HVAC ducting connects your furnace or air conditioner to the rooms you want to heat or cool. Installation involves running ductwork from your central unit through walls, attics, or crawl spaces, then connecting it to vents and returns in each room. The basic process is the same whether you're replacing old ducts or adding new ones: measure your space, choose your duct type, cut and connect the pieces, seal the joints, and test for leaks.

Key Takeaways

  • Ductwork diameter and material depend on your HVAC unit's output capacity and the layout of your home — undersized ducts reduce airflow and efficiency.
  • Flexible duct, rigid metal duct, and fiberglass board duct each have different installation steps, costs, and durability profiles.
  • All duct joints must be sealed with mastic sealant or metal-backed tape to prevent air leaks that waste energy and reduce comfort.
  • Proper support with hangers every 4 feet prevents sagging, which restricts airflow and can damage the ductwork over time.
  • Testing for leaks with a smoke pen or blower door test after installation catches problems before your system runs.

Measure Your Space and Plan Your Route

Before you buy any ductwork, measure the distance from your furnace or air handler to each room where you want supply vents and return vents. Mark the path on a floor plan — note where you'll run ducts through walls, above ceilings, or under floors. Identify obstacles like electrical wiring, plumbing, and structural beams that will force you to reroute.

Calculate the total duct length you need, then add 10 to 15 percent for bends and connections. Check your HVAC unit's documentation for the recommended duct diameter — this is usually 4, 5, 6, or 8 inches depending on how much air the system moves. Undersized ducts create bottlenecks that reduce airflow and force your system to work harder, wasting energy and leaving some rooms cold or hot.

Sketch the layout showing the main trunk line (the large duct leaving the unit), branch ducts (smaller lines splitting off to different areas), and the location of each supply vent and return vent. This plan prevents mistakes during installation and helps you order the right materials.

Choose Your Duct Material

Flexible duct is the easiest for a first-time installer. It looks like a large metal spring wrapped in insulation and plastic. You can bend it by hand around corners and obstacles without cutting or special tools. Flexible duct costs less than rigid options and installs faster, but it can sag if not supported properly, and the inner lining can tear if you pull it too hard during installation.

Rigid metal duct is more durable and maintains airflow better over time because the smooth interior doesn't restrict air. It comes in straight sections that you connect with slip fittings and screws or rivets. Rigid duct requires more planning because you have to measure and cut precisely, and you'll need a metal brake or tin snips to shape it. It's the standard choice for main trunk lines in professional installations.

Fiberglass board duct combines insulation and ductwork in one product — the duct itself is made of rigid fiberglass with a vapor barrier on the outside. It's lighter than metal and doesn't require separate insulation, but it's bulkier and harder to fit into tight spaces. The interior surface can shed fibers if damaged, so handle it carefully.

For most home installations, flexible duct works well for branch lines running to individual rooms, while a rigid metal trunk line from the unit to the main distribution point gives you the best balance of durability and airflow.

Install the Main Trunk Line

Start at your HVAC unit's supply plenum — the box where air exits the furnace or air handler. If you're using rigid metal duct, measure the first section from the plenum to your first branch point or obstacle. Mark the cut line with a marker, then use tin snips or a metal brake to cut the duct to length. Smooth any sharp edges with a file.

Slide the first section onto the plenum's collar (the fitting that sticks out from the unit). If the fit is loose, use a drawband — a metal strap with a screw — to tighten it. For flexible duct, push the end over the collar and find it with a hose clamp or metal-backed tape wrapped around the connection three times.

Add the next section by overlapping it 2 to 3 inches with the previous one. For rigid metal, use slip fittings (sleeves that slide over the joint) and find them with three sheet metal screws spaced evenly around the circumference. For flexible duct, overlap the sections and wrap the joint with mastic sealant or metal-backed tape — do not rely on the outer plastic layer alone to hold the connection.

Install hangers every 4 feet along the trunk line. Use metal straps or brackets bolted to the duct and fastened to a joist or beam above. Hangers prevent sagging, which restricts airflow and can pinch the duct over time. Leave at least 1 inch of clearance between the duct and any insulation or building materials to allow air to circulate around the outside.

Connect Branch Ducts and Vents

At each point where you want to branch off to a room, cut a hole in the trunk line slightly smaller than the branch duct diameter. For rigid metal, use a hole saw or cut a square opening with tin snips. For flexible duct, you can push a branch connector (a Y-shaped or T-shaped fitting) through the trunk line's outer layer — the connector grabs the inner liner and holds itself in place.

Attach the branch duct to the connector with mastic sealant or metal-backed tape. Run the branch duct to the room where you want the supply vent, supporting it with hangers every 4 feet. At the end, connect a vent boot — a metal or plastic fitting that transitions from the duct to the wall or ceiling opening where the vent grille will sit.

