As a heads up, your question posed will only draw opinions as "what's
required" in this regard is not crystal-clear.

That said, I would direct you to read the eQuest/DOE2 help entries for
constructions to understand and consider the difference between
"weightless" U-value only definitions and "weighted" layered
constructions. Read up in the mailing list archives for further
discussion/explanations.

Until you feel comfortable understanding what "weightless" constructions
entail, and where that will and won't cause a problem in the modeled
behavior, I would advise sticking with layered constructions and
defining material masses alongside their thermal transfer properties.

I personally work from envelope details for exterior constructions to
build from scratch, which the architect should have readily available.
You're right to feel concerned about "shortcuts"... Energy modelers who
rely entirely on others' calculations without performing their own
checks along the way can be sailing dangerous waters.

~Nick

NICK CATON, P.E.