Jesse Lizer’s new house will be in Climate Zone 6, where he can expect 7,400 heating degree days a year. High R-values in the building envelope are a high priority.
As Lizer explains in a Q&A post at GreenBuilding Advisor, there are three possible scenarios for constructing outside walls:
- A double-stud wall, between 10 in. and 12 in. thick, filled with dense-packed cellulose and sheathed with the Zip System OSB.
- A 2×6 wall framed 24 in. on center with 1/2-in. OSB or plywood sheathing, insulated with wet-blown cellulose and wrapped with two layers of foam on the outside of the wall — 1 in. of XPS and 1 in. of polyisocyanurate.
- A 2×6 wall with 1 in. of exterior polyiso over 3 in. of EPS, also with 1/2 in. sheathing and wet-blown cellulose.
In an unusually detailed post, Lizer lists two other important pieces of information: the cost of each option, and the nominal R-value of each wall system. The double-stud wall ($2.15 per sq. ft. in material costs) has an R-value of 40; the 2×6 wall with a total of 2 in. of foam ($2.05 per sq. ft.) is R-32; and the wall with a total of 4 in. of foam ($2.15 per sq. ft.) is R-41.
“Any big thoughts on the best approach?” he asks. “My thinking is leaning towards exterior thick foams would do a better job at sealing up the house from both vapor and air vs. the Zip and thicker wall. Since I will be building it (yes, I do have years of construction experience) I am thinking the 2×6 and thicker foam will be faster and easier to detail?”
That’s the topic for this week’s Q&A Spotlight.
The double-wall option
All three of Lizer’s proposed walls minimize thermal bridging, but the double 2×4 wall does so without the use of rigid foam insulation. Two conventional 2×4 walls are separated by an insulation-filled gap that dramatically reduces the amount of heat escaping through the framing. Building techniques are familiar and straightforward.
“Having built both double walls and exterior foam, I’d say the overall cost savings are with double wall, easily,” writes Dan Kolbert. “The interior walls are easy to build and go up quickly. Exterior foam is slow to install, expensive, and makes all the exterior details much harder and slower.”
Doug McEvers is another proponent of this approach. He calls it “the most effective option,” and adds a wall system that includes foam is not only expensive but more difficult when it comes to details such as attaching the siding.
To John Brooks, these factors make this type of wall more “buildable.”
From a pricing point of view, Lizer’s extensive homework reveals that for walls with high R-values, the system with double 2×4 walls has the edge.
“For R-35 and below, I have found that (pure materials cost) 2×6 24-in. o.c. with two layers of foam to be the same as double-stud 10-in. wall, in [dollars],” he writes. “However, the more R desired, then that starts to shift towards the double-stud option. R-40 and up probably cannot be beat, cost wise, for a double stud.”
The foam option
A layer of rigid foam on the outside of the building also reduces thermal bridging. While GBA senior editor Martin Holladay acknowledges that many builders are trying to avoid using foam for environmental reasons, it “aids durability significantly because it helps keep the sheathing and framing warm and dry.”
At a recent building science conference, Holladay saw a presentation of preliminary findings from a test building in British Columbia that had been sheathed with OSB. “There were photos showing mold growth on the interior side of OSB sheathing in walls without foam sheathing over the OSB – under some conditions (50% interior relative humidity),” Holladay writes. “The walls with rigid form over the OSB were clean and dry – no signs of mold.”
Holladay says builder Mark Gauvin, who presented the slides, argued that “exterior insulation keeps wood warm, keeps wood dry, reduces condensation potential, reduces thermal bridging, reduces energy losses – but most importantly increases durability.”
“In a way,” Holladay adds, “there’s nothing really new in these conclusions. But for me, they were a reminder of all the good things that happen when we install exterior rigid foam.”
Labor and other factors
One mystery in Lizer’s calculations is labor. Building a double-stud wall incurs higher labor costs, he writes, but at the same time adding double layers of rigid foam insulation, then taping the seams and adding strapping for siding and additional detailing for windows could make labor costs a wash.
Double 2×4 walls would, however, eat up more of the interior space of the house, he adds.
“The advantage thicker foams have is in your building footprint,” Lizer writes. “If you want your rooms the right size, the foundation needs to grow (add dollars to the double-stud number) while the foam adds to the exterior and can overhang the foundation if need be. Really thick walls also play against you some on taxes. Since they measure the exterior of your shell, you are being taxed for unusable square footage.”
If he goes the foam route, he’ll have to add the cost of the strapping plus the long screws that will be used to attach in through the foam. Screws alone could be $400.
Our expert’s opinion
We asked GBA technical director Peter Yost for his opinion. Here’s his reply:
I recently had pretty much this same discussion with a leading local remodeler here in Brattleboro, VT, Steve Mindel of Mindel and Morse Builders. His company has tried many different wall configurations and feels that the double wall is the most affordable and easily constructed high performance wall.
“If you install windows and doors on the exterior of double walls, the only details that change are interior jamb extensions,” remarks Steve. “For us, the exterior detailing of high performance walls with exterior rigid insulation really affects labor costs and the ‘fussiness’ of exterior work.
Mindel and Morse have struggled with the vapor retarder and its placement with double walls. “We like the idea of using MemBrain for these walls, given its variable vapor permeability.” (When “dry,” MemBrain has a perm rating of about 1, and when “wet,” has a perm rating of about 15). “It’s more about ready availability than cost for us when it comes to Membrain,” adds Steve.
“Peter and I have discussed poly-in-the-middle of double walls, Steve says. “But while the approach makes sense to us from a building science perspective, the issue is constructability and sequencing on the job site.”
Interestingly, when Steve builds double walls, he has not installed a dedicated interior vapor retarder (1 perm or less), but relied instead on two conditions: reduced vapor permeability of one coat of interior primer and two coats of paint; and installation of a heat-recovery ventilator (HRV).
“The paint layers don’t get us down to 1 perm on the interior, but close, and HRV operation during cold winter months keeps the interior relative humidity down in our homes,” states Mindel.
Just two closing comments for me:
- Provide every one of your clients with an electronic thermo/hygrometer so that occupants have feedback on their interior relative humidity during the winter.
- We need to figure out how to build inexpensive, less resource-intensive non-structural interior walls for our double walls, something along the lines of the SEE stud.