Forced air heating systems use heat ducts to distribute warm air throughout the home. The furnace, depending on its type and on the temperature of the intake air, generally produces air as hot as 150 degrees. This air is under pressure as it moves through the system. If the heating ducts are not properly sealed, they can cause air leakage and, in severe cases, can leak an equivalent of four to six complete house air exchanges per hour. When heat ducts are located in a crawl space, any heat loss will remain in a semi-conditioned area and may not be viewed as a complete heat loss. However, the heat that is lost from air leakage seldom finds its way into the living space above, resulting in a waste of energy. This causes too much heat loss from ducts located within the crawl space and results in not enough warm heat rising from the duct registers into the living space above. The furnace needs to run all the time in order to make up for this air leakage. Air that is delivered into the house via leaky heat ducts is much cooler than that of a properly sealed and insulated duct system.
The initial temperature of the air within the heat ducts is approximately twice that of the building it is heating. Typical duct insulation is around R5, which is about one-fourth of the building's wall insulation R value and about one-eighth to one-tenth of the building's ceiling insulation R value.
Radiant floor heating systems also require a means of controlling heat flow. Radiant heat is designed to heat the room above it, but not the space below it. The better the insulating values below the floor radiant system, the more heat will be directed into the zone it was designed to heat. A non-insulated or under-insulated radiant floor system is slow to deliver heat into the space it was intended for.