To get to code minimum compliance on a U-factor basis (=U0.026 max, = R38.5 "whole assembly") takes only about R35 if all the insulation is continuous, not thermally bridged by framing. With R15 fiber between trusses 24" o.c. it takes only about 4"- 4.5" of continuous roofing polyiso (R23 - R26) above the roof deck. A stackup of 4.5" of foam with a 5/8" nailer deck through screwed to the structural roof deck with pancake head timber screws can be finished off at the roof edges cleanly with a 1 x 6 facia board. This would be essentially Figure 4.
The material cost of R25 roofing polyiso is on the order of $2.50-$3.00 per square foot if going with virgin stock foam, but is less than a buck a square foot (cheaper than batt insulation) if using reclaimed foam. The nationwide dealer Insulation Depot has a facility in the Baltimore area selling used and factory-seconds rigid foam. They often advertise
here, as do a few others (
this is one of their adverts), and almost always has used polyiso in stock in various thicknesses. Similarly,
Repurposed Materials Inc has a facility south of Philly that regularly gets in used fiber faced roofing polyiso for cheap (but none listed at the PA facility currently at this hour.) Doing two (or even three) layers of thinner foam is generally better than one thick layer, since the foam expands & contracts with temperature & moisture cycles, and staggering the seams puts thermal breaks over the potential gaps.
A nailer deck of 5/8" OSB runs about 10 cents per square foot, 5-5.5" timber screws at 16 per 4x8 sheet adds another 50-60 cents per square foot, R15 fiberglass less than a buck a square foot, so using reclaimed foam it should come in around $2.25-$2.50 per square foot before labor. On simple roof lines without a lot of hips & valleys the labor isn't much. If there are bunch of gabled dormers or something it would require adjustments to the dormer siding and kick flashing and compound angle cuts to the foam, which starts to add up.
By comparison, R15 closed cell spray polyurethane would run about $2.50-$3.00 per square foot in my area, and 9"/R34 of open cell would run another $3.00 per square foot or more, something like $5.50-6.00, installed. For really complicated roof lines this is often the cheaper way to go, despite the higher environmental expense of the virgin-stock polymer and the HFC blowing agents for the closed call foam. There are more environmentally benign HFO blown closed cell foams out there, but 15-20% more expensive per R than the HFC245fa blown stuff in my area.