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Musings of an Energy Nerd

All About Larsen Trusses

A detailed history of John Larsen’s system for building thick superinsulated walls

Larsen trusses are non-structural. These lightweight trusses are tacked onto the sheathing after a house is framed and sheathed. In most cases, conventional 2x4 studs hold up the roof load, and the 2x4s are sheathed with plywood or OSB. The Larsen trusses get installed late in the construction schedule, after the roof is on.
Image Credit: Bruce Coldham

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.

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…

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36 Comments

  1. Bob Manninen | | #1

    use of sheathing over insulation
    Very nice article. I have a question as to the common practice of putting sheathing over the trusses. Are there are other common practices other than using sheathing? Could something like Rockwool be used for insulation (it's a little more moisture resistant than fiberglass and/or cellulose) and something like tar paper or a permeable building wrap be used instead? Or, is the sheathing used to prevent racking of the trusses.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Robert
    Robert,
    Some builders have installed housewrap and siding directly over the insulation, without sheathing the Larsen trusses. I don't recommend this practice, however, unless you are using a panel-type siding like T-111.

    Sheathing helps prevent the Larsen trusses from wiggling back and forth and improves the airtightness of the assembly.

  3. Doug McEvers | | #3

    Larsen Truss for energy retrofit
    It seems the Larsen Truss is the ideal candidate for superinsulating existing housing, more information on this system being used on retrofits would be welcome.

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Doug McEvers
    Doug,
    If you want more information on retrofitting houses with Larsen trusses, see if you can get a hold of a copy of the the April/May 1984 issue of Fine Homebuilding. (You might be able to buy it used online.)

    You should also check out the first link in the "More Information" box. It's a link to a paper by Neal Carter with many details on retrofits.

  5. David Argilla | | #5

    Neal Carter Paper missing figures
    Great Article! I was wondering how to do the corners, most figures of the larsen truss leave them out. The linked paper by Neal carter is missing figure 5 and figure 7. Think you can ask if they are available? Pictures/figures are very useful.
    Thanks,
    Dave

  6. J Chesnut | | #6

    great article
    I was first introduced to larsen trusses by Tim Eian just a couple of years ago when he brought me into his office to help him with his workload. Tim detailed them for a retrofit design and my first impressions were great idea but (1) is an engineer going to have to sign off on their shear strength for holding up cladding and (2) contractors are going to price this through the roof. According to this article field experience seems to show no need for the engineering stamp but there still is the problem getting competitive bids.

    We have moved towards the wall assembly with the I-joists acting as the larsen trusses and the first sheathing layer functioning as the air barrier and vapor retarder (with a redundant vapor barrier paint on the inside for good measure.) We then require vapor open fiberboard to be the sheathing layer outside the I-joists.

    I don't know if it is anymore cost effective but I'm interested in the idea of using Homasote as the outside most sheathing layer. It is a vapor open material made of recycled paper and is readily available in our market. The only drawback I can imagine when using this product is whether there is a problem if it gets wet during the construction process. I imagine it only being used with a siding material over a vent space and covered with a WRB. Any thoughts?

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to David Argilla
    David,
    Concerning the missing figures from the Neal Carter paper: the link I provided brings up a scanned copy of the original Neal Carter paper from the 1980s. This historic (and rare) document was scanned by Marc Rosenbaum, and I don't know of another copy. If any GBA readers have an intact copy of the document that includes the missing figures, please let me know.

    Concerning outside corners: here's what John Hughes wrote in his 1984 FHB article: "To cope with outside corners, we've found it easiest to prefabricate a corner truss from two straight trusses. One side of this right-angle truss is then nailed directly along one edge of the corner of the house. This leaves a gap on the other side of the corner that we bridge with short truss sections nailed horizontally to the wall every 2 ft."

  8. User avater GBA Editor
    Martin Holladay | | #8

    Response to J Chesnut
    J,
    Experienced builders have a low opinion of Homosote sheating because it is so flimsy and susceptible to water damage. Of course, Passivhaus builders who follow European methods are always in search of vapor-permeable sheathing options -- these builders are likely to be attracted to some of the cheesier American products like Homosote.

    Personally, I'd rather use diagonal board sheathing than Homosote.

  9. Robert Riversong | | #9

    Larsen or Wall Truss

    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.

