The continuous operating temperature service rating for Dow Styrofoam is 75 degrees C. (167 degrees F.); for BASF Walltite, a popular brand of closed cell spray polyurethane foam (ccspf), the rating is 80 degrees C. (176 degrees F.).

Distribution ducts would not normally reach those temps. But the main supply running from the furnace will. To be safe, I would wrap the distribution ducts first with foil-based insulation designed for that purpose. Even for the distribution ducts far from the furnace running in a garage ceiling, we wrap them and then spray ccspf.

Thank you.