Ductwork installation - and when sealing beats replacing

What is ductwork installation?

Ductwork installation means designing and fitting the supply and return duct system that carries conditioned air around your house - trunk lines, branch runs, registers and the return path back to the equipment. Full replacement is disruptive and expensive. In a great many homes the actual problem is leakage and poor design rather than worn-out duct, and sealing plus a few targeted modifications delivers most of the improvement for a fraction of the money.

Signs you need ductwork installation

• You are building, adding a room, or finishing a basement or attic that has no duct service at all
• The equipment is being upsized or changed and the existing ducts cannot carry the airflow it needs
• Rooms furthest from the air handler have never been comfortable, in either season
• Measured static pressure is high, and the new system is short-cycling or tripping its limit as a result
• The ducts are original flex duct that has sagged, crushed or torn where it runs through the attic or crawlspace
• You can feel air escaping at joints, or see dust patterns around seams in an unconditioned space
• The system is loud - whistling registers and roaring returns usually mean undersized duct, not a faulty blower
• There is a single central return trying to feed the whole house behind closed bedroom doors

Seeing something not on this list? Browse hvac problems by symptom →

How ductwork installation actually works

1. 1

   Test before you replace

   A proper assessment starts with measurement: static pressure across the air handler, room-by-room airflow, and ideally a duct leakage test. That is what distinguishes a duct system that is leaking from one that is undersized - two problems with very different price tags.

   Watch forA replacement quote produced from a visual look in the attic. Duct that looks tired is not necessarily performing badly, and duct that looks fine can be badly undersized. Without numbers, nobody knows which problem you have.

2. 2

   Manual J and Manual D design

   Manual J gives the room-by-room load; Manual D turns that into duct sizes, trunk layout and register selection. Sizing is what determines whether a room gets the air it needs at a velocity you cannot hear.

   Watch forBranch sizes picked by habit rather than calculation. "Six inch to every bedroom" is a rule of thumb, and rules of thumb are why so many houses have one room nobody can sleep in.

3. 3

   The return path - the half that gets forgotten

   Air that goes into a room has to get back out. Many older homes have a single central return, so closing a bedroom door pressurises the room, stalls the supply and pushes conditioned air out through the building envelope. Fixes include dedicated returns, transfer grilles or jump ducts.

   Watch forA quote that addresses supply only. Under-returned systems are extremely common and are one of the biggest causes of high static pressure, noise and uneven temperatures. Ask specifically what is being done on the return side.

4. 4

   Material choice

   Rigid sheet metal is durable, has the lowest friction and cleans well. Duct board is insulated and quiet but more fragile. Insulated flex is cheap and fast and perfectly acceptable in short, straight, properly supported runs - its bad reputation comes almost entirely from bad installation.

   Watch forLong, sagging, snaking flex runs. Every compression and every unnecessary bend adds resistance. Flex should be pulled taut and supported at intervals, not draped across joists like bunting.

5. 5

   Sealing and insulation

   Joints are sealed with mastic or approved tape - not standard cloth duct tape, which is not rated for the job and fails. Duct running through an unconditioned attic, crawlspace or garage needs insulating to code, because uninsulated duct in a 130°F attic gives away much of what you paid to cool.

   Watch forSealing quoted as an afterthought, or omitted. Sealing existing ductwork is frequently the highest return per dollar available anywhere in a home comfort project, and it is often skipped precisely because it is cheap.

6. 6

   Balancing and verification

   With the system running, airflow at each register is measured and dampers are set so the design airflow actually arrives in each room. Static pressure is re-checked against the equipment's rated maximum.

   Watch forNo balancing at all. An unbalanced new duct system can leave you with exactly the uneven temperatures you paid to fix, and the damper adjustments that would solve it take an afternoon.

What does ductwork installation cost?

New ductwork for a house typically runs $3,000 to $12,000 depending on the number of runs, accessibility and material. Partial modification or sealing an existing system is a fraction of that, and in many homes it delivers most of the benefit.

See the full ductwork installation cost guide - by material, size and region →

If the number is the problem, here are the ways people pay for it →