Image Credit: Bruce Coldham This detail drawing of a Larsen truss appeared in John Larsen's 1982 pamphlet, “The Larsen Truss System.” The lengths are shown in millimeters; 210 mm equals about 8¼ inches, while 600 mm equals about 23¾ inches.
Image Credit: John Larsen This illustration of Larsen trusses appeared in John Hughes' article, “Retrofit Superinsulation,” published in the April/May 1984 issue of Fine Homebuilding.
Image Credit: Fine Homebuilding This photo appeared in John Hughes' 1984 article. Polyethylene was stapled on the exterior side of the clapboard siding before the Larsen trusses were attached.
Image Credit: Fine Homebuilding In 1985, an article in Harrowsmith magazine featured this superinsulated house built by Alan Day and Sandra Day in Alberta. Equipped with R-40 Larsen truss walls, the Day house achieved an air leakage rate of only 0.89 ach50.
Image Credit: Harrowsmith This illustration of Larsen trusses appeared in a 1985 book by Ned Nisson and Gautam Dutt, "The Superinsulated Home Book."
Image Credit: The Superinsulated Home Book This photo of a house with Larsen trusses was used to illustrate Jim Young's “High Efficiency at Low Cost,” an article published in Fine Homebuilding #87 (Spring 1994).
Image Credit: Fine Homebuilding This illustration of Larsen trusses appeared in Bruce Coldham's article, “Six Proven Ways to Build Energy Smart Walls,” published in the December 2009/January 2010 issue of Fine Homebuilding.
Image Credit: Fine Homebuilding This illustration of Larsen trusses appears on a Web page maintained by the Building Science Corporation. Unlike many other energy experts, engineers at the Building Science Corporation are leery of Larsen trusses because they fear that the outermost layer of sheathing may be cold enough in winter to allow moisture accumulation and rot.
Image Credit: Building Science Corporation John Larsen used plywood gussets installed in dadoes cut into the vertical 2x2s; the gussets were secured with air-driven nails and glue. Other builders have chosen simpler details; these Larsen trusses on Topher Belknap's house have OSB gussets secured with drywall screws; there are no dadoes.
Image Credit: Topher Belknap These air-sealing details for a home with Larsen trusses were prepared by Coldham and Hartman Architects of Amherst, Massachusetts.
Image Credit: Coldham and Hartman Architects If you are using blown-in cellulose or fiberglass to insulate a Larsen truss walls, the insulation tends to migrate sideways, making dense-packing a challenge. One solution is to install air-permeable netting on each truss to separate the stud bays.
Image Credit: Coldham and Hartman Architects This detail drawing of a house with Larsen trusses was prepared by David Delaney of Ottawa, Ontario.
Image Credit: David Delaney Inspired by the Larsen truss system, some builders use 2x4s and stand-offs to create a thick wall for insulation when retrofitting an existing home. That approach is shown in this photo of Gordon Scale's house in Adolphustown, Ontario. While Scale did not use Larsen trusses, the idea is similar.
Image Credit: Gordon Scale Aidan and Johannah Van Dyk built a house near Ottawa that includes 9¾-inch-deep Larsen trusses.
Image Credit: Aidan and Johannah Van Dyk Architect Rob Harrison took this photo of wall framing at a Passivhaus project (the North residence in Olympia, Washington). Although Harrison refers to this type of framing as Larsen truss, the load-bearing framing is actually a wall truss, not a Larsen truss.
Image Credit: Rob Harrison Robert LaPorte of EcoNest offers workshops on building walls using a mixture of clay and fiber. The walls include framing inspired by Larsen trusses.
Image Credit: Eco Nest - Robert Laporte A German manufacturer, LignoTrend, manufactures wall trusses that include air-permeable netting to simplify the installation of blown-in insulation.
Image Credit: LignoTrend Although LignoTrench wall trusses are designed to be load-bearing, they can also be attached onto existing wall sheathing in a retrofit job, just like Larsen trusses.
