Could anyone help me determine what the relative cost savings might be to build an @5500 sq ft (including basement) ICF home with only basement and main as ICF, and use SIPS for the roof, using dormers for attic bedrooms? Versus, what the energy efficiency loss might be for not just having the second floor be ICF with an insulated attic? I realize the second option will of necessity be more expensive to construct, but was curious if the living space in the attic formed by a SIPS roof would be very comfortable to live in (compared to a stick frame)? Will it "ruin" the ICF house to build it one story as I describe if we can afford two levels? I have not lived in a house with finished attic space FYI.

The attic bedroom concept should be substantially less expensive than a full two story plan. Whether it will work for your aesthetic needs, I can't say. A lot will depend on your idea of comfort, and how you intend to use the space. SIPS vs stick frame is a non issue. SIPS win. I suggest you tour some homes that have the attic concept to get a feel for the space before you make any decisions.

As far as the "look" of liveable attic space, I love it in pictures, but didn't know how hot this space would get in the summer in Missouri and whether the SIPS roof would help to alleviate what I would guess might otherwise be a lot warmer than a 2nd story with attic above. Or maybe you already understood that in my question, sorry if you did.

Attic living space with SIPS roof panels should not be any harder to cool than a second story with conventional attic space above. Many times the temps in the unconditioned attic are higher than the outside temps, so the temp. diferential is actually greater than it would be with the SIPS.