I have a design in mind for a 1.5 story house with a 40' x 28' footprint, 9/12 gabled roof, and a large (16' across) shed dormer on each side extending from the ridge to 4' back from the exterior wall (So the dormers form a 16' x 20' rectangle together).  I was originally thinking 2x6 with Icynene, but I think ICF is the way to go for the walls, and possibly SIPS for the roof.

I would like to know what the options are for the dormers.  I don't think the roof would be too bad as it's just a gabled roof with shed roofs over the dormers.  But as for the walls of the dormers: one way would be to just build full height walls between the two.  That seems like kind of a waste, though, using SIPS for interior walls.  Would it be possible to only use SIPS for the exterior part and frame in partial walls under the roofline?  The "fronts" of the dormers would be mostly windows.

Even if it's structurally possible, would this cost way too much?  Part of the reason for doing a 1.5 story is to use spaces efficiently, but if it costs a lot more to do the upper story I'd have to think about going straight 1 or 2 story.  I don't know much about this stuff, so any insight is very much appreciated.

Thanks,
    - Ian.

Not sure if I picture the whole thing, but you can run SIPs on top of interior walls. You should also review some of the other posts and consider SIPs for your exterior walls. I am finishing a project with SIP over studs. The issue is the thicness of the dormers. It will take careful planning of your interior wall position to make sure your walls do not end up with an extra OSB skin of thickness. An easy and inexpensive solution is to run extra sheetrock on the interior walls that need to be thicker. Also check your engineering. You might need a continuous wall without a pivot point. Or start with a 2 x 6 interior and then set a 4" SIP for the dormer.