Image Credit: LignoTrend
A Larsen truss is a type of wall truss used to build a thick wall — thick enough to provide room for above-average amounts of insulation. It was developed in 1981 by John Larsen, a builder in Edmonton, Alberta.
In honor of the 30th anniversary of the Larsen truss, the time has come for a definitive article on the invention. This report includes an interview with the inventor of the Larsen truss, a history of its use, and a discussion of its advantages and disadvantages.
Defining a Larsen truss
A Larsen truss is usually site-built. Because the truss is not required to bear any roof load, its components are light. The original Larsen truss consisted of two parallel 2x2s connected by small rectangular gussets of 3/8-inch-thick plywood. The gussets measured 6½” x 8¼” each and were spaced 24 inches apart. A completed Larsen truss looked like a ladder with rectangular plywood rungs.
Although early Larsen trusses were 8¼ inches deep, they can be built to a variety of depths. Many builders have made 12-inch-deep Larsen trusses.
Larsen trusses are designed to be attached to the exterior surface of the wall sheathing of a new home. In most cases, these homes were framed with conventional 2×4 or 2×6 studs. Larsen trusses can also be used in retrofit work, in which case they are installed on top of the existing siding.
Many builders confuse Larsen trusses with wall trusses. If a truss is designed to bear the roof load, it is not a Larsen truss; it’s a wall truss. For example, some builders create double-stud walls with the inner studs bearing the roof load. They may connect the two rows of studs with gussets in order to allow the outer studs to cantilever off the foundation. Such trusses are properly called wall trusses, not Larsen trusses.
“Larsen trusses are installed on the outside of the wall sheathing,” explains Bruce Coldham of Coldham and Hartman Architects. “In most cases, after the trusses are installed, you then install another layer of sheathing on the outside of the trusses. A double-stud wall only has one layer of sheathing.”
Robert Riversong is a Vermont builder who has developed a type of wall truss that he refers to as a “modified Larsen truss.” However, because the inner chord of Riversong’s truss is load-bearing, and because his system does not include two layers of wall sheathing, it differs significantly from the Larsen truss system and is more accurately described as a wall truss.
If you aren’t sure whether you are looking at a wall truss or a Larsen truss, here are some distinguishing signs:
- In most cases, wall trusses bear the roof load and have just one layer of sheathing.
- In most cases, Larsen trusses don’t bear the roof load, and homes with Larsen trusses have two layers of sheathing.
An interview with John Larsen
John Larsen still lives in Edmonton, Alberta. After working for decades as a renovation contractor, he is now retired. When I recently called him up for an interview, I felt like a medical reporter offered the opportunity to interview Dr. Heimlich about the origins of his maneuvre.
MH: How did you get the idea for your trusses?
John Larsen: “Back in 1979 or thereabouts, there was a fellow in Jasper who wanted to build a superinsulated house. The site was up in the mountains, and we were working in the winter. The sun didn’t reach the bottom of the valley until 10:00 a.m. and it set at 3:00 p.m., so we had a short window to do work. We were building a house with double wall framing, and I was trying to think of a way to get the framers to frame a house quickly, so we could do all the heavy work quickly, without having to build two walls. I got the idea of a truss that you could just tack on the outside of a newly framed house, and away you go.
“After I had this idea, I think it was in 1981, I made a set of trusses and put them on a house and the system worked. The truss wasn’t structural, so it didn’t have to be engineered. It’s a stand-off truss, attached to the structural components of the building. We never did have it engineered. We would install them on the outside of houses, and they were incredibly strong — they could be used for ladders.
“I had a farm outside the city, and I set up a little manufacturing line to start making up the trusses. I had a big saw to rip 2x4s in half to make long 2x2s, and then I dadoed the 2x2s out and put the gussets in — 3/8-inch plywood gussets. We attached the gussets with brad pins driven by an air nailer and glue.
“The standard depth of the trusses was 8¼ inches. All the trusses were the same depth; I would stick to my standard size. They were sized for 8½-inch fiberglass batts.”
MH: Why did you bother to dado the 2x2s to receive the plywood gussets?
John Larsen: “The dado made it easier to install the insulation. The insulation comes a full 24 inches wide. Having the plywood in a dado made the trusses real easy to insulate.