    Most people are now referring to my modification as the "Riversong Truss", but the distinction is not as clear cut as Martin suggests. A structural truss is engineered to transfer gravity and wind loads through compression struts and tension ties to inner and outer chords. An insulation truss, whether Larsen or Riversong or any of the other varieties, is a non-load-distributing truss that is designed to carry only cladding and window loads on the exterior (and transfer wind loads to the structural frame).

    The primary modification of my system is to use the interior, load-bearing stud frame as the inner chord of the insulation truss by attaching an outer chord with gussets after the roof is in place. So the primary stud wall frame is the load-bearing assembly, just as on a conventional house, and the insulation cavity is extended outward. I always use a wide enough foundation to carry both inner and outer frame members so the tricky and often ugly cantilever is avoided.

    This hybrid system eliminates the extra layer of sheathing, uses half as much lumber for the deep cavity, creates a single insulation cavity to dense-pack (with cellulose), and puts the air barrier where I believe it belongs in a cold climate – on the inside, to stop the warm, humid air at its source – and makes it contiguous with the vapor retarder (VR primer). This does require the Air-Tight Drywall approach, but makes the entire construction process both quicker and far more resource-efficient (and hence less costly).

    The exterior can either be sheathed with diagonal boards or CDX (or fiberboard but never OSB), or simply enclosed with taped housewrap and clad with ¾" shiplap boards (pre-finished on all sides). This creates an extremely vapor-permeable assembly, with a high moisture storage and redistribution capacity, and virtually eliminates any moisture accumulation problems (assuming exterior detailing is well done). No rainscreen is necessary for this wall assembly as there are no moisture vulnerable sheathings and the entire assembly dries easily.

    My Riversong truss system can be seen here: https://www.greenbuildingadvisor.com/homes/thick-cocoon-cellulose-protects-superinsulated-house

  10. Robert Riversong | | #10

    Sheathing or Not

    Sheathing helps prevent the Larsen trusses from wiggling back and forth and improves the airtightness of the assembly.

    If the trusses are tied together at the bottom with a plywood plate and tied to the roof framing at the top, as well as connected laterally around doors and windows, there should be no "wiggling" and no need for sheathing for rack bracing.

    The primary issue that determines the need for sheathing (other than code officials) is the type of siding. I would not install clapboard (especially fiber cement) without a solid backing, but 3/4" thick wood siding can easily span 24" oc framing without distortion - even with cellulose dense-packed behind it.

    The fewer exterior layers, the more vapor-permeable the assembly. The only other reason to consider sheathing is to make future repairs or renovations easier.

  11. Thorsten Chlupp | | #11

    Thanks Martin
    Thanks for honoring John Larsen's work Martin!

    He surely is an icon of superinsulation and I got a great chuckle out of his advice to burn junk mail in the wood stove instead of designing to the coldest day of the year. I will remember that this winter when it hits 40 below and I don't have to fire up my masonry heater yet - what was I thinking? Overkill - but I am still enjoying the hell out of it.

    Exterior sheeting - you know the problems related to high R-Value walls and low perm on your exterior sheeting. I have seen plenty issues in very cold climates at least. Also let's not forget that ideally the inside Sd-value must be at least ten times higher than the outside Sd-value for optimal drying potential. And - there is certainly no wiggling going on without exterior sheeting. What to use? I concur with Homesate not being a good choice. High perm European fiberboard is still unobtainable but would be ideal. Tyvek works but is not ideal either for obvious reasons. We are testing out SIGA Majcoat (34 PERM) right now, which might be the right solution. And even in an assembly like this I am not giving up my air gap/rain screen between the membrane and the siding as it makes things work just so much better (Overkill again?). And I tried it both ways ...:-)

  12. Dan Kolbert | | #12

    Ship lap? Sheathing?
    Robert - have you found the ship lap holds up well? Are you buying stock product of making your own? I've seen enough ship lap curling at the laps that I get nervous, but maybe that's a function of poor prep.

    Thorsten - tell us more about the "plenty issues" you've seen.

  13. User avater
    Albert Rooks | | #13

    Good stuff!
    Great article Martin.

    Thanks for both the interview and depth. There is a lot to learn from the development history and It's really sad that the momentum stopped in the 80's. If it had kept up we would probably have companies like Ligno Trend in the US and be able to pull good load bearing, or non bearing Larsen Trusses "off the shelf".

