How to Replace Flex Ductwork: Attic and Crawl Space Runs

Replacing flex ducts yourself: $1,120–$1,518 in duct, mastic and fittings, 14–24 hours in the attic.

Before you start

Read the cost comparison first. This guide is for one job: replacing flex duct with flex duct, size for size, on the existing layout, in an attic or crawl space you can work in. That’s where the saving is — about $3,200 on a 1,500 sq ft house — and where the skill required is patience rather than trade knowledge.

It isn’t for a sheet metal trunk, for ducts inside finished walls, or for a system that has never delivered enough air to some rooms. Those need fabrication or a Manual D design, and a duct the same size as the one that didn’t work won’t work either. Get quotes for those.

What you need

For a 1,500 sq ft house: eight to ten 25 ft boxes of R-8 insulated flex duct in the sizes you measured, take-off collars, a bucket of duct mastic with a brush and mesh tape, UL 181 foil tape, nylon straps or inner-core zip ties, 1.5 in hanger strap, and new boots and grilles where the old ones are rusted. Tools: a strap tensioning tool, aviation snips, a drill, a utility knife, a respirator, headlamp and knee pads. The priced list is on the cost page.

The two things that decide how it turns out

Tension. Flex duct is a wire helix in a plastic liner. Pulled straight it moves its rated air; sagging, kinked or coiled it can lose a third or more. Every run gets pulled tight between its ends, hung every 4 feet, and the excess cut off. That one habit separates a system that works from one that’s worse than what you removed.

Sealing. The plenum collars, the liner-to-collar joints and the boots are where systems leak. Mastic, not tape, on every one, and two straps per connection — liner, then jacket. It’s slow, it’s messy, and it’s the part a contractor’s crew does without thinking. Do it the same way.

Step by step

1. Check the rules and the material

   Call the building department: does a homeowner need a mechanical permit for this scope, and can a homeowner pull it? If the house predates 1980, have the old duct tape and wrap tested for asbestos before disturbing it. Get a duct-leakage test if you're unsure the ducts are the problem — sealing may be all it needs.

2. Map and measure the system

   Sketch every run from the supply plenum to its boot: diameter (measure the inner liner), length, and which room it feeds. Note the returns. Check the plenum for cracks and the boots for rust. Count collars, elbows and boots you'll replace. Buy 10% extra flex, one collar per run, mastic, foil tape, inner-core zip ties or nylon straps, hanger strap, and new boots and grilles where the old ones are rusted or crushed.

3. Set up for the environment

   Spring or fall, early morning. Respirator, coveralls, gloves, headlamp, knee pads. Lay walk boards across the joists in an attic; drop cloths and a creeper in a crawl space. Turn the system off at the thermostat and the breaker. Have a helper — pulling and strapping flex is a two-person job on longer runs.

4. Remove the old runs one at a time

   Cut the straps at the collar and the boot, pull the old duct off, and bag it as you go — flex is bulky and old insulation sheds. Do one run at a time so you never lose track of which collar feeds which room. Scrape old mastic and tape off the collars and boots you're keeping; replace any collar that's dented or corroded.

5. Fit collars and boots

   Screw new take-off collars to the plenum where the old ones were bad, sealing the flange to the plenum with mastic. Check each boot is fastened to the framing and sealed to the drywall or subfloor with mastic or foam. If a boot is being replaced, cut the new one to match the register opening and screw it in from below.

6. Connect and pull each run

   Cut the flex to length plus a foot, no more — extra flex gets snaked and loses airflow. At each end, peel back the jacket and insulation, slide the inner liner 2 inches over the collar past the bead, mastic it, and tension a strap over it. Pull the insulation back over the collar and strap the jacket separately. Then pull the run tight between its two ends before you hang it.

7. Support the run

   Hang the duct from the framing with 1.5 in straps every 4 ft, within a foot of each end, so it sags no more than 1/2 in per foot. Keep bends gentle — no tighter than one diameter — and never let the duct rest on a joist edge, truss web or pipe. Where a run must cross another, the upper one goes over on a saddle, not crushed against it.

8. Seal everything

   Brush mastic over every collar-to-plenum seam, every liner-to-collar connection, every boot seam and any plenum joint you can reach. Use mesh tape over gaps wider than 1/8 in and mastic over the mesh. Foil tape the jacket seams closed. Let it cure before running the system.

9. Test the airflow

   Turn the system on and check every register by hand and, better, with a $30 anemometer or by comparing a sheet of paper's lift room to room. Weak air at one register means a kink, a crushed run or a loose liner. Check the attic or crawl space for any duct moving or whistling under pressure. Then schedule the inspection if you pulled a permit.

What would this cost you? The ductwork replacement calculator prices the DIY version with the tools you already own and a value on your time, next to a pro quote.