“I probably did about 50 houses myself using the trusses. Other people, the framers, would frame the house, and then I would come in to install the trusses as a sub-trade. It reduced the work of building a thick wall to a sub-trade.
“I sold information packages on the trusses — a brochure and a sheet of blueprints that you could insert into your plans for getting a permit and showing the builders. I also I sold a bunch of trusses to other contractors. I was making the trusses until the late ’80s. After that there wasn’t a big demand anymore.”
MH: What makes your system easier than building a house with double stud walls?
John Larsen: “With a double-wall house, you still have a problem with sealing the vapor barrier. What we liked about my system was, you could frame the house normally. Let’s say it’s a two-story house with an outside wall that’s 19 feet high. You could wrap the whole house with one piece of 20-foot wide polyethylene, and then put the trusses on top of that.
“Regular framers didn’t get the concept of getting a house airtight. It was hard to re-train them sometimes. But with my system, you could get regular framers to frame the house, and we just came in later and put on the vapor barrier and the trusses.”
MH: Did you use your trusses on retrofit jobs?
John Larsen: “I did 2 or 4 retrofits. When we looked at the truss system we realized it would work either way, for new construction or retrofits. For a retrofit job, you can put them right on top of the siding, as long as there’s enough structural support to nail them on properly.”
MH: Do you have any other stories to share about the early days of the superinsulation movement?
John Larsen: “Not really. My story is a simple one: going to Jasper and trying to do a double stud wall and realizing it was a lot of work, and trying to come up with a better way.
“Nowadays people are talking about net-zero houses. I’ve visited Peter Amerongen’s Riverdale net-zero house. My feeling is there’s a lot of overkill in a net-zero house. My version of energy efficiency is to conserve my own energy. It seems to me they are designing for the three coldest days of the year. Designing a house so it will keep you warm without any heat at minus 40 might be overkill. I say, design for 345 days of the year, and the other 20 days just burn some junk mail in the woodstove. I still have my farm out west, and these days I try to build with recycled materials. That’s being energy efficient in a different way — not using as many new materials.”
Magazine articles from the 1980s
Several magazines reported on John Larsen’s invention. For example, an article in the November 1983 issue of Popular Science, “Superinsulated House Trusses,” included an accurate description of a Larsen truss (see the sidebar, “An article from November 1983”).
AN ARTICLE FROM 1983
Excerpts from “Superinsulated House Trusses,” an article published in the November 1983 issue of Popular Science:“A superinsulated house needs at least a 12-inch space in the walls for fiberglass insulation. That gives an R-value of 35. The normal way to do it is to build double stud walls of 2x4s. But John Larsen, a contractor in Edmonton, Alberta, has come up with another way — and his requires much less site work. “His secret is a special wall truss he calls the Larsen truss. He makes the trusses in the shop by ripping 2x4s in half. He then cuts a series of dadoes in one side of each resulting 2×2. Then he joins two 2x2s with gussets of 3/8-inch plywood, each 6½ by 8 inches and spaced 24 inches on center. The gussets slip into the dadoes and are glued and nailed. At the building site, these trusses are nailed to standard 2×4-stud walls. (A polyethyelene vapor barrier and sheathing have been installed over the studs) … The trusses can also be installed over the siding of an existing house. “The Larsen truss sells for 55 cents (Canadian) a foot, but delivery beyond a few hundred miles of Edmonton is uneconomic. However, it is not patented, and it’s easy to build.”
The following year, John Hughes, a designer of passive solar homes and an associate of John Larsen’s, published an article, “Retrofit Superinsulation,” describing the use of Larsen trusses in the April/May 1984 issue of Fine Homebuilding. Hughes explained how builders could use 12-inch-deep Larsen trusses to add R-42 insulation to the exterior of an existing house.
Even back in 1984, energy-efficient builders understood whole-building concepts. Hughes wrote, “The new vapor barrier that is installed around the house will decrease the building’s air-change rate substantially. You might need to contact a home-energy consultant who can measure how tight your retrofit is with special pressurizing equipment [a blower door]. If your air-change rate is 0.4 per hour or less, you should install an air-to-air heat exchanger [an HRV] to exhaust stale indoor air and bring in fresh air from outside.”
