To make sure you're getting a good spray-foam installation, choose a certified contractor and run regular material tests.
This correctly applied closed-cell foam has been sprayed directly against the underside of the roof deck. To eliminate air leaks at the eaves, the foam has been extended beyond the top plate to form a seal against the attic floor. One minor omission: An additional shot of foam to each of the cracks between rafters and collar ties (arrow) would have prevented any possible air movement through that interface.
Defective B-rich foam, like that shown here, is characteristically soft and sticky and has a distinctive chemical odor. The material has not bonded to the sheathing beneath and can easily be pulled from the framing cavities by hand.
Closed-cell foam applied in lifts that are too thick is subject to overheating, which can lead to subsequent shrinkage and cracking — a problem that may not become evident until months after the applicator has packed up and left. The lighter-colored areas of foam mark the location of cracks that had been filled in an earlier attempt at repairing the still-ongoing shrinkage.
Cracks in the foam sprayed against the roof sheathing precisely matched melted areas of frost on the roof exterior in cold weather. Because such cracks may extend all the way to the sheathing, humid indoor air that moves through them may condense on the cold underside of the sheathing, promoting mold growth or rot.