This is branching off from the discussion that DL Bahler started, but I didn't want to get his thread too cluttered up with various side discussions.
The question is, what are the primary criteria that are important to all of you when selecting or considering an enclosure style?
As you may already have picked up I want to move away from SIP type enclosure and while I have some ideas they are a bit nebulous. Please let me know what your thoughts are on what I outline. I like ruthless criticism!
1. Ability to "breathe" through passive rather than active systems.
In a nut shell this would mean being able to dispense with centralized HRV systems and the like. I feel that an enclosure should be able to function within it's environment, not operate outside it. That being said, I think that properly placed and sized bath fans and stove hoods are a must to deal with large moisture loads.
2. Differentiation between drafts and desirable air movement.
This is sort of a tricky one, because it goes against modern building ideology of "tighter is better", but it is a critical distinction to make for #1 to work.
I will define a draft as an uncontrolled bulk air infiltration causing discomfort and causing an enclosure to behave unreliably.
What I would be going for would be controlled and predictable through construction methods and design. It should facilitate drying of the enclosure assemblies.
3. Longevity of materials and design.
I would like to use enclosure methods which will be able to keep up with the timber frame in longevity. This would entail finding materials with resiliency, or the ability to resist degradation even under less than ideal circumstances.
4. Sustainability and economy. The ability to use local resources when possible and and also to use homeowner labor effectively to keep their costs down (when they want to).
I suppose I could come up with more, but that will do for now.
Now on to implementation.
I think that my top choice that I have heard of is a light clay and wood chip wall enclosure.
I think that it satisfies 1 and 2 provided that is finished in such a way to not impede breathing; lime plaster or wood for example.
Criteria 3 should be met if basic details such as overhangs and and drainage are addressed. I like idea that the clay slip coats and isolates organic material from each other, rendering it much more resistant to fire, insects, mold, etc. The finished surfaces outside and in will be maintainable or replaceable (whether it is plaster or wood) without compromising the integrity of the enclosure.
Criteria 4 is easy in my area regarding raw materials. I have lots of clay and plenty of wood waste product (some even made by me!). It is also a system which would seem to lend itself nicely to using unskilled volunteer labor with a couple experienced people supervising. My immediate area is what is considered "economically depressed". I prefer to work within my community, building for people I know and live around. However, too often I find that I have to drive an hour or more away to work for people that have the money. It's important for me to find ways to incorporate the labor of motivated customers so they can have quality, beautiful houses.
My second runner up is, as Jay suggested, a form of wall and roof truss, often called the Larsen truss. Having a second option is important, not only for the roof of a chip and slip house, but because their are definitely customers that will demand something more familiar to conventional construction. Also it would be important to have a more production oriented option for further away projects.
They will be easily understandable to other trades, being very similar in form to stick framing.
As far as insulation goes I would lean toward mineral wool. I like the fact that it has good performance around water, unlike cellulose or fiberglass, as well as positive qualities around fire and mold which satisfy #3.
Criteria 1 and 2 are a little fuzzy for me here. It seems that typically people install an interior vapor barrier. Vapor barriers give me the willies as they seem to tend to become vapor trappers. Given the rigid yet permeable structure and it's hydrophobic nature, would it allow one to sucessfully use it without a vapor barrier? The devil would be in the details there I suspect. I also know that there are "smart" vapor barriers on the market by the likes of Pro-Clima, but I haven't looked into it.
I have local sawmills so I could get a lot of the material from small family operations using local trees. And while it might not be as friendly as chip and slip toward unskilled labor, it would still be more accessible toward sweat equity homeowners who are familiar with conventional construction than something like SIPS.