Location: Green Fret > Hjälmaren > Larsen Truss

Larsen Trusses


Wherein is described the construction, in 02002, of Hjälmaren, a timber frame larsen truss high post cape on the shores of the Sheepscot River. The pictures below are created from 3D models in an effort to make detail clear. They were rendered using Bryce 5. Clicking on a picture will bring up a larger version {L} (or a picture if I have a good one {P}).


A larsen truss is a system of adding large amounts of insulation to a house by putting light weight trusses around the perimeter, separate from the structural parts of the building. Ladder like trusses are installed around the exterior of the building (in my case the frame and drywall) sheathed and filled with insulation. It was developed by John Larsen, a builder from Canada, around twenty years ago, as a means of improving insulation levels in existing houses.

Foundation, insulation, sill {L}
model credit: Topher Belknap


The foundation is 10" thick to accommodate the timer frame, otherwise it is a fairly standard foundation. It is covered with a 2x10 sill (with sill sealer between). The timber frame sits directly on the sill. The foundation is waterproofed, has a perimeter drain pipe (inside and outside) draining to daylight. It is covered by 4 inches of extruded polystyrene (XPS) insulation for an R-value of about 20. The mass of the concrete is inside the heating envelope and adds measurably to the thermal mass of the house.

first floor joists
First floor joists {L}
model credit: Topher Belknap

First floor Deck

The first floor, joists extend beyond the foundation, cantilevering over it on all sides by about a foot. The boards parallel to, but inside the exterior joist, keep the insulation from going into the basement. The larsen truss sits on top of this cantilever. You can also see (or maybe you can't) that the perimeter insulation has been covered by a layer of cement board to protect it from UV exposure and physical abuse. This will eventually be covered by river rocks (which hasn't happened in reality yet either).

first floor deck
First floor deck {P}
photo credit: Topher Belknap

The first floor decking only goes out to the limits of the timber frame. As shown here it includes the finish floor of 1x6 tongue and groove pine, which was actually installed later in the process. The subfloor is Advantech. The model should show holes through which the frame posts will go (the picture does).

timber frame on the deck
Timber frame on the deck {L}
model credit: Topher Belknap

Timber Frame

The posts of the timber frame go through the decking and sit directly on the sill on the foundation (Interior posts sit on carrying beams on top of concrete filled steel posts on concrete pads). The timber frame is pine and constructed with traditional joinery and oak pegs. It went up in two days with a crane and crew of about 5.

The chimney can be seen rising through the hole in the decking (which will also hold the stairs to the basement). It has three flues, one for the wood stove (the primary source of heat), another for the tankless water heater, and one for future expansion (this last is sealed at the top of the chimney).

drywall over frame
Drywall over the frame {P}
photo credit: Topher Belknap


Here the blueboard (drywall with a higher resistance to water) has been put over the frame. The blueboard is the inner wall surface, but putting it over the frame was simple and ensured that there wasn't a problem with sealing the drywall against the timbers.

The vapor barrier goes over the entire house. Covering all the drywall, and incidently allowing it to be rained on without problems during the rest of the construction. The vapor barrier also goes down into the joists. There is still a vapor barrier gap around each of the floor joists, and from the sill down to the foundation. This is a concern and perhaps someone can find a better solution than I did.

The window and door openings are now cut into the drywall (not shown in picture). Since the outside corners don't even exist yet, the locations of the openings are taken off the centerline of the building. The openings area made larger than the rough opening would dictate, and the rough openings are made beveled to allow more light in.

trusses on walls
Trusses on walls {L}
photo credit: Topher Belknap


The truss are simply to keep the sheathing a set distance from the drywall, they don't support anything other than the windows, and they rest on the cantilevered floor. As a consequence they can be made very light. My builder used 2x3's with plywood struts holding them together. (Update 02008: I would now recommend that wood I-beams for this. 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.)

sheathing on trusses
Sheathing on trusses {P}
photo credit: Topher Belknap


Sheathing is nailed to the trusses in standard fashion, covered with Tyvec, and shingled. Shingles were chosen for several reasons, 1) they are locally grown, 2nd growth white cedar, they require little maintenance (no paint, no stain), and can be replaced in a piecemeal fashion, failure modes are noticeable, and they fit in the local ambiance (no one will look at a shingle house and feel the need to replace it because it is weird.


Cellulose insulation was blown into all the wall cavities. A lot of cellulose insulation. The insulation contractors had to come back the next day, since their truck didn't hold enough. The walls are roughly R-40.

Contrary to seeming prevailing practice, I did not put a lot more the insulation in the roof than the walls. The roof is also roughly R-40. I did not intend for the house to have much heat stratification, nor for it to have many penetrations of the vapor barrier in the roof. This means that the heat losses would be roughly the same for walls as for the roof. The government recommendations for insulation, are based on standard construction methods, so roofs get more because it is easier.

