Dryer Vent Ducting

Dryer exhaust duct moves hot, moist air and lint out of the house. During a normal drying cycle, up to a gallon of water may be drawn out of the clothes in the form of water vapor, and the purpose of the vent system is to transport that water vapor, and the lint that accompanies it, to a safe location outside the home. The single decision that matters most: material. Get it wrong and you’ve built a fire hazard into the wall.

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Here’s the short version. Use 4-inch smooth-wall rigid metal — galvanized steel or aluminum — for the main run. Dryer exhaust must vent to the exterior of the building, not into attics, crawl spaces, or interior walls. Skip the foil and vinyl entirely. Those aren’t shortcuts. They’re the reason houses burn.

The stakes aren’t theoretical. Statistics from the Consumer Product Safety Commission tie tens of thousands of house fires every year to faulty clothes dryer vent installations. Most of those trace back to bad material choices and lint that never had a chance to clear.

Why Material Is Everything

The wrong duct is the most common mistake and also the most dangerous. The most commonly seen improper type of dryer vent is flexible vinyl tubing — a type of plastic that can easily melt and lead to a house fire. This material, most often white and ribbed, tends to allow lint to readily accumulate, and lint is very flammable.

Mylar foil fools people because it’s shiny. Another improper material routinely installed is mylar foil tubing — a flexible ribbed shiny tubing that homeowners and contractors wrongly assume is metal because it’s shiny. Mylar foil tubing is not approved for use as a clothes dryer vent material.

PVC is no better. PVC is not approved for venting a clothes dryer and should not be used — PVC pipe can allow a static charge to build up, and this static charge can ignite the dryer lint leading to a fire.

What the code actually requires is specific. The IRC requires that clothes dryer vents be constructed of at least 0.016-inch thick rigid metal, have smooth interior surfaces, and shall not have sheet metal screws extending into the duct. The vent should meet the UL 2158A standard. Smooth wall matters because corrugation grabs lint and corrugation is exactly what foil and ribbed tubing give you.

Between the two acceptable metals: 26 or 24 gauge galvanized steel is the better choice because at some point you’ll need to clean out the ducts, so you want something that can withstand the abuse. Aluminum is the rust-free alternative — aluminum is available, which would eliminate any rust potential — though it runs thinner than steel.

Sizes and Components

Most dryers run 4-inch duct, and that’s the standard to build around. The pieces fall into three jobs: the transition behind the dryer, the rigid main run, and the exterior termination.

Component	What it does	Notes
Transition duct	Connects dryer to rigid wall run	UL-listed only, 8 ft maximum, never inside walls
Rigid main run	Concealed run through wall/ceiling	4-inch smooth-wall galvanized steel or aluminum
Wall cap / hood	Exterior side-wall termination	Metal, louvered or backdraft damper, no screen
Roof vent	Exterior roof termination	Purpose-built dryer roof cap, damper, no screen

The transition duct is the one exception where listed flexible metal is allowed — and only barely. A few manufacturers of mylar foil tubing have obtained a UL listing; these products specifically should be used as the transition duct ONLY between the dryer and the actual rigid metal dryer vent, not the full dryer vent. Keep it short. The transition duct should be as short as possible and no longer than 8 feet, and it must not run within a wall, floor, or ceiling covering since it will not be able to be visually inspected and can’t easily be cleaned.

Western Building Center’s dryer vent lineup covers these jobs across common sizes. The selection includes 4-inch aluminum 90-degree dryer elbows and a 3-inch version, offset 90-degree wall elbows in 4.5-inch, metal periscopes in 0-to-18-inch and 18-to-29-inch ranges for tight clearances behind the dryer, plus galvanized, white, and brown louvered vent hoods in 4-inch and 6-inch. There’s a brown replacement vent cap, a white louvered replacement cap, an over-the-top 5-piece kit, and a heat keeper vent kit. Roof termination caps come in metal options up to 12-inch diameter for the runs that have to go up.

Installation: Length, Joints, and Pitch

Even perfect material fails if the routing is sloppy. Shortest, straightest run wins. Best practice is the shortest, straightest route to an exterior wall or the roof, keeping vent runs under 35 feet whenever possible.

Every bend eats into that budget. For every 45 or 90 degree bend, you deduct length: subtract 2.5 feet for each 45 degree elbow and 5 feet for each 90 degree elbow. Two 90s and you’ve burned 10 feet before the first straight section.

Joints are where the rookies go wrong. Tape up all of the joints with UL approved tape. Do not use sheet metal screws. The reason is simple — sheet metal screws penetrating into the material can allow lint to get caught on the screws and possibly clog the vent over time. Foil tape or metal clamps, every time.

