T. Barker is planning a high-performance house in Climate Zone 7A, where the number of heating degree days reaches 10,000 a year and the January design temperature hits 20 below zero.
Although he is still tinkering with his plans, Barker is leaning toward a design with R-50 exterior walls to make the house as comfortable as possible and to reduce the cost of the mechanical systems. The question is how best to get to that insulation value.
In a Q&A post, Barker asks, “Are there any good reports using actual construction experience comparisons for the cost to build double-stud 2×4 (or 2×6) walls compared to single 2×6 with exterior insulation?”
What reading he’s done so far suggests that most double-stud walls are framed on 24-inch centers. “I would think for the minimal cost difference to stay 16 inches on center you get better drywall finish and shear wall capacity for 2-story construction,” he says.
What does the most recent research show in terms of material costs, construction labor, and pros and cons of each of these two options? Further, are there insulation materials on the market with very high R-value that might be a good fit for his house, such as silica aerogels? Those are the questions raised in this Q&A Spotlight.
Double-stud walls are more practical
Reaching R-50 with a 2×6 wall plus exterior foam takes a lot of foam, says Dana Dorsett, something on the order of 6 inches. That makes construction awkward, and may be a good reason to choose the double-stud option.
“A double stud wall is far more practical, since the distance between walls is flexible — two feet thick isn’t substantially more difficult than a foot,” Dorsett writes. “You have to pay attention to the thermal bridging of the subfloors and band joists, etc., and dense packing cellulose gets harder, making fiberglass (which unlike cellulose won’t sag or settle if you don’t quite hit the target density) perhaps a better option.”
As to Barker’s questions about the most up-to-date research, Dorsett points him to a study by the Building Science Corp. Even though it’s not quite as recent as Barker would like, Dorsett says, it still offers sound advice.
But, he adds, forget about aerogel. “Aerogel is ridiculously expensive, and would be totally wasted as cavity fill,” he says, “but there are commercial products designed for framing edge strips, the primary market being commercial construction with steel-stud curtain walls.”
It’s a matter of builder preference
The choice between a double-stud wall and one with exterior insulation is a matter of the builder’s preference, says Michael Maines.
“The ones who prefer double stud walls and don’t like working with foam say that double stud walls are the best value, and that installing exterior foam is difficult,” Maines writes. “The ones who like using foam say that double stud walls are twice the labor and twice the material, and that installing exterior foam is not a big deal. I’ve had this conversation with many different builders and most of them fall into one camp or the other.”
With that said, Maines has found that exterior foam is the best value when the goal is in the R-30 range. For R-values of 40 or higher, a double-stud wall is a more economical choice. The cost difference between an 8-inch-thick wall and an 18-inch-thick walls is just in the extra insulation.
“There are many variables in the equation,” he adds, “so the answer will depend on your specific situation.”
Nor does the choice between 16-inch and 24-inch on-center framing have a clear answer. While 16-inch framing doesn’t use significantly more material, it does use more, and the extra studs don’t make the building substantially stronger. (Builders of three stories or more, however, are required by the International Residential Code to be framed on 16-inch centers.)
Making the case for R-50 walls?
“Can I ask the big question,” says Walter Ahlgrim. “Why R-50? Given the current prices of fuel, materials and labor in most places, an R-50 wall is unlikely to justify its cost in terms of dollars and cents.”
If Barker is shooting for performance numbers required in a Passive House building, R-50 might make sense. Or Barker may live in an area where the cost of fuel is unusually high. He suggests Barker spend some time with a no-cost computer program called BEopt, developed by the National Renewable Energy Laboratory, to learn more about this options.
Packing R-50 worth of insulation into the walls, and R-100 into the roof, will save about $500 a year in heating and cooling costs, Barker replies, in addition to the C$20,000 in mechanical systems. These savings over 15 years will add up to at least C$25,000.
“Additional costs to superinsulate and seal a 2,200-square-foot house will be approximately C$35,000,” he adds. “I can live with the extra cost for better comfort, a quieter house, and being green (never thought I’d hear myself say that — LOL).”
