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How’s this for a wall assembly?

David McNeely | Posted in Green Building Techniques on

Drawing attached: 2×4 studs; 1″ x 2″ spacers—made from foam panels—glued to studs as required; plywood sheathing nailed to studs (1″ space provided by spacers); minimum 2″ closed-cell spray foam filling up the space between plywood and studs, and another 1″ of stud bays also. This would provide a thermal barrier; an air barrier; a vapor barrier; a lot of shear value and overall strength; if windows are nailed to the plywood it would provide a complete seal there also; it would provide a continuous seal around outside corners and at all rim joists. 2″ of closed cell foam = R-13; add another 2 1/2″ of dense packed cellulose for an R-22 assembly. If required a higher R value could be achieved by adding foam under siding or going to a 2 x 6 assembly.
Clearly some jigs would have to be made to speed up the assembly, but for the extra hassle it seems to solve a lot of problems and offer a lot of benefits. Am I crazy?

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Replies

  1. Expert Member
    Armando Cobo | | #1

    Here is a close up picture of your detail.

  2. TJ Elder | | #2

    David, there are a few problems here. First, in your drawing the foam strips run perpendicular to the studs, so there's just a 16" grid of contact points between the foam strips and the studs. That could never be enough support for a panel, whether that's structural sheathing or drywall. Second, there's no shear value to spray foam, it's just not dense enough. At best you might improve the effectiveness of shear plywood by essentially gluing the studs to the panel, which a closed cell foam might do. But the plywood must have direct contact with the studs, and if you really want to improve shear strength what you need is extra nails. You can double the usable shear strength of 1/2" plywood sheathing by using twice as many nails. Shear walls in seismic zones sometimes have nails as close as 2" on center at the panel edges.

    The next issue is your cellulose layer is too thin to be practical. There must be some depth in the wall profile so the material can flow vertically as it's being dense-packed, if this is blown dry fiber. Maybe if it's damp sprayed it could be installed this thin, but it doesn't seem worthwhile.

  3. David McNeely | | #3

    Reply to Thomas:
    I’m sorry I wasn’t more clear: I am proposing that the rigid foam pieces are spacer blocks, not furring strips—say 1" foam cut into 1 ½" x 2" pieces. The spray foam would thus be in continuous contact with sheathing.

    Regarding shear value, I’ve read several studies that indicate 2lb. foam significantly increases shear strength. Here is one that reports a resistance to a racking load that is about double compared to just studs and plywood, when stud bays are completely filled with 1 1/2lb foam:

    http://sprayfoamsaskatchewan.com/foam-insulation-strengthens.html

    I’ve also read reports that indicate the adhesive qualities of foam are significant although not often included in calcs. When I have time I’ll try to dig out one that is representative from BuildingScience.com.

    My proposed application combines these qualities: the 2lb foam between stud and sheathing would perform the function of extra nails, because the adhesion is nearly continuous; and strength is further increased by the foam overlapping the studs, creating a very rigid structure. I conceive this assembly to be like an SIP that is built on the jobsite, one that integrates the studs structurally. As you say, plywood attached to studs require lots of nails to create a shear panel. But SIPs do not derive their shear value from nails.

  4. TJ Elder | | #4

    David,

    Plywood sheathing will not brace the frame effectively if there's a spacer made of foam, because the nails will have to span through that 1" space. Under lateral load the nails will bend and allow the frame to wrack. In fact if you try to build a mock-up of this wall, I would guess you'll find it impossible to install the sheathing flat when it's not fully supported by the stud frame.

    And again, the foam blocks and spray foam are both too soft to resist significant shear load. If it's strength you're after, see what engineers recommend to brace a light frame--plywood and plenty of nails. Here's some information about shear walls: http://www.naffainc.com/x/cb2/Struct/Sheawall%20basics.pdf.

    Sticky foam may add some rigidity but I don't think many engineers would specify it for this purpose. As a material it's not inexpensive and has other downsides, including potential toxic outgassing and the fact that it becomes a mess to dispose of at the end of a building's lifespan. This is sometimes described as a "monstrous hybrid", meaning an amalgam of materials that can't be effectively reused or recycled because they're permanently stuck together.

  5. David McNeely | | #5

    Thomas,

    This is from the website you cited:
    “Foam Core Panels - This is a manufactured product in which solid structural foam takes the place of wood or metal studs between interior and exterior sheathing. In almost every way a foam core panel may be treated as you would a wood shear wall as far as shear-resistance is concerned.”

