Installing ducts yourself requires cutting, sealing, and securing metal or flexible tubing through walls and ceilings

Duct installation means running metal or flexible tubes from your furnace or air handler to each room in your home, then sealing the joints so air does not leak. The work involves measuring, cutting ductwork to length, connecting pieces with fasteners or mastic sealant, and securing the runs to joists or studs. Most homeowners can handle straightforward runs in open attics or basements; runs that require cutting through framing, navigating tight spaces, or working in crawlspaces are harder and may warrant hiring a contractor.

The difference between doing it yourself and hiring out comes down to three things: how many bends and turns your layout requires, whether you have access to the spaces where ducts run, and how comfortable you are working at height or in confined areas. A straightforward rectangular duct run from a basement furnace to a second-floor bedroom is manageable. A system that requires custom elbows, dampers, and sealed connections in a finished attic is not.

Key Takeaways

  • Ductwork runs from your furnace or air handler through walls, ceilings, and crawlspaces to deliver heated or cooled air to each room.
  • Metal ducts are rigid and durable but require cutting and bending; flexible ducts are easier to route but prone to kinking and require careful support.
  • All joints must be sealed with mastic sealant or metal tape to prevent air leaks that waste energy and reduce heating or cooling output.
  • Ducts must be supported every 4 feet along horizontal runs and secured to prevent sagging, which restricts airflow and damages the duct.
  • Most residential systems use a combination of metal trunk lines and flexible branch ducts, with dampers installed to control airflow to individual rooms.

Choosing between metal and flexible ductwork

Metal ducts come in rectangular and round shapes and are made from galvanized steel or aluminum. They are rigid, do not sag, and last 20 years or longer. Metal ducts require a sheet metal brake or a contractor's tools to bend and cut, and joints must be sealed with mastic or metal tape. If you do not own the tools, renting a sheet metal brake costs $40 to $80 per day, and cutting straight sections with a reciprocating saw and metal blade is slower but possible.

Flexible ducts (often called flex ducts) are plastic or foil tubing wrapped around a wire spiral. They are cheaper, easier to route around obstacles, and require no special tools to cut—a utility knife works. The downside is that flex ducts sag if not supported, kink easily if bent too sharply, and are more prone to puncture. Flex ducts also have higher friction inside, which means they lose more air pressure over long runs, so they work best for branch lines under 25 feet, not trunk lines.

Most residential systems use metal trunk lines (the main supply and return) and flex branches to individual rooms. This combination balances durability, cost, and ease of installation.

Measuring and planning your duct layout

Start by sketching your furnace or air handler location and marking each room that needs heating or cooling. Measure the distance from the furnace to the farthest room and note any obstacles—beams, plumbing, electrical wiring, or existing ducts. Measure doorways, crawlspace heights, and attic clearances to confirm you can physically move ductwork into place.

Calculate the duct size based on the airflow your furnace produces, measured in cubic feet per minute (CFM). Your furnace manual lists the CFM; a typical 80,000 BTU furnace produces 1,200 to 1,400 CFM. Duct sizing charts (available from HVAC supply houses or online) tell you what diameter or rectangular dimensions you need for that CFM. Undersized ducts restrict airflow and waste energy; oversized ducts are expensive and take up more space.

Plan your trunk line to run the most direct path from the furnace, then branch off to each room. Minimize bends and turns—each elbow or transition loses air pressure. If you must turn, use a 45-degree elbow rather than a 90-degree one when possible, and avoid tight radius bends in flex ducts.

Cutting and connecting metal ductwork

Metal ducts are sold in 5-foot and 10-foot lengths. Measure the distance you need to cover, then cut sections to length. A sheet metal brake bends the edges of flat metal into a tube shape; if you own or rent one, follow the manufacturer's instructions for your duct size. If you do not have a brake, buy pre-formed metal ducts and connect them end-to-end using slip couplings (sleeves that slide over the joint) or by overlapping the ends and fastening with sheet metal screws.

To connect two metal duct sections, slide a slip coupling over one end, then push the other section into it until the joint is tight. Drill three or four holes through both the coupling and the duct, then insert self-tapping sheet metal screws (size #8 or #10, typically 1/2 inch long) and tighten with a drill or screwdriver. Do not over-tighten—you only need the joint snug enough that it does not move.

After fastening, seal the joint with mastic sealant (a putty-like compound) or metal foil tape. Mastic is more durable and creates a better seal; explore it with a putty knife or your gloved hand, covering the entire joint on the inside and outside. Metal tape is faster but can peel over time if not applied smoothly. Let mastic cure for the time listed on the container (usually 24 hours) before running the system.

Installing and supporting flex ducts

Flex ducts come in rolls and are cut to length with a utility knife. Unroll the duct and lay it out straight before cutting to avoid kinks. When routing flex ducts, keep bends gradual—a minimum radius of one duct diameter. A 6-inch-diameter flex duct should not be bent tighter than a 6-inch curve; sharper bends crush the inner liner and restrict airflow.