Fuel is indeed a problem. There is no natural gas available on the site, and while propane is a possibility, it’s expensive. Plus, Barker sees no sense in running gas lines into the house when the future is clearly electric.
Several GBA readers question Barker’s estimates of how much money he will save in HVAC operating and installation costs. Will he really save $25,000 over 15 years?
“I wasn’t clear,” Barker says, “but I’m comparing R-50 ‘superinsulated, super-tight’ against the standard that most decent homes would be built to in this area today, which is about R-24 walls, ACH50= 3.0, etc. This happens to be about the quality my current home was built to 20 years ago, so I use electrical and utility usage from that house to make some of my comparisons. Will it end up being R-35 or R-40 instead of R-50? Maybe, but it will certainly be far more insulated than R-24.”
“At a whole-wall R-value of R-30 or more, the cost difference between R-30 vs. R-50 whole-walls or an R-50 vs. R-100 in the attic is often better spent on upgrading or fine-tuning the window options,” Dorsett adds. “Walter’s recommendation for maintaining a BEopt simulation as you adjust the design features is a good one. In most climates the heating and cooling loads can be cut to the level where point source heating/cooling can work without taking it to R-50 whole-wall and R-100 attic.”
Choices for exterior insulation
Barker has done some research on the cost of different types of exterior insulation and finds that mineral wool is the cheapest option. To reach R-50, he says, the cost of 12-inch-thick Rockwool would be about US$2.05 per square foot of wall area; EPS (13 inches) would be US$3.59 per square foot; XPS (10 inches) adds up to US$5.47 per square foot.
GBA editor Martin Holladay wonders why Barker might choose XPS for exterior rigid foam when most green builders prefer EPS or polyisocyanurate.
Barker says that he wants to stay away from polyiso because of its reduced R-values at low temperatures and, he suspects, some environmental issues.
Dorsett suggests he consider reclaimed polyiso. “Large industrial and commercial flat roofs are usually insulated with polyiso or EPS, which is often swapped out and upgraded during re-roofing,” he says. “The used foam is ‘gold’ for materials reclaimers — they make a good margin even when reselling it at less than 1/3 the price of virgin-stock foam.”
Plus, he adds, polyiso’s reduction in R-value at low temperatures is probably not as severe as Barker thinks.
Our expert’s opinion
GBA technical director Peter Yost adds this:
It’s surprising to have such a detailed discussion of wall performance without mentioning windows. This is especially true given the difficulty that windows introduce in either a 6-inch exterior foam walls or double-stud walls. What is their performance relative to the walls? What’s the increased difficulty of window installation in thick walls? There are lots of decisions to make, and some careful detailing to manage.
I decided to ask leading Passive House builder Steve Baczek what his preference would be for “superinsulated and super-tight” Climate Zone 7 walls. He provided the drawing at the top of this column, and added this:
“I’d love to tell you there is a ‘silver bullet’ wall assembly out there, but — sorry, I’m not aware of any.
“When it comes to enhanced wall assemblies, the choice is usually between a thicker frame (i.e. 2×8+) or a double wall assembly. I’ve done both numerous times and they each have their pros and cons. What I’ve found generally is that it really comes down to the builder’s perspective.
“I can make either assembly perform well, but builders apply the $ tag to it. If I asked 10 builders, I would likely get a 50/50 split in favor of each. We can debate a lifetime on this, but here’s one of my contentions: We worry so much about the R-value of the wall at the cavity, that we usually neglect the wall at the window.
“Understanding the impacts of whole wall R-value suggests heavy attention to exterior continuous insulation and, more importantly, to the windows. My stepping off point for a high-performance wall is usually a 2×8 24-inch on-center wood-framed wall with R-9 Zip sheathing. Everything is a bit enhanced, but the number of parts is the same as a code-built wall. So I am essentially paying extra for enhancements, not additional labor. This wall for my climate gets me into a R-40ish+ range. It also usually leaves some money for that very important window upgrade to leverage my whole-wall R-value.”