Nobody wants to go under the house. That is exactly why crawl space ductwork is the most neglected part of a Puget Sound heating system — and the reason we regularly find a supply run lying on the vapor barrier, dumping paid-for heat into a 45-degree crawl space while the homeowner upstairs is wondering why the back bedroom never warms up.
Here is what actually goes wrong down there, how to spot it from inside your house, and what the fix realistically costs.
Why Puget Sound crawl spaces eat ductwork
Most homes in Federal Way, Kent, Puyallup, Lakewood and Olympia sit on a vented crawl space with a dirt floor and plastic sheeting over it. That is a cool, damp, unconditioned space — and duct systems age faster in it than they do in a dry attic.
Four things do the damage:
- Moisture. Damp air corrodes sheet metal and takes the R-value out of fiberglass duct wrap. Wet insulation does not insulate; it just holds water against the metal.
- Age. Older flex duct — the mylar and wire kind — gets brittle. The inner liner tears, the outer jacket splits, and it sags between hangers until air has to fight through a series of low spots.
- Traffic. Every plumber, electrician, cable tech and pest inspector who has been under your house crawled over that duct. Crushed elbows and pulled-off boots are usually the fingerprints of somebody else's job.
- Rodents. Duct insulation is excellent nesting material. Rats chew the jacket, then the liner, then move in.
On top of that, a lot of these systems were assembled with cloth duct tape at the joints. It dries out and releases. When we find a duct completely disconnected from its floor boot, taped-only is usually why.
Warning signs you can spot without going under the house
Cold floors and one stubborn room
If a room farthest from the furnace or air handler never reaches setpoint while the rest of the house is fine, air is being lost on the way there. A cold floor right above the run points the same direction.
The crawl space smell shows up in the registers
That musty, earthy smell means the return side is pulling air out of the crawl space instead of out of your house. Return leaks are worse than supply leaks because they change what you breathe — not just what you pay for.
Dust comes back fast
Dust you just wiped reappearing in days is a classic sign of leaky returns drawing in crawl space and wall-cavity air.
Bills climbing with no change in habits
A duct system leaking a meaningful share of its air makes the equipment run longer for the same result. The thermostat still gets satisfied eventually. Your bill records the effort.
The system runs constantly in cold weather
On a 25-degree morning, long runtimes are normal. Nonstop runtimes with rooms still short of setpoint are not.
What the repair actually involves
There is no single "duct repair." There are four scopes, and honest pricing depends on which one your house needs.
| Scope | What it means | Typical outcome |
|---|---|---|
| ScopeReconnect and seal | What it meansDuct fell off a boot or a plenum joint let go. Refasten mechanically, seal with mastic or UL-181 foil tape. | Typical outcomeBiggest comfort gain per dollar. Cheapest fix available. |
| ScopeRepair one or two runs | What it meansA crushed elbow or a torn section gets replaced; the rest of the system stays. | Typical outcomeWell under full replacement cost. Often same-day. |
| ScopeRe-support and re-insulate | What it meansSagging runs get properly strapped and rewrapped where insulation is wet or missing. | Typical outcomeRestores capacity and cuts standby losses. |
| ScopeFull replacement | What it meansFlex duct 20 to 25+ years old, widespread damage, or a layout that never had enough return. | Typical outcome$3,000 to $8,000 standard markets; $4,000 to $10,000 for larger or more complex layouts. |
Custom ductwork work — commercial-grade sheet metal, fabricated for your house rather than pulled off a shelf — is the part of a heating system nobody sees and everybody feels. A full replacement typically takes 2 to 4 days. Simple modifications run 1 to 2 days and are often bundled into an equipment install so the crew is only under your house once.
Repair or replace: the honest dividing line
We would rather fix your ducts than sell you a system. Repair is the right answer when:
- The damage is isolated to one or two spots you can point at.
- The trunk lines are sheet metal and structurally sound.
- Insulation is dry everywhere except where the damage is.
- The system was reasonably sized and laid out to begin with.
Replacement earns its price when:
- Old flex duct is brittle and tearing in multiple places — patching it becomes a subscription.
- Insulation is saturated across the system, not just in one bay.
- Runs are undersized or the house never had adequate return air, so no amount of patching fixes the room that is always cold.
- You are installing a new heat pump and the ducts cannot move the airflow the new equipment needs.
That last one matters more than people expect. A heat pump moves a larger volume of cooler air than a gas furnace does, so duct restrictions that a 1990s furnace bulldozed through will show up immediately as weak airflow and disappointing comfort. Sizing math only works if the ducts can deliver what the load calculation asks for — the same reason we run a real load calculation on every estimate instead of guessing from square footage. Our full cost breakdown for equipment is in the heat pump cost guide for Washington.
What to ask any contractor who bids duct work
- Did you go into the crawl space? A duct bid written at the kitchen table is a guess.
- What is being replaced versus repaired? The scope should be listed run by run, not as one lump line.
- How are joints being sealed? Mastic or UL-181 foil tape with mechanical fasteners. Not cloth tape.
- What R-value is the new insulation, and how is it supported? Duct hangers at proper spacing keep flex duct off the ground and out of standing water.
- Is a permit being pulled? Washington requires a mechanical permit for most duct replacement and modification work. We pull it on every job.
What it costs to wait
Duct damage does not fail dramatically. It just quietly taxes every heating hour, all winter, every winter, while the rest of the house gets blamed for it. Homeowners spend thousands on new equipment and then discover the cold bedroom is still cold — because the problem was never the furnace.
Want a starting number before anyone visits? The instant estimate tool gives Good, Better and Best pricing for equipment in about 60 seconds. Duct scope gets confirmed at the free in-home visit, because that answer lives under your floor.
Talk to people who will actually crawl under there
Varsity Heating and Cooling is owner-operated out of Federal Way, serving 45 cities across King, Pierce, Thurston and Kitsap counties. We inspect duct condition as part of every estimate, price repair honestly against replacement, and back installations with a 10-year parts and labor warranty.
Call (253) 785-4622 — that is tel:+12537854622, Monday through Friday, 8 to 6 — or request your free estimate and we will tell you what your ducts actually need.