Fiber reinforced, Airseal 33 offers exceptional adhesion and flexibility for sealing all types of HVAC duct systems including: metal, fiberglass ductboard and flexduct. - High Solids Formulation - Formulated for Indoor & Outdoor Applications - Seals High Pressure HVAC Ducts Systems - Exceeds SMACNA Pressure and Sealing Classes - Excellent Mold & Mildew Resistance