Too big, too complicated, too expensive — all problems in Mike Sterner’s current home, and exactly what he’d like to correct in the new house he’s planning in northern Wisconsin.
Writing in a Q&A post, Sterner lays out his basic plan for a “pretty good house that finds that happy place between great energy efficiency and economy.”
The site is vacant farmland with a south-facing slope. Sterner’s woodlot has lots of pine and oak he intends to mill for use in his new house.
“I would prefer to stay away from anything too ‘techy’ and have a fairly simple approach to the build,” Sterner says. “It is also my preference that we use materials that are earth and human friendly, with a proven track record. We’re looking at a simple saltbox farmhouse with a single gable, a big covered porch on the east and south, and a shed roof on the back as a rear entry.”
The second floor will have a shed dormer, although Sterner is open to the suggestion of building a full second story it it would be a “vastly better approach” for ventilation and insulation.
His current stumbling block is choosing a construction method for the building envelope.
“I go back and forth between 2×6 framing with a 4-inch rigid foam wrap and a double-stud wall with blown cellulose in both cases,” Sterner says. “I don’t really want to use rigid foam because it is kind of nasty and a hassle to install siding over, but at the same time I am concerned about the double-stud wall cold sheathing and moisture problems. The double-stud wall seems to have questions to me, based on the articles I am reading here.”
He also wonders whether a house insulated to this degree would be able to handle a wood stove or masonry heater. He is considering radiant-floor tubing as a heat backup but would like to do most of the heating with wood.
Story-and-a-half designs are problematic
GBA senior editor Martin Holladay is no fan of 1 1/2-story homes, Cape-style homes, or homes with dormers. They tend to have complicated envelopes that are hard to insulate and air seal properly.
“If you want to live on two floors,” he tells Sterner, “build two floors. Then put a roof over the second floor.”
Steve Knapp, in the process of downsizing from 3,200 square feet to 1,500 square feet, would go even further: Forget the second floor altogether.
“We found that [multistory] levels end up segregating living areas into smaller and more isolated sections,” Knapp writes. “Putting needed spaces on multiple levels just makes the overall structure feel smaller if you don’t have much of a footprint to start with. Stairs also eat up a lot of real estate and drive up costs.”
Knapp also challenges Sterner’s choice of a full basement foundation. Although a basement is often called “free space,” he says, “there is nothing free about it.” It’s a challenge to create dry, finished, and comfortable space below grade.
The basement, Sterner replies, will be a walk-out, providing space for both a root cellar and a television where he can watch football. “A basement gives us a place for these things and utilities/mechanicals,” he says. “In my area, the basement is about double [the cost of] the slab, but for the square feet it seems very worth it to us.”
Choosing exterior wall design
As to the choice between 2×6 construction with a layer of foam and a double-stud wall, Sterner should rest easy. Either will work, Holladay tells him.
The decision, he adds, might come down to whether there is a well-regarded insulation contractor nearby who has experience installing dense-packed cellulose, and whether there is a local source for inexpensive, reclaimed rigid foam. If no experienced contractor is available, Holladay might be steered away from the double-stud wall approach.
According to Tim Rudolph, if building a house with materials that are “earth and human friendly” is a priority, then double-stud walls might be a better fit. Rudolph adds that too much insulation should not be a concern. “I built a straw-bale house with R-60 attic insulation,” he says. “One winter storm knocked out the power for three days, and we were still comfortable with the inside temperature between 65 and 70 degrees without power.”
Brian P., who says he built a home in New Hampshire a few years ago, “wouldn’t stress” about using the double-stud wall approach.
“It seems like if you follow best practices for either double-stud or exterior foam/Roxul, the wall system will work out,” he says. “It then comes down to cost and which system will be easiest for you to build.”
If Sterner does choose a double-stud wall, Brendan Albano adds, he might consider putting an air barrier and vapor retarder in the middle of the wall. “In many ways, it ends up being a lot like the wall with exterior foam,” he says, “where there is a barrier in the middle of the wall that is always warm, so condensation issues are avoided.”
