Is Double-Stud Wall Construction the Path to Efficiency on a Budget?
An owner-builder inherits a foundation and now needs a house
UPDATED with an expert opinion from Bruce King
Writing from Glacier, Washington, Karen Bean faces a home-building dilemma that confronts many thousands of people: what's the best way to insulate the walls of her new house on a modest budget?
She has $150,000 to spend on the two-bedroom, two-bathroom house, which she plans to build on a foundation originally intended for a traditional house. Although the concrete-block foundation is well made, it's not necessarily well matched to the double 2x4 walls she's hoping to use.
"Is double wall with blown fiberglass insulation worth the extra cost?" she asks in her Q&A post. "And can it be done on an already laid concrete block foundation? Or is there another equally warm option that I just don't know about?"
Bean's quandary is the subject of this week's Q&A Spotlight.
Suggestions for a truss wall system
Bean helps clear up some confusion over exactly what climate zone Glacier is in; it turns out that the site is relatively cold. Glacier is about 30 miles east of Bellingham, WA, and usually sees about 7,000+ heating degree days per year.
And that, says Robert Riversong, makes it a good candidate for a Riversong Truss house, an energy-efficient design he's been building in New England for many years. The 12-in. thick walls are filled with dense-packed cellulose, not fiberglass.
"I typically build with locally sourced, green, rough-sawn lumber, no exterior sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. and horizontal shiplap siding, with the Air-Tight Drywall system as the interior air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both.," Riversong writes. "This makes a highly breathable, highly insulative (R-45) wall with almost no thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. , and requires no more lumber than a conventional 2x6, plywood sheathed house. And it's about as green as can be done with conventional materials."
Rob Harrison is using a similar approach on a cabin not far away, in Wauconda, WA. Harrison hopes that his cabin will meet the PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. standard.
His walls will include 15-in. Larsen trusses over structural sheathing on 2x6 studs, all stuffed with dense-packed cellulose. "This yields a wall of R-70," Harrison says. "Roof assembly is R-85, floor over crawl space is R-75. Windows are U-0.13."
Both Harrison and Riversong think straw-bale construction might be another option, although Harrison cautions against it in certain parts of the state.
Plans first, foundation second
James Morgan probably wasn't the only one wondering why Bean is pondering wall systems when the foundation is already in the ground.
"Karen," he says, "I have to ask: how come the foundation is in place before you have decided on what wall system is to go over it? It can't be over-emphasized that good forward planning of your whole-house construction strategy is essential if you a) want to build green and b) do it on a tight budget."
It turns out that there's a good reason for this, even if it makes the project more challenging. As Bean explains the next time her dial-up modem agreed to work, the foundation was "inherited."
"The foundation is in because we inherited it," she says. "It’s a well-made foundation — concrete block, basement, nice crawl space — so I’m going with it. The original house was to be traditionally built. So I’m trying to figure out how to adapt what I have to something that would be more insulated than a traditionally built wall. If it’s not possible, I’ll live with it."
Is rough-sawn lumber a good way to save money?
Riversong typically includes rough-sawn lumber in his houses. In a heavily forested area that supports local saw mills, buying green, rough-sawn material can add up to some significant savings. Manufacturing costs and transportation costs are both lower.
Harrison, who is based in Seattle, isn't so sure. "I would strongly caution you against building with green lumber in western Washington," he says. "I caution against green lumber because the builders with whom I work here (who have been doing super-insulated houses for 30 years, back to before the BPA Super Good Sense days) have had trouble with it here in western Washington," he tells Riversong. "That is, based on their feedback I don't think you can just substitute green lumber in a conventional building assembly, and not have some issues. (Twisting, shrinkage, mold.) I'm sure green lumber works in your complete system, where you are. "
Green, rough-sawn material may be heavy, Riversong says, but it has a lot going for it, even if builders and local code officials don't always embrace it.
"The first principle of sustainability (which should be the basis of what is called 'green' building) is to use locally-sourced materials that are processed as little as possible," he says. "This not only limits transportation costs and embodied energyEnergy that goes into making a product; includes energy required for growth, extraction, and transportation of the raw material as well as manufacture, packaging, and transportation of the finished product. Embodied energy is often used to measure ecological cost. and global warming contribution, but also supports the local economy which sustains community and allows us to live responsibly within the annual output of our local environment."
Once green lumber is incorporated into a frame, it dries quickly and stays straight. Plus, Riversong says, "it cuts like butter" and nails with no resistance.
David Meiland has had similarly positive experiences with locally milled wood.
"We have excellent locally grown fir available here," he says. "The inspectors are familiar with this material and don't even blink, although maybe they're supposed to have you bring a lumber grader on the job to stamp everything. It is a little more expensive than commodity lumber and it's harder to get an order filled and delivered (the sawmills are small and manual here)."
On the issue of costs
Costs are the bugaboo of any building project, especially for owner-builders, and here there is no surefire way to keep them down.
As GBAGreenBuildingAdvisor.com senior editor Martin Holladay points out, "If you are an owner-builder, it's hard for people in other parts of the country to recommend an inexpensive way for you to build your wall. It depends on who is doing the work, whether you are paying for labor, how much time you have, and whether you are willing to scrounge for recycled materials."
But the use of rough-sawn lumber from a local mill is one way of holding down costs, he says, as is the possibility of picking up recycled polyisocyanurate insulation that's been salvaged from commercial roofing jobs. Ask around to see if any recycled foam is locally available, he suggests.
"If you are paying for the labor, it's hard to beat double-stud walls filled with cellulose insulation for a low-cost high-R wall," Holladay adds. "But even that option may still be too costly for your budget."
One way of making a project less expensive is by making it smaller, as Harrison suggests. "Have you considered somehow paring down the program?" he asks. "Let's say, doing one 'rush hour' bathroom instead of two full baths?"
Doing some of the work herself, or getting help from friends and neighbors, is often the best way of holding down costs, Riversong says, along with keeping the geometry of the house as simple as possible.
If Bean is having someone else design and build the house, she should plan on spending about $150 a square foot for something that's well made.
Our expert's opinion this week comes from Bruce King, a professional engineer and director of the Ecological Building Network:
Eliminating the plywood skin may be fine in New England, as Robert Riversong does, but unless Karen has otherwise allowed for seismic forces she really should keep it. Being on shaky ground really changes the ball game.
The Larsen trusses are easy: just think of the exterior line of studs as the bearing wall, and add inwards as needed for insulation. It doesn't matter if the interior studs don't align over the foundation, as they aren't taking any load. It's really conventional framing with thick insulated furring.
If you don't want to lose too much interior space, reduce the thickness of the cellulose (or fiberglass), and add a layer of rigid outside the plywood. But first read other articles and blogs on GBA about what can happen when a superinsulated wall doesn't allow enough "bleed" heat to dry the siding; superinsulation done wrong, especially in rainy Washington, can exacerbate moisture risks.
Straw bale construction is a perfectly viable option if you pay attention to keeping the bales dry during construction and thereafter. The plastering may seem like extra work, but it also gives you very very airtight buildings that you could otherwise only achieve with a lot of caulking and taping.
- Green Building Advisor
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