you build a wall for your home?

  5. GBA Editor
    Martin Holladay | | #5

    I would say that the two most common high-R walls are double-stud walls (usually 12 inches thick and insulated with dense-packed cellulose) or 2x6 walls with 2 to 4 inches of exterior rigid foam (preferably polyisocyanurate). The best insulation to fill the 2x6 stud cavities is probably dense-packed cellulose.

    For more information, see these articles:

    Building an Energy-Efficient Home on a Budget

    Choosing a Cost-Effective Wall System

    Is Double-Stud Wall Construction the Path to Efficiency on a Budget?

    Six Proven Ways to Build Energy-Smart Walls

    GBA Encyclopedia: Double-Stud Walls

  6. Vaseffa Fennick | | #6

    Thanks for your help. I will be reading and taking notes!

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