Exterior foam is not a hassle
Jim Tyler is in the middle of a similar project and tells Sterner he settled on the exterior foam option after doing his homework.
“I know what you are looking for: someone to tell you in plain terms which approach is better, and why,” Tyler says. “I also looked for this information, and there is some bad news — the answer isn’t out there. There are strong arguments to be made for each approach, and there are [somewhat] pesky details to deal with in making either approach work. The good news is that there are well established best practices for both exterior-foam and double-stud envelopes.”
Tyler found it easier to detail the foam layer on the outside of the building. But because he was milling his own framing lumber, he also had an incentive to go with the foam and not another stud wall that would require him to cut and mill that much more lumber.
“I’ll add that of all the things that are a hassle about a wall assembly with exterior foam, I don’t think installing siding over it is one of them,” he says. “The furring you will use to hold the foam in place will create air circulation behind your siding — a feature you should strongly consider regardless of your envelope construction if you are installing wood siding.”
Some concerns about foam
Michael Augustine does have some reservations about rigid foam insulation: how extruded polystyrene (XPS) burns, the environmental impact of producing and disposing of it, and its vapor impermeability.
“It apparently also shrinks a bit,” Augustine adds. “However, it has a high R-value per inch, and in my opinion the outer face makes for a pretty straightforward and effective WRB [water-resistive barrier] and air barrier when you use two layers with offset, taped seams. Roxul ComfortBoard addresses a lot of these concerns but at a lower R-value per inch. It comes in smaller pieces, is much heavier than foam, and has a cloth-like surface that doesn’t play nice with tapes.
“Some options would be to either use taped Zip System sheathing beneath the ComfortBoard as the WRB / air barrier, or install an additional building wrap product over the ComfortBoard. (I think the latter would be a really finicky detail to get 100% right.) There’s no magic answer,” he concludes, “but you already knew that.”
Dana Dorsett adds that in case of a fire, polyisocyanurate foam chars in place and has a higher kindling temperature than polystyrene. It is manufactured with a blowing agent that has a lower global warming potential than the blowing agent used to make XPS.
“Surplus reclaimed polyiso roofing insulation shows up pretty regularly, usually from building demolition,” Dorsett says, suggesting Sterner check Craigslist for possible sources of supply.
“Dealers near me typically have 3-inch polyiso in decent shape for $15-$20 for a 4×8 sheet,” Dorsett adds. “With reclaimed foam, the environmental hit has already been taken, but by re-using it you are adding to the benefit side of the cost/benefit balance. It’s as green as, or greener than, cellulose in that regard, and way greener than virgin-stock rock wool or fiberglass.”
Our expert’s opinion
Here’s what GBA technical director Peter Yost had to say:
Exterior foam on a 2×6 wall vs. a double-stud wall. GBA certainly has adequate guidance at this point on both of these assemblies to make a well-informed choice. Builders in my area generally feel a double-stud wall is easier and less expensive to build and particularly to detail at windows.
From a building science perspective, warming the water-resistive barrier, flashing, and exterior air barrier with 4 inches of insulation means a lot in terms of sustained energy performance, moisture performance, and overall durability.
One story vs. two. Having lived in both single-story and two-story homes (with and without a basement), I have to say that single-floor living with storage and mechanicals in a full basement seems like the best combination, especially as I make my way in to my 60s.
And detailing a basement to stay dry is just building with quality in this day and age. In this particular situation, the walk-out basement configuration seals the deal; it’s best of both worlds for a basement.
Dormers. There is a reason the Air Barrier Association of America airtightness thresholds are factors of ten: .02 for the material, .2 for the assembly, and 2.0 for the entire enclosure. (The units are liters per meter squared per second at 75 pascals.) It’s the inherent increase in complexity and places for air to leak as you go from material to assembly to full enclosure. So, the simpler and cleaner the geometry of your home, the less expensive it is to build and the easier it is to increase airtightness.