This is a good strategy for an unvented Cape Cod attic such as yours. Your contractor is correct; it is not necessary to fill the cavity with insulation all the way.
Here's how it works:
- The exterior side of the insulation is essentially the same as the outdoor temperature (let's say 20 F on a bad day).
- Similarly, the interior side of the insulation is essentially the same temperature as the home (probably 70 F).
- Within the insulation the temperature changes linearly, so on that cold day the very middle of the insulation is about 45 F.
- It just so happens that 45 F is the point at which normal indoor air will condense. In order for there to be condensation, moisture would have to permeate halfway through the insulation, which simply cannot happen because spray foam at these depths is an air and moisture barrier.
Caveat: this logic only applies to impermeable insulation such as spray foam. Insulating the roof deck with a permeable product like fiberglass batts could very well lead to condensation on the roof sheathing surface.
Bottom line: you don't need to fill the cavity all the way, although you should put in at least R-38 for energy efficiency.
For more information:
Check our home foam insulation Q&A to see what other homeowners and contractors are saying about foam insulation, condensation, etc.