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Buying Lumber for Double-Stud Framing

gstan | Posted in Green Building Techniques on

Is this the time to revise your plans and go double wall instead?
Lumber prices are falling – a lot – futures are down over 60% and
still dropping. Normally, I think double wall only makes sense in
climate zone 6 and above but this coming summer lumber may
get cheap enough to justify it in CZ5.

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  1. Tim_O | | #1

    I was interested in comparing costs of different high performance wall assemblies a few months back (lumber already low-ish). I put together a spreadsheet using pricing from Menards for just about everything. Specialty items I actually got quotes or dug around the internet. Broke it down by cost per sqft of wall area. Options and pricing were as follows:
    1) 12" double wall from 2x4s - $3.71
    2) 2x8 + ZipR6 - $4.48
    3) 2x8 Tstud - $3.80
    4) Mooney Wall from 2x8s - $3.46
    5) Bonfiglioli Wall from 2x8s - $3.26
    6) 2x6 with 2" Polyiso (I broke my rules here and referenced local salvage foam) - $3.82
    And the reference point:
    7) 2x6 + OSB (no interior strapping or membrane) $1.69

    All assemblies 1-6 should achieve true R of 30 or so. Except the double wall being ~ 48 at 12". I used an Intello membrane on the inside and Mento 3000 on the outside or Zip. All assemblies include 16" on center 2x3 strapping so there are no penetrations of the air barrier. Overkill assembly? Probably. All assemblies use dense pack cellulose for insulation. We are intending to owner build, so a bullet proof assembly is my goal, and the service cavity I'm hoping is a detail that enables that. I did not include sill/top plates and probably some other odds and ends that would likely be pretty similar between these assemblies.

    So with that said... while there are some price differences, it's pretty insignificant between all the multitude of options. What will play a much larger factor is labor I would think. 2x8+ZipR may be the most expensive, but it might also be the least expensive labor since it's the closest to "normal" techniques.

    I'm interested in seeing what happens over the next few years now that the 2021 IBC says R30 minimum for climate zone 4+ (R20+5 or R13+10 also being options). Of course, it takes a while for states to adopt the latest code...

    1. buildzilla | | #2

      wow, that's some pretty cool info you worked up there!

      since you are on a roll, how would u feel about adding a few that i was considering:

      8) 10" double from 2x4
      9) 2x6 with 1.5" comfortboard-80
      10) 2x6 with 3" comfortboard-80

      related question, for exterior insulation, did u attempt to factor in longer fasteners, or extra labor for window-bucks? (my strategy was to use thermal-bucks with outie windows which would incur extra cost of thermal-buck and adhesive)

      also, menards stocks t-studs?

      1. Tim_O | | #3

        10" vs 12" double wall is the same cost. Tiny savings on cellulose, but not much.

        I'm not sure the price of comfort board, I haven't looked very hard though.

        No, no accounting for window bucks since it's hard to count that by sqft. But I did count for long screws for the polyiso.

        Tstuds I got a quote from the manufacturer for the 2x8 bare naked Tstud. They are basically $2.50/foot. So $40 per 32sqft wall section at 24" on center. Doesn't account for corners.

  2. gstan | | #4

    Although futures are down over 60% (year to date) for large quantities,
    corresponding price reductions at your local supplier probably won't
    trickle down for several months yet - but it may well be considerably
    cheaper by next summer.
    I don't know where you are, of course, but it has been my experience
    that a lot (maybe most) building departments adopt new versions of the
    code only when it's politically expedient and then they feel free to modify
    as they see fit (I was a fat butt bureaucrat for years in one of the largest
    counties in the U.S. - ran the engineering dept & survey dept. & GIS dept.).
    Maybe your county, state, or whatever is better - anyway GOOD LUCK

  3. buildzilla | | #5

    gotcha tim. so those prices exclude labor (e.g. dense-pack install)?

    if so, they seem pretty close from a material perspective, i guess the challenge is figuring out associated labor with each strategy (cost and sourcing of labor would also vary by location)

  4. buildzilla | | #6

    i think comfortboard got caught up in supply-chain shenanigans, so much more expensive than a couple of years ago, i'm seeing ~1.30/sf for 1.5" (r6) at lowes

  5. buildzilla | | #7

    can anyone provide datapoints on installed cubic-foot price of dense-packed-cellulose?

    i'm guessing installing in a double-stud wall is more labor than installing in a 2x6 wall taking into account any possible cavity isolation techniques like running web across the gaps.

    material is easier to peg, trying to get closer to labor diff between a double-stud wall and a single-wall with exterior-board/zip-r, is more involved.

    another potentially significant aspect is the probability of pulling it off well with available labor.

    in other words:

    - how difficult is getting dense pack right
    - what does your local source of installers look like (good ones could b booked out)
    - what would the overall impact be of a less than stellar install
    -- for instance, with a double-stud assembly, a dense pack fail would have more impact, than with a 50% exterior board assembly

    i think these r the questions on everyones mind when considering wall-assemblies, and most people can't approach with a "money is no object" position

    1. Expert Member
      MALCOLM TAYLOR | | #8
    2. Tim_O | | #9

      That was sort of my point with the post too, the other factors are probably much larger to consider. Labor rates likely will be a factor of how comfortable the framer is with your proposed wall assembly. Or if you want to do some yourself, Mooney or Bonfiglioli become better options with a traditionally framed wall.

      I think my dense packed estimate was a rough installed price, but I can't remember actually... It's not that expensive really.

      For the comfortboard option, just add the price of comfortboard to the generic option 7 I made. Maybe $0.15/sqft for screws.

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