Duct leakage testing finds defects in ductwork which can hurt the efficiency of an HVAC system. To have an energy efficient home, all systems must operate at peak performance. Leaky ductwork can increase utility costs in a home by as 
much as 30%. A duct blaster test helps to determine if ductwork is correctly installed and sealed. By keeping ductwork properly sealed, more warm or cool air reaches the intended areas of your home providing the most comfort at the lowest cost. At the same time, air which should be vented to the outside of your home, does so without escaping into the living areas.
To perform the test, a duct blaster fan is attached and sealed to one of the air return vents in the home. Once all of the other supply and return vents are temporarily taped off, the fan is used to force air into the system. Because this air cannot escape through the sealed vents, the pressure created forces the air to escape through any imperfections in the ductwork joints. By pressurizing the ductwork to the same standard for each home (in this case 25 pascals) ductwork system performance can be scored by measuring the air flowing through the fan while maintaining a constant duct pressure. The greater the amount of air needed to pressurize the ductwork, the leakier the system.
The National Comfort Institute says that the ductwork in the average home in the US has leaks at a rate of 30%. For new homes using current sealing methods, a leakage rate of less than 10% should be easily obtainable.
Problems with Leaky Ducts
Leaky ductwork can lead to many problems with a home. The obvious one is the loss of efficiency in the heating and cooling system. Beyond that, there are other concerns. Leaky ducts can emit dust and odors into your living space which should be vented to the outdoors. Additionally, ducts which leak moist air can lead to problems with mold and mildew. Finally, warm air which escapes into an unheated attic space can create issues with ice dams along the eaves of a home.
Proper Duct Sealing
A proper duct sealing job never involves the use of duct tape. Duct tape will eventually become dry and brittle and lose its ability to seal. Correct duct sealing involves the use of a thick layer of mastic at all duct connections. The job is messy but the result is the most effective and permanent you will find. Some contractors use what is called a “belt and suspenders” approach. This involves using two layers of mastic, one from the inside as the ducts are being connected and another from the outside after the sections are in place.