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Community and Q&A

Double stud framing insulation

R Ferd | Posted in Green Building Techniques on

We are building our full time retirement home in zone 5 in Lakeside Arizona. We want to use a double studded framing system to take advantage of the thermal break option.. What are our best options for insulation? and what are the R values of each option listed. See attachment:
1:Open Cell Foam Insulation 2×4 primary Dense Pack Cellulose / wool insulation for 2×4 secondary wall
Option 2:All dense Pack cellulose or wool
Option 3 Use 2×6 with either open cell foam or dense pack cellulose or wool 5 ½ inches
Option 4 2×8 with either open cell foam or dense pack cellulose or wool 7 ½ inches
foam insulation.
Which option would you recommend and why

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    R Ferd,
    I think that dense-packed cellulose insulation makes the most sense if you are set on framing with double-stud walls.

    But there are lots of questions here, including these: (1) Are you sure that a double-stud wall makes sense for your project? (2) Are there insulation contractors in your area who are experienced at dense packing cellulose in double-stud walls?

    For more information, see How to Design a Wall.

  2. R Ferd | | #2

    No we are not set on double stud walls just learned about the thermal break option. Our framer has no experience in this. We are moving into a climate that gets as cold as 7 degrees - 90 degrees. So we want to be as efficient as possible. I do not know if any of the insulators have worked with double studs but it seams to be a good option for the climate. Would we get the same r value by framing with a 2x6 with the dense packed cellulose? If we use a 2 x 6 wouldn't we loose the thermal break concept?

  3. User avatar GBA Editor
    Martin Holladay | | #3

    R Ferd,
    Once again, I urge you to read the article I linked to, How to Design a Wall. As is explained in that article, you can interrupt thermal bridging through the studs of a 2x6 wall by installing a continuous layer of exterior insulation (rigid foam or mineral wool).

    When designing a house, there is no such thing as "as efficient as possible." You need to look for cost-effective solutions that work for you climate and your budget. The designer of your home will use energy modeling software (and an understanding of construction costs) when making these judgments.

    If insulation contractors in your area aren't familiar with dense-packing double-stud walls, I don't advise you to choose that approach.

  4. User avatar
    Dana Dorsett | | #4

    Dense packing a double studwall adequately requires some experience. Open cell foam is probably a safer bet for a double studwall unless you have a cellulose contractor who has done a number of double studwall or Larson Truss walls and can guarantee that it has sufficient density everywhere.

    A 2x6 wall with dense packed cellulose or open cell foam comes in at about R15 "whole wall" give or take, after factoring in the thermal bridging, and the R-values of the wallboard, siding, sheathing, interior & exterior air films, etc. At the same wall thickness a 2x4 studwall with open cell or cellulose with 2" of rigid exterior polyisocyanurate foam comes in north of R20 whole wall. With 3" of polyiso it'll be north of R25 whole wall.

    A 2x8 framed open cell or cellulose wall no insulating sheathing wall would also come in around R20 whole-wall, but a significantly fatter wall.

    With a double-stud you can pick the thermal performance by the thickness.

    Take a look at the stack-ups and whole-wall performance of the walls described in this document, as well as the discussions for each regarding costs, risks, and build-ability:

    Case #4 is a double studwall with 9.5" of cellulose coming in at a whole-wall R of ~R30 ( see Table 3). The performance would be about the same with 9.5" of open cell foam. The discussion for Case #4 starts on p.37 (p.41 in PDF pagination).

  5. R Ferd | | #5

    We are thinking of a 2 x 6 wall with closed cell foam between the studs. On the outside 1.5 inch polyisocyanurate foil faced. Buck out the openings with 2x4. Smart-side lap siding exterior. One half inch OSB shear.
    Do we need a Tyvek barrier? Does the foil face replace the Tyvek?
    R value on Polyiso seems to range between 7.9 and 9.3 depending on the manufacturer. That seems to be quite a difference for the same 1 1/2" foam?
    There is a un-vented crawlspace and attic. Two inches of closed foam in the crawlspace. Closed foam on the underside of the roof deck, and blown in insulation on top of the ceiling.
    This seems to be our best option in terms of initial and monthly costs?

  6. User avatar
    Dana Dorsett | | #6

    Closed cell foam between the studs is a waste of high environmental impact foam. Despite the much higher center-cavity R, the closed cell foam only adds ~R2 to the whole-wall performance due to the thermal bridging.

    A higher performance lower impact approach would use open cell foam or cellulose in the wall cavities, and adding a half-inch to the thickness of the exterior foam. See:

    Bottom line: Save the high R/inch foam for where it can be applied in continuous layers where it can reap the full benefit, and is not thermally bridged by framing.

    A 2" shot of closed cell foam in the crawlspace doesn't quite make code-minimum for a zone 5 foundation. The IRC 2015 calls out R15 continuous insulation. A minimalist 2" + 2" EPS insulated concrete form (ICF) would get you there at ~R16, and the exterior foam of the ICF would align nicely with the framed wall's foam if the sill plate is placed at the edge of the foundation.

