Mobile Home Dryer Vent: It Has to Exit the Skirting
If your clothes dryer discharges into the crawlspace under your manufactured home rather than out through the skirting, that is not normal and it is not harmless. The federal model installation standard, 24 CFR 3285.505, requires the dryer exhaust to be vented to the outside of the skirting. A dryer puts several pounds of water into the air with every load, and lint travels with it. Both stay under there.
This is one of the few problems in this whole subject you can check in about two minutes, from outside, without touching anything.
The check, before any of the explanation
Run the dryer. Walk outside. Then look for the exhaust in this order.
Look along the skirting for a hood, flap or louver at the height of the dryer's wall. A correctly terminated dryer vent ends outside the skirting in a fitting with a movable flap, and when the dryer is running the flap is open and you can feel warm, damp air coming out of it. If you find that, you are done.
If you find nothing, feel for warm air at the skirting joints and vents nearest the laundry area. Warm, humid air pushing out through the general skirting vents, rather than out of a dedicated hood, means the duct is discharging into the crawlspace and the air is finding its own way out.
Open the skirting access door and look, without going in. From the doorway, on a dry day, you are looking for a flexible duct that ends in mid air, a duct lying disconnected on the ground, or a visible drift of lint on the ground sheet or the frame. You are looking only. Do not crawl in to investigate. The next section explains why that limit is not a formality.
Look at the ground under the laundry end of the home. Persistent damp soil under one end, when the rest of the crawlspace is dry, is a familiar sign of a duct discharging under there.
If any of those come back positive, the fix is real work and the rest of this article is about who does it and what they need to know.
Where you stop, and why this one has a hard line
The space under a manufactured home is not an attic. It holds the water supply lines, the drain lines, the electrical crossover between sections, the heating ducts, and on many homes the gas line to the furnace or water heater. It is a low, dark, restricted space with limited routes out.
Reworking or reconnecting a dryer duct under there is work for a licensed manufactured-home installer, or for an HVAC or appliance contractor who works on manufactured homes. This article will not give you a procedure for doing it yourself, because a procedure for that job is a procedure for putting an untrained person in a confined space alongside a gas line and live wiring, and no amount of care in the writing changes that.
There is one more reason, and it is specific to this repair. A duct that has been discharging into a crawlspace for months or years has been depositing lint the whole time. Lint is not dust. It is fine, dry fabric fiber and it is combustible, and it accumulates in exactly the places you cannot see from the access door. Disturbing an accumulation like that, in a confined space, with a light in your hand, is the wrong way to find out how much of it there is.
If you find a disconnected or missing duct, close the access door and call someone. That is the whole of the homeowner's job on this one.
What the installation standard actually says
The federal model installation standard for manufactured homes, 24 CFR 3285, is the document behind the requirement. Section 3285.505 is the crawlspace ventilation section, and it is the same section that sets the venting rule most owners have heard a garbled version of.
Two things live in it that belong together.
The dryer exhaust is required to be vented to the outside of the skirting. It is not permitted to terminate in the crawlspace.
Crawlspace ventilation is set as an area ratio, not a vent count. The minimum net area of ventilation openings is one square foot for every 150 square feet of the home's floor area, dropping to one square foot per 1,500 square feet where a vapor retarder covers the ground under the home, with openings on at least two opposite sides and as high above the ground as practicable.
The reason both sit in the same section is the reason this article exists. Crawlspace ventilation is designed to carry away the moisture that gets under a home in the ordinary course of things: ground moisture, humid outside air, the odd small leak. It is not designed to carry away a laundry appliance. Putting a dryer under there is not a small addition to the load. It is a category change, and no amount of correctly sized venting is the answer to it.
One caution about the standard, and it applies to everything in this cluster. 3285 governs how a home is installed. What happens to it afterward is governed by your state's manufactured housing installation program, your county or city building department, and, if you are on a leased lot, the park. The federal text tells you what the correct condition is. It does not tell you who inspects it on your lot or what your local rule adds. That part you look up.
The two problems, and they are different problems
People who find this in their own home usually understand one of these and not the other.
The moisture problem
A clothes dryer is a machine for turning water in fabric into water in air. All of that water leaves in the exhaust stream. Directed outdoors, it disappears into the weather. Directed into a crawlspace, it condenses on the coldest surfaces it can reach, which under a manufactured home are the metal chassis members, the ducts and the underside of the floor.
That is the same condensation cycle that produces the classic complaint of a frame dripping in dry weather. The difference is that a dryer is a recurring, scheduled, high-volume source. Ordinary crawlspace moisture problems build slowly. This one arrives in loads.
