John Holscher has done enough research to know there are many ways of building and insulating an energy-efficient home. Options include double-stud walls, 2×6 walls with rigid foam on the exterior, and structural insulated panels.
Now he has to figure out which one makes the most sense for his Cape-style home in New England.
“So many options, so little time,” he writes in his Q&A post.
“I’d also like to avoid (as much as I can) petrochemical-based products and use as many natural and/or recycled products as possible,” Holscher adds. “I understand the argument that using foam insulation in the end saves more oil than is used to produce it, but if I can get equal or better results using natural/recycled, all the better.”
First, start with a smaller, simpler house
“The most cost-effective approach to an efficient home is to build only the space actually required for basic shelter, which is generally no more than half of what the typical American believes they need,” writes Robert Riversong.
No matter what materials you use, or how the house is built, using fewer resources by building smaller lowers both operating and maintenance costs, Riversong says. “A half-sized house can have half the R-value and still be energy efficient.”
Riversong’s recommendations for the most cost-effective approach are simplicity over complexity. “The most cost-effective building system for a temperate climate?” he asks rhetorically. “Structural straw bale with earthen plaster, cordwood masonry with earthen floor, or scribed log construction, with no interior plumbing or wiring, a simple woodstove or Rumford fireplace, a shallow well with windlass bucket or gravity-fed spring and a rain barrel, and a pleasant composting outhouse.”
James Morgan likes the “small” part, but not all of Riversong’s details. “The very first concern for an aspiring green builder should be to build no larger than necessary,” Morgan writes. “Sadly, most of the featured ‘green homes’ we see, including even the plans featured on Sarah Susanka’s ‘Not so Big’ Web site, set expectations of well over 2,000 s.f. for a modest three-bedroom home.
“Oddly, as a result of poor planning and fussy detailing, many of these oversized homes feel anything but spacious.”
Double-stud walls or a Larsen truss?
“When it comes to high-R walls, the most cost-effective option in most areas is a double 2×4 wall with a total thickness of about 12 inches, insulated with dense-packed cellulose,” writes GBA senior editor Martin Holladay.
But in areas with very high labor costs, he adds, it may be less expensive to build 2×6 walls covered on the exterior with 2 in. to 3 in. of polyisocyanurate foam.
Riversong still designs double-stud walls for some customers, but he prefers another approach. “I’ve modified the Larsen Truss to make a 12 in. thick, nearly thermal bridge free wall, out of locally-sourced rough-sawn (often green) lumber, which uses no more wood than a standard 2×6 house,” he says. “And all above grade insulation is dry cellulose with borates, the very best of all the commonly available insulation options (short of straw bale and earthen plasters).”
Riversong also includes a link explaining his approach to building.
The house he describes can be heated with a cord of wood, even in New England.
Insulating the roof
Holscher is planning on using the second floor of his Cape for bedrooms, so what he needs is an insulated cathedral ceiling. What’s the best way of tackling that?
Here, says Holladay, the best option isn’t necessarily the most cost-effective. “As far as I’m concerned, the best option would be a [structural insulated panel] roof covered with a layer of 2-in.-thick rigid foam above the SIPs, with the top layer of foam lapping over the SIP seams,” Holladay wrote. “Then 2×4 sleepers, eaves to ridge, for ventilation. Then another layer of roof sheathing (ideally plywood or boards, not OSB), followed by roof underlayment and the roofing of your choice.”
Holscher asks, Why not a thicker SIP, without the extra layer of foam?
First, says Holladay, adding 2 in. of polyisocyanurate foam to a 12-in. panel gets the R-value up to 56. And second, the failures in SIP roofs in Juneau, Alaska, occurred at the seams, where effective air sealing is tough. Adding the foam layer on top of that solves the problem.
Graham Mink, who is building a double-stud wall house in Vermont, had another idea. “For the cathedral roof we used 2×12 rafters, then gussetted 2x4s to the interior to create an extra 12 in. cavity,” he writes. “This 23-in. cavity will then be dense-packed like the walls. We chose to go unvented in order to align the air barrier (Zip system roof sheathing) with the insulation layer. A vented assembly can be done this way as well. I am not sure if its the best way, just the way we are doing it.”
Hang on a minute, says Rivesong. An unvented, non-breathing roof is “not the best partner for cellulose.”
His suggestion: “Rather than extending the frame outward with a truss chord, extend the rafters inward with an additional non-structural framing member under each rafter connected by gussets (or webs) to reduce thermal bridging and create a deep enough insulation cavity to still leave room for venting under the roof sheathing, which would require continuous baffles from eave to ridge.”