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Q&A Spotlight

Choosing the Right Wall Assembly (2015)

An owner-builder planning a house in Nebraska wrestles with details for a cost-effective, practical design. Is a 2x8 stud wall the way to go?

A double stud wall is one way of leaving lots of room for insulation, and it may be more practical and less expensive than trying to build walls with 2x8 studs. This wall is part of a townhouse project in Boston built by Urbanica under the city's E+ Green Building Demonstration Program.
Image Credit: Urbanica

Southwest Nebraska sounds like the kind of place that gets all kinds of weather: hot and occasionally humid summers, cold winters, and by many accounts lots of wind. This is where Nicholas C will be building his house, and the question is, how?

He’s done so much reading on the subject that he’s now confused by the number of options he has. Getting it right is important because Nicholas plans on living in the house for a long time.

His best thinking so far? A 2×8 stud wall framed on 16-inch centers and insulated with blown-in cellulose, then wrapped in 2 inches of rigid foam insulation.

Spacing studs farther apart would mean a savings in lumber, and less thermal bridging through the framing. But for purposes of hanging drywall and cabinets, 24-inch on-center framing doesn’t interest him.

“How does my method sound?” he asks in a Q&A post at GreenBuildingAdvisor. “I am aware spray foam offers better insulation, but I believe it may be too costly.”

Other possibilities he’s considering include using more rigid foam inside the stud bays, or sealing walls with a thin coat of spray foam and then filling the remainder of the stud bays with cellulose (what’s called the “flash and fillt” method).

“Is my method flawed?” Nicholas asks. “Have I combined too many methods into one big mess?”

That’s the focus for this Q&A Spotlight.

Walls framed with 2x8s are really heavy

For one thing, warns Maine builder Dan Kolbert, those 2×8 walls will be very heavy, and more expensive than building a double 2×4 wall.

“I think you’ll find your idea is perhaps the least cost-effective method,” he writes.

Kolbert can buy an 8-foot 2×4 for $3.25, and a 2×8 of the same length for $7.45, so building two 2×4 walls is about 14 percent cheaper than a single 2×8 wall in terms of lumber. What’s more, the 2×8 wall will be a lot heavier than a 2×4 wall, so Nicholas will need a bigger crew to lift the walls into place, or he’ll have to resort to wall jacks or even a crane to tip them up.

And then there is the question of getting all the R-value he wants.

“If you do two separate walls, you can make them however far apart you want,” Kolbert says. “We have typically built double walls with a total thickness of 11-12 inches. You’d be breaking the thermal bridge with the exterior foam, but you could get rid of the expensive and chemically nasty foam by just making your wall thicker. Plus exterior foam leads to a lot of extra labor for siding and window treatment.”

Later, Kolbert adds, “And framing with 2×4 walls is fast, cheap, easy. The double wall flies up. You could easily go to 24-inch [on-center] on the double wall, since it’s not doing anything structural. You want to stick to 16-inch o.c. on exterior walls.”

Think about more exterior foam, not wider studs

GBA senior editor Martin Holladay likes the idea of wrapping the exterior walls with rigid foam insulation. But if the goal is to build a well insulated wall, he adds, it would make more sense to beef up the foam thickness rather than opt for 2×8 studs and more insulation in the stud bays.

“In this type of wall,” Holladay writes, “the rigid foam is doing most of the insulating. (The problem with inserting insulation between the studs is that the studs act as thermal bridges, degrading the performance of the wall.)

“A 2×6 wall with exterior rigid foam makes a lot of sense,” he continues. “If you want a wall that performs better than a 2×6 wall, what you need to do is to add thicker foam (in other words, go to 3 inches of foam). You’ll get much better results doing that than upgrading from 2×6 studs to 2×8 studs — and the thicker foam is a much better use of your money.”

Nicholas, however, still isn’t sure about the exterior foam option. Although his original plan called for 2 inches of rigid foam on the outside of his sheathing, it’s not common practice in his area, and if he could skip the foam altogether and still get a well insulated house, it sounds like he’s favor that approach.