What it does over time is not dramatic and that is why it is missed. Wet insulation in the underbelly loses most of its value and stays wet. The bottom board sags and eventually tears. Subfloor near the laundry end goes soft. Every one of those gets diagnosed as a leak, because a leak is what people look for, and the actual source is running twice a week in the hallway.
The lint problem
Dryer lint is combustible and a dryer exhaust stream carries a steady supply of it. Vented outdoors, it disperses. Vented into a crawlspace, it settles, and it goes on settling.
We are not going to publish a figure for how much, how fast, or how many fires this causes, because we have not opened a source in this session that we can quote and date, and a number we could not attribute is worse than no number. What is worth saying without a figure is what any appliance manual and any fire service will tell you: lint accumulation near an appliance is treated as a fire hazard, and the reason dryers come with instructions to vent outdoors and clean the duct is that hazard, not tidiness.
The two problems compound, which is the part almost nobody says out loud. Damp lint packs down and holds moisture against whatever it is resting on. So the moisture problem is not simply happening alongside the lint problem. It is being helped along by it.
For the authoritative version of the fire risk, read your own dryer's manual, which states the manufacturer's venting requirement for that machine, and the U.S. Consumer Product Safety Commission's material on clothes dryer fires. Those are the two documents worth having in front of you when you talk to a contractor, and they carry the numbers we are declining to invent.
The three places this actually goes wrong
The duct was never connected. This happens at installation, and it happens most often on homes that were moved and reset. The duct passes through the floor, and the outside half, the part that runs through the crawlspace and terminates through the skirting, was never installed or was never reconnected. Nothing indoors looks wrong. The dryer works.
The duct came apart. Flexible ducting under a home is exposed to temperature swings, to animals, and to anyone who has ever worked down there on the plumbing. A joint separates, and the duct now ends wherever it separated. This is the most common finding, and the symptom that gives it away is that a dryer which used to work well now takes noticeably longer to dry a load, because a duct that has come apart is often also crushed or kinked at the break.
The termination is blocked or was never a termination. A vent hood whose flap is stuck shut with old lint, or a duct that simply ends flush at the skirting behind a solid panel, both send the exhaust backward into the crawlspace. From outside, the skirting looks finished. That is the case people miss for years.
Who to call, and what to tell them
The trade depends on where the fault is, and you will not always know which before someone looks.
A licensed manufactured-home installer is the right call when the duct runs through the crawlspace and the fix involves working around the chassis, the underbelly or the skirting. This is the trade that knows the installation standard and knows what else is down there.
An appliance or HVAC contractor who works on manufactured homes is the right call when the fault is at the machine end or in the wall behind it. The qualifier matters: a manufactured home's floor, duct routing and clearances are not the same as a site-built home's, and a contractor who has never worked on one will say so.
Your state's manufactured housing installation program is who to ask if the home is newly installed or newly reset and you think it was never connected. This is an installation defect, and states run these programs precisely to handle that.
Your county or city building department is who to ask whether a permit or an inspection applies to the repair where you live.
If you are on a leased lot, tell the park office too, before anyone starts. Park rules commonly govern who may work under a home and what has to be reinstated afterward, and finding that out after the skirting is off is a bad day.
When you call, say four things: the make and model of the dryer, whether you found a hood outside the skirting or nothing at all, whether you saw a disconnected duct from the access door, and whether the ground under the laundry end is damp. That is enough for a competent contractor to bring the right parts.
What we are not going to tell you
No duct length, diameter, material specification or maximum number of bends appears in this article. Those come from your dryer's own installation manual, because they are set by the machine, and a general figure copied from another house would be a figure your dryer does not obey.
No screening, sealing or securing detail for the crawlspace end appears either. That is installation work under the home, and this article stops at the access door for the reason given above.
No cost. Prices vary by region, by how much duct has to be replaced, and by whether skirting has to come off. Any number here would be invented.
The short version
Run the dryer, walk outside, and look for warm damp air coming out of a hood in the skirting. If it is not there, your dryer is very probably discharging into the crawlspace, and it has been putting both water and lint under your home for as long as that has been true. The installation standard says it has to exit the skirting, and the reason is both of those loads at once.
Do not go under the home to fix it. Look from the access door, close it, and call a licensed manufactured-home installer or a manufactured-home-experienced appliance contractor. Bring your dryer's manual to the conversation, because it carries the requirements for your specific machine, and this article deliberately does not.