This lowers the efficiency of these ducts relative to those in interior walls and floor spaces. How ducts perform in basements depends on how the basements are used. If the basement is conditioned, then a basement home is similar to homes where the equipment is in a furnace closet and the ducts are in the conditioned space. The cooling performance of ducts in basements is usually higher than attic ducts. Attics get hot in summer, so attic ducts are least efficient for cooling just when the the highest cooling loads occur.
Summer cooling in basement houses contributes less to peak loads on electrical utilities.
Sizing Up the Monster Basements are usually not well-vented to the outside, so much of the energy lost by the ducts will be recovered through a reduction in the house conditioning load. The ducts were modelled to account for leakage, conduction losses, radiative losses, and the change in temperature of the ducts during equipment cycling. The fraction of energy recovered will obviously depend upon the insulation levels of the basement walls and ceilings. Insulating the ceiling will reduce the fraction of energy recovered but decrease energy use. Insulating the basement walls will increase both quantities.