Ductwork installation means designing and building the supply and return ducts that carry air between your furnace, air conditioner or heat pump and every room. You need it when ducts are missing, damaged, leaking badly or too small for the airflow your system needs. Often you don't need all-new ducts: sealing leaks, fixing crushed runs and adding a return can solve the problem for a fraction of the work. A proper job starts with a Manual D design and ends with static pressure and leakage tests.
Signs you need new or repaired ductwork
Duct problems usually show up as comfort and noise complaints that new equipment won't fix.
| What you notice | Likely duct cause | Typical fix |
|---|---|---|
| Hot or cold rooms, mostly at the ends of runs | Undersized branches, long or kinked flex, leaks, no balancing | Reroute or resize branches, seal, add balancing dampers |
| Whistling grilles, loud rush of air, doors that slam | Too few or undersized returns, high static pressure | Add or enlarge returns, transfer grilles |
| Dust returning soon after cleaning | Leaky return ducts pulling in attic, crawlspace or wall air | Seal returns with mastic; replace if badly deteriorated |
| Rising bills, system runs long | Supply leaks dumping air into the attic or crawlspace | Seal and insulate, or replace badly leaking sections |
| Weak flow at certain registers | Crushed, collapsed or disconnected flex duct | Reconnect, support and pull tight, or replace that run |
| Sweating ducts or water stains below | Torn insulation or vapor jacket in a humid attic or crawlspace | Reinsulate; replace if the duct has rusted or soaked through |
| Popping or banging when the blower starts | Thin metal flexing under pressure, often from high static | Add returns, stiffen or replace the section |
What you can check before calling anyone
You can learn a lot in an hour with a flashlight. Turn the system off before you handle ducts, wear a mask and gloves in insulation, and step only on joists in the attic.
- Look for disconnected and crushed runs. In the attic, basement or crawlspace, check for flex duct that has slipped off a collar, lies flattened under stored items or sags in long loops.
- Check the seals. Old cloth duct tape that has dried and peeled means leaks at the joints. Mastic that's intact is a good sign.
- Check the insulation. Ducts in attics and crawlspaces should be insulated with the vapor jacket intact. Torn jackets or bare metal mean energy loss and condensation risk.
- Count your returns. Many older homes have a single central return. Note whether bedroom doors cut off airflow when closed.
- Feel the airflow. Compare the air from each register with the system running. Note weak rooms and noisy grilles.
- Change the filter. A clogged filter mimics duct restriction.
Leave the furnace and air handler cabinets closed, and don't move or reroute gas lines, flue pipes or wiring near ducts. If you spot crumbling, chalky wrap on old ducts, don't disturb it; older duct insulation can contain asbestos and should be tested first.
Repair, seal or replace: how to decide
Seal and repair when the ducts are mostly sheet metal in sound shape, the layout reaches every room and the problems are leaks, a few crushed branches or a missing return. Replace when flex duct is breaking down across the system, the ducts are badly undersized for the airflow the equipment needs, the layout no longer fits the house after remodels, or you're adding central air to a home that never had ducts.
Duct cleaning is a different job. It removes dust and debris but doesn't fix leaks, sizing or crushed runs. See air duct cleaning and when it helps before paying for it to solve an airflow problem.
When replacing equipment, have the installer measure static pressure before deciding. Many residential furnaces and air handlers are rated for about 0.5 inches of water column total external static pressure, and readings well above that point to ducts that are choking the blower. The full HVAC system installation guide covers how ducts fit into an equipment project.
How new ductwork should be designed: Manual D
ACCA Manual D sizes ducts from two inputs: the airflow each room needs, which comes from a Manual J room-by-room load calculation, and the pressure the blower can deliver. The designer lays out trunks and branches, sizes each run, and accounts for the resistance of fittings, filters and the coil. Good design also includes:
- Enough return air, ideally a return in each bedroom or transfer grilles so closed doors don't starve rooms.
- Short, direct branches with few sharp turns.
- Balancing dampers on branches so airflow can be tuned after install.
- Ducts inside conditioned space where possible, since attic ducts lose heat to the hottest and coldest part of the house.
Sheet metal, flex or duct board: which goes where
| Material | Best use | Strengths | Limits |
|---|---|---|---|
| Sheet metal (galvanized steel) | Trunks, plenums, long runs, returns | Smooth interior, durable, holds shape | Higher labor; must be sealed and insulated outside conditioned space |
| Flexible duct | Short branch runs to registers | Inexpensive, quick to route around obstacles | High friction; performance drops fast when kinked, sagging or too long |
| Fiberglass duct board | Plenums and trunk boxes in some regions | Built-in insulation, quieter | Fibers can erode if damaged; harder to clean; not for wet areas |
Flex duct works when it's pulled nearly taut, cut to length, supported at intervals the manufacturer specifies (often no more than about 4 to 5 feet), and never crushed by framing or straps.
