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?