Proper duct work is one of the most important green building technologies for energy efficiency, health, and comfort in a home. Unfortunately, proper duct work is also one of the most overlooked parts of an efficient mechanical system, and installing it is often considered an entry-level job requiring little experience or training.
Planning duct work, however, is part art and part science, and it should be modeled by computer software to ensure that each room receives the right amount of air supply and return. A duct system should adhere to the same principles as fluid dynamics, that is, radius turns and as few bends and obstructions as possible.
Duct work should also have as few gaps as possible. While all duct work should be properly sealed with either mastic or foil tape, sealed gaps aren’t designed for long-term performance. Joint sealants eventually fail, and properly installed duct work shouldn’t be compromised by the failure of joint sealants.
The placement of cold air returns is another consideration that affects both health and the system’s efficiency. Usually, cold air returns in a duct system are located in the framing openings. The problem is that these areas are impossible to clean, and they become excellent incubators for dust and microbes.
Also, framing chases have gaps and openings, and these suck air into them as well, so using framing chases as cold air returns decreases the efficiency with which air is drawn from the right locations. A properly ducted cold air return helps a system to operate efficiently and to provide better air quality.
Placing duct work in unconditioned spaces should also be avoided. Even if the ducts are insulated, it is usually not at a high R-value, and they will continuously lose energy to the unconditioned area. When duct work must be run in crawl spaces and attics, try bring these areas into the conditioned space. Barring that, build a well-insulated chase around the duct work. This helps those ducts lose less energy and keeps the rooms they serve at a more comfortable temperature.