The duct system in your home or building circulates warm and cool air supplied by your space conditioning unit (air conditioning and/or heating). Space conditioning accounts for as much as 40% of your energy bill, according to the California Energy Commission (CEC). Duct losses can account for more than 30% of energy consumption for space conditioning, especially if the ducts are in an unconditioned space such as an attic. A duct system that has been properly sealed can safely and efficiently keep your home or office feeling fresh and comfortable year-round ¡ª and save you money too.
Ducts are the network that connects your furnace, air-conditioner, or heat pump to the air vents throughout your home or building, delivering heated or cooled air to make you more comfortable. Small holes or gaps in your duct system are common and can make your system inefficient by leaking this conditioned air to the outside and forcing your system to work harder to provide the desired temperature.
An enormous waste of energy occurs when cooled air escapes from supply ducts or when hot attic air leaks into return ducts. Recent studies indicate that 10 to 30% of the conditioned air in an average central air conditioning system escapes from the ducts.
For central air conditioning to be efficient, ducts must be airtight. Hiring a competent professional service technician to detect and correct duct leaks is a good investment, since leaky ducts may be difficult to find without experience and test equipment. Ducts must be sealed with duct "mastic," metal-backed tape or aerosol sealant. The old standby of duct tape is ineffective for sealing ducts; it cannot withstand the heat and will fail.
Aerosol sealant, or Aeroseal, is a patented technology that utilizes tiny particles of sticky material that are blown through your duct system and attach themselves to the edges of leaks. This technique can save enormous amounts of time by allowing leaks to be sealed in places that are hard to reach or cannot be seen.
Obstructions can impair the efficiency of a duct system almost as much as leaks. You should be careful not to obstruct the flow of air from supply or return registers with furniture, drapes or tightly fitted interior doors. Dirty filters and clogged evaporator coils also can be major obstructions to air flow.
For optimum efficiency, insulate your ducts. The large temperature difference between attics and ducts makes heat conduction through ducts almost as big a problem as air leakage and obstructions. Ducts in attics should be insulated heavily in addition to being made airtight. Duct insulation should be rated at least R-6. See the Insulation Guide for information about R-ratings.
ENERGY STAR offers a duct system leakage specification which states that duct leakage should be no more than 10% of the total supply. Visit for more information.
Leaking ducts can decrease the overall efficiency of your heating and cooling system by as much as 20%. Duct sealing increases efficiency and lowers your energy bills. The typical home could save up to $150 annually. Commercial buildings could save even more. And, many utilities provide rebates for upgrading your duct system. Payback periods range anywhere from two to four years based on the cost of diagnostics and repair.
Energy generation and use is the single largest contributor to air pollution. If you are wasting up to 20% of your heating and cooling systems' performance through leaky ducts, you are using more energy to get the same job done. By sealing your ducts and reducing the amount of energy necessary to comfortably heat and/or cool your home or building, you will reduce the amount of air pollution created from generating the energy.
Hire a good contractor. A knowledgeable contractor can perform tests on your duct system to determine if duct sealing will make you more comfortable and efficient. When improving your duct work, your contractor should:
Identify any leaks with diagnostic equipment.
Seal ducts with mastic, metal-backed tape or aerosol sealant - not duct tape.
Test airflow out of the registers after ducts are sealed.
Insulate your ducts to keep air at the desired temperature.