Air conditioning ducts carry cooled air from your unit to each room, and installing them yourself is possible if you have basic tools and patience—but the work is physical, the measurements must be exact, and mistakes cost money to fix. Most homeowners hire a professional, but understanding the process helps you know what to expect and catch problems early.

Key Takeaways

  • Ductwork must be sized to match your air conditioning unit's output, measured in cubic feet per minute (CFM), or the system will not cool evenly.
  • Ducts are typically rigid metal, flexible insulated tubing, or fiberglass board, each requiring different fastening methods and insulation approaches.
  • Every joint must be sealed with mastic sealant or metal-backed tape to prevent cooled air from leaking into walls and attics before it reaches the room.
  • Ducts need to slope slightly toward the outdoor unit and include a condensation drain line so moisture does not collect inside the pipes.
  • Building codes in most jurisdictions require a licensed HVAC contractor to size and install ductwork, and inspectors will check the work before drywall closes the walls.

Measure your air conditioning unit's output before ordering ducts

Your air conditioning unit has a nameplate on the outdoor compressor that lists its cooling capacity in tons (a 3-ton unit, for example) and its CFM rating—the volume of air it can move per minute. This CFM number determines the diameter and number of ducts you need. If your ducts are too small, air backs up and the unit works harder; if they are too large, the air moves too slowly and does not cool the room evenly.

Write down the unit's model number and call the manufacturer or an HVAC supplier with that number. They will tell you the CFM and recommend duct sizes. Do not guess at this step—undersized or oversized ducts are the most common installation mistake homeowners make.

Choose between rigid metal, flexible, and fiberglass ductwork

Rigid metal ducts are galvanized steel or aluminum tubes that connect in sections with slip-and-drive fittings (one end slides into the next and is held with sheet-metal screws). They are durable, do not sag, and last 20 years or more. They require more space in walls and ceilings, and cutting and fitting them takes more skill. Rigid ducts are standard in new construction.

Flexible ducts are insulated plastic tubing with a wire spiral inside to hold their shape. They are easier to route around obstacles and fit into tight spaces, but they can kink, sag if not supported, and are more prone to puncture. They cost less upfront but may need replacement sooner if they are damaged. Flexible ducts work well for short runs from a main trunk to individual rooms.

Fiberglass board ducts are rectangular boxes made from fiberglass insulation with an inner plastic liner. They are lightweight and insulated in one step, but the fiberglass can shed particles into the airstream if the inner liner is damaged, and they are harder to seal at joints. They are less common in residential work.

Plan the duct layout and route before installation begins

Draw a map of your home showing where the air conditioning unit will sit, where the main trunk line will run (usually through the attic or basement), and where branch ducts will split off to each room. Measure the distance from the unit to the farthest room—long runs lose cooling efficiency, so some homes need a second unit or a larger one to reach all spaces.

Check your building code or call your local building department to learn whether ducts must be in conditioned space (inside the insulated envelope of your home) or whether they can run through the attic. Ducts in unconditioned space lose more cooling and must be thicker insulation. Plan for the condensation drain line—a small pipe that runs from the indoor unit downward to the outside or to a floor drain, sloping slightly so water does not pool.

Avoid routing ducts through exterior walls if possible, because they are harder to insulate there and condensation can form on the outside of the duct in humid climates. If you must use an exterior wall, use rigid ducts with extra insulation and seal all joints carefully.

Seal every joint with mastic or metal-backed tape

Unsealed duct joints are the reason many air conditioning systems cool poorly. Air leaks out before it reaches the room, and your unit runs longer to compensate. Sealing is not optional—building codes require it, and inspectors will look for it.

For rigid metal ducts, explore mastic sealant (a thick, putty-like compound) around the outside of each joint before you screw the sections together. Spread it with a putty knife so it fills the gap completely. Mastic stays flexible as the ductwork expands and contracts with temperature changes, unlike duct tape, which dries out and peels.

