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Building Science

The Benefit of Larsen Trusses

Adding depth to exterior walls is a smart way to add lots of insulation

Almost all of the work I perform is on houses that predate the Civil War. As I’ve written about here before, doing the right thing by them in terms of insulation isn’t always straightforward. In fact, doing the right thing on any kind of energy retrofit of an existing house almost always means navigating some trade-offs.

The lid is usually relatively easy. You head into the attic and either spray foam the hell out of the roof, or you air seal the attic floor and then blow in a couple of feet of cellulose. Of course, it’s a little more complicated than that, but these energy improvements don’t require gutting exterior walls and figuring out how to air seal and insulate a wall assembly that probably doesn’t even have sheathing.

What if I were to tell you there’s an easier way?

Insulate the outside

Forty years ago, in 1981, the same year that Fine Homebuilding magazine came out, a Canadian fellow named John Larsen came up with a way to insulate houses from the outside. Larsen wasn’t just Canadian, he was from Edmonton, Alberta—a city that can claim the harshest winters south of the Arctic Ocean. The man was solving a real problem.

Larsen built simple trusses that he nailed to the outside of the house to provide added depth for insulation. His original design joined 2x2s together with 8-in.-wide strips of ½-in. plywood glued or nailed into dadoes. He would nail these to the house’s studs over the wall sheathing, completely separating the insulation layer from the interior walls. This simplified the insulation installation process significantly, with no need to weave batts around pipes and wires.

The trusses ran down to the mudsill, allowing for simple insulation of the…

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34 Comments

  1. Brian Wiley | | #1

    This is a great overview of the concept and process. It also seems particularly well-suited to people that are trying to avoid foam/petroleum products in their home.

  2. Doug McEvers | | #2

    Harold Orr and company added superinsulation to the outside of walls to existing homes in Canada. They went right down to the footing of homes with basements and superinsulated the foundation wall as well. As I remember they used a Larsen Truss or something quite similar.

  3. Robert Swinburne | | #3

    We are doing a simplified Larsen Truss on new builds here in Vermont. With subsequent projects we are able to evaluate and simplify. To the point where it uses less material and less labor than even double stud walls for an r40 wall. and with the bonus of getting the air barrier into a mid-wall position.

    1. Dan Kolbert | | #4

      Would love to see the numbers, Bob. I've looked at I-joists for new construction a few times and could never figure out how to make it work cheaper than double stud. The cost of the i-joists alone put it out of contention.

      1. Robert Swinburne | | #5

        will send you something via email. - Also, not I-joists. too expensive.

        1. Karl B (Zone 6A) | | #30

          Bob, if you're willing to post details about your replacement I-joist method here, I'm sure Dan and I aren't alone in being interested.

          EDIT: Actually, was this the detail you mentioned on the BS & Beer show episode "Designing a High-Performance House with the PGH Approach" at 44:44 - 45:43?
          (link: https://youtu.be/cXDaGG-v_60?t=2684)

    2. toothman2020 | | #31

      Robert,

      What do you like about the air barrier in the mid-wall position?
      Thanks

  4. Paul Kuenn | | #6

    Thanks for the input and adding Palin, Idle and Chapman in for the laugh. I had a crew like that once. Nodded their heads yes a lot but didn't understand the concept at all... I'd like to try the truss sometime on my old projects but I get the recycled EPS so cheap from roofers. Though, I am growing very tiered of the white beads everywhere and mostly stuck to me.

  5. Brian Wiley | | #7

    Is the Larsen truss treated like any other exterior continuous insulation in terms of the prescribed climate zone ratios?

    I get that a big reason for using them is the ability to make them thicker, and therefore warmer, than rigid typically allows for which would put you out of any sort of cold-sheathing danger. I’m just curious relative to property setbacks and other constraints that might come up.

    1. Charlie Sullivan | | #8

      There's no need to follow those ratios--they are about impermeable insulation vs. permeable insulation, but since the fluff in the truss is permeable, there's no issue.

      1. Benjamin Bogie | | #16

        That’s not correct, the ratios have nothing too do with the type of insulation material being used but with maintaining the sheathing layer above dew point temperature. A vapor permeable exterior insulation will be more forgiving of not following the minimum ratios for the climate zone but otherwise the IRC recommendations should be followed as a minimum.

        1. Expert Member
          Malcolm Taylor | | #17

          Ben,

          Are you sure? The ratios are trying to keep the sheathing above the dew point in walls that can not dry to the 0utside. Walls with inadequate permeable exterior insulation may not keep the sheathing above the dew point, but neither do walls with no exterior insulation, which are deemed safe by the code. The only meaningful distinction seems to be whether they can dry outwards.

          Here is Martin's take on it:
          https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
          And:
          https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness

          1. Tyler Keniston | | #18

            This strikes me as one of those grey areas where we all wish for easy rules, but the reality is probably that it just depends (that, and assessing risk).

