Larsen Truss vs. Joe Lstiburek’s Ideal Double-Stud Wall Design
I’m designing a house for Rochester, NY, zone 5. It’s going to be a single floor with a finished basement. The roof will be constructed with raised heel trusses and insulated with cellulose. I don’t want air-tight drywall, so I’m planning on using a layer of plywood or osb to form the ceiling air barrier.
I’ve would like a cellulose-insulated double-wall. The two designs that seem to fit well with my roof are “Joe Lstiburek’s Ideal Double-Stud Wall Design” and the “Larsen Truss”. They both allow an interior structural wall with an air barrier that can seal with plywood on the bottom of the truss.
Is there any reason to pick one design over the other? They seem to use similar quantities of materials. The Larsen trusses seem to involve more labor. However, the plywood gussets look like a stronger structure than Lstiburek’s cantilevered plywood supports.
I look forward to your thoughts.
–John
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
The 'best' system is most often the one which a person has experience with. Here's mine.
By 1980 we had been designing energy/resource efficient hoimes for some time. We had tried many variations to make thick, tight walls and reduce thermal briding. Some were pretty strange. With the introduction of truss joists we had been imagining using them, but the cost kept us away. In 1980 we designed a home in Jasper in the front range of the Rockies west of Edmonton. Since we didn't know who would be the contractor at the time of designing it we fell back on what we considered the least imaginative but most easily grasped wall option - a double stud wall. Anyone could handle it. But it turned out that the contractor that took on the job was not anyone - he was John Larsen. John wanted to make his own truss joist to tack onto the outside of the bearing 2x4 wall. That was the first 'Larsen Truss' wall home. John went on to fabricate these trusses for anyone doing a new or retrofit project. They quickly became quite popular.
We started doing our own variation and used them on many homes. In 1982 I built my own home using them. Most commonly we would put up the sheathed 2x4 wall, wrap it in poly, tack on the 8" trusses, install house wrap, strap and install siding. This can all be done on the ground if there's crane opportunities available. I would then blow cellulose from the attic space down the void of the trusses.
After about a decade with this as our 'go to' wall option we simplified it by just using a 12" 'Larsen truss' with a 2x4 bearing chord and 2x2 opposing chord. The 2x4 could be inside or outside bearing. This system was quicker and simpler with less material and labour. A big benefit was that we could blow cellulose in the entire cavity with netting and see what we were doing. Before it was a bit of religious conviction leap of faith.
We glue and nail the gussets on the trusses (as opposed to dadoing them as John had done) and you can load most any siding on them. They're very strong. And they can be long - I've done 20' balloon framing with them - and they end up straighter than any lumber you will buy.
Usually - but not always - the exterior is sheathed. I continue to read with interest the discussion given to the conerns with cold sheathing. We build quite heavily insulated wall systems in chilly Alberta and so I should be sleeping poorly. But I'm not. I think of all the tract 2x6 housing around here with indequate interior air barriers and, because of our climate, extended periods of cold sheathing (even with that bridging). That's a combination that invites curious concern. But I haven't heard of nor observed problems. Of course, we have generally dry winters.
That's my reflections. I don't believe you need to get too complicated to still get a 'pretty ideal wall'.
Michael,
Thanks for adding to GBA's documentation of the history of the Larsen truss. It's a fascinating story.
John,
I assume that you have read GBA's article on Larsen trusses -- the article that includes an interview with John Larsen. Here is the link: All About Larsen Trusses. Whether you have seen the article before or not, you might want to check out the comments posted on that page.
You may also want to read this article: The Klingenberg Wall.
Larsen trusses have advantages and disadvantages, but they work just fine. One thing I have learned over the years: every builder has strong opinions on wall construction, based on their experience and prejudices. The best wall is the one that the builder has researched and is confident building. The decision is up to you.
Some time ago, I posted a question on one of those "cold sheathing" related threads, and I don't recall getting any satisfactory explanation. While a double wall typically has much more water-absorbing insulation in it vs a "code wall," so that the potential reservoir of moisture thus is greater, I don't see why the "cold sheathing" problem should be called that. If there is no insulation outside of the sheathing, just the siding, then the sheathing of both the double wall and the "code wall" will be close to outside air temperature at steady state. Given that the sheathing and siding do have perhaps R1 of insulation value, then the inside surface of the sheathing of the double wall will be less than 2 degrees (F) colder when it's zero outside, well within the day/night or even hourly temperature changes. Otherwise the only difference between the two walls will be the much lower heat loss of the double wall. So why the "cold wall" tag on the double wall? That sounds misleading.
Dick,
Your question has been much discussed. Other than climate factors, there are at least three important factors affecting the moisture content of wall sheathing in winter: (a) the location and size of air leaks; (b) the R-value of the insulation in the wall; and (c) the rate of vapor diffusion through the wall from the interior to the exterior.
For years, building scientist Bill Rose has been emphasizing the importance of factor (b) -- the factor that gave rise to the "cold sheathing" shorthand. As you noted, "The only difference between the two walls will be the much lower heat loss of the double wall." This lower rate of heat loss makes the sheathing colder; whether the temperature of the sheathing is cold enough to make a difference -- cold enough to raise the moisture of the sheathing compared to walls with less insulation -- is subject to some dispute.
Field studies have documented that the sheathing in a double-stud wall has more moisture in February than the sheathing in an "ordinary" wall. This phenomenon also shows up in hygothermal modeling programs like WUFI. So what's going on?
It turns out that vapor diffusion also plays a role (in addition to the fact that the sheathing is colder than usual). For more information on this issue, see The Return of the Vapor Diffusion Bogeyman.