Aeroseal vs Mastic: Which Duct Sealing Method Fits Your Home?

Mastic and Aeroseal both stop air from leaking out of your ducts, but they work in opposite ways. Mastic is a paste applied by hand to joints you can reach. Aeroseal is an aerosol sealant blown through the duct system to close leaks from the inside. Most homes get the best result from a mix, because each method covers the other’s blind spot. Which one fits yours depends on where your ducts run, how big the gaps are and what you are willing to spend.

TL;DR

• Mastic is applied by hand to seams and joints you can reach. It is the right tool for large gaps, boots, returns and the area around the air handler.
• Aeroseal seals from inside the duct and can reach leaks in walls, ceilings and other places nobody can get to. It cannot close large holes or disconnected ducts.
• In a federally documented North Carolina study, leakage fell by an average of 90% with Aeroseal plus manual sealing and 59% with manual sealing alone.
• Never use cloth duct tape. Use mastic, or a UL 181A or 181B rated tape on clean, dry surfaces.
• Have a combustion safety test done after sealing if you have a gas- or oil-burning appliance.

Table of Contents

• How each method seals a duct
• Side-by-side comparison
• What the research found on effectiveness
• What it costs
• How to choose for your house
• Signs your ducts may be leaking
• Safety checks after sealing
• Perspective
• Get your ducts checked in Miami-Dade
• Questions to ask before you hire anyone
• FAQ
• Sources

How each method seals a duct

Mastic: a paste applied by hand

Mastic is a thick, gummy sealant that is brushed or troweled over duct seams, joints and connections. It dries into a flexible, durable coating. For joints near the air handler or furnace, the federal guidance is to use mastic with fiberglass mesh, which bridges small gaps and gives the paste something to hold on to.

The catch is access. A technician has to get a hand and a brush to the joint. Ducts in an open attic or a mechanical closet are usually fine. Ducts buried in a soffit, a wall cavity or under blown-in insulation are a different story, and in those places mastic or tape may not be possible.

Aeroseal: sealant blown through the ducts

The aerosol approach came out of Lawrence Berkeley National Laboratory in the 1990s. As the federal measure guideline describes it, the registers are blocked and sealant is blown through the system, and the sealant builds up around seams and gaps until they close. Air escaping through a leak carries the particles to it, which is why the method finds holes nobody knew were there.

Because it works from the inside, Aeroseal can be effective where ducts cannot be accessed. That is its real advantage. The same guidance is clear about its limit: it cannot seal large holes or disconnected ducts.

Side-by-side comparison

The last two rows come from one study of affordable housing units. They show the relationship between the methods, not a quote for your house. We cover both figures below.

What the research found on effectiveness

The most useful hard numbers come from a Building America report on air distribution retrofits. In its North Carolina study, duct leakage fell by an average of 90% with Aeroseal plus manual sealing and 59% with manual sealing alone.

Read that carefully. The 90% figure is for Aeroseal combined with manual sealing, not Aeroseal alone. The same report notes that gaps larger than ⅝ inch and accessible boots, returns and air-handler areas are sealed manually before the aerosol step. In practice, the question is rarely “aeroseal or mastic.” It is “what do we seal by hand first, and is the rest worth sealing from the inside.”

What about durability?

Manufacturers of aerosol systems publish long lifespan claims. We would rather you judge by the material and the conditions than by a number on a brochure. Attics in South Florida see extreme heat, and duct sealants live through that cycle for years. Whatever method is used, ask the contractor what the product is rated for, how it was applied, and what warranty covers the work. Our piece on HVAC repair warranty checks lists the contract details worth reading before you sign anything.

There is one hard rule on materials. The EPA’s duct sealing brochure says never to use duct tape on ducts because it is not long-lasting. The federal sealing guidance goes further: do not use fabric or rubber duct tape. If tape is used at all, it should be UL 181A or UL 181B rated and applied to clean, dry, dust-free surfaces. Mastic, or fiberglass mesh with mastic, remains the standard answer.

What it costs

Prices vary by house, duct length, access and region, so treat published figures as context rather than a quote. The best sourced comparison is again the North Carolina affordable housing study: manual sealing cost $275 to $511 per unit, while Aeroseal-treated units cost $700 per unit. That report was published in 2014, so today’s prices will be higher, and those were modest multifamily units rather than a large Miami single-family home.

A few things push the price of any duct sealing job up or down:

• Duct length and number of registers. More runs and more boots mean more work, by hand or by machine.
• Access. A clean, open attic is quicker to seal by hand than a cramped one.
• Prep work. Large gaps and disconnected sections need manual repair first.
• Testing. Measuring leakage before and after adds time, and it is the only way to know the job worked.

We publish a flat-rate approach on repairs: you see the price before work starts. Our diagnostic fee starts from $89, and the quote for any duct repair follows from what the diagnosis finds.

How to choose for your house

Choose mastic when the leaks are visible and reachable

If your ducts run through an open attic or a utility closet and the problem is loose joints, a pulled-apart boot or a gap around the air handler, mastic is the direct fix. It handles large gaps that an aerosol cannot, and it is the right tool for the connection points where leaks are often the worst.

Choose Aeroseal when the leaks are out of reach

If your ducts are in walls, soffits or ceilings, or buried under insulation you do not want to dig out, sealing from the inside is the only way to reach every seam. It makes the most sense once the big problems have been fixed by hand and what is left is a scatter of small leaks you cannot see.