            I found this info interesting: http://cchrc.org/media/SEAR_snapshot.pdf

            Note both that source^ (cold climate based) and Martin's relaying of Joe Lstiburek's rethinking of minimum foam rules include mention of interior poly and proper interior air sealing.

            I imagine this also depends on if one sheaths the Larsen trusses. Are people typically sheathing them, or just strapping?

          2. Expert Member
            Malcolm Taylor | | #19

            Tyler,

            I don't think that there is any doubt exceeding the IRC minimums with either type of insulation yields a safer wall. What I'm still not sure of is Ben's assertion there is no difference between the two.

            What I'm struggling with is this: If a wall is safe with no exterior insulation, then why would adding a highly permeable exterior layer affect that? Whereas we know we can make the same wall unsafe with impermeable foam.

          3. Benjamin Bogie | | #20

            Maybe I was being a bit staunch with my statement. There’s certainly as much risk factoring in the equation related to what the cavity insulation is vs the permeability of the exterior. I’m finding myself leaning a bit more to the side of caution when making recommendations as so many other factors can push a marginally risky assembly into the danger zone and it’s hard to express all those risks all the time.

          4. Expert Member
            Malcolm Taylor | | #21

            Ben,

            It's entirely academic for me, as like you I'm not a fan of exterior foam, and I'm as conservative as y0u about incorporating risk int0 building assemblies, so I'd do as you suggest and at least hit the minimum ratios just to be safe, whatever type of exterior insulation was used.

          5. Tyler Keniston | | #24

            "If a wall is safe with no exterior insulation, then why would adding a highly permeable exterior layer affect that?"

            It probably won't negatively affect it, I agree. I was just pointing out that at least one source claims it's not completely simple (their recommendation apparently being to perform a hygrothermal analysis of the wall). I'm sure they are being conservative and science-headed about it. This seems to be something we see with double stud walls as well, correct? (Few issues in practice, but modeling that indicates potential problems. Clearly some forces are at work that the modeling does not account for.)

            I can't completely make sense of their 'data', but apparently adding 1.5" of rockwool made for a more dangerous wall than adding 3". Furthermore, the 3" rockwool wall was safer than the baseline (no exterior insulation), while the 1.5" was worse than baseline.

            Adding ANY amount of exterior insulation will indeed 'warm' the inner (existing) sheathing, and the conventional wisdom is that ANY warming is inherently safer. But does warming it some small degree guarantee that moisture loading potential has been 'significantly' reduced? Is the relationship linear (warmer by X degrees = condensation loading potential –X) or could there be a non-linear relationship dependent upon interior dewpoint conditions and yearly exterior temps?

            Even slightly warmer sheathing is not worse than colder sheathing, but the catch is that adding exterior insulation does add SOME vapor flow resistance (even if relatively high perm), and probably reduces convective drying at the interface of the original sheathing (depending on the drying characteristics of the original cladding).

            This highly unscientific graph shows what I mean (the line shapes, etc. are completely made up mind you—just relaying an idea). And of course many other factors (air-leakage, interior vapor retarders, etc.) are at play.

          6. Charlie Sullivan | | #26

            Tyler, thanks for that link. That's a really interesting study I hadn't seen. The metric they are using is number of days in the danger zone, meaning a combination of wet enough and warm enough to grow mold. The 1.5" of mineral wool doesn't make it worse as far as how wet it is. It just makes it warmer during that time that it's wet, such that mold can grow. That's a good heads up that sometimes just thinking though making sure things can dry well isn't the whole story--it's OK if the are damp as long as they stay cold while they are damp, and can dry before they get warm.

          7. Tyler Keniston | | #27

            Charlie,
            Good eyes and good point (figure 3 in that link helps make that evident). This all seems very climate dependent.

            Though that suggests to me that if you are going to 'make sheathing warmer' you also have to be sure you are making it SUFFICIENTLY drier, not just marginally drier, since the increased temp actually increases mold/rot risk. That is what I was getting at with my kindergarten graph: when do such lines cross? (I really should have had mold potential on the y-axis and some additional parameters). It doesn't seem we can say with blanket statements, so it comes down to risk assessment or modeling (or better yet, real-world empirical evidence. Ben seems to be working on those fronts, which is awesome).

          8. Expert Member
            Malcolm Taylor | | #28

            Tyler,

            Thanks, that's a good take-away.

          9. Derek Maxwell | | #32

            This discussion had me thinking today about double stud walls and like Tyler mentioned the concerns shown in modeling but not happening seen in practice often. In a double stud wall or Larsen truss system the sheathing can get wet and its no big deal if it is under a vented rain screen, the air flow will help it dry if it ever gets too wet. If moisture gets into the cellulose its going to be ok most of the time too because it can distribute the moisture. The problem with exterior insulation is that transition point from one material to the next in the center of the wall where it is far from fresh air that can dry it. The exterior insulation creates a space that can be a trap even if it is permeable and thus creates a risk that does not exist in a wall without exterior insulation.

