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Double stud wall, or better/fewer windows?

Keith Andrews | Posted in Energy Efficiency and Durability on

My wife and I are in the beginning stages of planning our next home build here in Maine and have been going back and forth with how we should spend our money.  On one hand, I would love to build with a double stud wall and try and reach r40-r45, but on the other hand, we are seeing some higher than expected estimates for our other costs which may limit us in how we approach our wall assembly.

If we can’t afford the extra expense for the raw material, insulation, and labor to construct the double wall, would there be a “next best thing” that we could do?  One thought was to just build with a traditional 2×6 wall, insulate with rock wool, or spray foam and add 1-2″ of rigid foam on the exterior (Zip r6?) and spend more money on better quality windows (triple-glaze), while minimizing windows on the north side and anywhere else we can.

I should mention we will be a slab on grade, 4″ of foam under the slab, two-story home, truss rafters, vinyl siding (metal in places for accent), open concept on the first floor, bedrooms on the second, with heat pumps for heating, and some type of HRV system.

Thanks in advance!

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    An IRC code min wall in Maine would be 2x6/R20 + R5 continuous insulation, or 2x4/R13 + R10.

    Where in Maine? (It matters- the climate ranges from the warm edge of zone 6 to the middle of zone 7.)

    R5 isn't sufficient for dew point control on R20 fiber in Maine, but R10 is sufficient for R13 or R15. It takes R11.25 for dew point control on R20 in a 2x6 wall. A 2x6/ R23 rock wool wall with 2" of foil faced polyiso would be marginal on dew point, but would work if using 2" Dow Thermax, which doesn't need to be derated for cold temperatures the way most polyiso does. The 2x6/R23 + R13 polyiso wall comes in around R30 "whole wall" after thermal bridging.

    In southern and coastal Maine a U0.20-U0.25-ish double low-E double pane can work, beating code min by a substantial amount. The second low E coating is on the surface in contact with the indoor air, and the glass temp will be lower than with a triple pane, which in the interior or in far northern Maine can result in copious window condensation during the coldest weather. A double low-E double pane is quite a bit cheaper than a triple pane of comparable performance.

    1. Keith Andrews | | #2

      Thank you for the information. We are building in Coastal Maine, Freeport to be exact. Budget is certainly a factor with the build so if we could do it with some sort of hybrid approach such as double pane low-e argon on the South and West and triple glaze on the North and East side, I'm happy to do that.

      1. Expert Member
        Dana Dorsett | | #3

        In Freeport a double low-E double pane is fine (for all sides of the house).

        With argon fill Cardinal Glass LoE-180 + i89 runs about U0.20 at center of glass , and is higher solar gain than typical off the shelf single low-E argon code-min windows. That's an option with several double-pane window vendors, including value-priced regional player Harvey (who refer to it as "Roomside Low-E"):

        https://www.harveybp.com/support-center/what-is-low-e/

  2. Expert Member
    Rick Evans | | #4

    Keith,

    Just a couple of thoughts:

    Be sure to apply for energy star ratings. We got back almost $6,000 for our pretty good house in NH.

    Regarding windows, it's not just about the number of panes- air leakage also plays a role. European tilt and turns have multiple locks (mine has 9 per window). I've seen triple pane windows from Marvin and Anderson that only have two locks. While heat loss probably isn't dramatic with these windows, there may be comfort issues. Talk to the guys at Pinnacle Windows in Augusta. Our Logic windows were cheaper than mid level Anderson windows at home Depot.

  3. Stephen Sheehy | | #5

    Don't forget to use reclaimed foam under the slab. You'll save a few thousand dollars to use for better windows or more insulation.

  4. thrifttrust | | #6

    R21 fiberglass batts are half the price of R23 mineral wool batts.

  5. Expert Member
    Peter Yost | | #7

    HI Keith -

    Interesting dilemma: putting money into windows vs opaque walls. In my own home, although it was a retrofit not new construction, I really wish I had put in fewer and higher quality windows. In the long run, windows that deliver less maintenance, less air leakage, greater comfort is worth every penny. Take a look at Matthews Brothers in your neck of the woods (https://mathewsbrothers.com): "local" and they do have triple-pane windows.

    Peter

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