Not for the concealed run — that must be smooth-wall rigid metal. A short listed transition duct between the dryer and the wall is permitted, but it cannot pass through a wall or ceiling.
Published · Updated · by Dryer Vent Cleaning For You
Metal not vinyl, tape not screws, and a length allowance most second-floor laundry rooms quietly exceed. The rules that actually matter.
Colorado jurisdictions adopt versions of the International Residential Code, and while the adopted edition and local amendments vary from county to county, the dryer exhaust provisions have been broadly stable for years. These are the rules that come up on almost every job.
This is a plain-English summary to help you understand your own house — always check with your local building department for the edition and amendments that apply to you.
Duct material
Exhaust ducts must be smooth-wall rigid metal, minimum 4 inches in diameter, with joints running in the direction of airflow. Vinyl (the white plastic hose) and thin flexible foil are not acceptable. They sag, they tear, they trap lint in every ridge, and they burn.
If you have vinyl or foil duct in a wall, ceiling or crawlspace, that is worth replacing regardless of how clean it currently is.
The transition hose is different
The short flexible section connecting the dryer to the wall duct is a transition duct, and it has its own rules: it must be listed and labelled to UL 2158A, limited to about 8 feet, and it must not pass through a wall, floor or ceiling. It should be visible and accessible for its whole length. Semi-rigid metal is the sensible choice.
Maximum length and elbow deductions
The default maximum is 35 feet from the dryer connection to the outlet, reduced for every bend. Typical deductions:
- 45-degree bend — deduct about 2.5 feet
- 90-degree bend — deduct about 5 feet
So a run with three 90-degree elbows has an effective allowance of roughly 20 feet of straight duct. Manufacturer instructions can be more restrictive than the code, and where they are, the manufacturer wins. This is exactly why long second-floor laundry runs need more frequent cleaning — many of them are at or beyond the practical limit.
If your duct is long enough to be marginal on paper, treat six-monthly cleaning as the baseline rather than annual.
No screws inside the run
Joints must not be fastened with screws or any fastener that extends into the duct. A protruding screw tip is a lint hook, and a lint hook becomes a blockage. Foil tape and external clamps are the correct method. We find screwed joints regularly in older Englewood and south Denver homes where the laundry was retro-fitted.
Termination
- The duct must terminate outside the building — never into an attic, crawlspace, soffit or garage
- It must have a backdraft damper, and must not have a screen at the terminal
- It needs adequate clearance from openable windows, doors and air intakes
That “no screen” provision is in the code for the same reason we argue against retro-fitted mesh: screens trap lint and defeat the vent.
Where homes commonly fall short
- Vinyl or thin foil duct concealed inside a wall or ceiling
- Screwed joints in the concealed portion of the run
- A duct that terminates in the attic or crawlspace rather than outside — the source of a lot of mysterious moisture problems
- A transition hose crushed behind a dryer that has been pushed too close to the wall
- A terminal fitted with an aftermarket screen
None of these require a permit to put right, and most are quick. See dryer vent repair and replacement for what each one involves.
Not sure what is behind your wall? A camera inspection with a written report tells you exactly what you have — $79, waived when booked with a cleaning.
Call (720) 545-5983Request a quoteNeed this done rather than explained?
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Good Questions
Related Questions
Is flexible foil dryer duct allowed by code?
What is the maximum dryer vent length?
Generally 35 feet, reduced by about 5 feet for each 90-degree bend and 2.5 feet for each 45. Manufacturer instructions may be stricter and take precedence.
How often should a dryer vent be cleaned?
For most households, once a year. If you have a large family, pets that shed, or a duct run longer than about 15 feet, plan on every six months. Homes where the dryer vents through the roof also tend to need more frequent service.
What are the signs that my dryer vent is clogged?
Clothes still damp at the end of a normal cycle, drying that needs two or three cycles, a dryer that is hot to the touch, a burning or musty smell, a laundry room that gets hot and humid, lint gathering around the outside vent hood, or a vent flap that no longer opens when the dryer runs.
