Most homeowners never think about ductwork until a contractor is halfway into the crawlspace with a flashlight. Then they get told one of two things: "we can seal this up" or "this needs to be replaced." Those are very different numbers, and the difference between them is not a judgment call — it is a checklist.
Here is the checklist we run in Federal Way, Kent, Puyallup, Tacoma and everywhere else we work, so you can tell whether you are being sold a repair or a rebuild.
What each job actually is
Duct sealing
Sealing addresses leakage. Air escapes where sections join: at the plenum, at branch takeoffs, at boot connections under floor registers, at old flex clamps that have loosened. The fix is mastic — a thick gray paste brushed over the joint — sometimes with fiberglass mesh across wider gaps, plus UL 181 foil tape and mechanical fasteners where flex meets metal. Cloth duct tape is not a sealing material. It dries out and falls off, faster in a damp crawlspace than anywhere else.
Sealing usually rides along with re-insulating and re-supporting: pulling sagging flex back up onto straps, replacing insulation that has gotten wet, closing off abandoned runs.
Duct replacement
Replacement addresses the system itself — its condition, its sizing, its layout. New sheet metal trunk, new insulated branches, new boots, sometimes a re-designed return path. Ductwork replacement runs $3,000 to $8,000 for a standard home and $4,000 to $10,000 for larger or more complex layouts. Targeted repair or modification costs less, because it is scoped to specific sections rather than the whole house.
The five-question test
Run these in order. Each "yes" pushes you toward replacement.
- Is the duct crushed, torn, or disconnected? A flex run that a plumber stepped on, or a boot that fell off the floor register twenty years ago and has been heating the crawlspace ever since. Sealing does not fix a collapsed duct.
- Is there mold staining or water damage on the insulation? Puget Sound crawlspaces run humid. Wet duct insulation stays wet, and once the liner is contaminated, you are not brushing mastic over that in good conscience.
- Is it flex duct older than 20 to 25 years? Older flex inner liners get brittle and split along the wire helix. You can seal the ends perfectly and still lose air through the body of the duct.
- Is the system undersized for the airflow you need? This is the one people miss. A heat pump moves more air, more of the time, than the old furnace did. Sealing an undersized duct system makes a restriction tighter, not better. Static pressure goes up, the blower works harder, and comfort gets worse.
- Is there suspect tape or wrap on 1950s-70s ductwork? Some older duct joint tape and wrap materials contain asbestos. That is a testing-and-abatement conversation before anyone touches it, not a mastic job.
Zero or one "yes" answers? Seal it. Two or more? Price the replacement honestly before you spend money twice.
Why leaky ducts cost more here than people assume
In a lot of Western Washington homes, the ducts run through an unconditioned crawlspace or attic. Every cubic foot of air that leaks out of a supply duct down there is air you paid to heat, delivered to the spiders. Every cubic foot a leaky return pulls in down there is cold, damp, dusty crawlspace air pushed through your living room.
That second one matters more than most homeowners realize. Return-side leaks are why some houses smell musty when the system kicks on, and why the filter looks filthy a month after you changed it.
Symptoms that point at ducts, not equipment
- One or two rooms never keep up, no matter what the thermostat says
- Dust returns within days of cleaning
- The system runs long cycles but the house feels drafty
- Registers in far bedrooms barely move a tissue held in front of them
- Crawlspace or attic smells show up indoors when the blower starts
If your equipment is newer and you still have these symptoms, spending money on the equipment again will not fix them.
The bundling advantage
The cheapest time to do duct work is while a crew is already in the crawlspace installing something else. Plenum transitions get rebuilt anyway during a changeout. Access is already open. The permit and inspection are already scheduled.
So if you are within a couple of years of replacing a furnace or moving to a ducted heat pump, get the duct scope priced with that job rather than as a separate visit later. It is one mobilization, one inspection, one day of disruption instead of two.
Want a starting number before anyone comes out? The instant estimate tool gives you Good, Better and Best starting prices for a heat pump, mini-split, furnace or AC install in about 60 seconds. Duct scope gets confirmed in person, because nobody can price a crawlspace from a web form.
What an honest duct assessment looks like
When we look at ducts, we are checking five things, and the estimate should tell you what we found in each:
| What we check | Why it decides seal vs. replace |
|---|---|
| What we checkPhysical condition | Why it decides seal vs. replaceCrushed, torn, disconnected sections cannot be sealed |
| What we checkInsulation and moisture | Why it decides seal vs. replaceWet or moldy liner means removal, not patching |
| What we checkSizing and static pressure | Why it decides seal vs. replaceUndersized ducts throttle a new system regardless of leakage |
| What we checkReturn path | Why it decides seal vs. replaceToo few or too small returns starve airflow |
| What we checkLayout and reach | Why it decides seal vs. replaceRooms with no supply need added runs, not sealant |
If a bid says "replace all ductwork" with no explanation of which of those failed, ask. If the answer is specific — "the north branch is separated at the takeoff, the flex to the two back bedrooms is split, and the trunk is one size small for 3.5 tons" — that is a real diagnosis.
What we tell homeowners to do
- Ducts sound, symptoms mild: seal, re-insulate, re-support. Cheapest real fix in HVAC.
- Ducts sound but one room is starved: targeted modification or an added run, not a whole replacement.
- Old flex, moisture damage, or sizing problems: replace the affected sections or the system, and do it during your next equipment install.
- Suspect asbestos wrap: stop, test, abate. Nobody should be brushing mastic on that.
Any utility incentive for duct work depends on your program and, critically, on the utility that bills your meter — not your county. Rebate money in our area is aimed primarily at qualifying heat pump installations. Programs change, so we confirm your current eligibility and handle the paperwork.
Get a real answer about your ducts
We pull the permit, we crawl the crawlspace, and we tell you which of the five checks your system failed — including when the honest answer is "seal it and keep your money."
Call (253) 785-4622 or request a free in-home estimate. Owner-operated out of Federal Way, licensed VARSIHC766RJ, 10-year parts and labor warranty on everything we install. Mon-Fri 8-6.