    The R/inch of polyisocyanurate varies with density and blowing agents. For exterior sheathing applications for most vendors it would have to be derated to ~R5/inch (R7.5 for 1.5" , R10 for 2") in your climate & stackup. But Dow claims to have found the magic mix that does not suffer from cold temperate derating issues the way most polyiso does. See figures 2 & 3 in this document:

    Note, the temperature scale is the mean temperature through the foam layer which will be higher than the outdoor temperature in winter.

    Insulate either at the roof deck or the attic floor, not both. In zone 5 insulating at the roof deck requires a minimum of 40% or more of the R-value to be on the exterior side of the fiber insulation, whether above the roof deck or below, with the fiber insulation tight to the foam (or the roof deck, depending on where the foam is located in the stackup. It is substantially cheaper to specify a deep energy heel truss to accommodate 20-22" of cellulose for an R65-R75 attic than it is to do a code-minimum R49 foam + fiber hybrid at the roof deck.

    If the ducts are going to be above the ceiling, specifying a plenum truss to keep all of it inside the insulation and pressure boundary of the house is also cheaper than insulating at the roof deck.

  7. User avatar GBA Editor
    Martin Holladay | | #7

    R Ferd,
    Once again, I advise you to read How to Design a Wall. That article explains why you don't want to install closed-cell spray foam between the studs along with exterior rigid foam. (Dana -- who is giving you good advice -- explained it as well.)

    Q. "Do we need a Tyvek barrier? Does the foil face replace the Tyvek?" For more information on this issue, see Using Rigid Foam As a Water-Resistive Barrier.

  8. R Ferd | | #8

    We are leaning towards the 2X6 framed walls with open cell foam. In addition, two inches of polyiso.
    as a thermal barrier. We're going to frame, shear, and cover the walls with Tyvek until we sell our existing residence. The project may sit for a few months. Resuming construction should we use 2" foil faced polyiso or no foil faced? We can certainly use felt paper rather than Tyvek for house wrap?
    At this point with opposition with our framer, we'd like to do more than 2X6 open cell walls, but that is what he is comfortable doing.
    I'm thinking of using the 2" polyiso and set it on top of the 8" CMU stem? Could 1" EPS serve as a thermal break?
    We appreciate the articles, but doesn't help us enough to show, explain, and justify our desires on our project.
    Thank you

  9. User avatar GBA Editor
    Martin Holladay | | #9

    R Ferd,
    Q. "Should we use 2-inch foil-faced polyiso or no foil faced?"

    A. Either foil-faced polyiso or polyiso with a different facing will work. If the foil facing is adjacent to an air space (as it would be if you install furring strips on the exterior side of the polyiso), then the foil facing will add about R-1 or R-2 to the R-value of the adjacent air space.

    Q. "We can certainly use felt paper rather than Tyvek for housewrap?"

    A. Yes, either material is acceptable as a water-resistive barrier.

    Q. "I'm thinking of using the 2-inch polyiso and set it on top of the 8-inch CMU stem?"

    A. I'm not sure what you mean by "set it on top." If you are thinking of installing polyiso on the exterior side of a CMU foundation wall, you should know that polyiso cannot be used in contact with soil.

    Q. "Could 1-inch EPS serve as a thermal break?"

    A. I'm not sure what you mean by "serve as a thermal break." If you want to install a continuous layer of rigid foam on the exterior side of a 2x6 wall in Climate Zone 5, then the minimum R-value for that rigid foam is R-7.5. If that's the purpose you are thinking of, then 1 inch of EPS isn't enough. For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  10. User avatar
    Dana Dorsett | | #10

    Rigid foam can't go between the sill plate and the top of the foundation wall without the engineering analysis, since the foam would have the full weight of the house bearing down on it. Use an EPDM sill gasket instead, which tend to seal fairly well, and is an excellent capillary break keeping any ground moisture wicking up the stemwall from getting into the foundation sill.

    For the insulating sheathing, it can be either foil or foam faced. If there is an air gap between the foam and the siding, a foil facer is preferable, giving it another ~R1 of average performance.

    Using #15 felt instead of Tyvek is fine.

    The stem walls /crawl space will need at least R15 of continuous insulation to meet current IRC 2015 code:

    That R15 for the crawlspace walls can be all on the interior, all on the exterior or split beween them. With 2" polyiso on the above grade framed walls it makes sense to put 2" of EPS on the stem walls (R8) so that the exterior planes of the foundation foam and framed wall foam align, with Z-flashing directing any moisture coming down the #15 felt (or Tyvek) to the exterior side of the EPS. The EPS can be protected from the elements /finished with a cementicious EIFS type material such as Quikrete Foam Coating. The interior side of the stem wall would also get 2" of EPS, bringing the total to R16.

  11. R Ferd | | #11

    I'd like to nail the 2" polyiso to the Tyvek covered OSB and then nail the lap siding to the polyiso. I could extend it down the rim joists to the 2X8 sole plate. The lap siding will extend 2" below the top of the CMU.
    We'll use closed cell tin the crawl space.

  12. User avatar GBA Editor
    Martin Holladay | | #12

    R Ferd,
    If you want to install lap siding over 2 inches of rigid foam, I recommend that you install vertical furring strips on the exterior side of the foam rather than trying to nail the siding directly to the rigid foam with long nails.

    Before you proceed with your plan, make sure that the manufacturer of the lap siding approves of that type of installation.

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