What are the correct methods to vent a downdraft or range hood to avoid crawlspace moisture?

Short answer

Never vent a downdraft or range hood into a crawlspace. Run a smooth metal duct to the building exterior—through an outside wall or roof—and terminate with a listed wall/roof cap that has a backdraft damper. If the duct must pass through a crawlspace on the way out, keep it sealed, insulated, supported, and sloped to the exterior. Follow manufacturer specs and local code.

Why crawlspace venting is a problem

Kitchen exhaust carries steam, grease, and combustion byproducts. Dumping that into a crawlspace can cause mold, wood rot, odors, pest attraction, and corrosion. The IRC (M1503) requires kitchen exhaust to discharge outdoors—never into crawlspaces, attics, or garages.

Approved ways to vent a range hood or downdraft

• Through an exterior wall: Shortest, most reliable route. Ideal if the range backs to an outside wall.
• Through the roof: Common for island hoods or when exterior walls aren’t nearby. Use a roof cap designed for kitchen exhaust.
• Through a crawlspace to an exterior wall: Acceptable if the duct is continuous smooth metal, sealed, insulated in unconditioned areas, supported, and terminates above grade outdoors. Do not terminate inside the crawlspace.
• Recirculating (charcoal) mode: Filters odors but does not remove moisture and grease to the outdoors. Not a fix for crawlspace moisture concerns.

Key specs and planning

Follow the hood/downdraft manual first. Where not specified, these best practices usually apply:

Duct material:      Smooth-wall metal (galvanized, stainless, or aluminum), 26–28 ga
Flex duct:          Avoid (only use listed short transitions if allowed by manufacturer)
Minimum size:       Match collar — commonly 6 in round or 3.25 x 10 in rectangular
Do not reduce:      No neck-downs; upsizing is fine, reducing is not
Elbows:             Use long-radius; minimize count (each 90° ≈ 10 ft equivalent)
Max length:         Per manual (often 35–75 ft equivalent)
Slope:              1/4 in per foot toward exterior on horizontal runs
Support:            Every 4 ft with metal strap/hangers; protect from damage in crawlspaces
Sealing:            Foil tape (UL 181) + mastic on seams; no cloth “duct tape”
Dampers:            Backdraft damper at the hood/blower and at the termination cap
Insulation:         R-8 or better in unconditioned spaces to limit condensation
Termination:        Listed wall/roof cap for kitchen exhaust; no mesh screens that clog with grease
Clearance:          Keep away from soffit/attic vents; typically ≥3 ft from doors/windows (check local code)
Makeup air:         Required ≥400 CFM (IRC M1503.6); interlocked, tempered

Step-by-step: Wall termination (most common)

1. Plan the route
   • Choose the shortest, straightest path to an exterior wall. Avoid more than two 90° elbows.
   • Verify duct size equals the hood outlet. Plan for makeup air if CFM ≥400.
2. Mark and cut openings
   • Inside: Trace the duct path and locate studs/joists with a stud finder. Avoid plumbing/electrical.
   • Outside: Choose a termination location that clears trim and utilities. Drill a pilot hole and use a hole saw or reciprocating saw for the wall cap opening.
3. Assemble and dry-fit duct
   • Use snap-lock rigid duct with long-radius elbows. Keep seams accessible for sealing. Maintain a slight slope to the outside.
4. Seal and secure
   • Fasten with sheet-metal screws at joints (keep screws short to avoid protruding into airstream more than necessary). Seal seams with foil tape and mastic.
   • Support every 4 ft with metal straps. In a crawlspace, keep ducts off soil and protected from damage.
5. Insulate (if unconditioned path)
   • Wrap with R-8 duct insulation; tape and mastic the vapor barrier to prevent condensation.
6. Install termination cap
   • Use a kitchen-rated wall cap with integral damper (no grease-catching screen). Bed the flange in exterior sealant and fasten to siding/masonry as appropriate. Flash as needed.
7. Connect to hood/downdraft
   • Install a backdraft damper (if not built-in). Use a short listed transition if needed. Seal connections.
8. Test
   • Run the fan, verify strong airflow at the cap, damper operation, and no rattles. Check for leaks with a smoke pencil.

If routing through a crawlspace

• Use continuous smooth metal duct, fully sealed.
• Maintain slope to the exterior and insulate R-8+ to avoid condensation.
• Penetrations through floors and rim joists should be air-sealed with fire-rated sealant where required.
• Support with metal straps, and keep the duct a few inches above the vapor barrier/ground.
• Terminate above grade on an exterior wall with a listed cap and damper.

Tools and materials

• Tools: Drill/driver, hole saws, reciprocating saw or oscillating multi-tool, aviation snips, tape measure, level, stud finder, caulk gun, ladder, PPE (gloves, eye/ear protection).
• Materials: Rigid metal duct and long-radius elbows, listed wall/roof cap with damper, foil HVAC tape (UL 181), duct mastic, metal straps/hangers, R-8 duct insulation, exterior sealant, sheet-metal screws, backdraft damper (if needed).

Safety and code considerations

• Shut off power to the hood circuit while working. Verify there are no hidden utilities before cutting.
• Roof work requires fall protection; consider a pro for penetrations.
• Do not combine with other exhausts (dryer/bath). Each appliance needs its own duct.
• High-CFM hoods (≥400 CFM) often require an interlocked makeup air kit; without it, backdrafting of water heaters/fireplaces can occur.

Pro tips

• Upsize one duct size for long runs or multiple elbows to reduce noise and static pressure (only if the hood allows upsizing).
• Inline or external blowers can reduce kitchen noise and improve performance on long runs.
• Keep filters clean and check the exterior damper annually; stuck dampers raise static pressure and cause condensation.

Common mistakes to avoid

• Venting into a crawlspace, attic, or soffit.
• Reducing duct size below the hood collar.
• Using corrugated flex duct that traps grease and kills airflow.
• Skipping insulation in unconditioned spaces, leading to dripping ducts and moisture issues.
• Too many tight elbows and long runs without checking equivalent length limits.
• Using mesh screens at the termination (they clog with grease).

Costs and when to call a pro

• DIY materials: $100–$400 (duct, cap, insulation, sealants). Makeup air kits: $300–$800+.
• Pro installation: $400–$1,500 for wall termination; add $300–$600 for roof work; core-drilling masonry can add $250–$500.
• Call a pro if you need a roof penetration, structural modifications (joists/rim), makeup air design, or if gas appliances could backdraft. For island downdrafts crossing joists or slab, consult a contractor to protect structure and meet firestopping requirements.