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Zone 4A walls

Chris_King | Posted in Energy Efficiency and Durability on

Hey all,
Have 2×4 walls that I have been slowly ripping open to rip out old fiberglass and add R-15 Roxul. Hoping to start exterior insulation this year and was told by one contractor the easiest approach would be to buy windows for a 2×6 wall and just add 2” of exterior foam. Do you think 2” of polyiso on exterior plus the interior Roxul is enough insulation that I could in theory hit net zero with enough solar panels or would it be more economical to pay extra for more for the window work around insulation on the exterior to 3” and pay up for window framing work compared to extra panels?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Do you think 2 inches of polyiso on exterior plus the interior Roxul is enough insulation that I could in theory hit net zero with enough solar panels?"

    A. You haven't provided us anywhere near enough information to answer that question, which needs sophisticated energy modeling to answer. (If you intend to install your PV array on your roof, one of the many variables is the unshaded area of your south-facing roof slope. You didn't tell us that area.)

    One well-respected group of consultants, the Building Science Corporation, recommends that builders in Climate Zone 4 aim for R-25 walls. That isn't necessarily the answer to your question, of course, but it's a starting point for discussion. For more information, see this article: R-Value Advice from Building Science Corporation.

  2. Chris_King | | #2

    Thanks Martin for the link I’ll check it out. Figured it would be a more in depth answer with a t lot of variables was just hoping for a guideline which BSC seems to have.

  3. Expert Member
    Dana Dorsett | | #3

    With enough solar panels an uninsulated tent can hit Net Zero.

    A good starting point would be Table 2 on p.10 of this document:

    ote those are "whole-assembly Rs. A 2x4/R15 wall + sheathing & siding + wallboard comes in at about R11 whole-wall. Adding 2" of polyiso brings it up to the R21-R24 range, depending on product and local climate.

    With a reasonable house shape (=not 27 corners with 10 dormers) and reasonable shading factors (not in the deep woods on the north side of a hill) it's usually possible to hit Net Zero with a PV array that fits on the house.

  4. Jon_R | | #4

    Net Zero isn't likely to be optimal in terms of the environment, $ or any combination of these. So I'm curious why you are interested in it.

    Consider using BEopt for optimization.

  5. Chris_King | | #5

    Thanks guys for the info. It’s a long ranch east west facing but enough roof space to make it work. Jon in regards to net zero I want to be as close as possible that it makes economic sense. Another factor is I live in a small village in the pine barrens of nj and when power goes out for us it’s out for awhile like during sandy. Due to a small amount of people in our area we are the bottom of the work order. So for storms like sandy we were out for two weeks. So the closer to net zero I can get the less money on batteries I will also need in the future.

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