Useful hint: 2x2s can warp
The following year, Harrowsmith magazine published an in-depth article on Larsen trusses, “Suitable for Framing: Superinsulation Made Simple,” in its April/May 1985 issue. The author of the article, Len Milne, interviewed John Larsen, who provided tips to prevent warping. “You shouldn’t cut more 2x2s than you can assemble right away, and if you are not going to use the trusses immediately, they should be strapped or wire-tied into tight bundles,” Larsen explained. “Keep them dry and out of direct sunlight. Badly warped trusses are not much fun to work with when you are doing the install.”
One of the Larsen-truss homes described in the Harrowsmith article, a 4,500-square-foot home built by Alberta school teachers Alan Dan and Sandra Day, achieved a high degree of air tightness. A blower-door test revealed that its air leakage rate was only 0.89 ach50.
Site-built Larsen trusses usually vary from the classic details developed by John Larsen. Most builders don’t bother dadoing the chords to receive the plywood gussets, for example.
John Hughes preferred to follow Larsen’s recommendations. “The trusses we use have 2×2 chords and intermittent webs of 3/8-in. plywood,” Hughes wrote. “The plywood webs are dadoed into the chords. Most builders in our area use an 8¼-in. deep truss. Adding these to a sheathed 2×4 stud wall yields a frame that’s close to 12 in. thick, not including the new siding. This is a convenient thickness because the plywood used to line the rough openings for windows and doors can be ripped from 4×8 panels with little or no waste.”
Ten years later, in another Fine Homebuilding article (“High Efficiency at Low Cost,” FHB #87, Spring 1994), Jim Young wrote, “As Larsen originally built them, the trusses were made from 2×2 vertical chords that had been dadoed to accept 1-ft. squares of 3/8-in. plywood spaced every 2 ft. or 3 ft.
“I made the trusses a little differently: first, by using 2x3s straight from the lumberyard instead of ripping 2x2s from larger stock, and second by skipping the dadoes in the truss chords. The carpenters … attached 1-ft. squares of ½-in. plywood to the sides of the 2x3s with waterproof yellow glue and ring-shank nails.”
Young’s article includes many helpful details. “Trusses were nailed on the walls through the 2×3 chords into headers and rim joists [and were spaced] 2 ft. o.c.,” Young wrote. “Along top and bottom outside edges, the trusses were joined to each other with short lengths of 2x3s toenailed at each end into the 2×3 chords. Outside corners were a little tricky. An L-shaped assembly made from 2x6s was attached to trusses on both sides with 2x3s to make a solid outside corner. To anchor each truss at the top of the wall, the uppermost plywood gusset was extended 1½ in. and nailed to the adjacent roof truss. At the bottom of each truss wall, sections of plywood were nailed in from the bottom to complete the box that would hold the blown-in fiberglass insulation.”
Aidan Van Dyk, a Canadian software engineer, built a Larsen truss house near Ottawa in 2006. Because he wanted to space the fasteners used to attach each Larsen truss 4 feet apart (vertically) rather than 8 feet apart, Van Dyk planned ahead: he included horizontal blocking at the midpoint of his 2×4 wall. Van Dyk’s 9¾-inch-deep trusses hold R-31 fiberglass batts.
Many builders who build their own Larsen trusses end up scratching their heads when it comes to outside corners. In The Superinsulated Home Book, published in 1985, authors Ned Nisson and Gautam Dutt provide a caption to an illustration showing an outside corner on a house with Larsen trusses: “Floating external corner: Floating 2×4 — attach [the 2×4] first to [the projecting] sheathing, then apply and nail second sheathing [from the intersecting wall] to it.”