Choose both when you want the best measured result

That is what produced the 90% figure in the study. Seal what you can reach, then treat the rest. If you are weighing the aerosol step, ask for a leakage test before and after so the improvement is a measurement, not a promise.

Consider replacement when ducts are failing

Sealing cannot rescue ducts that are crushed, collapsed, disconnected in many places or full of damage. If a technician finds that, replacement may cost less over time than repeated patching. Our guide on AC repair vs. replacement in Miami covers how to weigh that call.

Signs your ducts may be leaking

Leaky ducts rarely announce themselves. These are the clues we hear about most:

• Some rooms never cool down while others get cold. See our guide to uneven cooling in your house for the other causes to rule out.
• Energy bills climb with no change in how you use the system.
• The system runs long cycles and still struggles on hot afternoons.
• Dust builds up fast, or the air feels humid indoors.
• Visible gaps, loose connections or crumbling old sealant at joints in the attic.

Not all of these point to ducts. Airflow that is simply out of balance produces similar symptoms, and air balancing is a separate fix. A proper inspection tells you which one you have before you pay for sealing you may not need.

Safety checks after sealing

Tightening a duct system changes how air moves through the house. For homes with gas- or oil-burning appliances, the federal guidance says duct improvements should include a combustion safety test after sealing to confirm those appliances are not backdrafting. Ask any contractor you hire whether they do this, and ask for the result. If you have a gas water heater or furnace, a licensed HVAC technician or your gas utility is the right person to confirm it.

After the work, it also helps to confirm airflow at each register and to give the house some fresh air. Basic steps like ventilating and filtering indoor air help clear any lingering smell from the job. If a smell or irritation persists, tell your contractor.

Perspective

Homeowners often ask us which product is “better.” We think the better question is which problem they actually have. A leaking duct is a symptom, and the right fix starts with finding out where the air is going and why.

Our approach is to diagnose first, quote a flat rate before any work starts, and keep the repair tied to what the inspection shows. Our factory-certified technicians look at the whole system: ducts, air handler, airflow and humidity. In Miami-Dade, where an air conditioner runs most of the year, a few percent of lost air is a few percent of lost comfort every day.

We would rather you understand your options than guess. If you want the ducts checked, our ductwork inspection and repair service is where we start, and our work is backed by a one-year warranty on repairs.

Boreal

Get your ducts checked in Miami-Dade

If your rooms cool unevenly or your bills keep rising, start with a diagnosis. Our factory-certified technicians inspect the system and give you a flat-rate price before work begins, with same-day service seven days a week across Miami-Dade. The diagnostic fee starts from $89, and it is listed on our AC repair page.

If ducts are one part of a bigger question, our maintenance plans are priced at $24.95 (Essential), $47.92 (Comfort) and $83.33 (Concierge) per month. They are built to catch problems before they become breakdowns. Planning a full system replacement? Installation and replacement is listed at $6,000 to $14,000 on our installation page, and a new system is a natural moment to fix the ductwork too. For efficiency ratings on a new unit, see SEER2 vs SEER in Miami.

Questions to ask before you hire anyone

Whoever you choose, these questions separate a careful job from a rushed one:

1. Will you test leakage before and after? A measurement is the only proof the sealing worked.
2. What gets sealed by hand first? Large gaps and disconnects must be fixed manually, with mastic or properly rated materials.
3. What materials will you use? The answer should never be cloth duct tape.
4. Do you run a combustion safety test? Essential if any appliance burns gas or oil.
5. What is the warranty, and what does it cover? Get it in writing, with the price fixed before work starts.
6. Are the ducts worth sealing? An honest contractor will tell you when replacement makes more sense.

FAQ

Is Aeroseal better than mastic?

Neither is better in every case. Aeroseal reaches leaks in hidden ducts that mastic cannot, while mastic closes large gaps and disconnects that Aeroseal cannot. In the North Carolina study, the best result came from combining them: a 90% average leakage reduction, against 59% for manual sealing alone.

Can I use duct tape instead of mastic?

No, not standard cloth duct tape. The EPA says it is not long-lasting, and federal guidance says not to use fabric or rubber duct tape on ducts. Use mastic, or a UL 181A or 181B rated tape on clean, dry, dust-free surfaces.

How much does Aeroseal cost?

It depends on the size of the system and the access. In a 2014 North Carolina study of affordable housing, Aeroseal-treated units cost $700 per unit, compared with $275 to $511 for manual sealing. A single-family home in Miami-Dade will usually differ, so get a written quote after an inspection.

Do I need a safety test after sealing ducts?

If you have a gas- or oil-burning appliance, yes. Federal guidance recommends a combustion safety test after duct sealing to make sure appliances are not backdrafting. A licensed HVAC technician can run it as part of the job.

Can Aeroseal fix a disconnected duct?

No. It cannot seal large holes or disconnected ducts, so those have to be repaired by hand first. After that, the aerosol step can deal with the smaller leaks that remain.

Sources

• Measure Guideline: Sealing and Insulating of Ducts in Existing Homes (U.S. Department of Energy, Building America)
• Air Distribution Retrofit Strategies for Affordable Housing (U.S. Department of Energy, Building America)
• BSC Information Sheet 602: Duct Sealing (U.S. Department of Energy, Building America)
• Duct Sealing brochure (ENERGY STAR)
• Duct Sealing (U.S. EPA)

Recommended

• Air Balancing in Homes: Signs, Fixes and Costs
• Electronic Air Cleaner vs Media Filter: Which Is Right?
• Uneven Cooling in Your House: Causes and Fixes