Bath, Kitchen & Dryer Exhaust
~10 min read · Confirm every exhaust terminates outdoors — and why attic termination is a defect.
Every exhaust fan has one legitimate destination: outdoors. Bath fans into attics, kitchen recirculators pretending to vent, and dryer ducts of vinyl accordion are the findings — each a moisture, grease, or lint story with a known ending.
Bath fans
Bath exhaust must duct to the exterior — through roof or wall caps with dampers — never into attics, soffit cavities (where soffit intake vents suck it right back), or 'toward a vent.' Attic-terminating bath ducts paint their own finding: a frost/mold bloom on sheathing above the duct's end. Riders: duct length/kinks (smooth, short, insulated in cold attics to prevent in-duct condensation raining back), damper function, fan actually moving air (tissue test), and switching (timer/humidistat upgrades noted).
- Outdoors only — attic/soffit termination is the classic defect
- Insulate cold-attic runs; keep them short and smooth
- Mold bloom above the duct end tells the story
Kitchen and dryer
Kitchen: ducted range hoods exhaust grease-laden air out through smooth metal (grease + long duct = fire load; recirculating hoods filter-and-return — legitimate appliances but noted as non-exhausting). Dryer: the duct rules are firm — smooth rigid or semi-rigid METAL, short with few bends, cleanable, terminating outside with a damper and no screen (lint mats on screens); plastic/vinyl flex duct is a fire finding (lint ignites; flex traps it), and foil accordion is barely better; terminations into crawls/attics/garages dump lint and gallons of moisture per load. Lint accumulation at terminations and behind the appliance is the maintenance evidence.
- Dryer: smooth metal, short, damper, screenless outside
- Vinyl/plastic dryer duct = fire finding
- Interior terminations = moisture + lint dumps
System interactions
Exhaust math meets house physics: powerful hoods and fans depressurize tight homes and can backdraft natural-draft appliances (the makeup-air conversation for big range hoods); multiple exhausts vs available infiltration is a note, not a calculation, at inspection level. Whole-house and radon fans get destination-and-function checks in the same spirit: air taken from the house must exit the building, and something must replace it.
Worked example
The upstairs bath fan's flex duct snakes 20 feet across a cold attic and stops a foot short of a gable vent; sheathing above its end wears gray-black bloom; the dryer below vents through white vinyl accordion into the crawl space, where lint drifts like snow; and the new 900-CFM range hood makes the water heater's draft hood spill when both run. Report the exhaust systems.
Bath: 'toward the vent' is not 'outside' — the duct exhales into the attic; cold-attic condensation plus moisture feeds the bloom (mold finding + duct correction: shortest route through roof/wall cap, insulated). Dryer: two findings in one — vinyl accordion (fire-material finding) and crawl termination (moisture + lint bed = biological growth and fire load); replace with smooth metal to an exterior screenless damper. The hood: 900 CFM in a tight house out-pulls the water heater's natural draft — spillage is CO entering the house on laundry day; makeup-air provision referral. Three fans, three violations of the same one-line rule: house air, once exhausted, leaves the building — and its replacement must not come down the flue.
Common exam pitfalls
Accepting 'near the vent' terminations.
Pointing at a gable or soffit vent is attic-dumping — ducts connect to exterior caps, physically.
Passing vinyl dryer duct as common practice.
Common and fire-rated are different — plastic flex is a named fire finding; smooth metal is the standard.
Ignoring depressurization from big hoods.
High-CFM exhaust can backdraft combustion appliances — spill test with the hood running; note makeup air.
Outside means outside: metal for lint, caps for steam, and makeup air before the flue runs backwards.
Recap
- Bath fans: exterior caps, insulated short runs, never attics/soffits
- Mold blooms mark attic terminations
- Dryers: smooth metal, screenless exterior damper; vinyl = fire finding
- Kitchen: ducted grease out via metal; recirculators noted
- Big exhausts can backdraft appliances — makeup-air note
- Exhausted air leaves the building, full stop
Prove it: 10 questions on this topic
Every lesson ends with a ten-question check in the free course — your progress syncs between the web and the EstatePass app.
Studying for the National Home Inspector Examination? Track every lesson free — progress syncs with the app.
Start freeMore in Insulation & Ventilation
Study smarter in the free dashboard
- Every lesson tracked, synced with the iOS app
- Ten-question checks after each lesson
- Lesson videos, flashcards and mock exams
No credit card required.