The article has been updated successfully.Homes are not static systems, and conditions change as homes age. Tape adhesive dries out and caulking erodes. Many systems have supply registers in each room, but only one centrally located return register for the whole home. When we close doors for privacy, air in that particular room can't reach the return register—but the supply register is still bringing in conditioned air.
The delivered air has to go somewhere, so air gets forced out any space available. Meanwhile, enough air isn't entering the return duct, so unconditioned air from the attic, basement, garage, or crawl space gets sucked in through weak spots, cracks, or crevices. This situation can be avoided by having supply and return ducts in each room, or by providing an air pathway between the room and the main body of the home.
Such a pathway can be created by adding vents in doors or walls, or by installing a jumper duct or transfer vent that connects vents in the ceiling of each space. Also, keep furniture clear of air registers and return air vents. Anything that interferes with air circulation will make the system less efficient and potentially lead to problems.
source: Wendell A. Porter, Hyun-Jeong Lee, and Kathleen C. Ruppert, The Duct System, UF/IFAS FCS3263, 2008.
Additional Resource
UF IFAS Energy Efficient Homes: Energy Efficient Homes: The Duct System - http://edis.ifas.ufl.edu/FY1024
U.S. Department of Energy – Better Duct Systems for Home Heating and Cooling - http://www.nrel.gov/docs/fy05osti/30506.pdf
U.S. Department of Energy – Improving the Efficiency of Your Duct System - http://apps1.eere.energy.gov/buildings/
