Duct-Sealing Strategies for Retrofits

A side-by-side field study of two duct-sealing techniques shows that both methods are effective and save energy with relatively short payback times.

Duct sealing can be difficult, costly and disruptive to deal with in a retrofit situation. The Advanced Residential Integrated Energy Solutions (ARIES) Collaborative conducted a field study to compare two techniques: manually applied sealants and injected Aeroseal aerosol. Their goals were to understand and compare the cost and effectiveness of these two approaches and to identify the logistical and technical issues that might affect large-scale implementation in low-rise multi-unit residential public housing complexes.

Air Handlers

Mastic was used to hand seal air handlers from
the outside.

Side-by-Side Testing

This study took place in 40 units in two housing developments in North Carolina. The developments included one- and two-story units that utilized central air conditioning and natural gas heating. The ductwork included both flex ducts and metal ducts, attic and floor ducts, and ducts inside and outside of conditioned space. The air handlers were all located in conditioned space.

Half of the units were treated with hand sealing, while the other half were treated with injected Aeroseal aerosol sealant.

Scope of Hand Sealing:

  • Register boots were sealed to the floor or to the ceiling.
  • Return plenums were sealed inside with mastic.
  • Air handlers were sealed from the outside with mastic.
  • Where accessible, the ridges trunk ducts and the trunk to flex duct connections inside attics were sealed with mastic.

Scope of Aeroseal Sealing

Aeroseal sealant could reach ducts that were inaccessible by hand.Return ducts were too small to use the Aeroseal system, so these were hand sealed. Air handlers and junctions between registers and walls, ceilings and floors were also hand sealed. (Note: In many cases the Aeroseal can be used to seal return systems. A wide connector in the duct system can split the air stream towards the return and the supply so both are treated at once.)


On average, hand sealing resulted in an almost 60 percent reduction in duct leakage to the outside, whereas the Aeroseal system showed a 91 percent reduction—a 32 percent improvement over hand sealing alone.

Return and supply flows, as measured by a fan-powered flow meter, increased after both treatments. Return flow increased by an average of 7 percent, but increases were higher with the Aeroseal system. One unit showed decreased flow afterwards; this was likely due to damage or compression of a duct in the attic.

While Aeroseal does seal ducts to a much tighter level than hand sealing, about 70 percent of the leakage reduction in the Aeroseal units was due to hand sealing at the returns, registers and other locations.

plastic tunnel

The plastic tunnel required to inject the Aeroseal sealant into the duct system can be awkward to set up in small spaces.

Energy savings and cost. Whole-house source energy savings ranged from 3 to 7 percent for the various units. The cost for hand sealing ranged from $275 to $511 per unit. The cost was higher for single-story units because workers had to climb into the attics to address the leakage, whereas the ducts in the two-story units were located in between floors and so were inaccessible. The sealing in those units was confined to the registers and air handler and at the return. The Aeroseal treatment cost a flat rate of $700 per unit.

Return on investment. An analysis of utility bills from one year before and one year after the retrofits shows that on average, the units enjoyed a 15 percent reduction in heating and cooling energy use. Hand sealing resulted in a shorter simple payback: just over two years versus 4.7 years for the Aeroseal treatment.

Lessons Learned

Both the Aeroseal method and manual sealing stop leakage. Because the Aeroseal method is less familiar, the researchers noted several advantages, challenges and recommendations for readying the system for production-scale use.

Advantages. This system allows sealing of inaccessible ducts. It also avoids some of the hassles of manual sealing, which involves removing duct insulation, cleaning ducts, applying masking, waiting for it to dry, reapplying insulation, kicking ducts loose and other quality control issues.

Challenges. Small units made set-up challenging. The humid environment created some difficulties in keeping globules of sealant dry on the outside, which caused some clogging issues.

Recommendations. Because the set-up and clean-up are so involved, most of the time the Aeroseal equipment was not actually actively sealing ducts. Using a Y connector to serve multiple units at once would save time. The researchers also recommend using a scaled-down system for small units.

Precise Control

A computer monitors the flow rate, temperature and humidity levels of the air stream, indicating adjustments that need to occur so that the Aeroseal sealant is neither too dry nor too wet when delivered.

The Aeroseal Process Aeroseal, created from work done at Lawrence Berkeley National Laboratory in 1994, seals ducts from the inside of ductwork. The Aeroseal sealant particles are partially dried before they enter the duct system, so they will not stick to the duct walls. The sealant consists of a vinyl material suspended in a water solution. The Aeroseal process puts escaping air under pressure and causes polymer particles to stick first to the edges of a leak, then to each other until the leak is closed.

The set-up is straightforward but somewhat time consuming. Supply registers are sealed with compressible foam plugs to contain the sealant inside the duct and prevent it from dispersing into the building. The sides of the coil and other HVAC equipment are sealed off to protect from sealant material. Aeroseal equipment is attached together using a plastic tunnel, or duct. This is used to give the sealant time enough to dry before entering the ducts. A computer controls and monitors the process.

Presenters: Jordan Dentz, Advanced Residential Integrated Energy Solutions (ARIES), and Francis Conlin, High Performance Building Solutions, Inc.