Are double-stud walls a fire danger?

One of Nicholas’s reservations about building double-stud walls is the potential for violating fire safety codes, presumably because an air gap between the walls could act like a flue in the event of a fire — in much the same way that balloon-framed walls once did before platform framing introduced fire stops at each floor.

“The fire safety issue I am regarding is the fact there has to be something above so there is not just a void between the two walls that opens to the attic,” Nicholas says. “Perhaps a piece of plywood, fire Sheetrock, or a top plate? I do not know about the exact code here, but I also believe I have to have ‘fire blocking’ inside any exterior wall.”

Actually, Kolbert says, there are no fire issues. “Think of the framing as a house built with 2×4 walls,” he says. “Then you just build another wall around the perimeter of the house before your interior partitions. No fire issues — the decks on each floor block things.”

Malcolm Taylor adds that in his area, building codes allow the insulation to act as a fire stop, so no continuous top plate is required. “But,” he adds, “it’s probably a good idea to see what your code says.”

The type of foam is important, too

If Nicholas ends up choosing a wall assembly that incorporates rigid foam, he’ll have a number of options, including polyisocyanurate (polyiso for short), extruded polystyrene (XPS), and expanded polystyrene (EPS). They offer different insulating properties, and have different environmental impacts.

Polyiso, for example, has a nominal R-value of about 6 per inch, Holladay tells Nicholas, but it doesn’t perform as well in cold weather, so it should be “derated” to R-5 (or less) per inch when it gets cold.

Instead of using 2 inches of polyiso, Nicholas might use a sandwich of polyiso on the interior and EPS on the exterior.

The type of foam Nicholas was originally considering is Foamular, made by Owens Corning, because it’s available locally.

That, Charlie Sullivan says, is XPS, and the problem with XPS is the blowing agent used to make it has a global warming potential 1,000 times that of carbon dioxide. “That’s a good reason to opt for EPS instead,” Sullivan says. “EPS has a global warming impact that is 200 times lower, and is lower cost. Lower cost per inch thickness, and also lower cost per unit R-value.”

In addition, Sullivan says, Nicholas may be able to save some money by buying EPS that’s made into insulation locally, rather than buying at a big box store. (Sullivan refers him to a directory of EPS manufacturers posted by the EPS Industry Alliance.)

Reclaimed XPS is another option, adds Stephen Sheehy. It’s often used to insulate commercial roofs, and typically isn’t reused when the roofs are replaced. “It is priced at about half the cost of new,” he says, “and other than being dusty is in great shape. Just Google ‘reclaimed foam.’”

Great idea, Nicholas replies, but reclaimed foam may not be available in rural areas with low population densities — like the part of Nebraska where he intends to build.

Our expert’s opinion

GBA technical director Peter Yost adds these thoughts:

I thought that I would add to my own cut on this question by polling members of our Sustainable Energy Outreach Network Building Science Guild here in southeastern Vermont (and being Climate Zone 5b compared to Zone 6 makes the discussion even easier).

Without exception, our builders characterize the 2×4, staggered-stud, double wall — insulated with cellulose cavity fill insulation — as the most economical, efficient, and easiest-to-build high performance wall, for the following reasons:

  • If you can easily build and erect one 2×4 wall, you can easily build and erect another.
  • The materials — 2x4s and cellulose insulation — are inexpensive, about as environmentally benign as you can get, and available everywhere.
  • It’s easy to run mechanicals between the two walls.
  • The wall overcomes thermal bridging.
  • The non-structural wall can be framed on whatever framing centers you want and made from whatever you want (light gauge steel or 2x3s, for example).
  • The lack of exterior rigid foam simplifies door and window installation.