Support flex ducts every 4 feet along horizontal runs and every 3 feet on vertical runs using metal or plastic straps rated for ductwork. Strap the duct to the top of joists or studs, not the sides, so gravity does not pull the duct down between supports. Do not use wire, rope, or zip ties—they can cut into the duct over time. Avoid compressing the duct with straps; tighten just enough to hold it in place.

Connect flex ducts to metal trunk lines or to registers using metal collars or clamps. Slide the flex duct over the collar, then tighten a hose clamp or metal band clamp around the connection until it is snug. Seal this joint with mastic or metal tape, the same way you would seal a metal-to-metal joint.

Sealing joints and preventing air leaks

Air leaks in ductwork waste 15 to 30 percent of your heating or cooling output, so sealing is not optional. Every joint—where ducts connect to each other, to the furnace, to registers, and to dampers—must be sealed.

Mastic sealant is the industry standard. It is a thick, putty-like compound that you explore with a putty knife or gloved hand. Spread it over the joint, working it into gaps and cracks. A typical joint takes a 1/8-inch-thick layer. Mastic sticks to metal, flex, and plastic, and it remains flexible as the duct expands and contracts with temperature changes. One gallon covers roughly 100 linear feet of joints.

Metal foil tape (also called HVAC tape) is faster but less reliable. It works best on smooth, flat joints. Clean the surface with a dry cloth, then press the tape firmly over the joint, overlapping by at least 2 inches on each side. Smooth out air bubbles as you go. Tape can peel at the edges if not applied carefully, especially in attics where temperature swings are large.

Do not use duct tape (cloth tape with rubber adhesive). It degrades in heat and loses its seal within a few years. The industry has moved away from it for good reason.

Installing dampers and registers

Dampers are sliding gates inside ducts that let you control airflow to individual rooms. A damper is installed in the flex branch duct that runs to each room, usually within 6 feet of the register. Slide the damper into the duct, then find it with the fastening tabs or screws provided. The damper handle sticks out of the duct so you can open or close it by hand.

Registers (the grilles you see on walls or ceilings) are connected to the end of the flex duct with a metal collar. Push the duct over the collar, tighten the clamp, and seal the joint with mastic. Screw the register frame to the wall or ceiling, then snap the grille into the frame. Make sure the damper inside the register (if it has one) moves freely.

Return air ducts carry air back to the furnace. These are typically larger than supply ducts and may run through a central return plenum (a large box) before entering the furnace. Return ducts must be sealed the same way as supply ducts, and they should not be routed through unconditioned spaces like attics or crawlspaces, because they will lose temperature and reduce efficiency.

Common mistakes and how to avoid them

Undersizing ducts is the most common error. A duct that is too small creates high air velocity, which causes noise and pressure drop. Check your furnace CFM and use a sizing chart—do not guess. If you are unsure, go one size up rather than down.

Failing to seal joints is the second most common mistake. Even small gaps add up. Seal every joint, including where ducts connect to the furnace and to registers. A leaky system loses efficiency and may not heat or cool evenly.

Kinking flex ducts and not supporting them properly causes sagging and airflow restriction. Unroll flex ducts straight, support them every 4 feet, and keep bends gradual. Do not compress flex ducts with straps or lay them on sharp edges.

Running ducts through unconditioned spaces without insulation wastes energy. If ducts must run through an attic or crawlspace, wrap them with fiberglass insulation (typically R-6 or R-8) and seal the insulation seams with mastic. This keeps the air inside the duct at the temperature you want it to be when it reaches the room.

Frequently Asked Questions

Can I install ductwork myself if I have never done it before?

Yes, if your layout is straightforward—a straightforward trunk line from the furnace to a few rooms with minimal bends. Start with one or two rooms to build confidence. If your system requires custom elbows, dampers in tight spaces, or work at height, hiring a contractor is safer and faster.

What size duct do I need for my furnace?

Duct size depends on your furnace's CFM output, listed in the manual. A 1,200 CFM furnace typically uses a 12-inch round or 10-by-6-inch rectangular trunk line. Use a duct sizing chart from an HVAC supply house to match your CFM to the correct diameter or dimensions.

How do I know if my ducts are leaking?

If some rooms are much warmer or cooler than others, or if your heating and cooling bills are high, ducts may be leaking. You can feel for air escaping at joints, or hire an HVAC contractor to perform a blower door test, which pressurizes the ducts and measures leakage.

Do I need to insulate my ducts?

Yes, if ducts run through an attic, crawlspace, or garage. Wrap them with fiberglass insulation rated R-6 or higher and seal the seams with mastic. Ducts in conditioned spaces (inside walls or finished basements) do not need insulation.

What is the difference between mastic and duct tape?

Mastic is a putty-like sealant that remains flexible and lasts 20+ years. Duct tape is cloth with rubber adhesive and fails within a few years in heat. Mastic is the correct choice for ductwork; duct tape should not be used.