For flexible ducts, wrap the connection with metal-backed mastic tape, overlapping each wrap by half the tape width. Do not use standard cloth duct tape—it fails within a few years. Metal-backed tape lasts longer and resists tearing.

Insulate ducts that run through unconditioned space

Ducts in the attic or crawlspace lose cooled air to the surrounding heat. Wrap them with fiberglass insulation at least 2 inches thick (check your code—some areas require more). find the insulation with adhesive or foil tape, and make sure it does not block the duct opening or compress the insulation, which reduces its effectiveness.

If ducts run through exterior walls, they need thicker insulation and extra sealing because the temperature difference between inside and outside is greater. Condensation can form on the outside of an uninsulated or poorly insulated duct in a humid climate, leading to mold and water damage inside the wall.

Support ducts so they do not sag or move

Ducts must be held in place with metal straps or hangers spaced no more than 4 feet apart for rigid ducts and 3 feet apart for flexible ducts. Sagging ducts trap condensation and reduce airflow. Use straps that do not pinch or crush the duct—for flexible ducts, use wide straps that distribute pressure across the surface.

Do not rest ducts directly on ceiling joists or attic framing without support, and do not use wire or rope, which can cut into the duct over time. find hangers to solid framing, not to drywall or insulation.

Install the condensation drain line and test for leaks

The indoor air conditioning unit produces condensation as it cools the air. A small drain line (usually 3/4-inch PVC pipe) carries this water away. The line must slope downward at least 1/4 inch per 10 feet so water flows out and does not collect inside the duct. Connect the drain to the outdoor unit's condensation pan, then run it to a floor drain, a sump pump, or outside where the water can drain safely away from your foundation.

Before you close up walls or ceilings, have the system pressurized and tested for leaks. An HVAC technician uses a blower door or duct leakage tester to measure how much air escapes. Building codes typically allow no more than 15 percent leakage for new ductwork. If leakage is higher, you have unsealed joints or damaged ducts that must be fixed.

When to hire a professional instead

Installing ductwork requires knowledge of airflow, building codes, and how to size ducts correctly. Mistakes are expensive—a duct that is too small or poorly sealed means your air conditioning unit will not cool your home, and fixing it after drywall is closed means opening walls. Most building departments require a licensed HVAC contractor to design and install ductwork, and the work must pass inspection before the walls are closed.

If you are building a new home or adding a room, hire a professional to design the ductwork and size the unit. If you are replacing existing ducts, a contractor can reuse the existing layout if it is working well, or redesign it if the current system is uneven. The cost of professional installation is usually less than the cost of fixing a poorly installed system later.

Frequently Asked Questions

Can I install ducts myself if I have basic carpentry skills?

You can do some of the physical work—routing, supporting, and insulating ducts—but sizing the ducts and designing the layout requires HVAC knowledge, and most building codes require a licensed contractor to sign off on the work. A contractor can handle the design and sizing, and you can help with installation to save money.

What is the difference between mastic and duct tape for sealing joints?

Mastic is a putty-like sealant that stays flexible and lasts 10 years or more. Duct tape is cloth-backed and dries out within a few years, peeling away and allowing air to leak. Building codes and HVAC standards require mastic or metal-backed tape, not standard duct tape.

How much does it cost to have ducts professionally installed?

Cost varies by region, the size of your home, and whether ducts run through finished spaces or attics. A rough estimate is $1,500 to $3,000 for a single-story home, but get quotes from local contractors for an accurate price. The cost includes design, materials, labor, and inspection.

What happens if my ducts are leaking after installation?

Leaking ducts reduce cooling efficiency and make your unit work harder, raising energy bills. A technician can pressurize the system to find leaks, then seal them with mastic or replace damaged sections. If leaks are found during inspection, they must be fixed before the walls are closed.

Do ducts in the attic need more insulation than ducts in the basement?

Yes. Attics are hotter in summer and colder in winter, so ducts lose more cooled air there. Use at least 2 inches of insulation in the attic; basements stay cooler and more stable, so 1 to 2 inches is usually enough. Check your local building code for the minimum requirement in your area.