    Having watched two site made Wall Truss (bearing) jobs it seems that site built trusses are not efficient enough to last as a practice. It comes down to the fact that a site built truss is usually handed. The gussets being glued and nailed on the outside rather than center dadoed makes another sequencing step in keeping track of how the gussets fit window and door openings. It's cumbersome to handle the bottom plates when they cantilever over exterior foam.

    It seems clear to me that the issues would be solved by a good off the shelf product like the original John Larsen model: Non handed with centered gussets and the addition of "new models" of load bearing wall trusses with standard framing members inside in a few wall depths.

    It's interesting to note that Ligno Trend does offer both non, and load bearing trusses. They don't use a ply or OSB gusset, but instead use dimensional solid stock. Since there are no metal fasteners, you can pull a length and can cut it straight, or at an angle, at will. it's a great design and with more clients & projects demanding super-insulated envelops, we'll have some "hardy soul" venture into the business of offering them someday soon ( I hope).

    Attached are more pictures that I shot of the Ligno Trend trusses. (I've not cropped them. sorry for the poor quality). You can see the joinery and build-up method. they are really good looking components. I wonder how many builders would really be interested in the level of quality of Ligno Trend or the original John Larsen center dadoed truss?

  14. Thorsten Chlupp | | #14

    "Plenty Issues!?"
    Issues? Martin has blogged about this plenty. John Straub cautions about risky “cold sheeting” often. Brilliant Bill Rose educates us with his great work about sorption and basic laws of materials and moisture – “Cold stuff is wetter than hot stuff”. My ever curious mind has led me to take the time to physically inspect wall assemblies of old and new buildings for many years. And once too often I have seen wet plates and mold growth – and not just on old homes but also barley two year old 5-Star+ build homes. Bad and ugly stuff, conveniently hidden away behind a pretty wall. I am of course fortunate and live and build in a very extreme and unforgiving climate which create the worst conditions for building assemblies. Issues I can see in two years might take ten years somewhere else (or maybe they never develop? I am doubtful about that…).
    Plywood or OSB are a class 2 vapor barrier with a PERM under 1 in a dry cup state. And it is mainly made out of wood and will react to environmental conditions of its closest surrounding and try to find an equilibrium. Great features on the warm side – inside – not so good on the cold exterior.

    Take a basic analogy: Go for an easy run on a cold winter day. What will you wear? First we try a thick layer of fleece. Off we go, we are warm, we sweet and next thing we know there is a layer of frost on the outside of the fleece – but who cares, we are comfy. But bummer, it gets windy and now we are shivering and freezing cold. We need to stop that wind quickly to be able to warm up again – so let’s layer a windproof rain jacket! Great, now we are warm again and can run on. A mile later however we don’t feel so comfy anymore and will notice that a layer of ice is forming between our fleece and the inner jacket layer. Yikes! Okay, we are smarter than that. We get rid of the frost layer, take the fleece of and place the jacket under the fleece (VB!)– no moisture can get out, nothing can freeze and all will be good. Brilliant. Down the trail we go, warm and frost free – but ugs…it becomes uncomfortable pretty quick. Ok, time to quit, obviously running in the cold sucks. But wait, maybe there is a better solution after all…mmh? Screw the rain jacket, let’s use a permeable windbreaker on the outside. Now, that wasn’t that hard was it?

    Wet sheeting and walls are bad. Mold is bad. Dry rot is bad. We can be ignorant - but stupid? We know the answers for the longest time but choose nevertheless to build flawed systems which have to fail as they defy simple laws of nature. Our health is priceless … that is about the least we should consider when we design and build buildings in which we will spend a lot of time in and that should last a min of 80 years.

    Anyways, time to go for a run ….

  15. Robert Riversong | | #15

    Ship Lap
    Dan,

    I've used rough air-dried hemlock that I had milled 3-sides into 13/16" thick shiplap, and I've used standard pattern #105 spruce drop siding - with no problems. I use solid-color latex stain and pre-stain all six sides before installation with a second face coat immediately after installing each "lift".

  16. David Argilla | | #16

    Using 1x4's
    Any thoughts about using 1x4's sandwiching the gusset for outer leg of Larsen truss? I have a bunch of 1x4's stored away.

  17. User avater GBA Editor
    Martin Holladay | | #17

    Response to David Argilla
    David,
    If I understand correctly, you propose using two 1x4s instead of one 2x2 as the outer vertical member of a Larsen truss. These 1x4s will be separated by about 1/2 inch -- the thickness of the plywood gusset.

