The Quonset's curved steel arch is what makes it cheap to build and hard to insulate - there are no flat cavities for batt and no easy way to fasten rigid board to a continuous curve. Closed-cell spray foam is the natural solution because it's the only insulation that conforms to the arch in one seamless layer.
Why foam and not the alternatives
- It fits the curve: sprayed directly to the inside of the arch, following every contour - no framing, no gaps.
- It stops the sweat: a bare Quonset condenses heavily; closed-cell's vapour control ends the dripping that rusts the shell.
- It stiffens the shell: the rigid layer adds structure to thin steel.
Planning the job
Decide the use first - a heated shop wants more R than cold storage that only needs condensation control - then spec thickness to match. It's a pro job on a big curved surface.