My R-40 walls for instance lose around 7.3 MBTUs over the course of a winter; the roof another 5.5 MBTUs. A comparable house built of 2x4 stick construction (with R-9.6 whole wall) would lose 32 MBTUs through the walls alone. No amount of insulation in the roof would make them even. 2x6 stick (with 13.7 whole wall) would lose 22 MBTUs, still almost twice what I lose through walls and roof.

looking through the truss
Window framing (still needs plaster and trim)
photo credit: Topher Belknap


The openings for the windows and doors are splayed on the inside. Thus the opening is wider on the inside than the outside, allowing more light in and preventing that fortress feeling. The rough openings are made of two-by lumber on the sides and sheathing material on the top and bottom. They were then built into the trusses as those were built. After window installation, the openings were trimmed out, first spray foam insulation (great stuff door & window) was used to fill the gap between the rough opening and the window, then the vapor barrier was completed by glueing a piece of 6-mil plastic between the window and the piece behind the drywall; and finally covered by beadboard (1/4 inch thick). Yet to be done is a standard trim around the circumference.



The house walls are R-40, which means that I lose only about 12% of my heat loss through the walls. A larsen truss has no real limit on how much insulation can go in the walls. I made mine 1 foot thick as it was a round number (honestly, it was easier to describe to my general contractor that way), and I thought thicker walls would make windows and door trim too difficult.

Whole wall performance

Whole wall performance is the percentage of theoretical insulation is actually achieved in practice (or at least, when measured in a lab). For instance, 2x6 walls 24 inches on center with fiberglass insulation has a theoretical R-value of 19, the whole wall performance of 13.7 for 83.7% rating. Larsen trusses get a rating of 95.3%. You end up getting most of that insulation you paid for. Note: the 95.3% is for a wall constructed differently from mine, which contains more wood than mine; I would expect my house to have a higher whole wall percentage.

Continuous vapor barrier

The vapor barrier was very easy to install on this house. The only tricky parts are the sill and the window and door openings. Otherwise the vapor barrier surrounds the blue board with no gaps. Putting it up was easy for my builder, the blueboard and the vapor barrier took no more than a day or two for two workers.

looking through the truss
Looking through the trusses
photo credit: Topher Belknap


Foot thick walls

The walls are a foot thick, there is no getting around that. Among other things, I suspect that this is raising my taxes slightly (I think they calculate by exterior dimensions around here). I actually like the way the windows look. It is not quite up to window seat size, but you can sit and look out at the view. The shelf in front of each window is a huge place for plants (some quite large, like the lemon tree), and other stuff. This is, of course, a mixed blessing as stuff seems to accumulate in such places.

Wiring difficulties

Timber frame houses, in general, have problems with locating wire runs. The larsen truss didn't exacerbate these, but it was still a problem that needed attention. One solution often used is to run the wires outside the frame before the outside walls are put up. This did not appeal to me as it would violate open building principles and make changing or adding new wires extremely difficult. I decided to run a wide base board all around the exterior walls to hold both wires and outlet boxes. This allowed all the wiring to remain inside the vapor barrier and insulation envelope. They are about the size of baseboard heating units, but since they are between posts, the aren't too much of a problem when placing furniture. Adding new wires is simple, just unscrew the baseboards from the wall and run the wire. The timberframe posts have grooves cut into them on the outside to pass the wires.

Unknown technology

Neither my general contractor, nor my builder had ever heard of a larsen truss. 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).

Insulation settling

In 02008, I was able to examine the entire house with a borrowed infrared camera. 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). I would recommend anyone using trusses to use wood I-beams with a solid webbing to keep insulation from migrating during application. In addition, I would recommend a dense pack process.

Why no SIPs?

Most timber frame houses use Structural Insulated Panels (SIPs) as a wall structure. There are a number of reasons why I did not (some rational, some less so). First, was price, my general contractor said he could build a larsen truss for a lot less. Second, I had concerns about the insulation, I think cellulose is more environmentally friendly than EPS or other foams in SIPs. Third, although they make 10 inch thick panels with R-value around 40, they were not readily available in my area. Fourth, 10 inch thick panels would require similar detailing around the windows and doors, but I thought that it would be more difficult to accomplish with panels. Fifth, my general contractor had concerns about using them, specifically about attaching shingles to the outside. Sixth, I had some concerns about creep. This last not based on anything except rumors and lack of contrary evidence. SIPs were definitely my second choice however.

I also looked at Bensonwood open-built panels which, at the time, looked like a good option, but they would not sell to me unless I also bought a frame from them.


At the end of This Old House projects they always ask, 'would you do it again, knowing what you know now?' For me, the answer is yes. The larsen truss part of my house is a complete success, in my opinion. The deep recesses of the windows and door provide room for all sorts of things: large house plants, new seedlings for the garden, assorted stuff lacking a better home, the occasional sunset gazer. The level of insulation they hold keeps my energy consumption very low. And lastly, it gives me a great feeling of pride, that this was something that I achieved by sticking to my principles and learning all I could.

P.S. If you have a larsen truss home, I would love to hear from you.