Most experts recommend the installation of a plywood “bottom plate” connecting the bottoms of the projecting Larsen trusses. In “Six Proven Ways to Build Energy Smart Walls” — still another Fine Homebuilding article discussing Larsen trusses, this one in the December 2009/January 2010 issue — author Bruce Coldham wrote, “The truss exterior is not supported by the foundation and is sealed with a plywood bottom plate. A plastic air barrier and a vapor-retarding membrane are wrapped around the entire outside of the sheathed 2×4 frame and are sealed to the sill plate or directly to the concrete foundation wall.”
Almost everyone who writes about Larsen trusses is tempted to comment on their surprising ruggedness. “The Larsen trusses are very strong,” wrote Jim Young in 1994. “When nailed to the house, they became a sort of ladder or scaffolding that wouldn’t budge, even when we jumped up and down on them. Although the original purpose of the truss system simply was to hold insulation, it became apparent that it adds a lot of rigidity to the building envelope, too.”
Whole-wall R-value is 38.5
In 1997, engineers at Oak Ridge National Laboratory measured the whole-wall R-value of a classic 8-inch-thick Larsen truss wall insulated with 8″ fiberglass batts attached to a sheathed 2×4 wall insulated with R-11 fiberglass batts.
As reported in “Clear-Wall and Whole-Wall R-Values for Tested Wall Systems,” an article published in the March/April 1997 issue of Home Energy magazine, the wall had a very respectable whole-wall R-value of 38.5 — better than might be expected by those who dismiss the insulating ability of fiberglass batts.
Poly in the middle of the wall? What’s up with that?
Twenty-first century builders may be put off by John Larsen’s descriptions of wrapping the outside wall sheathing with a very wide roll of polyethylene. After all, in the 30 years since Larsen developed his trusses, the reputation of poly has slipped a few notches.
It’s worth remembering, however, that the pioneering developers of superinsulation techniques in Alberta and Saskatchewan were all using polyethylene as an air barrier in the early 1980s. In a cold, dry prairie climate, interior poly works well. Other air-barrier methods — specifically, the Airtight Drywall Approach and the current darling of energy-efficient builders, taped exterior sheathing — came later.
In his 1984 Fine Homebuilding article, Hughes explained why it was perfectly acceptable to wrap poly around a house with Larsen trusses: “Research in Canada has shown that as long as there’s at least twice as much insulation (in R-value) on the cold side of the vapor barrier as on the warm side, no significant condensation will occur in the wall under normal circumstances.”
Advantages of the Larsen truss system
For builders of new homes — at least those who followed the traditional Canadian recommendation to use interior polyethylene as an air barrier — one of the most obvious advantages of Larsen trusses is that they greatly simplify the installation of the poly.
“When we were using double walls, we used to run the polyethyelene air barrier on the inner plane of the wall,” said architect Bruce Coldham. “More recently, we began putting the air barrier on the outside, by taping the sheathing, and we’ve gained confidence with that approach. However, we used to have an interior air barrier. With a double wall, that required a tortuous folding the polyethylene around the second floor rim joist. But putting up the poly is simple and clean with the Larsen truss system. It has a nice look about it. It’s aesthetically pleasing.”
Putting up with the learning curve
Larsen trusses have several disadvantages. One that is frequently mentioned is the steep learning curve. The problem was mentioned back in 1985 by Nisson and Dutt in The Superinsulated Home Book: “The main disadvantage of the Larsen Truss and similar systems is that they are new and very unconventional. They are commercially available only in western Canada and will usually have to be site-built. The first installation for any builder will undoubtedly be slow and probably quite expensive.”
More than two decades later, the problem persists. “Neither my general contractor, nor my builder had ever heard of a Larsen truss,” said Topher Belknap, a green consultant who built a Larsen truss house in Edgecomb, Maine. “I had to educate them, and we had many discussions about various details. This required not only knowledge on my part, but also a firm belief that this is what I wanted. It also required paying for someone else’s learning (both in terms of uncertainty and lost time).”
