there are restrictions in "sub-floors" which I think you are refering to. We did a Job and we had to actually "modify"
the luminaire to have the required dimension between luminaire and adjacent structure. Its not much, I think 3"? The manufacturer of the luminaire has specs for how close the luminaire can be to insulation, subfloors etc. those incandescents get very hot.
you'll get a code reference here pretty quick I imagine