    I wouldn't recommend it, if only because it's hard to nail sheathing or siding to the 3/4-inch-thick edge of a 1x4.

  18. Dan Kolbert | | #18

    Simplifying
    Thanks, Robert. Having done several fairly high tech projects over the past several years we're trying to simplify our details (and houses). Nice to see you back.

    And I agree with Martin - doubled 1x4 sounds only marginally more trustworthy than single 1x4.

  19. David Argilla | | #19

    SIding
    Ah yes, that's an important component. Didn't think it through, thanks.

  20. User avater
    Ted Clifton | | #20

    Why not just Insul-lam?
    The Larsen Truss looks like a lot of work for a retro-fit, with a lot of potential moisture issues. Why not just screw Insul-lam™ or Nail-Base™ (now sometimes called Retro-fit SIPS) panels to the existing wall? One operation, and you are insulated and ready for siding. The EPS insulation can be ordered up in any thickness you desire, we have used up to 9 1/4" of EPS, for R-40, outside of an R-15 HD f/g batt in a 2x4 wall. Costs a lot less, and does a better job, air-tight, also, no poly required. R-55 total wall construction.

  21. Thorsten Chlupp | | #21

    Insul-Lam? x-Factor
    Factor x64 in Embodied energy might be a motivator against the foam...
    Costs in a high R-value wall is another reason: foam is more expensive.
    If out-insulating with foam be sure to stay in your safe ratios. This is extremely important. No. 1 quick fix to boost wall R-value I see in new construction and retrofits is adding 2"of foam on the exterior. No moisture buffering and drying potential...guaranteed to fail in my climate.

    Picture from a two year old wall assembly in very cold climate; from inside - out:
    1/2"sheetrock, 6 mil VB, 2x6 Std, R-21 Fiberglass batt, 1/2"CDX, Tyvek WBR, 2"Foil faced EPS, Vinyl Siding. Mold, Bottom plate with very high moisture contend = unhappy wall.

  22. Skylar Swinford | | #22

    Larsen Truss "Chainsaw" Retrofit Animation
    Martin,

    You included one of Rob Harrison's photos in your article, but you left out his awesome animation of a Larsen Truss "Chainsaw" retrofit!

    For those that are interested check it out: http://www.flickr.com/photos/robharrison/5337590411/

    Thorsten, I know you make the most of your short building season so thanks for taking the time to impart your wisdom.

    Riversong, good to have you around again!

  23. Skylar Swinford | | #23

    Lamdaplus Larsen and Passive House Conference Proceedings
    In addition to the Lignotrend truss system that Albert posted (I think that was my favorite booth at the conference), I saw a photo of the Lamdaplus System during a presentation:
    http://lambdaplus.de/page.php?seite=lambda
    https://youtu.be/m28lnW1IrLY

    Also, if you missed the 2011 PHI Conference in Innsbruck, I recommend picking up a copy of the "Conference Proceedings 15th International Passive House Conference 2011" at the link below (Don't worry the book is in English).
    http://www.buchhandel.de/?caller=vlbPublic&strFrame=titelsuche&Schlagwort=Passive%20House or search elsewhere with ISBN: 978-3-00-034396-4

    This 600+ page bible is a compilation of all the conference presentations. Books from past conferences are also available.

  24. User avater
    Albert Rooks | | #24

    A questionable admission
    To a building material nerd such as myself, there is nothing sexier than a high quality Larsen Truss. (Either wall or classical non bearing.)

    There. I said it.

  25. Robert Riversong | | #25

    Back at Ya
    Ted Clifton: "The Larsen Truss looks like a lot of work for a retro-fit, with a lot of potential moisture issues. Why not just screw Insul-lam™ or Nail-Base™"

    Reason: a lot of potential moisture issues.

    A cold-climate wall system should ideally:
    - be relatively air-tight (and easy to keep air-tight over time)
    - limit outward vapor diffusion (mostly controlled by mechanical ventilation)
    - buffer (safely store, redistribute and release) excess diurnal and seasonal moisture
    - breathe (be vapor permeable) to the outside with no mid-wall vapor stops
    - have as few discrete layers as possible (improves breatheability and moisture release)
    - contain no highly moisture-vulnerable materials (such as OSB)

    The focus on cold sheathing is a diversion (unless the cold sheathing is also highly moisture vulnerable, such as OSB), because cold and wet is not a problem - only warm and wet (and aerobic) can lead to mold and decay organism growth. If sheathing picks up some moisture during the winter but can dry out quickly in summer or from solar radiant drive, then it's not a problem. Cellulose so effectively stores and redistributes moisture that it protects sheathing and framing from reaching high (dangerous) moisture content.