There are some caveats:

  • Windows and doors: pushed to the exterior, flanged units are pretty much a “no-change,” but some homeowners find the aesthetics of this challenging. But when placed in any other location in a thick-wall profile, there are details to work out.
  • Which wall is the structural one? If the interior wall is structural, you can extend the second one out over the foundation exterior plane to align the above-grade wall with the rigid insulation on the outside of the foundation. In terms of sequencing, it is easier to make the exterior wall structural so you can build the interior, non-structural wall more at your leisure, and after the building is weather-tight. And then the alignment of barriers for the roof is just a consideration of which wall has the air barrier.
  • Recommendation/requirement for vented cladding? In Climate Zone 5b, you can get away with a Class III interior vapor retarder if you use vented cladding.

Although the question of hygrothermal performance did not really come up (hygrothermal performance being commensurate management of moisture along with energy), we do have to more carefully consider moisture management with these deeper, more energy-miserly walls. So a few more considerations:

  • Carefully read this blog by Martin Holladay.
  • All of the discussion above presumes that the wall has continuous bulk water barriers and air barriers; let’s not add the moisture concerns of flashing leaks or air-transported moisture to the mix.
  • The thicker the deep wall and the colder the climate, the colder the exterior sheathing in the winter and more likely condensation on this first condensing surface. Consider a more forgiving exterior sheathing (plywood, or even board sheathing) and an interior smart vapor retarder go along with the factors of wall depth and climate.

6 Comments

  1. Kye Ford | | #1

    Double Stud Wall
    I am very curious as to how double stud wall assemblies will hold up in the future. The high moisture content in exterior sheathing layers has deterred me from jumping on the cheaper wall assembly band wagon.

    Time will tell....and it may get expensive...with repairs....

    More rigid installation!

  2. David McNeely | | #2

    Why not EIFS?
    I often wonder why EIFS seems excluded from the discussion on this website. It has many advantages:
    4" of foam instead of 1" adds very little to the bid; it eliminates one trip around the house for the rain screen furring; drainage details are worked out, including drip edges above windows; one subcontractor does it all; it is cheaper than most other sidings. Making the sheathing serve as both air and moisture barrier, and locating the windows and doors there, seems like an efficient choice. Filling a 2x4 wall with cellulose adds up to about R-25.

  3. Alex House | | #3

    My suggeston for a future blog post
    I too am wrestling with double stud wall issues, in particular how to resolve the cold sheathing issue when combined with a masonry layer resting on a brick ledge - the foundation wall now needs to get thicker.

    One possible solution I referenced in this thread https://www.greenbuildingadvisor.com/community/forum/green-building-techniques/39021/can-brickstone-veneer-coexist-double-wall-construct is moving the sheathing layer to the inner surface of the external wall. I found this http://www.ecohome.net/guide/interior-sheathing-better-air-vapour-barrier" linl to a project which is doing this form of construction.

    The problem I'm having is finding more discussion of this voodoo-art. In my opinion this is a topic that should be discussed in detail, the pros and cons and particularly the nitty gritty methods of framing and a blog post gives you lots of room to pull together information from many sources.

    Moving the sheathing into the middle of the wall nicely solves the wet wall problem but at what trade-off.

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Alex House
    Alex,
    Good suggestion. While it doesn't address all of the issues that you bring up, the following article may be useful: The Klingenberg Wall.

  5. Jason Hyde, Peterborough 6A | | #5

    I still like the sheathing on
    I still like the sheathing on the outboard face of the inner (load bearing) wall. You can insulate it, or keep it clear as a service cavity. Then, rather than adding foam, add an outer wall (studs/TJIs) and fill with DPC/mineral wool.

    Been done many times, here is one example. http://www.cchrc.org/sites/default/files/docs/Thorsten_walls_report.pdf

    cheers

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to David McNeely (Comment #2)
    David,
    I agree with you that EIFS often makes sense. It is the best way by far to insulate an old brick building, for example.

    However, when it comes to new construction, I don't agree that "EIFS is cheaper than most other sidings," nor that "4 inches of foam instead of 1 inch adds very little to the bid." In fact, many areas of the U.S. have few EIFS contractors, and prices can be high.

    There's another problem with EIFS: the wet-wall fiascos in North Carolina made many home buyers leery of EIFS homes. That fact makes it difficult to sell an EIFS home in some parts of the country.

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