I read related discussions, such as: greenbuildingadvisor.com/question/spray-foam-against-underside-roof-deck-or-install-vent-baffles and greenbuildingadvisor.com/question/spray-foam-directly-on-underside-of-roof, but they don’t quite answer my question.

Ignoring issues of shingle deterioration, thus considering only condensation / mold issues…

A rep from the largest and most reputable spray foam insulator in my region guided my renovator in insulating his parents’ attic.  In their case, spraying directly on the underside of the attic was appropriate. The same rep was horrified to see that the renovator had sprayed directly on the underside of the roof deck in my cathedral ceiling.

What’s the difference? 1. Their attic was not a cathedral ceiling. 2. I think their attic was not being converted into a living space – no heat source.  I don’t know if their attic was a storage space or just an open attic.

When is it okay to spray foam on the underside of the roof deck?  

(I can’t ask the rep to clarify because he is away from work for a few months.)

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Optional details
(if the question is too general without them):

— The spray foam depth is generally 2″ to 3″.

— The product sprayed was a 2-component kit from Home Depot: Touch’n’Foam, which is almost-2-pound (per cubic foot), almost-closed-cell foam, approved for use in cavities in the USA but not in Canada.  (In Canada it’s only approved for use as beads around windows.  It’s been that way for years.  This factor is not a great concern to me but Dow’s rep on the HomeDepot.ca site wrote in at least a dozen comments that it is not approved for cavities – perhaps that’s just for bureaucracy’s sake.)

— room is a converted attic, 20′ x 10′.  There is no polyethylene vapor barrier on the ceiling.  (Some sources say this foam at sufficient thickness (perhaps 2″) qualifies as a vapor barrier, others disagree.)

— I’m in climate zone 5b, but I’m interested in general answers as well as answers specific to my case.