Finally, an area where I have some experience! Not that I consider myself an expert, but through my work (remodeling in the Boston area) I have access to many bona fide experts in the field of building performance, or building science, or whatever you'd like to call it. And I've heard this topic debated from many angles.When it comes to a wall, how might one reduce or eliminate thermal bridging? I mention this at the outset as a reduction of thermal bridging is one of the cornerstones of Passiv Haus.
In a long winded answer to Chris' question above... The theoretical goal is to have a continuous layer of insulation that surrounds the conditioned space on all six sides. Since most common (read: affordable, mainstream) insulation materials have little structural integrity, the challenge is to integrate the structure and the insulation allowing each to do its job without getting in the way of the other, and without exposing the materials to conditions that make them fail. You can put all the insulation on the inside, but then the structure is left cold, which can lead to durability problems, you sacrifice interior space, and interior partitions are difficult to support (especially floors). You can put all of the insulation on the outside, and the challenge becomes supporting and protecting the insulation. In practice we usually end up using a combination of approaches and living with some thermal bridging which we try to minimize. Tough spots are penetrations like windows and doors, and to a lesser degree the intersections between foundations and walls, and walls and roof.
To minimize thermal bridging in conventional wood framed walls (rather than mass walls like brick, stone, etc.) the approaches I'm familiar with include double stud walls- basically two walls built next to each other but not touching, with insulation in between; truss walls (larsen truss, TJI, open web)- which are like double stud walls but the two walls are tied together with minimal amounts of material for strength and low conductivity; and staggered stud walls, where wide top and bottom plates (like 2X6 or bigger) are used with narrower studs (say, 2X4s) which are offset so that the studs carrying the exterior sheathing and cladding are not in direct contact with studs carrying the interior finishes. There are various ways to put the insulation outboard. The approach we end up using the most in our residential work (though I'm not saying it's the best!!!) is insulating the cavity then attaching polyisocyanurate foam board outboard of the structural sheathing, secured with wood strapping and long screws.
Sadly I have almost no experience with "alternative" insulation techniques. Antoine- I'm eager to hear how your hempcrete project goes! Please take lots of photos!