When connecting fibrous glass duct systems to sheet metal components such as equipment flanges, fasteners such as sheet metal screws and washers are used to carry the mechanical load. Mastic and glass fiber fabric are used to seal the connections at these points. However, approved pressure-sensitive aluminum foil tape may be used to seal fibrous glass ducts to sheet metal when operating pressure is less than 1 in. w.g. (250 Pa) and when sheet metal surfaces are cleaned in accordance with tape manufacturers' instructions. Closure details may be found in Section IV of the NAIMA Fibrous Glass Duct Construction Standard.

The use of non-listed closure materials or techniques voids the UL 181 Class 1 Air Duct rating.