I am researching/planning a small, single-story house for maximum insulation without using toxic chemicals

I will not use standard chemical foam insulation, but I am interested in aircrete and soy foams.

Anyway, I was wondering if anyone has done research on exterior foam panels like vs spray in foam inside walls. Dana1 had a good post on both: http://www.greenbuildingtalk.com/Forums/tabid/53/aff/22/aft/79280/afv/topic/Default.aspx

The reason I think that 2" of external foam panels might be able to hold it's own against 2x4 walls filled with spray foam is because of the thurmal bridging of the wood posts. Each 16" or 24" cavity is filled with spray foam, then there is 1 1/2" of wood transferring cool.

My Idea would then be to use small wood or strips on the outside of the foam panels to attach the actual siding. So the total wall would look like this (outside -> inside):

[siding] + [furring strips] + [airgap] + [2" foam (1" x 2 overlapping seams)] + [osb/plywood sheathing] + [2 x4 walls] + [sheetrock]

vs

[siding] + [osb/plywood sheathing] + [2 x4 walls filled with foam] + [sheetrock]

Even with the exterior foam, it would be wise to insulate inside the stud cavities. One inert option for cavity insulation is stone wool batts such as those manufactured by Roxul. Google for more info.  And of course it is hard to beat damp sprayed cellulose in the stud cavities for the best bang for your buck.

I have some thoughts to contribute. Not as an expert, but just because I've considered doing the same thing. While I don't think the thermal bridging through wood studs is huge, it's definitely a penalty. Wood is a better insulator than concrete. The external rigid insulation is better for comfort and energy performance in every way, it just requires careful installation of flashing around doors and windows. You have to provide drainage for any water that condenses or gets behind siding. If you install a lot of rigid say 2" or more you might be looking at having to install strapping which adds time and expense to the installation. In my next new build, I will be installing rigid exterior with dense packed cellulose inside a double stud wall. I think it's the best wall money can buy for comfort and durability.

Even heavy fiber cement siding is OK at 4" of foam. The primary force transfer is friction between the siding, the foam and then the wall - it's not a lever acting on the fastener. Even more so if you use adhesive.

The external rigid insulation is better for comfort and energy performance in every way, it just requires careful installation of flashing around doors and windows.

I didn't think about the extra steps I might need for the flashing around door and windows.

You have to provide drainage for any water that condenses or gets behind siding.

That is why the siding is not nailed straight through the foam (touching it) but is instead nailed to the small wood peices leaving a 1/2 or more air gap between the siding and the foam to allow hot air in the summer and moisture in the winter to run down the foam to the dirt.

If you install a lot of rigid say 2" or more you might be looking at having to install strapping which adds time and expense to the installation.

What do you mean "strapping"?

Thanks for that link Jonr! It's a very helpful article.