    OSB, foam board and spray foams limit or prevent moisture diffusion - both liquid and vapor - while fiberglass insulation has no moisture buffering ability. These are not good materials to use in a highly insulated wall assembly. Even plywood limits vapor diffusion, but has the salutary ability to increase its (wet cup) vapor permeance as it becomes more saturated. OSB does not.

    And polymeric housewraps can become problematic since they present a condensation surface often adjacent to sheathing and will trap liquid water. 15# felt diffuses liquid water as well as water vapor, and it's shingled application style allows better drainage and better integration with door and window flashings.

  26. User avater GBA Editor
    Martin Holladay | | #26

    Do walls have to breathe?
    Most building scientists would dispute the statement that "A cold-climate wall system should ideally ... breathe (be vapor permeable) to the outside with no mid-wall vapor stops."

    A properly designed wall with an adequate thickness of exterior rigid foam will not have "a lot of potential moisture issues." On the contrary -- ongoing studies continue to show that such walls are among the driest and most problem-free cold-climate walls ever studied.

  27. Bill Dietze | | #27

    Building Science Corp. Comments
    Martin,

    In the text of the blog, you write
    " A document on the Building Science Corporation Web site states, “The [Larsen] 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."
    But isn't the wall in the BSC article something other than a Larsen Truss? A Wall truss perhaps - there is no inner sheathing, so do the BSC comments apply? Seems not.

    Bill D.

  28. User avater GBA Editor
    Martin Holladay | | #28

    Response to Bill Dietze
    Bill,
    Good catch! I have edited my blog to reflect your observation. Thanks for your comment.

  29. Steve Young | | #29

    Wood beams in Larsen structures
    I am starting to look into a Passive house and the Larsen Truss system sounds very interesting.
    I was thinking of a wall like this, from inside to outside.
    Drywall ADA - with vapour perm paints
    2x4 insulated "Chase " wall, with dense pack cellulose, sealed electrical boxes and, piping gaskets.
    OSB taped (and primed?) AIR BARRIER
    2x6 24" OS structural wall, filled with dense pack cellulose
    A permeable rigid sheathing of some sort. (Beaverboard-like substance?)
    2x14 I-beans filled with Cellulose, again)
    Another permeable rigid sheathing
    furring strips and siding

    Overall, does this sound OK? Any suggestions for easily obtainable, and inexpensive permeable sheathing? If I use wood I beams, do I have to worry about the OSB? Or should I assemble my own full height "C" beams (no dado) with plywood and a couple 2x's?

    PS I love this site. The articles, and subsequent discussions have been a real eye-opener as I wrestle with trying to come up with a super-insulated wall. Things have gotten a lot more complex since my days in Edmonton, Alberta in the late 80's when I last undertook a significant home insulation project.

  30. User avater GBA Editor
    Martin Holladay | | #30

    Response to Steve Young
    Steve,
    Q. "Any suggestions for easily obtainable, and inexpensive permeable sheathing?"

    A. There are several options, including diagonal boards (1x6s, for example) or structural fiberboard (for example, Buildrite fiberboard or G.P. structural fiberboard).

    Q. "If I use wood I beams, do I have to worry about the OSB?"

    A. Your question is unclear. There are several possible reasons to worry about OSB, but I'm not sure which worry you are referring to.

    Q. "Should I assemble my own full height "C" beams (no dado) with plywood and a couple 2x's?"

    A. I'm not sure what you mean by C-beams. If you want to build Larsen trusses, detailed information is provided in the article on this page.

  31. Steve Young | | #31

    Clarification
    Thank you for the links for the fiberboard.

    With regard to OSB, I am concerned that it will get very cold in the I beam of the larsen truss. I have read many times on this site and other's that OSB does not like to get cold because it absorbs water and expands/degrades - eventually failing. Are there other aspects of OSB that I should be worried about?