Pitch the horizontal runs so condensation drains out, not back into the dryer. All horizontal runs should pitch 1/4-inch per foot to the outside, which helps avoid moisture from sitting in the pipe or draining back to the dryer. A small detail that pays off through a Montana winter, when moist exhaust hits cold duct and wants to condense.

Where It Ends: Wall vs. Roof

Termination is half the job. The duct has to dump outside — venting a clothes dryer into a garage, basement, attic, or anywhere else inside the home can lead to excessively high humidity levels, mold, and an increased fire risk.

The exterior cap needs a damper and no screen. Install a backdraft damper to prevent pests and debris from entering, and avoid using screens or mesh covers as they collect lint and violate most codes. A screen seems like a smart idea until it’s packed solid with lint. For optimal airflow, the vent cap should be made of metal, include a damper, and be located at least 12 inches off the ground.

Placement keeps it clear of trouble. Dryer ventilation systems should only terminate to the exterior and have a proper dampered exterior cover to help prevent water, birds, and insects from entering — and the exterior cover should not have a screen since it will cause lint build up and block the vent over time. Keep it away from doors, windows, and the AC unit, and somewhere snow and dirt won’t bury it.

That last point hits hard in snow country. Side-wall termination beats roof termination when you can get it, and the reasoning is plain. The roof is a poor spot to terminate clothes dryer vents because it’s difficult to clean and it’s a major contributor to ice dams — clothes dryers should be vented through the side of the house, preferably within reach from the ground.

Here’s the ice dam mechanism, which any Montana builder already understands intuitively: when clothes dryers vent through the roof, they melt a bunch of snow, and after the snow melts below the vent it’ll probably freeze again farther down on the roof — and if this happens enough, an ice dam will form that’s large enough to cause roof leaks. A dryer pushes warm, wet air constantly. That’s a snow-melting machine pointed at your shingles.

Cleaning access seals the argument. The terminals for clothes dryers need to be cleaned on a regular basis — terminals covered with lint cause the dampers to be stuck open. A wall cap you can reach from a stepladder gets cleaned. A roof cap two stories up does not.

When the roof is unavoidable — second-floor laundry rooms force this more often than not — use a vent built for the job, not a generic cap. Purpose-built dryer roof vents like the DryerJack address the airflow and cleaning problems. Shorter duct runs created by venting straight through the roof can improve drying efficiency and lessen lint buildup, and the DryerJack is engineered specifically for dryers to meet or exceed code requirements. One model is built with snow zones in mind: Model 477 fits metal, low-profile tile, and asphalt shingle roofs with pitch between 3/12 and 12/12, while the Model 486 is the extra clearance version, taller for higher profile tile or moderate snowfall zones. And whatever goes through the roof gets flashed right — seal the penetration to keep water out.

Frequently Asked Questions

What size dryer duct do I need? Four inches is the standard for most dryers. Vent material should be 4 inches in diameter for most standard dryers. Build the entire run — elbows, periscope, cap — around that diameter so airflow stays consistent.

Can I use the flexible foil duct that came with my dryer? Only as a short transition behind the dryer, and only if it’s UL-listed. The transition duct should be as short as possible, no longer than 8 feet, and must not run within a wall, floor, or ceiling covering. The concealed run through the wall must be rigid metal.

How long can the duct run? Keep runs under 35 feet, and subtract 2.5 feet for each 45 degree elbow and 5 feet for each 90 degree elbow from that total. Always confirm against the dryer manufacturer’s manual, since that figure can vary.

Why no screen on the vent cap? Lint. If you’re venting a dryer, the cap should not be ordered with a screen — a screen can catch the lint from your dryer causing a fire hazard. Use a louvered hood or backdraft damper instead.

Steel or aluminum for the main run? Both meet code. Steel takes cleaning abuse better — 26 or 24 gauge galvanized steel is favored because at some point you’ll need to clean out the ducts and you want something that can withstand the abuse. Aluminum trades some strength for rust resistance.

Should I vent through the roof or the wall? Wall, whenever the layout allows. Roof terminations are difficult to clean and a major contributor to ice dams. A ground-accessible side-wall cap gets cleaned and won’t melt snow onto your shingles. If a second-floor laundry forces a roof run, use a dryer-specific roof vent and flash it properly.

Can I screw the duct sections together? No. Tape the joints with UL approved tape and do not use sheet metal screws, because screw points snag lint and clog the duct over time.