I think this is what you are looking for.
It's not a thermal barrier that's required for the kraft facings,
Thats for foam, section 314
SECTION R316
INSULATION
R316.1 Insulation. Insulation materials, including facings,
such as vapor retarders or vapor permeable membranes
installed within floor-ceiling assemblies, roof-ceiling assemblies,
wall assemblies, crawl spaces and attics shall have a
flame-spread index not to exceed 25 with an accompanying
smoke-developed index not to exceed 450 when tested in
accordance with ASTM E 84.
Exceptions:
1. When such materials are installed in concealed
spaces, the flame-spread and smoke-developed limitations
do not apply to the facings, provided that the
facing is installed in substantial contact with the unexposed
surface of the ceiling, floor or wall finish.
2. Cellulose loose-fill insulation, which is not spray
applied, complying with the requirements of Section
R316.3, shall only be required to meet the smoke developed
index of not more than 450.