Fiberglass is naturally fire resistant, but faced insulations will contribute to flame spread unless flame-resistant materials are used.
Batts and rolls are available with facings of asphalt-coated kraft paper, aluminum foil, or plastic film already attached. The facings extend over the sides of the insulation to provide strengthened flanges that can be stapled to wood framing to hold the insulation in place where recommended by the manufacturer. (Some faced products may be pressure-fit between framing without stapling.)
The kraft paper or standard foil vapor retarder facings on many blanket insulation products are combustible and should not be left exposed. Gypsum board is one such covering material. Special care must be taken to keep open flame and other sources of heat away from the facing. In addition to the home’s habitable rooms, garages, storage rooms, utility rooms, and laundries are areas where faced insulation also should be covered. If in doubt, always consult individual manufacturers’ installation instructions. It is also important to repair damaged vapor retarders by covering the damaged area with scrap vapor retarder material and taping it in place or, in the case of small rips, by using duct tape or polyvinyl tape.
The National Institute of Standards and Technology (NIST) recently published a study that demonstrated that the permeance of the asphalt-impregnated kraft facing used as a vapor retarder on batt insulation can rise dramatically under very humid conditions. These results suggest that in very moist areas such as shower rooms, kraft paper alone may not be sufficient to keep moisture out of the walls. A better strategy is to install a continuous polyethylene vapor retarder.10
Some blanket products are available without these facings. If unfaced fiberglass insulation is used, a separate vapor retarder should be applied after the insulation is installed.
Special low-flame-spread vapor-retarder facings are available that can be left exposed. (Flame-resistant facings are labeled FS25, or flame spread index 25.) Sometimes, the flame-resistant cover can be purchased separately from the insulation. Fiberglass blanket products also are available for basement walls that can be left exposed. These blankets have a flame-resistant facing and are labeled to show that they comply with ASTM C665, type II, class A.