Sealing Heating Ducts with Mastic FAQs

Q. What is mastic?
A. A group of high strength adhesive compounds used in the building and construction trades. Usually applied by trowel, brush, or caulking gun. There are three general classifications: water-based; solvent-based; and two-component curing systems. Water-based is the safest and most easy to use and performs as well, in some instances better, than the other two classes.

Q. Why is sealing ductwork important?
A. Leaky ductwork often accounts for 10 to 30 percent of total heating and cooling costs. For an average home, leaky ducts can waste hundreds of dollars each year. While the increase in energy costs is significant, protecting health and safety is the most important reason to seal ducts.

Q. Why is duct leakage so common?
A. Most connections are simply not sealed. There is also a problem with poor quality materials being used to seal ducts. Duct tapes do not provide a permanent seal. Their adhesive dries out. Experts recommend duct sealing with mastic.

Q. If ducts are insulated, do they need to be sealed?
A. Yes. Insulation does not stop air leaks. Look for dirt streaks in duct insulation, they're a sign that air has been leaking from the ducts.

Q. Are certain types of ducts more air tight?
A. Studies show that all types of ductwork can have problems with air leakage. Mastic works to seal metal, flexible, and fibrous ductwork.

Q. What are the most important areas to seal?
A. High priority leaks:

  • Disconnected components
  • Connections between air handling unit and the plenums
  • All seams in the air handling units, and plenums, takeoffs, boots, and other connections, especially site-built items.

Q. What is the VOC on Mastics?
A. VOC = volatile organic compound. Water is an inorganic compound. The products in question are water-based and the volatile compound is water. So the VOC would be infinitesimally small (not recordable).

Q. How many lineal feet will a 10.5 oz. tubes cover?
A. One 10.5 oz. tube will cover approximately 50 lineal feet at 1/4" bead.

Content provided by RCD Corporation