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Hybrid 2×6 wall construction with 3.5″ of closed-cell spray foam between studs and 1″ of XPS outside the sheathing

JhpCHR8VLu | Posted in Energy Efficiency and Durability on

My question is if there are any ideas that would work in standard 2×6 wall construction to augment the benefits of spray foam insulation?

My initial intent when building this house was to upgrade from the traditional fiberglass batt and vapour barrier insulation method to 3.5″ of BASF closed cell spray foam in the 2×6 walls. There are other techiniques that offer better technical solutions but I have already had 2 reputation builders in the area walk away as they are not experienced nor want to get involved with the detail it takes to build a proper tight home. Therefore, following traditional prairie construction techniques turns out to be one of the important factors when selecting spray foam. (Dense pack cellulose is completely unknown here)

The downside is that there are many thermal bridges and the whole house heat loss is still not ideal. One thought is to add 1″ of XPS styrofoam to the outside of the sheathing to provide the thermal break across the entire envelope. The problem is that then the 1/2″ plywood sheething is sandwiched between two foam layers, can not dry and is susceptable to damage. (We are planning on using fiber cement siding with Tyvek Drainwrap as our house wrap.)

From other threads on this site it is apparent that applying exterior foam is a good idea but must be thick enough to keep the temperature gradient/dewpoint within the foam. The 4 inches required in our climate makes constructability a problem wheras one inch really has very little impact on window and door openings (cost) and attaching the siding. If you could spray the cc foam to the 1″ of XPS (deleting the exterior sheething) and have it stick it might be worth considering but there are likely other problems with doing this besides the fact that the builder I am working with now suggested that previous experience with building unsheethed walls meant extra work to keep the walls square when raising them and didn’t sound too keen. Maybe a dimpled membrane bewteen the foam and sheething would work like a rainscreen to allow the 1″ of foam but is this shorthcircuiting the insulation?

Back to the original question if there are any ideas that would work in standard 2×6 wall construction to augment the benefits of spray foam insulation….Maybe just go to a 2×8 wall and flash and bat but still does not eliminate the thermal bridging. Double stud walls are also an option that the builder was okay doing but besides the cost there is still some thermal bridging/short circuiting at the rim joist.

For the ceiling I am planning on spraying 2 inches of cc foam and blowing in 14 inches of cellulose or fiberglass. I am leaning towards cellulose so that if 10 years down the road the roof begins to leak water I will at least be able to look in the attic and see it happening by looking for the crust. Otherwise, how would you ever know and could be just breeding mold in your attic with no air flow at the interface between spray foam and blown in insulation where the water will collect. .

We are building our new house on a farm near Lloydminster Alberta Canada and are looking to build a tight house that will last a lifetime as this is the last home we will live in before our kids send us to the old folks home. It is a 2700 sq ft bungalow with walk out ICF basement and 900 sq ft attached garage on ICF footings (slab heating in basement and garage, forced air upstairs and dedicated dual core HRV).

If we do it wrong and have an issue with mold or toxin or chemical reaction then there is no easy solution as we do not have the option of selling and moving. My wife and sister both have hyperactive immune system symptoms so it is of very high concern. Any suggestions or guidance would be appreciated.

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  1. JhpCHR8VLu | | #1

    Sorry, the title should read between 2x6 studs not between 2x6 joists. I was too excited to get to the rest of my post I guess.

  2. GBA Editor
    Martin Holladay | | #2

    Lots of narration -- so much narration that the questions, if any, get lost.

    Q. "My question is if there are any ideas that would work in standard 2x6 wall construction to augment the benefits of spray foam insulation?"

    A. To reduce thermal bridging, your choices include: (a) Adding exterior foam sheathing (without plywood or OSB), (b) Adding 2x2 horizontal interior strapping, (c) Adding rigid foam on the interior of your walls.

    Q. "Maybe a dimpled membrane bewteen the foam and sheething would work like a rainscreen to allow the 1" of foam but is this shorthcircuiting the insulation?"

    A. A dimpled membrane in that location would improve drying potential slightly, but might undermine the wall's themal performance.

    Q. "Double stud walls are also an option that the builder was okay doing but besides the cost there is still some thermal bridging/short circuiting at the rim joist. "

    A. That's the best suggestion you've come up with so far.

    Q. "We are building our new house on a farm near Lloydminster, Alberta, Canada."

    A. Ah, finally! A clue about your climate!

    Q. "Any suggestions or guidance would be appreciated."

    A. Go with double-stud walls -- although it's a pity that no one in your area knows how to install dense-packed cellulose. You can always insulate your rim joists with closed-cell spray polyurethane foam.

    [And by the way -- I changed "joists" to "studs" in the thread title...]

  3. JhpCHR8VLu | | #3

    Thank you Martin, Sorry about the diatribe. I realized after I posted that there is more than one line of questioning and should have broken up the questioning in more than one post.
    The Building Science website shows a couple of double stud wall configurations. One uses spray foam with dense packed cellulose could this be modified in the flash and bat style?... Trying to get good insulative value and economy without cellulose is tricky.
    The offset frame wall they show looks like one of the best technical solutions for insulation as it also covers the rim joist but there is not enough structural detail in their basic brochure to even get a quote from a builder that is not familiar.
    One builder we talked with will build to a "R-2000" standard (older Canadian standard) which includes offset 2x4 stud walls but built on 2x6 plates so there is only an inch or so of isolation and uses cc spay foam at the same 3.5 to 4 inch thickness. Definitely an improvemnet over everything else in the area that is being built and the additional cost for the double wall was about $5k for the whole house so in my mind it was easily justified. Is this enough offset for a thermal break in our climate?
    Finally, and I know that this is a loaded question but is spray foam falling out of favour with green building? Some comments on these threads seems to be sceptical about the hype and so am I as I am about to invest nearly 30k into insulation and have to live with the results for ever but I don't have many options.

  4. JhpCHR8VLu | | #4

    By the way, I have learnt more from this website than any other source and I appreciate that you have taken the time to reply.

  5. GBA Editor
    Martin Holladay | | #5

    Q. "Is spray foam falling out of favor with green building?"

    A. I think so -- yes. Most closed-cell foams use blowing agents with a high global warming potential, and that is a matter of concern.

  6. jbmoyer | | #6


    I have skimmed over this thread so I may missed a couple of things.

    I would bag your original idea of closed cell spray foam and insulated sheathings. The OSB/plywood sheathing will be sandwiched between two low perm materials (2+ inches of closed cell spray foam starts to become a complete vapor barrier) and will have a hard time drying when it gets wet by the almost inevitable leak in the wall assembly.

    Martin's suggestion of using double walls and dense packed cellulose is good one, but If you don't have seismic issues in your area, and your builder can handle it, I would recommend the Larsen or Riversong truss for your exterior walls. This will reduce the thermal bringing at the rim joist.

    As to foam products being green? You will find countess hucksters out there that will promote foam as green. But in new construction it makes sense only in below grade applications, air sealing small penetrations, and perhaps at the rim joists. Using copious amounts of foam in above grade walls and roof assemblies makes ZERO sense in a green home.

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