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Cost comparisons of R40 wall designs

Jaero | Posted in General Questions on

I am planning a new house to be built in southwestern PA (climate zone 5).  I’m following the 10, 20, 40, 60 rule of thumb for insulation R values, but I’m finding it hard to select an R40 design for the above grade walls.

The wall I most liked was a 2×8, staggered stud wall with 3.75″ of cc spray foam in the cavities against the exterior sheathing, then 3.5″ of high density fiberglass bats.  This would achieve R36+ for the insulation, reduce much of the thermal bridging and would benefit from the air sealing provided by the spray foam.  However, the cost estimate I received for that much spray foam was pushing $32k.  There must be a lower cost alternative.

In all the many threads that I’ve read on GBA and elsewhere, I don’t recall seeing a cost comparison for various wall designs.  Does that exist somewhere? 

For new construction and targeting R40, what is the most cost effective wall design?  Opinions…

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Replies

  1. Zdesign | | #1

    Are you factoring in labor for your assembly or only looking at material cost and doing a self install. I looked at my assembly as cost of material per 100 sf to come up with a material cost per SF. Keep in mind your 35% of rigid foam on the outside for Zone 5 for thermal bridging. My cost effective assembly worked out to be 2x6s 24 oc w/R23 Rockwool, Zip sheathing, and then Kool Therm 2" K12 framing board over the top for a total of R40. Going to 2x8s requires more exterior insulation making it less cost effective and the cost of Zip R9 or 12 far exceeds the cost of the Kool Therm board.

    1. Jaero | | #3

      Thanks for the reply. I am not familiar with Kool Therm framing board; I will check it out.

      Cost of labor has to be considered. There will be little self-install work - at least with insulation.

      Part of the problem is that the 2 builders that I have been in discussions with have no experience (or interest it seems) in installing exterior board insulation. While I have considered wall designs with exterior insulation, I have not found a builder willing to build them.

      So the wall I had been favoring has 2x8 plates with 2x4 staggered studs (12" oc). All the insulation would then be inside the stud cavities. This would only address the thermal bridging of the studs; thermal bridging of the plates would not be addressed.

  2. Aedi | | #2

    There is an old building science corporation article that compares different high performance walls: https://buildingscience.com/sites/default/files/migrate/pdf/BA-0903_High-R_Value_Walls_Case_Study_rev_2014.pdf

    Closed cell spray foam is consistently the most expensive option, and IMO not a good choice in new construction.

    The two most common sane/cheap options for high performance walls are:

    -2x6 24" O.C. with R19 batts, taped OSB sheathing, and a few inches of exterior foam insulation (2" for a ~R30 wall, 3" for ~R36, and 4" for ~R42)
    -2x4 double stud (10" for ~R35, 12" for ~R40)

    If reclaimed foam is available in your area, then exterior foam is probably your best bet. You can find plenty of information on these assemblies on GBA.

  3. Expert Member
    Dana Dorsett | | #4

    >"I am planning a new house to be built in southwestern PA (climate zone 5). I’m following the 10, 20, 40, 60 rule of thumb for insulation R values"

    Why? That's arguably excessive for a zone 5 location (that will probably be zone 4 before long...)

    Table 2 in BA-1005 suggests something a bit less than that for zone 5, and even zone 4 might be closer to whats financially rational with current technology heat pumps and the lower cost of energy (even renewable energy) than 10 years ago when that was published:

    For zone 5 that table would be more like a 7.5, 15, 30, 65 rule (with R4 windows)

    For zone 4 it would be 7.5, 15, 25, 60 rule, (with R3.3 windows)

    Their zone 6 numbers would be pretty close to the Pretty Good House numbers that I'm assuming you got the "10, 20, 40, 60" (with R5 windows) from.

    Rather than bending over backwards while jumping through hoops for an R40 whole-wall shoot for R30, and be OK with R25.

    >"The wall I most liked was a 2×8, staggered stud wall with 3.75″ of cc spray foam in the cavities against the exterior sheathing, then 3.5″ of high density fiberglass bats. "

    I hate it- especially for the the excessive amount of closed cell foam that is installed between studs. With that approach the sheathing can only dry toward the exterior, and that much foam is an affront to the environment for the paltry amount of thermal improvement it provides. The staggered stud approach also still has significant thermal bridging at the top & bottom plates & subfloor.

    At about the same wall thickness 2x6/R21 studwall with 2" of polyiso on the exterior has about the same performance, uses less than half the polymer, keeps the structural sheathing warmer/drier/ mold-free, and is easier to build. That comes in around R30 whole wall if going with 24" o.c. studs, and single top plates to reduce the framing fraction. With more traditional framing it's about R27 whole-wall.

    A 2x4/R15 wall with studs 16"o.c. with 3.5" of exterior polyiso does even better a bit north of R30 whole-wall, but it 3.5" foam is a bit harder to build than 2" foam, and almost as much polymer as that 3.75" closed cell staggered stud thing.

    The BA-0903 document Aedi linked to is a good reference, with an excellent discussion of the performance and degree of difficulty for each approach. In Table 2, take a look at wall 2a & 2b. Wall 2b has a center-cavity R of about R38, and a whole-wall R of about R35. Swapping in 3.5" of polyiso for the 4" of XPS , R21 for the R19s would be about the same performance level. But there isn't much rationale for R35 in climate zone 5, (soon to be zone 4, at least within the lifecycle of a house.)

    https://buildingscience.com/sites/default/files/migrate/pdf/BA-0903_High-R_Value_Walls_Case_Study_rev_2014.pdf

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