What's the safest way to reroute a dryer vent to avoid moisture problems and meet code?

Short answer

Reroute your dryer vent using 4-inch smooth rigid metal duct on the shortest, straightest path to the exterior wall, sloped slightly downward toward the outside, with sealed joints (foil tape, no screws), a listed transition duct at the dryer (max 8 ft), and a proper exterior termination with a backdraft damper and no screen. Keep total equivalent length within the dryer manufacturer’s limit (often 35 ft minus elbows), insulate any runs through unconditioned spaces, and never vent into attics, crawlspaces, garages, or soffits.

What code and best practice look like

Dryer vent rules come from your local code (often IRC/IMC) plus the dryer’s installation manual. The stricter rule wins. The common targets:

Duct material: 4" rigid metal, smooth interior, min 0.016" (28-gauge) steel or equivalent
Transition duct: Listed UL 2158A flexible (single length, max 8 ft), at the dryer only
Max equivalent length: Often 35 ft, subtract 5 ft per 90° elbow, 2.5 ft per 45° (or per the dryer manual)
Termination: Outside only, backdraft damper, no screen, 4" opening
Slope: About 1/4" per foot downward toward the exterior termination
Joints: Foil tape (high-temp), no screws penetrating the airstream
Support: Rigid duct supported every 4–6 ft and near elbows

Prohibited: Vinyl/plastic flex duct; venting into attics, crawlspaces, garages, or soffits; screens at termination; reducing below 4"

Planning the reroute

• Choose the shortest, straightest path to an exterior wall. Wall termination is usually preferred over roof to reduce leaks and maintenance.
• Avoid more than two 90° turns if you can. Use long-radius elbows to reduce flow resistance.
• Add up equivalent length: for example, a 20 ft straight run + two 90° elbows (2 × 5 ft) = 30 ft equivalent. That’s typically okay under a 35 ft limit.
• Through unconditioned areas (attic, crawlspace, cold garage), either reroute through conditioned space or plan to insulate the rigid duct to reduce condensation.

Tools and materials

• Rigid 4 in dryer duct (26–30 gauge), long-radius 90° and 45° elbows
• Listed UL 2158A transition duct (flex) for the dryer-to-wall connection
• Exterior dryer vent hood with backdraft damper (no screen), 4 in
• Aluminum foil tape (UL 181 or high-temp rated), stainless/band clamps
• Pipe straps/hangers, screws for strapping, and a few for the termination flange (not inside the airstream)
• Stud finder, tape measure, torpedo level
• Drill/driver, 4–1/4 in hole saw or masonry core bit for masonry walls
• Aviation snips, oscillating multi-tool or jab saw for drywall
• Caulk/sealant (exterior-grade), fire-rated sealant if passing through a rated assembly
• PPE: gloves (sharp edges), eye protection, dust mask/respirator

Estimated cost: $75–$250 in materials depending on run length and whether you add a recessed dryer box or insulation wrap. Time: 2–6 hours for a typical DIY reroute.

Step-by-step guidance

1. Plan and measure
   • Map the route and calculate equivalent length. Confirm it’s within your dryer’s allowance. If not, revise the route before you cut.
2. Prep and safety
   • Unplug the dryer. For gas dryers, close the gas shutoff valve and disconnect the flex connector after relieving pressure. Move the unit out.
3. Cut the termination opening
   • Use a stud finder to avoid framing. Drill a small pilot hole from inside, then cut a 4–1/4 in hole from outside with a hole saw (add a slight downward angle). For brick/concrete, use a core bit and rotary hammer, or hire a pro for coring.
4. Run rigid duct
   • Dry-fit rigid sections with long-radius elbows. Maintain a gentle downward slope toward the exterior—about 1/4 in per foot—so condensation drains out.
   • Join sections with crimped ends in the airflow direction. Tape seams with aluminum foil tape only. Do not use screws inside the duct.
   • Support every 4–6 ft and near elbows using metal straps. Keep the duct 4 in diameter the whole way.
5. Install the wall cap
   • Caulk the flange to the siding, fasten the hood, and seal edges. Ensure the damper moves freely. No screens—lint will clog them.
6. Connect the dryer
   • Install a recessed dryer box if space is tight. Connect a single length of listed UL 2158A transition duct between the dryer outlet and the in-wall rigid duct using band clamps. Keep it as short and as straight as possible.
7. Insulate where needed
   • In unconditioned spaces, wrap the rigid duct with insulation (e.g., R-6 to R-8) to reduce sweating and condensation.
8. Test and verify
   • Restore power (and gas if applicable, leak-check with soapy water). Run the dryer on air-fluff and check for strong airflow at the exterior and full damper opening. Listen for rattles, fix any sags, and confirm joints are tight.

Safety considerations

• Gas dryers: After reconnecting, leak-check with soapy water at fittings. No bubbles = good. Never use an open flame for leak testing.
• Electrical: 240V circuits can be hazardous. Unplug or switch off the breaker before moving the dryer.
• Cutting: Watch for hidden wiring or plumbing in walls. Use PPE—sheet metal edges are sharp.
• Fire-rated assemblies: If penetrating a rated wall/ceiling (e.g., garage separation in some homes, multi-family dwellings), use proper firestopping and follow local requirements.

Tips for best results

• Prefer wall termination over roof. If roof termination is unavoidable, use a dryer-rated roof cap with a damper and flashing, and expect more maintenance.
• Use long-radius elbows and minimize turns to preserve airflow and drying performance.
• Label the duct location or keep a simple sketch for future service.
• Clean the duct annually; more often if you do heavy laundry or have pets. A straight, rigid run makes cleaning easier.

Common mistakes to avoid

• Using plastic or thin foil flex inside walls—these are a fire and lint hazard.
• Terminating into attics, soffits, crawlspaces, or garages—this creates moisture and mold problems and violates code.
• Exceeding max equivalent length—dry times go up and lint risk increases.
• Using sheet metal screws through the duct—lint snags and clogs form. Use foil tape and band clamps instead.
• Allowing sags or negative slope—condensation will pool and leak back toward the dryer.

When to call a professional

• You must penetrate masonry or a roof, or pass through fire-rated assemblies.
• The only feasible route exceeds manufacturer length limits and may require a listed dryer booster fan—these need correct sizing, a pressure switch, and proper access.
• You need to relocate gas lines, modify dedicated electrical circuits, or you’re uncertain about hidden services in walls.

A carefully planned, rigid-metal, short-and-straight reroute with proper termination and slope will keep moisture out of your home, meet code, and help your dryer run efficiently.