HVAC ducts likes this one in the attic of this Harpers Ferry WV are designed to deliver conditioned air (heated or cooled) from the HVAC system into the rooms of the home below. In the summer attic temperatures and reach up to 120 degrees or higher and in the winter attic temperatures can fall well below freezing. That means the conditioned air could be travelling through a duct that is either baking in the heat or has been sitting in freezing cold temperatures as if it were outside. These harsh conditions can work against the efforts of the HVAC system and result in the conditioned air being less cool or hot than it needs to be to make the rooms of the home comfortable. It also causes the HVAC system to run more than it should, running up energy bills. Encapsulating the ducts in spray foam helps insulate them from being impacted by external temperatures.