Air Leakage
Air leakage through duct seams and holes will result in the loss of a portion of the air flowing through the duct and a proportionate reduction in the heating or cooling effectiveness of the system. Depending on the seriousness of the problem, the ductwork can lose up to 1⁄3 of the air supply in this manner. It is usually a matter of poor workmanship and can be corrected by sealing the seam cracks and holes by caulking or soldering.
Duct Insulation
Ducts are insulated to prevent excessive heat loss or gain. If the ducts are used to convey heat, excessive heat loss from the ducts will reduce the efficiency of the heating system. The reverse is true of ducts used in cooling systems. If the ducts are not insulated, they will absorb heat from the air around them, and system performance will be impaired.
To maintain the proper level of performance in a heating or cooling system, the following ducts should be insulated:
• Supply ducts running through spaces that are neither heated nor cooled (for example, attics, basements, garages, crawl spaces).
• Long supply ducts (particularly those over 45 feet in length).
• All ducts located on the outside of buildings.
• Cool-air return ducts passing through hot areas (for example, furnace and boiler rooms, kitchens).
Round ducts are insulated with flexible fiberglass insulation. Both flexible and slab (board) insulation are used for rectangular ducts. The latter is made of spun fibrous glass wool. These lightweight, semirigid panels are available with a variety of facings (for example, 0.0025 embossed aluminum foil) for appearance and functional use.
Flexible and slab insulation are also produced as duct liners for absorbing duct system noise. One or both sides are coated to reduce air friction loss and bind surface fibers. They also insulate thermally.
Flexible insulation is secured in place with light-gauge wire ties. Slab insulation is secured to the duct surface with adhesive or mechanical clips.
It is recommended that a vapor barrier be placed between the insulating material and the duct surface to prevent the formation of condensation. The vapor barrier should be used when the temperature of the air inside the duct is lower than the dew-point temperature of the air surrounding the duct.