    "C-Beams". Larsen went through the trouble of rabitting the 2x2 to insert the plywood plates. Rather than going through that detail, I would simply attach the plywood to the sides of 2 2X2's with outdoor screws and construction adhesive), making a stretched "C" shape.
    I have concerns about dense packing these cavities, therefore, I might run the plywood the entire length of the truss - no open area between cavities. (although, I read about a mesh product in another blog at this site that is used to "harness" celulose - Insulweb, I believe).

  32. User avater GBA Editor
    Martin Holladay | | #32

    Response to Steve Young
    Steve,
    Many Passivhaus builders, including Katrin Klingenberg, have framed thick walls with I-joists. However, if you are worried about the OSB, why choose that route? The article on this page explains how to build Larsen trusses with plywood instead of OSB.

    It's possible to skip the step of dadoing the 2x2s if you want to simplify the construction of your Larsen trusses; the article and photos include examples of builders who skipped the dadoes.

  33. Steve Young | | #33

    Response to Martin H
    Thank you, again
    Yes, I have been in KK's house, along with the other PH's in Champaign-Urbana. I don't know whether she has had any problems with the I-beams' OSB over the years (I didn't know enough to ask at that time - it was all very new to me). I liked the wall used on the Stanton Farm, but without the expense that they incurred.
    I am trying to develop a wall that I can build on my own. I would contract the framing of a 2x6 walled house (using advanced framing, of course), then I could tackle the Larsen Truss system, additional sheathing, the inside OSB airbarrier and inside chase wall. At first, I considered the true Larsen Truss, but it seemed like a lot of extra work to make an I-beam that I could purchase. Then I remembered all the talk about how OSB has issues with cold weater, and figured that I would ask for other folk's expert opinion.
    These Blogs/Forums have been very educational. Every time I read something, it leads to another topic or detail. My tabs list keeps getting bigger and bigger.

  34. Mat Marcum | | #34

    Hello, All:

    I've got an 1887 home that is 2-course, structural brick that I'm completely renovating. The brick is in good condition with the usual issues that require pointing, but the bodies of the brick have no burst or crumbled so, it seems like a perfect candidate for Larsen trusses. I have a few questions, though;

    Off the top of my head, the most reliable air & vapor barrier for this circumstance is closed cell spray foam. Would it be better to use a 10mil poly draped and taped with a liquid applied window and door flashing as a tie-in? Then, just increase the depth of insulation to accommodate the missing 2" of spray foam?

    Is there a common alternative to the classic Larsen Truss that utilizes a different type of member on the wall-side of the truss? Something like a metal framing stud so that the surrounding area can be better insulated rather than having the entire contact area behind the wall-side stud being non-insulated? Perhaps a couple tiers of 2'x as stand-offs and then attach the LTs to that?

    I'm planning to use 4'x8' wood grain finish sheets by Hardie as the base of a board and batten system, and then attaching their 1'x for the verticals. That's a lot of weight. Should I create a small, perimeter footer and build a wall atop that? I'm reluctant to attach so much weight to just a two course brick wall of this age. Without the reliability of new brick and solid mortar, I don't want to risk the additional weight pulling my walls outward over time, or the bricks to which the Larsen's are attached being "Jenga'd" by the heavy-faced wall.

    Thanks for any/all advice/direction y'all are able to provide.

    1. User avater GBA Editor
      Martin Holladay | | #35

      Mat,
      Your suggested retrofit approach is unusual, and I don't recommend it. Every Larsen truss job I've ever heard of has been used on a wood-frame building, not on a building with structural brick walls. Your suggested approach raises structural issues, so if I can't dissuade you, your first visit should be to an engineer.

      The usual method for installing exterior insulation on an old brick building is to use continuous rigid foam, followed by synthetic stucco (a method called EIFS) or the siding of your choice. Here are links to two case studies of this type of retrofit:

      "Exterior Insulation for an Ugly Brick Building”

      “Deep-Dish Retrofits"

  35. Mat Marcum | | #36

    Hey, Martin:

    Thanks so much for your prompt reply! I have not seen any other bricks doing Larsen trusses, but I greatly dislike the appearance of stucco and my neighborhood association would never allow that to move forward as its not in the covenant to permit stucco. So, I'm stuck with brick, grey-tone stone or a fairly large selection of vertical siding. It's one of those "you can have any color you'd like as long as it's black" deals. Per your suggestion and an apparent lust for pain, I made some calls to structural engineers and I'm looking forward to meeting with one of them in the next few days. I'll try to share any interesting news on here in case anyone else would like to flog themselves. Thanks again!

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