ULTRA FIRE RESISTANT – ThermoMFG FIRE-RESISTANT THERMO-PAN™ is a high-quality “No Noise” sheet metal alternative for constructing return air ducts. No duct noise. Satisfied homeowners. Excellent for open web trusses. Lightweight and FIRE-RESISTANT THERMO-PAN™ is a high-quality “No Noise” sheet metal alternative for constructing return air ducts. No duct noise. Satisfied homeowners. Excellent for open web trusses. Lightweight and easy to install. Will not sustain combustion. Airspace created provides R-Value = 5.