Who says it is OK to foam a roof, for any odd contractor or home owner? We should fear roof rot within twenty years, unless foaming is with perfect completeness and perfect maintenance, forever, of a vapor barrier over the expanded living space. No one can offer that guarantee. A roof, foamed, rotted, is ruined.
Instead, Raise The Roof. Look for agreement with the statement "raising a roof does better weatherization than insulating the roof ." Practically, it is less a quest to save energy if a bungalow attic has been converted to living space, and is uninhabitable especially in Summer. Claim comfortable living space, and more of it.
Raise a roof, Then weatherize an ordinary attic . Be smart though, about second floor spaces outboard of new and existing attic walls. The excellent builder here sealed none of the large openings between not-conditioned floors, and the walls and ceilings below. Whimsical bungalows will remain challenging of weatherization. Simple cape construction has much hope of rewarding and fairly easy cures, in modified roofs.
As a weatherization contractor working alone, and not as manager of a crew, I have insulated a roof, only a few times. Foaming a roof is not an option, in my opinion. The task with batts is a serious one and will usually be only-wishful thinking of a home owner. As growth of weatherization activity it is not likely in one's budget.
I want to illustrate my general approach, and immediately think of this example of a nice home in the prosperous Irvington neighborhood of Portland, Oregon. The link is a Google Photos album.
This afterthought child's play room in a kneewall closet, was cozy only a few months each year. It was mostly uninsulated, drafty, intolerable in high Summer and deep Winter. Here I begin demolition of flimsy paneling. The work followed thorough insulation of the upper attic and all other kneewall closet floors, with imperfect treatment of all challenging heat bleeds of bungalow construction. Work done in 2008 is reported here in 2018, as example of best-effort coping with difficult-to-insulate bungalow construction, vs. better practice of raising the roof as an ordinary attic over a full second floor.
My method is durable, unlike found flimsy layup of paneling with nails. And yet, it is not honest treatment of all second floor heat bleeds.
Brittle stiff paper behind the paneling removed, a surprisingly full fill of cellulose on this gable wall only, tumbles down. See that insulation blown from outside with ending any hope the exterior sheathing is air tight, failed to treat very leaky non-insulation about the rough frame of a remodel vinyl window. Insulation was almost entirely bypassed by convection and large leakage-movement of conditioned air, between buckled stiff felt paper, and the flimsy wood-panel sheathing of the knee walls and ceiling.
See that I had previously insulated the ordinary floor of the upper attic, and had air sealed and stuffed floors of knee wall closets.
I am quite displeased with my leaving of perhaps-useless paper-faced mineral wool batts upon the second floor attic walls.
See my placement of R15 kraft-faced batts with full filling against roof sheathing. In 2x4 space thinner insulation as with joke "R13" - - must not be commanded to cool the roof. Allowed convection in foolish and ineffective deference to composition shingle warranty, ruins any insulation value. R15 was little enough, to be achieved. See that some odd roof pitches challenge the imagining of how to place batts. Did I do right?
Know now that the kraft facing is nearly useless as an air and vapor barrier. I want both.
Apply then, Tenoarm 6-mil virgin polyethylene vapor and air barrier. Lose my shirt with the immense difficulty of setting strong GP Densarmor drywall. Accept meager payment that was larger than the customer wished. Learn that the room was nice for a family with growing-up little girl, now grown. Family moved on, out of Oregon. A heat bleed remains in the triangular wall to left in the photo above, where a difficult, expensive trapezoidal door, was not afforded although framing of the door was provided, just needing a cut through the drywall.
Might the new home owner now wish to raise the roof, for more living space, in a home where rent-out of portions is in great demand? That would be beyond my skills and interest. Just sayin'.