OT-- Concrete homes

I live in (and build) a concrete home(s), and not just the foundation, everything. You're asking about insulating concrete forms, ICFs. I recommend them for DIYers. A number of conventional contractors are sold on them too, for the labor saving.

Not something I use as they put insulation on the inside of the mass, as well as the outside where you want it. This decreases the thermal advantage of the mass. Some manufacturers have lied about R value, preferring to talk about "effective R value". Also, you can't use a normal concrete vibrator. But for anybody who won't/can't use normal concrete formwork, it's far better than any other kind of wall.

Only big drawback to concrete walls is remodelling. Plan it right ahead of time. The problem that does occur for novices with ICFs is lack of bracing at the time of the pour. The manufacturer will make recommendations, follow them.

I use mass, much more than you're considering, for almost all of our heating/cooling needs. The system I adapted is called Passive Annual Heat Storage (book came out in 1983). Mass, properly used, will help immensely in making a house comfortable. Ever talk to someone living in a log house? They generally tell me how well insulated the house is. Well, that's almost always wrong. Comfortable, yes, well insulated, almost never. It's mass working there. Slow to heat up, slow to cool down. Ours takes half a year to heat up, half a year to cool down. Annual storage, which requires a LOT more mass than you asked about.

If you put insulation between the mass and your interior, you lose a lot of the benefit of the mass. That's what Oak Ridge determined too. But as I said, I still recommend ICFs over most other choices.

R value is the total of the parts. 2" foam, 6" concrete, plus 2" foam. Total will depend a great deal on the foam, XPS or EPS. R26 is not an ususual claim. Anybody who claims R50 lies, or has foam I haven't seen. With the mass, the wall can have lower R and still be as comfortable as a low-mass, but better insulated, wall. Then there's infiltration to consider. Concrete's pretty immune.

We live in a 4166 degree-day climate. Madison is 7863, Milwaukee 7635. I use 2" of XPS on our exteriors. I'd use more there. Only by doing a total heat loss calculation can you really know what the benefit of additional insulation might be.

Plumb and straight are what the bracing does. Absolutely necessary. Not unusual for a novice user to assume you can drop the mud straight down and that there's no need for bracing. That's the guy who's frantically looking for shoring lumber from anybody in the neighborhood so he can finish his pour.

This is a LOT more than anybody else where probably wanted, so if you really want to hear more, I suggest it go off-list.