What a ductwork installation involves, step by step
- Assessment and design. Load calculation, Manual D layout, and static pressure readings on the existing system.
- Permit. The contractor pulls the mechanical permit.
- Removal. Old ducts that are being replaced are disconnected and removed.
- Plenums and trunks. Supply and return plenums are fitted to the equipment and trunks hung with proper support.
- Branches and boots. Branch runs go to each register boot, with balancing dampers where needed.
- Sealing. Every seam, joint and boot connection is sealed with mastic or UL 181 listed foil tape, not cloth duct tape. Boots are sealed to the drywall or subfloor.
- Insulation. Ducts in attics and other unconditioned spaces are insulated. Energy codes commonly call for R-8 on attic supply ducts, with the vapor jacket taped closed.
- Testing. Static pressure is measured with the system running, and a duct leakage test pressurizes the ducts with a calibrated fan to measure how much air escapes. Many energy codes require that test on new systems where ducts run outside conditioned space.
- Balancing and inspection. Dampers are set so each room gets its design airflow, and the inspector signs off.
Coordinating new ducts with a central AC installation, step by step or the furnace installation guide lets one design cover both.
What affects ductwork installation cost
For typical ranges, see the ductwork installation cost guide. The biggest drivers:
- Scope. Sealing and a few repairs sit at the low end. Adding returns is middle tier. Whole-house replacement or ducts for a home that never had them sit at the high end.
- Access. Open attics and unfinished basements are easiest. Finished ceilings, low crawlspaces and walls add labor and drywall repair.
- Materials. Sheet metal costs more to fabricate and install than flex.
- Size of the home and number of runs.
- Removal, insulation and testing. Hauling away old ducts and meeting code insulation and leakage requirements.
- Permits and local labor rates.
How to vet a duct contractor: questions and red flags
Ask each bidder:
- Will you do a Manual J and Manual D design, and share it?
- What static pressure did you measure, and what will it be after the work?
- How will you seal joints, and will you seal the boots to the drywall?
- What insulation level will you use in the attic or crawlspace?
- Will you do a duct leakage test, and give me the result?
- Who pulls the permit? Are you licensed and insured?
Red flags:
- No measurements, just a quote based on square footage
- Sizing ducts to match the old ones without checking airflow
- Cloth duct tape as the main sealant
- No permit, or an offer to skip it
- Cash only, or pressure to sign the same day
- Pitching duct cleaning as a fix for leaks or undersized ducts
Frequently Asked Questions
How long does ductwork installation take?
Replacing ducts in an accessible attic or basement often takes one to three days. Whole-house replacement in a finished home with tight access, or adding ducts to a home that never had them, can take a week or more. New construction is fastest because the ducts go in before the walls close.
Is flex duct better than hard duct?
Neither wins everywhere. Sheet metal has the smoothest interior and lasts longest, so it suits trunks and long runs. Flex duct is cheaper and easier to route for short branch runs, but only performs well when it's pulled tight, well supported and kept short. Most good systems use sheet metal trunks with short flex branches.
Do I need to replace ductwork when getting a new HVAC system?
Not always. Ducts in good condition that are sized for the new system's airflow can be reused after sealing. Have the installer measure static pressure and inspect the ducts first. If static pressure is high, rooms are uneven or the ducts are crushed or disconnected, fix them as part of the equipment project.
How long does ductwork last?
Sheet metal ducts can last for decades if they stay dry and sealed. Flex duct wears out sooner, especially in hot attics, where the outer jacket and insulation break down. Condition matters more than age: crushed runs, failed tape, torn insulation and rising dust are better signals than a date.
Do I need a permit for ductwork installation?
Usually, for new ducts or a full replacement. Local building departments treat it as mechanical work, and many energy codes require sealing and sometimes a duct leakage test. The contractor normally pulls the permit. Small repairs like resealing a joint generally don't need one.
Can I seal my own ductwork?
Yes, for ducts you can reach safely. Brush water-based mastic or press foil tape with a UL 181 listing over seams and joints, then refit insulation. Avoid cloth duct tape, which fails quickly. Turn the system off first, and leave sealing inside walls or a full redesign to a professional.