    I am proposing that my assembly is acting as a foam panel that is adhered to the studs, providing the benefits of both for a fraction of the cost of SIPs.

    The foam spacer blocks are acting as stand-offs and would keep the plywood flat. They would be spaced at intervals easily spanned by the plywood, allowing the nails to be driven anywhere on the stud.

    Regarding foam being “too soft to resist significant shear load,” this is from Lstiburek (http://www.buildingscience.com/documents/insights/bsi-048-exterior-spray-foam/):
    “We have done lots of tests. We have found that you can pretty much hang an SUV off the wall using screw stand-offs and SPF.” This was in the context of using screws only to support the lateral loads of the cladding. Since my assembly also benefits from the direct adhesion of the foam to the plywood, the strength would be even greater.

    I appreciate your concern that eventually disposing of 2" of spray foam would be messy at the end of the lifespan of the structure. I find myself considering it because of its utility. In his essay on “The Perfect Wall,” Lstiburek especially likes “The Clever Wall,” which he says is clever specifically because it uses 2lb. foam to serve as water control layer, air control layer, vapor control layer, and thermal control layer. To these benefits I believe I am adding shear strength with this assembly (but not water control).

    Finally, I haven’t read anywhere that toxic outgassing is a concern after the curing period of a few days. I’d appreciate you sharing your cites for this concern.

  6. David Argilla | | #6

    I'm not an engineer or a contractor, but looking at your diagram and references I have some questions. It looks like the reference to hanging an SUV off of standoffs was for the dead loads of the cladding, not active shaking. Is there a difference?
    The screws in the reference were also run through a solid sleeve, the increasing surface area would spread the forces out a little more than you will see if you just have a nail through foam. Are you not concerned that during high lateral forces (especially shaking back and forth) such as you would see in an earthquake or high winds that the nail would incrementally deform the foam, to the point that your nail would eventually start to bend?

    Also, your diagram shows only foam attached to one panel. Isn't a foam core panel made up of a layer of foam sandwhiched between 2 panel?
    Just curious...

  7. David McNeely | | #7

    David Argilla,

    Imagine gluing 1" plywood to ½". Now glue the other side of the 1" to a stud wall—not just with a bead of construction adhesive but with the real stuff. Now cut 1" plywood to fit perfectly between each stud and from plate to plate, and glue it all together. Add some 16d nails going up the studs. This wall would be an absolutely stiff membrane and nearly indestructible. Can you imagine the forces required to make those nails bend?

    In my proposal I’ve replaced the two pieces of 1" plywood with SPF. I understand SPF to have more than adequate adhesive properties, and the nails wouldn’t even be required. Take a gander at this website to see how simply using a bead of SPF to glue studs or rafters to the exterior membrane improved resistance to hurricanes by two–three times:

    http://nesbuildingsolutions.com/images/Hurricane%20Article.pdf

    Your rhetorical question about an SIP having two panels was of course true. Are both panels required to provide shear value? No: even shear panels engineered for seismic resistance have only one sheet of plywood.

    So: split an SIP such that it has 2” of foam, and magically ooze it onto a stud wall so that it goes around the studs and plates with terrific adhesion (336 sq.in. gluing surface per 8’ stud). Now replace the styrofoam of the SIP with 2lb. SPF—much denser, stiffer, and better R value. It still looks like an incredibly solid membrane to me. And it still has those great qualities of vapor barrier, air barrier, and a thermal break that even spans corners and rim joists. And you don’t have to worry about all those persnickety details of innie or outtie windows : ).

  8. John Brooks | | #8

    David M,
    I will give you 10 points for the Outie Window... Go Outies!

    You are going to have a hard time convincing Thomas J, David A. and the Code official that your stratagem is equal or better than SIP or conventional Shear wall Bracing.

    I also see a problem with "Build-Ability"
    have you tried to build a mock-up? ... I think it is going to be a fussy method.

    In Addition...you still have a "Turd" of a wall assembly.
    COLD SHEATHING
    which may not be so terrible ;--)
    https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/19969/apples-turds-mineral-wool

    So.... if you must build a turd-of-a-wall.....
    I think a Double Stud or Hatched wall with Airtight Exterior Sheathing would be much more Build-Able .....and certainly Not-So-Foamy

    Why is your way better than a double wall with Airtight Sheathing and or Airtight Drywall?
    Why do you need a Vapor Barrier?
    If you're going to set up netting to blow cellulose ... why not get your money's worth and go ALL Cellulose?

    Or an Arctic Wall (fussy but not-a-turd-wall)

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