I didn't say that cork would be R4 if it is 1/4", I was responding to your giving it an R value of 0.5, which means it would be more conductive than steel. You probably would have been pretty close though if you added the insulation value to the "1" baseline to come up with something slightly less than R=1.5. Easy math error, so no worries.
What I was more trying to get at is that we hit a practical limit quickly with the thin walls of our vans. The roof and wall beams are fairly thick steel and only a couple inches deep. Building studs are very thin steel with more depth. Fortunately it doesn't take much to create a thermal break by covering the steel with a thin layer and that is shown in the following two graphics. As you can see in the first one, the initial 1/8"-1/4" is really important.
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This next graphic is heat modeling various ways to design a wall for a building, like you mentioned, and one aspect of this is how it shows that heat transfer is not smooth through the construction. It sounds like your house will be a hybrid, similar to #5. Our vans are more like #4 when we add almost any thermal spacer (yellow) over the steel beams, so the values in the above chart become the yellow layer in the graphic below for cases #3, #4, & #6. Again, we are working with thinner wall depth and thicker steel material in the vans, so there is less we can do about the beams and filling the space between the beams becomes more important. btw - This graphic nicely shows the value of Orton's approach of bolting the interior metal to his plywood (violet).
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If I were going to live in Alaska or at a ski slope every weekend, I would be more willing to sacrifice usable width in the van, but how much becomes a personal choice and could become impractical quickly. If the wall panel is 1/2" tongue & groove wood and there is an inch of insulation over the steel beams, multiplied by the two walls, then there would be 3" lost from the width of the usable space. Using a half inch of insulation and quarter inch wall panels means losing 1.5".
For my use mostly as a pickup truck that I can comfortably sleep in into the low 40s/mid 30s, I'm in the middle of installing 1/4" insulation over the beams to get the thermal break and tougher 3/8" wall panels, so I'm giving up only 1.25" of the total interior width. Most of the insulation comes from insulating the big flat surfaces between the steel beams with the sound deadener, poly-iso, and reflectix (again similar to Orton) and the beams will have a thermal break like the yellow in #4 above.
Hope this helps.
Bob / CW
