Direct answer
Vent a bathroom fan to the outdoors through a dedicated exhaust duct and exterior termination (wall or roof cap). Use smooth metal duct sized to the fan, keep the run short and straight, insulate ducts in unconditioned spaces, slope the duct slightly toward the exterior to shed condensation, and seal all seams. Do not vent into an attic, crawlspace, or soffit where moisture can be drawn back into the home.
Why it matters
A properly vented fan removes moisture and odors, preventing mold, peeling paint, and damaged framing. Building codes require bathroom exhaust to discharge outdoors. Poor venting (into an attic or long undersized duct) can cause condensation, wood rot, and ice buildup in cold climates.
Plan the route
- Choose the shortest path to the exterior with the fewest turns.
- Prefer a sidewall termination if it avoids a long run or working on a steep roof.
- Avoid venting through the soffit. Moist, warm air can be drawn back into the attic through intake vents.
- Verify you can meet the fan manufacturer’s maximum equivalent length (see below) and local code.
Sizing and layout quick specs
- Fan sizing: at least 1 CFM per sq ft of bathroom floor area (minimum 50 CFM). Larger rooms or enclosed showers may need 80–110+ CFM.
- Duct diameter: 4 in for up to ~80 CFM; 5–6 in recommended for 100–150 CFM to reduce noise and backpressure.
- Slope: ~1/8–1/4 in per foot toward the exterior termination.
- Equivalent length (rule of thumb—check your manual): keep total under ~25–35 ft for 4 in duct; 90° elbow ≈ 10 ft (4 in) or 15 ft (6 in).
Materials and tools
- Duct: 26–30 gauge smooth galvanized or aluminum (preferred). Short section of UL 181 listed semi-rigid for difficult alignments ok.
- Exterior cap: wall or roof cap with backdraft damper (no fine insect mesh that clogs with lint).
- Backdraft damper (often built into fan; add inline damper if needed).
- Insulation: R-6 to R-8 duct wrap with vapor barrier for any duct in unconditioned spaces.
- Sealing: UL 181 foil HVAC tape and water-based duct mastic.
- Fasteners: sheet metal screws (#8 x 1/2–3/4 in).
- Support: metal hanging strap or duct supports every 4–6 ft (and within 1 ft of each elbow).
- Sealants: polyurethane or high-quality exterior caulk, spray foam for interior air sealing, roofing sealant for roof penetrations.
- Tools: drill/driver, 4–6 in hole saw, reciprocating saw or oscillating multi-tool, aviation snips, caulk gun, utility knife, tape measure, stud finder, ladder. For roof work: flat bar, roofing nails, hammer, roof cement/ice & water membrane.
- PPE: gloves, eye protection, dust mask or N95 for attic work, fall protection for roof.
Estimated cost: $80–$250 in materials; 3–6 hours DIY depending on access and termination type.
Step-by-step
1) Measure and size the system
- Confirm fan CFM and required duct diameter from the manufacturer. Upsize duct if the run is long or has multiple elbows to keep noise low.
2) Plan the termination
- Sidewall: pick a location at least 3 ft from doors/windows and 10 ft from intake vents when possible. Avoid locations where vapor will stain siding or ice up walkways.
- Roof: choose a spot near the ridge but below it. Avoid valleys. Use a proper roof vent cap with flashing.
3) Mount the fan and start the duct
- Install or replace the fan housing per instructions. Air-seal the housing to the ceiling with caulk or foam to prevent attic air leakage.
- Attach duct to the fan collar: slide on, secure with 3 sheet metal screws, then seal with mastic and foil tape. Ensure an internal or inline damper is present and swings freely.
4) Route the duct
- Run smooth metal duct through the attic or joist bay with the fewest bends. Keep radius of bends large; avoid tight kinks.
- Maintain a slight slope toward the exterior termination to drain condensation.
- Support the duct every 4–6 ft with strap. Avoid letting it rest on sharp framing.
5) Cut and install the termination
- Sidewall: drill a pilot hole from inside, then cut a 4–6 in hole (match your cap). Slide the cap’s duct stub through, bed the flange in exterior sealant, fasten, and caulk the perimeter. Add a short piece of trim flashing if needed for laps in siding.
- Roof: from inside, drill a pilot hole. On the roof, remove shingles around the hole, cut the sheathing, install the roof cap with flashing under the upslope shingles and over the downslope. Seal nail heads and shingle cuts. Follow the cap maker’s instructions closely.
6) Connect, seal, and insulate
- Attach the duct to the cap stub with screws, mastic, and foil tape. Check damper swing.
- Wrap exposed duct in unconditioned spaces with R-6/R-8 insulation, tape seams tight to the vapor barrier.
- Air-seal all ceiling and top-plate penetrations with foam or caulk to stop warm, moist air from bypassing the fan into the attic.
7) Test
- Run the fan. Verify strong airflow with a tissue test at the grille and watch the exterior damper open. Check for leaks at joints. Listen for rattles (often a sign of backpressure or loose fasteners).
Tips for best results
- Oversize the duct (e.g., use 6 in for a quiet 110 CFM fan) to reduce noise and improve performance.
- Use a countdown timer switch to keep the fan running 15–30 minutes after showers.
- In cold climates, choose an exterior cap with a gasketed damper and low profile to reduce wind-driven backdrafts and frost.
- If you must use flex duct for a short section, keep it under a few feet, fully stretched, straight, and supported.
Common mistakes
- Terminating in the attic, garage, or soffit.
- Long runs of 3 or 4 in duct on a high-CFM fan, causing noise and poor flow.
- Too many sharp elbows and no slope, leading to condensation and drips.
- Skipping insulation on ducts in unconditioned spaces.
- Relying only on duct tape (it fails). Use mastic and UL-rated foil tape.
When to call a pro
- Steep, high, or complex roof penetrations (metal, tile, low-slope roofs).
- Multi-story runs, masonry wall penetrations, or limited access attics.
- If you find vermiculite or suspect asbestos, stop and consult an abatement professional.
- If local code requires mechanical ventilation calculations or permits you’re not comfortable handling.
Done right, a properly vented bathroom fan is quiet, moves moisture out fast, and protects your home from condensation damage for years.