In another thread about REMOTE wall systems, a Larsen-Truss system was mentioned. I like the idea of cellulose insulation, and, equally important for me is the ability to construct it on-site with local rough-sawn lumber. While looking for examples of Larsen-Truss walls on timber frames, I came across this link to a home - named Hjälmaren by the owner - in mid-coast Maine. The frame was built by Connolly & Co. (is John a participant in this forum?), with builder Bruce Cummings doing the truss work. As the owner describes, he had to educate the GC and the builder as neither had heard of a Larsen Truss prior to this project. To integrate the Larsen-Truss walls with the timber frame, a 2x deck was built atop the foundation, extending a foot outside the foundation walls. The timbers were inserted between the joists and rest atop the foundation walls, leaving a one-foot perimeter support on which to build the walls.

The example is one option for integration of a Larsen-Truss wall system with a timber frame, but I'd like to find more. I'd be curious if any of you on this forum have feedback (on this house or the Larsen-Truss walls in general), or any other examples to introduce into the discussion.

Best,
~ Kevin

I think Will Beemer used a Larsen system on his home (or was it a Heartwood project?)


Does the Larsen truss offer any shear transfer from the frame? Often in the work I do we rely on the SIPs or the wall system to take wind load / shear.

A friend is entertaining a double offset 2x framing. 2x10 plate and sill and 2x4 studs will be offset except @ windows / doors, which will be framed with 2x10. It will be filled with wet cellulose and possibly have Hi-R 1 1/2" foam on the exterior, over sheathing. Rain screen / strapping over the foam for siding. This doesn't solve the shear issue unless we get creative - but for his needs it will work.

As far as I can tell, the Larsen-Truss is simply a form of double-wall framing that uses plywood gussets to space the walls - 2x4 for the load bearing walls and 2x3 for the outer frame. The wall system used in the Maine house example (initial post in the thread) does not carry any loads and is all 2x3 - merely vertical members to hold the insulation, sheathing, windows, doors, and siding. Would the plywood sheathing (and, to some extent, the drywall) offer any shear resistance?

The issue with the plywood, assuming it is on the outermost layer of framing, is that it is now 12" away from the load bearing structure (the frame), so you have a lever arm without a solid connection between the two frames.

It may not matter in some cases - but whenever I've been involved with alternative systems like this (just a few times) the engineers always want to either see the frame designed for lateral / shear loading - or the wall system set up to take those loads.

Yes, the drywall is technically doing something, but I've never seen it included in any sort of calculation.

I doubt that shear loading will be an issue for the house I'm planning - a small, 20 x 20 x 2-story home. I'm inclined to think that a wall system wrapped in sheathing will be adequate resistance to shear.

What is the most common way to support SIP's on a timber frame? I've seen examples where the posts are supported inside the foundation walls (going all the way to footings in the basement in some cases) with the SIP's resting on sills atop the foundation walls. At first glance that seems a bit unnecessary. Just curious at this point as to what the "best practices" are.

I've often had the deck run over the foundation, then set the SIPs on the deck. With a 6.5" SIP wall and a 10" foundation you have half your post over masonry, and you solid block in the deck as needed. If you have a need you can pilaster up in the basement or build a bump out in the foundation. Depends on loading, etc.... as my usual response.

I'm curious as to what the common/best practice is for thick walls. A Larsen-truss type wall is 12" thick, for a total thickness of 20" once an 8" post is included. I've seen one where a 2' wide perimeter of plywood was laid atop 2x sills. The deck was built to the width of the foundation walls with 1' of the plywood extending out from beneath the deck to support the walls. Granted, the trusses are not carrying any load, but it just seemed a bit flimsy.

What is the method used for a straw bale / timber frame house?

I am going to use a somewhat extreme example to illustrate something here.

I have seen plenty of examples of very old timber frames where the walls have absolutely no direct support whatsoever from the foundation. These are cases where the first floor of the building is built of stonework -so it serves as the foundation- and the second floor is timber framed, but jettied on all four sides well past the stone wall.

Now here is my point. I believe it is possible to have an outboard enclosure system that has no direct support from the foundation at all. The frame would sit on the foundation and the the shell would be 'jettied' out beyond. This could save tremendously on the cost of building an oversized foundation, and also there are 2 huge advantages that this makes possible.
The exterior shell can go down along the foundation past the sills, this can in turn provide excellent insulation in a very hard to insulate spot. It also totally isolates the sill from water ingress. The only 'sill' involved in such an outboard system would be a piece of wood enclosing the bottom. This would obviously have to be used with a wall system especially adapted to this.

I have seen SIPs hanging off the frame and running past the sill. This does insulate the floor framing.

This does not work if you need the shear planes of the panels or walls to be able to push against the foundation for wind loading...

That certainly makes the most sense, as with SIP's. I'm trying to sort out the best way to do it with a wall system that's composed of 12" thick trusses sandwiched between T&G interior wall and exterior sheathing/cedar shake siding.