Sealing refers to closing off any gaps or leaks in the duct system to prevent the direct loss of heated air into unheated spaces. Sealing is done with mastic or wraps at all duct joints and seams. Some sealing is done poorly and may be ineffective, older tapes especially may have become torn, loose, or cracked. (see "locating leaks" for more information)
Insulation for ductwork is the creation of a thermal barrier (an insulation wrap) to reduce the convection loss of heat inside the ducts to the surrounding air.
The photo at left shows uninsulated ducts running through an unheated subfloor crawlspace. There is some sealing of the duct joints provided by the white wraps (that may contain asbestos in this case, see more information below), but other joints are not sealed. This system will allow significant heat loss through both leaks and convection.
The photo at right shows an insulated duct running through an unheated attic space. The insulation makes it impossible to visually assess the quality of the duct sealing, but the insulation will make this duct far more efficient than the duct on the left.
The photo on the left shows a newer, continuous insulated duct running through an unheated subfloor crawlspace. These ducts are manufactured with insulation in place and in continuous rolls. They are cut to length at the site so there are no joints except at the furnace and register connections. These ducts are the typical installation at new homes or older homes where the ducts are replaced.
Duct insulation is often loose or damaged (cats and raccoons love to tear at it). This photo (right) shows retrofit duct insulation that has become loose. It also shows that "Duct Tape" is inappropriately named since it performs very poorly in the warm and dirty environment of ducts and crawlspaces.
Sealing and Air Quality Issues
Ducts located in areas such as crawlspaces, garages, attics, or unconditioned basements can pull air from those spaces through gaps and leaks into the duct system and deliver it to the living areas. Return air ducts in particular create a powerful suction that draws in surrounding air at any leaks. That air may contain contaminants such as mold, radon, hazardous or irritating fibers from insulations or asbestos, or fumes from stored items. Good duct sealing is needed in those areas to protect indoor air quality.
Testing Ducts and Locating Leaks
Generally all duct joints and seams outside of heated living spaces should be properly sealed. In existing homes, however, it can be difficult to assess the presence or condition of duct sealing. There is a type of test called "Duct Blasting" where a home performance contractor will pressurize your duct system to measure and locate duct leakage. This can be a very valuable tool, and there are incentives available for duct blasting and subsequent sealing (www.energytrust.org).
Asbestos Duct Wraps and Safety
Many older duct systems have joints (or the entire duct) sealed with a paper-like duct wrap that may contain asbestos (see photos below). Generally these wraps are safe as long as they are not damaged or disturbed in which case hazardous fibers can be released. Testing is available from local laboratories to verify if particular wraps contain asbestos. There are products and procedures that can be used to seal or encapsulate the wraps to help prevent damage.
If the wraps do become damaged, or the wraps or ducts need to be removed, steps should be taken to prevent any release of fibers. Consult with an asbestos abatement specialist and more information can be found at numerous websites including www.epa.gov/iaq/homes/hip-concerns.html
The left photo shows a duct wrap that is likely to contain asbestos. This wrap appears to be in good condition.
The photo on the right shows loose and damaged wraps that may contain asbestos. Testing is recommended to verify asbestos and if it is present special measures are needed to minimize further release of asbestos fibers.
Related Links
http://www.energystar.gov/index.cfm?c=behind_the_walls.btw_ducts Energy Star information and photos on duct sealing.
http://blog.oregonlive.com/homesandgardens/2008/11/leaky_ducts_know_the_signs.html Oregonian story on duct sealing.