While studying up on the idea of a light framed enclosure, and while pondering the idea of a larsen truss enclosure I had a thought come to me, and I present it here for evaluation.

The principle behind the Larsen Truss is to prevent thermal bridging by not having any wood connect both sides of the cavity.

With this in mind, an idea came to me.

What if one was to build a double frame enclosure, where there are 2 light frames with studs 2' OC, staggered, so that the studs of the inside frame set between the studs of the outside frame.

The size of the studs could vary, it could be a double 2x4 wall or perhaps a double 2x2 or 2x3 wall, and of course a double 2x6 for super insulation. It would seem likely that the easiest method to execute this would be to have each wall section with its own plate, and the two units connected either by a double plate at the top, or strapping connecting the two plates.

One possibility is also that the two frames could be separated as well as tied to each other with a layer of perhaps 1/2" foam sheathing, or OSB could be used here if there are structural concerns that need dealt with such as shear stress.

2 2x2 frames connected and separated with an intermediate layer of OSB would be plenty sturdy to support insulation and wall enclosures, and by increasing the size of the studs you increase the r-value of the assembly. Very easily adapted to budget in this way.

Check out Andrea's article in the latest edition of Timberframing...

I did, that's part of what got me thinking.

I think that a staggered double light envelope could be a great solution, it has the lack of thermal bridging inherent in the Larsen Truss, but with intermediate sheathing it would still be able to provide lateral bracing against shear stress.

the biggest matter is to determine how much this would cost...

I just finished a job where we did a double stud 2x4 wall, 24" OC, with separate bottom plates, and a continuous top (top) plate. The walls were laid out, framed, and raised separately, then tied with a 2x10 top plate. A timber plate was set over that with timber trusses. Total wall framing thickness was 9.25, with dense-pack cellulose. The windows and doors were flush to the outside, then widened by 3" to the inside framing. That is, the windows and doors were set in the outside 2x4 framing, they were trimmed on the outer 2x4 as well, and then the inner wall stepped 3" wider to allow for the inside trim and the swing of the doors (although we changed to outswing doors anyways). The drywall returns are bullnosed. We trimmed after drywall (all cherry and beech from the property), meaning window stools and casing had to be scribed to the GWB. I've heard of people trimming before drywall, but it would be a challenge to protect all of the wood.

were the stud walls staggered? In case I am not being clear here, what I mean by that is that if you were to view an elevation of the wall, there would be a stud every 1 foot, but each individual frame has a stud every 2 feet, so there is no thermal bridging across the wall.

The reason I thought to have a double top plate tied together with plywood strips is for ease and speed of assembly and raising. Since the stud frames would serve no structural purpose, this would be perfectly acceptable. If the walls were structural, then I would want a common top plate as well.

to me, the biggest improvement is the intermediate sheathing layer, which makes the whole thing a very rigid framework, and totally solves the problem of shear strength demanded by modern codes.

DL, where is this intermediate sheathing layer? Can you post a sketch?

certainly, here is a very rough sketch i whipped up:


direct link: http://i538.photobucket.com/albums/ff345/HiddenOrder/scan0001-1.jpg

this of course only shows the basic wall layout, no detail door/windows/corners/etc.

The trick with that will be to get the inner layer of sheathing to tie through a stud or blocking to timber...

Are you drilling large holes and filling with foam or wet cellulose? The inner cavity is easy, the outer one - not so much.

There are a variety of ways one could fill the cavity I suppose

around here, fiberglass is still the dominant method of insulation, and after that I would suppose dry blown cellulose

I am weary of foam, because it does not breath. In Central Indiana that is a huge huge problem, moisture will will will cling to it, it happens every time. But that's not our discussion so lets not go there!

For insulating the system, I would prefer to consult with an insulation expert that I know rather than try to figure it all out myself.

there is also the possibility that the intermediate sheathing would stop short maybe a foot or 8" or some such from the top plate, allowing the outer cavity to be filled with blown in insulation from the inside.

The method I am considering is a hybrid insulation, with cellulose blown into the outside cavity and fiberglass used in the inside cavity. large factors in this are ease of installation, and also ease of wiring, etc. in the inner cavity.

I know fiberglass doesn't have the highest r-value in the world, but I think in a hybrid system like this it could help lower the costs of the system quite a bit.

Now of course the whole story changes if you use the method that involves attaching the drywall to the timber frame before attaching the light frame to the outside. i'd have to think that through a bit...

One thing, perhaps it should be noted that anything I post on these forums is free to be used, experimented with, modified, or to use as inspiration for your own ideas, unless I should specifically request otherwise. So if anyone would like to use any of these ideas in any way feel free to do so, no need to ask. I would like to know if and how my ideas are used though.