Bath Fan Duct Run Costs: Length, Terminations, and Shared Runs
The fan unit is the cheap part of the job -- the duct run is where installs get expensive. Rigid duct runs $8 to $14 per linear foot, exterior terminations add $40 to $800 depending on roof, soffit, or wall, and multi-bath shared runs trade one fan against extra branch duct. The duct math decides the quote.
Orange County, CA · Updated 2026 · Costs shown as ranges — your quote will vary by contractor and scope
Duct length pricing: $8-$14 per linear foot
Six-inch rigid galvanized duct runs $8 to $14 per linear foot installed, and flexible insulated duct is a few dollars cheaper per foot. Every foot past the minimum adds material plus the labor to hang it with proper slope and support -- long attic crossings are where a $400 install drifts toward $900.
Shorter is cheaper for reasons beyond materials: every bend adds equivalent length in resistance, and installers price elbow-heavy routes higher. The cheapest duct run is a short, straight shot from the fan to the nearest exterior surface. Bathrooms far from any exterior wall are the classic expensive route.
Termination adders: roof vs. soffit vs. wall
Roof termination is the priciest: a through-roof fan route totals $500 to $1,500 all-in (fan, duct, flashing, roofing labor), and a standalone roof vent runs $300 to $800 depending on the roof material. Tile and steep roofs cost more than comp shingle. The roof cap itself is $40 to $120 with a backdraft damper.
Wall termination is the budget option: a wall cap at $40 to $120 plus a short horizontal run, with far less labor than a roof penetration -- no roofing work, no flashing. Soffit termination sits in between on cost but is restricted in some jurisdictions: some codes bar dumping exhaust into the soffit because the air can recirculate into the attic -- confirm locally before planning around it. Cost ranking: wall cheapest, soffit middle, roof highest.
Multi-bath shared runs: one fan, two bathrooms
Two adjacent bathrooms can share one duct trunk via an inline fan ($100 to $310 unit) or a Y-branch off one fan's run -- the shared trunk saves a second termination and second fan, while each branch adds $8 to $14 per foot of duct plus a grille takeoff. Shared runs make sense when the baths sit back to back with short branches.
The math: a second full fan install is $240 to $550; a shared-run branch is often $150 to $400 in extra duct, fittings, and labor. When the branches are long or the baths are far apart, the shared trunk loses its edge and two fans win on both cost and airflow. Inline fans earn their keep on shared runs by sitting mid-duct where one ceiling fan cannot reach both rooms.
What pushes the duct bill up
Attic access is the silent driver: a walkable attic makes duct runs quick; a low, tight attic -- or none at all -- means drywall cutting, fishing, and patching, which can double the duct labor. Long horizontal runs need more hangers, more sealant, and more time.
Roof type and distance round out the adders: the farther the fan from an exterior surface, the more duct and the more labor. Tile roofs, steep pitches, and multi-story drops to a wall cap all add time. Then the coastal OC labor premium of 10 to 20 percent applies to the labor portion of every adder -- which is why two identical duct routes quote differently by zip code.
Bath fan duct run cost adders (2026)
| Item | Typical range |
|---|---|
| Rigid duct, 6" galvanized | $8 – $14 /linear ft |
| Flexible insulated duct | $5 – $10 /linear ft |
| Termination cap (wall or roof), w/ damper | $40 – $120 |
| Through-roof fan route, total | $500 – $1,500 |
| Standalone roof vent add-on | $300 – $800 |
| Shared-run branch (duct + fittings + labor) | $150 – $400 |
| Inline fan unit (for shared runs) | $100 – $310 |
| Second fan vs. shared branch | $240 – $550 vs. $150 – $400 |
| Duct labor adder, complex route | $150 – $400 |
DIY vs. hiring a pro
The duct run is rarely DIY-friendly: flex duct looks easy but sags, kinks, and loses airflow when hung wrong -- and a bad duct run is invisible until the fan underperforms. Terminating through a wall is the feasible DIY edge for experienced hands: cut the hole, set the cap, seal it. Roof penetrations, flashing, and anything in a tight attic belong to pros. Save the DIY money for the fan swap itself; spend it on a pro for the duct.
What drives the price up or down
- Duct length -- $8-$14/ft adds up fast on long runs
- Termination type -- wall cheapest, roof priciest
- Roof material -- tile and steep roofs cost more to penetrate
- Attic access -- tight or no access means drywall work
- Shared vs. separate runs -- branches vs. a second fan
- Location -- coastal OC labor premium 10 to 20 percent
How to keep the cost down
- Take the shortest straight route to the nearest exterior surface
- Terminate through a wall where the bathroom allows it
- Share a trunk only for back-to-back baths with short branches
- Keep bends few -- every elbow costs money and airflow
Frequently asked questions
How much does it cost to run duct for a bathroom exhaust fan?
Rigid duct runs $8 to $14 per linear foot installed; flexible is a few dollars cheaper. A short straight run adds little; a long attic crossing with elbows can add several hundred dollars in duct, hangers, and labor.
Is it cheaper to vent a bath fan through the roof, soffit, or wall?
Wall is cheapest: a $40 to $120 cap plus a short run, no roofing work. Roof is priciest: $500 to $1,500 for a through-roof route total, or $300 to $800 for a roof vent add-on. Soffit sits in the middle but is restricted in some jurisdictions -- confirm locally.
Can two bathrooms share one exhaust fan duct?
Yes: a shared trunk with an inline fan ($100 to $310 unit) or a Y-branch works for back-to-back baths -- a branch adds $150 to $400 in duct and labor vs. $240 to $550 for a second full fan install. Long branches or distant baths favor two separate fans.
How long can a bath fan duct run be?
From a cost angle, longer runs cost more per foot ($8 to $14) and lose performance per elbow -- which is why installers price elbow-heavy routes higher. The practical advice: keep runs short and straight, and terminate at the nearest exterior surface.
Does attic access change the duct cost?
A lot. A walkable attic makes runs quick; a tight or inaccessible attic means drywall cutting, fishing, and patching, which can double duct labor. No attic access is the single biggest reason a simple install's duct bill surprises.