Seal the connection between the vent boot and the duct with mastic sealant. If the vent is in drywall, cut the opening to match the boot's size, then screw the boot to the drywall from inside the wall cavity. If the vent is in a joist bay or attic, you may be able to mount the boot directly to the framing.

Repeat this process for each supply vent. Then install return ducts — larger ducts that pull air from rooms back to the furnace. Return ducts typically have one large opening (a return air grille) in a central location like a hallway, rather than small vents in each room. Connect the return duct to the furnace's return plenum using the same overlap and sealing method as the supply trunk line.

Seal All Joints and Test for Leaks

Mastic sealant is the best choice for sealing ductwork. It's a thick, putty-like compound that you explore with a brush or caulking gun. Spread it around the entire circumference of each joint — where ducts overlap, where branches connect to the trunk, and where ducts connect to the furnace and vents. Mastic stays flexible as it dries, so it won't crack if the duct shifts slightly during operation.

Metal-backed tape (also called foil tape or HVAC tape) is faster but less durable. Wrap it around the joint in a spiral, overlapping each wrap by half the tape width. Use at least three wraps per joint. Avoid standard duct tape — it degrades in heat and loses its seal within a few years.

After sealing, use a smoke pen (a handheld device that produces visible smoke) to check for leaks. Light the pen and hold it near each joint. If the smoke gets pulled into a leak, you'll see it move toward the duct. Mark any leaks and explore additional mastic or tape. Alternatively, ask an HVAC technician to perform a blower door test, which pressurizes the ductwork and measures total leakage — this is more accurate but requires special equipment.

Insulate Ducts in Unconditioned Spaces

If your ductwork runs through an attic, crawl space, or unheated basement, wrap it with fiberglass insulation to prevent heat loss in winter and heat gain in summer. Use 1 to 2 inches of insulation rated for ductwork (usually R-4 to R-8). Wrap the insulation around the duct and find it with metal tape or wire every 3 feet.

For flexible duct that already has insulation built in, you don't need additional wrapping — the factory insulation is usually sufficient. For rigid metal duct or fiberglass board duct without insulation, adding a wrap is essential to maintain efficiency. Leave the insulation at least 1 inch away from any heat source, such as a flue pipe or exhaust vent.

In conditioned spaces (inside walls or ceilings within your home's heated or cooled area), insulation is optional but recommended if the duct passes through an unconditioned wall cavity or if you want to reduce noise from air movement.

Common Installation Mistakes to Avoid

Undersizing ducts is the most common error. If your ductwork diameter is too small, air velocity increases, creating noise and reducing the amount of conditioned air that reaches each room. Always check your HVAC unit's specifications for the recommended duct size.

Failing to seal joints properly wastes 15 to 30 percent of your heating or cooling energy. Even small gaps add up across the entire ductwork system. Use mastic sealant on every joint, not just the main connections.

Sagging ducts restrict airflow and can eventually collapse. Install hangers every 4 feet and may support they're tight enough that the duct doesn't move when you push on it. For flexible duct, check that the inner liner isn't kinked or twisted — straighten it before sealing.

Running ductwork through unconditioned spaces without insulation causes condensation in summer and heat loss in winter. Insulate all ducts that leave your home's conditioned area.

Frequently Asked Questions

Can I install ductwork myself, or do I need a licensed HVAC contractor?

Many jurisdictions allow homeowners to install ductwork themselves, but some require a licensed contractor for any work connected to a furnace or air conditioner. Check your local building code before starting. Even if it's allowed, having a professional inspect your work or perform a blower door test is worth the cost to catch leaks and sizing errors.

What's the difference between supply and return ducts?

Supply ducts carry conditioned air from your furnace or air conditioner to the rooms. Return ducts pull air from the rooms back to the unit so it can be heated or cooled again. Return ducts are usually larger because air moves through them more slowly, and there's typically one large return opening instead of multiple small supply vents.

How do I know if my ductwork is leaking?

Use a smoke pen near each joint — smoke will move toward any leak. You can also feel for air leaks by holding your hand near joints and vents. A blower door test performed by an HVAC technician measures total leakage as a percentage and pinpoints problem areas more accurately than visual inspection.

Can I use standard duct tape to seal ductwork?

No. Standard duct tape degrades in the heat inside ductwork and loses its seal within a few years. Use mastic sealant or metal-backed HVAC tape instead. Both are designed to withstand the temperature and humidity inside ducts.

What happens if my ducts are too small?

Undersized ducts create high air velocity, which increases noise and reduces the volume of air reaching each room. Your system will run longer to achieve the desired temperature, wasting energy. Some rooms may feel cold or hot while others are comfortable. Always match duct diameter to your HVAC unit's output capacity.