Another possible disadvantage: if you decide to fill your Larsen trusses with blown-in insulation, it may be hard to find an installer with enough experience to do the job well. “In 2008, I was able to examine the entire house with a borrowed infrared camera,” Belknap wrote. “Doing so pointed up many places where the insulation had settled (or never got) and there were now gaps in the insulation coverage. This required reapplying insulation (blown in from the inside).” When I talked to Belknap about the issue, he said, “The problems I had with the insulation were attributable to the fact that the cellulose would drift from one bay to the next, and some of the bays would never get filled.”
Experienced installers of dense-packed insulation have developed techniques to fill deep walls, including Larsen-truss walls and double-stud walls. However, depending on the skills of the insulation installer may be risky; some architects and builders prefer to install air-permeable netting on each Larsen truss to separate the stud bays (see photo below), a technique that facilitates insulation installation.
Another issue is the controversy about many types of thick walls, sometimes called the “cold OSB” problem. A document on the Building Science Corporation Web site states, “The truss wall system can achieve a very high whole wall R-value … and would be perform well in extreme climates provided the air barrier was detailed perfectly minimizing the high risk of air leakage condensation durability issues. The Larsen truss is time consuming to construct and susceptible to premature enclosure failures resulting from poor construction and detailing. … This wall system has greater risk for severe air leakage condensation since the sheathing is considerably colder than standard construction.”
However, there is a contradiction in this document; while the text refers to problems with Larsen trusses, the illustration depicts a wall truss, not a Larsen truss. It’s hard to tell whether these BSC criticisms actually apply to Larsen trusses, or whether they only apply to wall trusses.
One of the principals at the Building Science Corporation, Dr. John Straube, explained: “With a Larsen truss, cellulose, and sheathing on the outside, the sheathing is actually colder than ambient air because of radiation transfer. … Therefore there is a condensation risk for the exterior sheathing in this situation.”
Not all experts are as worried about the cold OSB problem as John Straube, however. To learn more about the issue, see How Risky Is Cold OSB Wall Sheathing?
Would you do it again?
Aidan Van Dyk is unequivocal. “I was quite pleased with the Larsen truss system,” he told me. “I’d do it again.”
However, some builders who have used Larsen trusses have moved away from the system. According to architect Bruce Coldham, “At the Moomaw house [in Williamstown, Mass.], using Larsen trusses turned out to be more expensive than my contractor imagined. I think the lesson we learned is that Larsen trusses tend to be more expensive than a double wall.”
Topher Belknap also learned a few lessons. “If I had to do it again, instead of using site-built trusses, I would just buy I-joists,” Belknap told me. On his Web site, Belknap wrote, “The trusses did represent a substantial chunk of labor (on site) and may have caused some problems with the insulation. I-beams are now commonplace.”
Wall trusses in Europe
At least one German company, LignoTrend, is selling wall trusses that can be used for load-bearing exterior walls in new construction or for retrofit jobs (attached to the exterior of existing wall sheathing like Larsen trusses).
The trusses are sold with air-permeable webbing already installed to facilitate the installation of blown-in insulation (see photos below).
Foam-free superinsulation retrofits
There is one type of project where Larsen trusses still make a lot of sense: a superinsulation retrofit job for a client who wants to avoid the use of foam insulation. If you have crossed spray foam and rigid foam insulation off your list of acceptable green products — if you’re trying to stick to insulation products like cellulose, blown-in fiberglass, or mineral wool — then Larsen trusses will work for you.
“Larsen trusses are a fine way of retrofitting,” says Bruce Coldham. “If you aren’t using foam — if you are using an insulation that is only R-3.5 or R-4 per inch — then you have to construct a containment cavity to get the R-value you need.”
Such retrofit jobs are expensive, of course. In many climates, predicted energy savings won’t be high enough to justify an R-40 retrofit job. John Larsen addressed the payback issue back in 1982, in a pamphlet called “The Larsen Truss System,” and his analysis is still relevant. “The question most often asked by people considering a superinsulated house is, ‘Is all this extra insulation and tight vapor barrier worth the extra cost entailed?’ In a location with high energy costs and a large winter heating load, the answer in an unqualified yes. In areas with low-cost fuel still available, or in areas with a smaller winter heating load, the answer appears less certain.”
Last week’s blog: “Job Sites in Maine, Part Three.”