  6. Derek Maxwell | | #9

    Let's say someone is going to to do this without removing the siding and the house is original interior plaster and lath. Is there any concern about the lack of a vapor barrier? Or would you just wrap a vapor barrier over the existing siding?

    1. Charlie Sullivan | | #10

      I would be most concerned about the lack of a good air barrier and would consider a self adhesive membrane over the existing siding as a way to achieve that. With that, the whole sheathing, siding, membrane would act as something of a vapor retarder as well, which is worth thinking about carefully for specific set of materials and climate, but it's likely to be fine.

    2. Expert Member
      Malcolm Taylor | | #11

      I'd take the siding off.

      1. Expert Member
        Michael Maines | | #12

        If you're going through the trouble of adding Larson trusses on the exterior, the extra step of removing the siding is not much extra work--why wouldn't you remove it? Old windows and doors are virtually never flashed to modern standards so removing the siding will also let you tie those in to the WCL and ACL.

      2. Charlie Sullivan | | #15

        Despite my first comment outlining how to make it work out with the siding on, I agree that taking the siding off would usually make a lot more sense. The only exception I can think of is something like T1-11 that's siding and sheathing in one, in which case you could go either way, but if you remove it, you would need to replace it with new sheathing.

  7. Rick Taron | | #13

    I agree. Take off the siding.

  8. Expert Member
    Malcolm Taylor | | #14

    One thing that would worry me about retrofitting Larson trusses to an older wall that wasn't opened on either side would be the possibility of hitting services - specially wiring- with the fasteners.

  9. Derek Maxwell | | #22

    Yes thanks for the replies on the siding I also agree and I think Malcolm your point is very good and brings up another potential that if siding comes off there is easy access to wall cavities to do minor electrical or hvac work without much interior disturbance. My original idea was based on having many turn of the century homes in the Twin Cities that could use an economical energy upgrade. The idea of being able to show up day one on site and be almost immediately into adding on trusses would be great. Of course there is the eaves and roof to deal with but I think one could come up with a simple efficient system for that too. But as many have pointed out above there are always risks that need to be addressed so no free lunch. Too bad we can’t just grind that siding up and turn it into recycled fiber insulation boards for a foam free project.

    1. Expert Member
      Malcolm Taylor | | #33

      Derek,

      I certainly see the appeal. What soured me on leaving extraneous layers either on walls or roofs was being the guy who does the next round of work on the project. Early in my career I did a lot of renovations of old building in Ottawa. It wasn't unusual to open roofs and find old ones underneath supporting the new framing. None of the problems had been dealt with, just covered over. The OCD part of me couldn't stand leaving them - to my financial detriment.

  10. Expert Member
    Peter Engle | | #23

    A word of caution about those deep horizontal plywood sills. I've now seen a number of houses where they rot out in short order. We seem to have become dependent on self-adhesive rubber membranes as some sort of magic solution, believing the marketing hype about them being self-sealing. Well, they do seal around fasteners when they're new. But once the fasteners rust out, they leave a hole right through the membrane for water to enter the wall. The attached photo shows a short sill of about 2" that was penetrated by two staples and two screws. The staples held the second layer of WRB and the screws held the foam insulation in place. All of the holes were leaking and there were mushrooms growing out of the wall after only 2 years in service. In this particular case, there was also a seam in the membrane on the horizontal section that never sealed, allowing even more water into the walls. Just a complete mess. Sloping those sills is harder but well worth the effort, and using sill finishes that allow rapid drainage and don't require ANY penetrations in the membranes is also an important component of a bulletproof installation.

    1. Doug McEvers | | #25

      Builders of many years ago seemed to build with nothing more than a roofing felt WRB and never had the type of bulk water intrusion we see today. As I think back to some of those wood windows the bottom sill was always sloped allowing for the water to drip away from the siding surface. Most windows today have a nailing flange and the bottom sill is flat. Rain can then flow along the bottom of the window jamb horizontally and find its way behind the siding. Worse yet I have seen exterior clad windows sashes on the north side of homes here in Minneapolis rot at the bottom. The clad area of the window frame under the sash is flat for a portion and water sits there often. The high moisture conditions can lead to a failure of the wood window sash.

      1. Expert Member
        Peter Engle | | #29

        Doug,
        Not entirely true. My own 1894 Victorian has felt in some places, just rosin paper in others. It had bulk water intrusion below most of the windows, but no damage to walls or sheathing. This is because there was no insulation in the walls and lots of airflow to dry things out. Builders of many years ago got lucky that the drying potential was greater than the wetting potential. Our problem now is that we've dramatically reduced the drying potential without adequately reducing the wetting potential. Our very efficient walls are, unfortunately, far less forgiving of bulk water leakage.

    2. Andy Engel | | #34

      Thanks for that info!

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