It'll work well as long as you spray ALL the interior steel that has a conductive path to the exterior - roof, purlins, beams, verticals - everything. Steel is such a good conductor that any left exposed will condense ("sweat") during cold weather.

A friend did exactly what you are describing with great results. ICF walls, 8/12 pitch steel roof on purlins with 3" of urethane over all the steel. In fact, all the framing is steel, no stinkin termite bait anywhere. The upstairs rooms are framed with steel studs and have batt fiberglass primarily for sound attenuation. The whole house - all 2800 square feet of it - is kept comfortable with a single 2 1/2 ton ac system, in Texas.

Sfair,

One thing to check on is the adhesion factor on the foam you choose to use and make sure that the steel was not treated with a thin coat of oil to reduce corrosion. If it was then you will need to degrease it.

ICF Contractor