I have been looking to build a 30’x30’ detached garage on my property here in Tucson, Arizona and have had my heart set on using SIPs for everything until I started pulling together estimates for the work I need done. To start off with, the project will be built into a hill that basically changes elevation from 0’-0” at street level to ~8’-0” at where the back wall of the garage would be. I am designing the building around the desire to be able to add a second story later and utilities down the road. No cutouts for windows or doors are needed except for the very front. Both the excavator and concrete contractor don’t see any real problems with being able to make the cut into the hill and laying the foundation. Designing future utilities into the project is no problem either because I work in the MEP field.

 My problem lies in that the concrete guy is wanting to build a cantilever retaining wall to retain the dirt then backfill which requires an insane amount of concrete in the footings. Having a 30' SIP span anchored into an 18" or 24" footing seems more reasonable. I rather not use CMU walls because they have such a large thermal mass which would be a pain to overcome since the garage will not be heated/cooled except when I am using it for any length of time. As far as I understand it, I could use MgO SIPs on the sides of the garage touching earth, but the literature detailing the tension strength of the panels in this situation is scant at best. Thermapan is about the best information I can find, but Canada is a bit far from me. For southern Arizona, where we rarely hit below freezing for more than a few days and moisture is not very prevalent, what would be the best bet?

If you tried to use SIPs as a retaining wall you would need steel I-beam pilasters @ about 4ft on center bolted into the slab/footing to stop the wall caving in. CMU retaining walls rely on all the steel rebar embedded in them that's tied into the footing. SIPs have no qualities without significant structural reinforcement that make them useful as retaining walls.

I hope your 'concrete guy' is a structural engineer, because otherwise your wall and foundation may end up underdesigned and fail or be overdesigned and cost too much.

This sounds like a situation better suited for an ICF wall, at least the back and sides subject to earthen backfill. The front could be stick framed since it will most likely be mostly garage doors. The ICF walls would then easily support a second story of SIPs if so inclined in the future.