Is Running the Duct Work Under the House Efficient?

Ductwork Efficiency

• The efficiency of a home’s ductwork layout depends on where ducts intersect with living spaces and the condition of the ductwork in terms of potential leaks and gaps. Heating and cooling systems are designed to work within closed-air environments, meaning the same amount of air is constantly recycled, or reconditioned. This closed-air feature helps to improve a system’s efficiency, since existing air has already been conditioned. Conditioned air results once a system extracts built-up heat (in the summer) or cold (in the winter) from the air in a home and replaces it with cool air or warm air. As a result, ductwork layouts tend to work more efficiently when positioned inside the home’s structure.

Insulation

• As underground ductwork runs within an unconditioned air space, temperature differences can cause additional energy efficiency loss within an underground ductwork system. Temperature differences appear as warm air flowing through duct lines encased in cold earth or cement. Without proper insulation, heat loss occurs as cold interior duct surfaces absorb warm air flows. Insulation materials also work as a back-up barrier in cases where leaks or gaps form in underground ductwork lines. Without insulation, existing leaks introduce unconditioned air into the heating system, which makes the furnace work that much harder to maintain temperatures within the home.

Types of Ductwork

• The type of ductwork material best suited for underground lines depends on the type of environment ductwork lines will run through. Homes in the process of being built may incorporate running ductwork through a cement slab foundation area, through a crawlspace area or through a soil-exposed area. The materials used to make the ductwork will determine how well the system holds up under different underground conditions.

  Ductwork materials come in three general types, which include flex duct, rigid duct board and metal. Flex duct consists of three layers made up of a plastic interior lining, insulation materials and a reflective external surface. Rigid duct board has a fiber glass lining surrounded by a foil-like material. Rigid duct board may also include a fungicidal lining on the duct’s interior surface. Metal duct materials consist of bare, galvanized steel wrapped in insulation with a vapor-protected coating.

Warm Climates

• Homes in warm climate regions typically expend greater amounts of energy for cooling purposes than heating purposes. Under these conditions, installing an underground ductwork system becomes the more energy-efficient option. As heat rises and cold air sinks, an underground ductwork system benefits from existing ground temperatures and from the natural flow of cold air molecules. Also, while installing ductwork within the conditioned areas of a home helps to reinforce the closed-air system effect, heat build-up tends to increase toward the upper areas in a home’s structure. This means that the likelihood of energy losses increases as ductwork lines extend toward the top areas of a home.