Ideally ducts should be located in conditioned space, such as within a dropped ceiling, between floors, in an insulated basement or crawlspace, or in an unvented attic that is insulated along the roof line. If ducts are located in an unconditioned space, such as a vented attic or vented crawlspace, they should be sealed and insulated to prevent heat loss due to air leaks and conduction and to provide some protection against harsh conditions.

Sealing

Duct leakage is a double hit on the utility bill: 1) duct leaks are an uncontrolled loss of conditioned air to the outdoors and 2) duct leakage drives building infiltration. For example, if a home had a 2.5-ton (30,000 BTU/H) cooling system moving 1,000 CFM (cubic feet per minute) of air and the ducts had 10% leakage (which is typical in code-built homes), the leakage rate would be 100 CFM. Each cubic foot of air carries with it 30 BTUs/H, so 3,000 BTUs of conditioned air would be lost to the outdoors each hour.

Duct leakage is an infiltration driver; it can negatively or positively pressurize the house depending on where the ducts are leaking, pulling outside air in through cracks in the building envelope or pushing conditioned air out. If the duct leakage is in the supply-side ducts, the house will be negatively pressurized compared to outdoors. If all the leakage is on the return side, the building will be positive with respect to outdoors. The field technician should understand this concept to help accurately diagnose the dominant sources of duct leakage.

Insulation

Whenever ductwork is located in unconditioned spaces, thermal insulation with a vapor barrier is a must to prevent unnecessary heat gain or loss through the duct walls and to prevent condensation from forming on the ducts themselves. For the insulation to work properly, it must be fully aligned and in contact with the walls of the duct system. A typical vented attic with a dark shingle roof can reach summer temperatures of 140ºF. At the same time, the dew point temperature in the attic will be about the same as it is outdoors. In humid climates "duct sweating" can become a significant problem if the ducts’ thermal and vapor barriers are not properly aligned along the entire length of the ducts.

Metal ductwork is constructed using sheets of galvanized steel. Metal ducts are either rectangular or round in shape. Metal ducting consists of straight lengths, elbows, connectors, junction boxes, and register boots that are purchased or fabricated in the HVAC contractor’s shop and fitted together at the job site. A compact duct layout design with short, straight runs and minimal bends provides the best performance from an air flow standpoint and also reduces the number of lengths and seams that need to be sealed and insulated.

For more on duct layout, see No Kinks or Sharp Bends in Flex Duct Installation.

Duct installation standards and installation guidance are also provided in Manual D Residential Duct Systems 2009 Edition published by the Air Conditioning Contractors of America.

Metal ducts for residential HVAC systems are typically insulated on the exterior using a "duct wrap," such as fiberglass blanket insulation with a foil-faced vapor barrier.  Duct wrap comes in four-foot-wide rolls 50 to 100 feet long and in R-values that range from R-4 through R-8.  Before applying duct wrap, sheet metal duct should be clean, dry, and tightly sealed at all joints and seams. The 2009 IECC (Section 403.2) requires that supply ducts located in unconditioned space be insulated to at least R-8 (return ducts can be insulated to R-6). 

Ducts should be sealed with duct mastic and/or metal tape that meets the requirements of the Underwriters Laboratory UL-181, UL-181A, or 181B (CEC 2005). Regular cloth-backed duct tape should not be used because it can dry out and fail quickly. Mastic is a thick, gooey, non-hardening substance that is spread onto the duct seams with a paintbrush or a putty knife. For additional strength with seams that are wider than 1/8 inch, cover the gap with fiberglass mesh tape then apply the mastic.

How to Seal and Insulate Metal Duct

1. Install the metal ducts. Fit pieces together at joints by sliding the wider pipe over the narrower connecting collars. Use screws to secure connections. Wipe ducts with a dry cloth to ensure they are clean and dry especially at all joints and seams.

2. Use mastic to seal the seams along the lengths of the pipes, the seams in the elbows, the joints where the elbow and straight pieces are joined, where ducting is joined to the air handler, junctions between trunk ducts and branch ducts, and at duct boots.

   

3. Measure foil-faced blanket insulation to fit around the circumference of the pipe with two inches of overlap. Wrap the insulation around the duct with the facing to the outside so the 2-inch flap completely overlaps the facing and insulation at the other edge of the insulation. Insulation should be snugly butted but not wrapped too tightly, which would compress the insulation and reduce its R value. Follow the manufacturer’s "stretch-out" guidelines, which specify how much insulation to use, based on the pipe circumference, to ensure that the insulation thickness is maintained to achieve the rated R-value (see Table 1). The insulation should be a minimum of R-8 for all supply ducts and at least R-6 for all return ducts.

4. Overlap the insulation and staple along the seam every 6 inches at the "tape flap" using outward-clinching staples.
5. Wrap another piece of insulation around the next section of duct, overlapping two inches at the end of the first piece of insulation. Staple along the lengthwise seam and at the overlap. Continue overlapping and stapling until the entire length of duct is covered. After stapling, cover and seal all tape flaps and overlaps with UL-181 metal tape or mastic (mastic is recommended).
6. Make sure insulation covers all pipe connections to trunk duct, junction boxes, and boots.