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.

I sketched up a similar system for a friends TF here in VT. Double wall, 2' OC. staggered, with the window / door framing passing through the whole depth. 2x4 with 2x10 top and bottom plates.

He ended up getting a great deal on SIPs, so he wrapped the house with foam and OSB and has been happy ever since.


Insulation - I'd consult with an expert and try to use wet cellulose. I like the idea leaving the inside open... makes lots of things easier. Just space the framing off the frame with a suitable dimension - for drywall 5/8" works, for plaster / etc. 3/4" or more, depending on the substrate.

That extra layer of OSB or plywood will likely make this pretty expensive. Have to think on that bit. You are essentially building 2 walls now.

I never understood hanging the drywall on the frame. It gets wet, damaged, etc.

ya I much better like the idea of offsetting the frame and 'inserting' the drywall. I just suspect there are some people who might want to do it their way, but oh well, they can use a different system!

the extra layer of plywood is my biggest concern, I know it is going to up things quite a bit. the biggest reason it is there however is for shear strength.

Being isolated by insulation on both sides, it wouldn't need to be very high quality stuff, or overly thick. You could get away with 3/8" cheap plywood without any trouble, or maybe even rigid foam board would work (as well as add r-value). It's never going to get rained on, so warping isn't at all a concern, not to mention that it is nailed to a stud every foot.

I think your center layer of sheathing needs to be the strong stuff. The further away from the frame, the harder it is to get sheathing to work in shear. You have a long lever arm when the sheathing is 10 inches or more away, and we are complicating things by having offset studs. You'd likely need solid studs or blocking behind the posts and plates at some regular intervals.

Depending on your siding, you could get away with rigid foam and strapping as the outelayer, or maybe 3/8 or 1/4 inch plus strapping. It only needs to hold in the insulation.

perhaps you are right, it might be looking at the problem backwards. that would certainly make sense

either way, with the intermediate sheathing you would not have the weathering concerns that you have with exterior sheathing.

there are stick frame houses built with nothing more than rigid foam or cellotex as the sheathing, no plywood. While I would be scared to death to do that for obvious reasons, I wouldn't have any trouble with putting a foam layer as the outer sheathing if you already have a good layer of structural sheathing somewhere else.

The biggest thing to remember here is the cost of the sheathing itself isn't the only consideration. You've also got to keep in mind the cost of installation. This would depend entirely on your local conditions, is it cheaper for you to use a system with a little more labor and save on materials? Or is it better to save on labor and get better (and therefore fewer) materials?

Thanks for the help bmike, it's exactly what I was hoping for. It's best to have someone else's perspective to help you work out all the bugs..

The studs were staggered (outside wall flush to outside corner, inside wall flush to inside corner), but it could be done either way because they're not touching. I set the double mudsills down, each wall section, and then a 2x10 cap plate. I avoided a single mudsill because of the crappyness of PT SYP.

If you want to do your light double wall system, then I'd turn the inside studs horizontal (especially on one of your germanic frames with loads of intermediate nailing surfaces). Also, I'm no engineer, but I don't think that it really matters (in this case) where the sheathing is. The enclosure is it's own structural system. But if you want to stick something relatively inexpensive in the middle, then you could use Thermoply or Dow SIS.

the enclosure technically is it's own structural system, to a degree. But, as has been pointed out in the Larsen Truss thread, we also rely on our enclosures a lot of times to attain the shear bracing needed to appease local building codes, which is difficult to attain in a tradition timber frame which is designed to flex.

With such in mind, I agree with bmike that the intermediate layer, the one closest to the TF, should be able to provide this bracing, and the outer layer can be whatever is handy or suited to the job.

Out of curiousity, why do you propose the inner studs be horizontal?

Conceivably, could doing so provide adequate lateral bracing, and thus eliminate the need for structural sheathing to resist shear? If so, this is a great idea, as it would eliminate the need for any costly sheathing, and the intermediate layer could be replaced with foam board.

I tend to use a lot of long, complex sentences, in a way which is not proper for English grammar. o well.

with horizontal interior frames, it would be easy to tie them into the TF. What you could do is have the horizontal frames connected to a vertical stud at each post, which is then screwed to the post or otherwise fastened (with the proper offset for the interior finish)

This way, your horizontal studs would need to be as long as the space between TF posts.

It would be advisable to have blocking in the middle to keep the horizontal 2x4's from sagging.

something like this what you had in mind?


I tried to show the vertical studs behind the posts.

quick explanation,
cutaway to show that there is intermediate sheathing of some sort, mostly to separate the insulation cavities from each other

outer cavity blown in insulation, or perhaps it could be an interface for a clay-based outfill. inner cavity fiberglass

one disadvantage to horizontal nailers instead of studs is that you will have to frame in supports for electric boxes and such likes

however, with the long running cavities chasing wires would be extremely fast and easy, no need to drill through a stud every 16 inches!

Overall, these two factors probably about even out.

another thing is that special care would have to be taken to position a nailer in an appropriate spot where cabinets are to be hung.

the framing would be tricky around windows, unless you would be relying solely on the exterior part of the frame to support them, in which case it would actually be quite easy.

you could switch that up depending on what your siding is... horizontal outside for board / batten...

true, but wouldn't we then be getting back into the need for structural sheathing?

sandwich the sheathing. it is then closer to the frame, which it needs to be.

i need to email andrea about her wall design article and get the skinny on the shear capacities of the various systems.

what I was meaning, mike, is that with horizontal nailers on the inside crossing between the posts, you achieve an admirable amount of shear strength by linking the posts together, which is reinforced by having the nailers attach to studs at the posts, and having them tied together in the middle.

If you make the outside horizontal instead, then you don't get any shear benefit worth noting from the framework, and instead you will have to incorporate a sandwiched structural sheathing. With horizontal inside frames, the only real purpose the intermediate sheathing would serve is to separate the 2 cavities for insulation purposes, provided the horizontal framework is somehow fastened to the vertical studs wherever they cross for rigidity (toenailing would work just fine).

At least this is how it seems to me, I may be totally wrong about the shear benefits given by horizontal framework.

It should also be noted that this system is not designed for application to a German style frame, but is designed for a standard American frame. If I were applying it to a German frame, the same concerns regarding shear strength would not apply.