Dryer Vent Condensation: Why Water Collects

Where the water comes from

A washer that has finished spinning still leaves a lot of water in the clothes, and the dryer's job is to turn nearly all of it into vapor and push it outdoors. The exhaust leaving the drum is warm and close to saturated. As long as it keeps moving and stays warm, the vapor rides out of the house with it.

Trouble starts when that air touches a surface colder than its dew point. The vapor turns back into liquid on the inside of the duct, exactly the way a glass of iced water sweats on a summer day. The colder the duct and the longer the air spends inside it, the more water drops out before the air reaches the hood.

Why winter and cold spaces make it worse

In a heated room the duct sits at roughly room temperature, and little water forms. Once the duct passes into an unheated attic, a crawlspace, an attached garage or a cavity against an outside wall, its surface drops toward the outdoor temperature. On a freezing day, the exhaust cools fast and drops its moisture along the way.

Roof terminations are the most prone. The air has to rise, it slows as it climbs, and water that condenses near the top runs back down the duct toward the dryer. Short runs straight through a wall near the machine rarely have much trouble, because the air is outside before it has time to cool.

Routes in older houses where we see it

Flat-roofed rowhouses in New York and Philadelphia often send the dryer duct up through the top floor into the cockloft, the shallow unheated space between the top ceiling and the roof deck, and out through a roof cap. That space can be close to outdoor temperature all winter. Water forms there, runs back along the duct and shows up as a stain on the top-floor ceiling.

Basement laundries bring a different version. A duct that runs along a cold stone or block foundation wall, or exits under a front stoop after a long horizontal stretch, cools along its length. Any section that dips between hangers becomes a trap where water sits.

Signs of a condensation problem

Most people notice the result before the cause. These are the usual clues, roughly in the order they appear.

• Damp, clumped lint in the hose or at the hood instead of dry fluff
• Water dripping from duct joints or from the bottom of the transition hose
• Stains on a ceiling under an attic or cockloft duct run
• Rust streaks on metal duct or on the outside hood
• Water pooling in the drum or around the back of the dryer
• Icicles or frost around the outside hood on cold days

Rain or condensation?

Not every drip is condensation. A broken hood or a roof cap without a working flap lets rain straight into the duct, and a roof leak around a roof cap can look much the same from below.

Timing is the easiest way to tell them apart. Condensation appears on cold days while the dryer is running or shortly after, collects at joints, sags and the low end of the run, and leaves the lint damp and matted along the whole duct. Rain shows up during or after storms whether or not the dryer has run, concentrates near the hood or roof cap, and wets the lint mostly near the outlet. The first calls for sealing, pitching, supporting and insulating the duct; the second for a new hood or cap, or flashing repair around the roof opening.

Why it matters

Wet lint sticks to the inside of a duct far more readily than dry lint. Condensation speeds up clogging, a clogged duct slows the air, and slower air leaves more time for the exhaust to cool and drop water. The cycle feeds itself until drying times climb and the duct is packed with a damp felt.

Water that escapes from joints stains plaster, soaks insulation and can lead to mold and rot in framing. Water sitting in a sag rusts the duct from the inside until it perforates, and then the leak carries lint into the ceiling cavity too. On a gas dryer, a duct choked by wet lint can also push exhaust gases back toward the laundry room, one more reason to keep carbon monoxide alarms working.

How urgent is it?

A few drops at the hose on the coldest mornings, with dry lint everywhere else, is a minor problem worth fixing before next winter. Water reaching a ceiling, a sag holding standing water, or lint that comes out of the hood as wet clumps is more pressing, because the duct is clogging faster than it should and the water is already doing damage somewhere.

Stop using the dryer and get the vent looked at if the duct has come apart at a joint, if water is dripping onto electrical fixtures or wiring, or if drying times have jumped along with the water. Those combinations usually mean a heavy, wet blockage.

What you can check without opening anything

Exposed duct in a basement or laundry room is easy to look at. Run a load on a cold day and watch the joints and the bottom of the hose for drips. Check whether any stretch of duct sags between its hangers, and whether joints are held with screws, which poke into the air stream and catch lint.

Look at the ceiling under any concealed run for brown rings or soft spots, and look at the outside hood from the ground for rust streaks, ice or a flap that stays open. Leave roof caps, cockloft access and opening ceilings to someone with the right gear.

How it gets fixed

The fix starts with the route. A shorter run to a closer exterior wall leaves less time and less cold surface for water to form, and it is worth asking whether one exists before patching a long run. Where the route has to stay, the duct should be smooth rigid metal with joints sealed by foil tape rather than screws, supported so it cannot sag, and pitched so any water that forms drains toward the outside instead of pooling or running back to the dryer, following the dryer maker's instructions.

Sections that cross attics, cocklofts, crawlspaces or garages are commonly wrapped with insulation to keep the air warm until it leaves. The hood or roof cap should be one made for dryer exhaust, with a flap that opens fully and closes against rain. The duct also has to be cleaned, since a run full of wet lint keeps the cycle going.

The technician starts at the dryer, disconnects it and looks at the transition hose for trapped water and crushed spots, then follows the route as far as it can be seen, noting sags, screws, unsealed joints and any section crossing cold space. The run is cleaned from the dryer to the outlet, wet lint plugs are pulled out rather than pushed further along, and the hood or cap is cleared and checked for a working flap. You get a list of what would stop the water on your particular route, from a few hangers and foil tape to insulation or a new exit through a closer wall, and you choose what to do.