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A mobile home vapor barrier is a sheet of polyethylene laid on the soil under the home. It is not the membrane stapled to the underside of your floor — that is a different thing with a different job, and confusing the two is the single most common mistake in this subject.
The federal installation standard says when you need one, what it has to be, and how it goes down. It also contains a number that most owners have never heard and that changes how much skirting ventilation you need by a factor of ten.
The rule
24 CFR 3285.204, “Ground moisture control”, in HUD’s Model Manufactured Home Installation Standards:
- When it is required. If the space under the home is to be enclosed with skirting or other materials. The trigger is enclosing the crawl space, not the soil type.
- Material. A minimum of 6-mil polyethylene sheeting, or equivalent.
- Coverage. The entire ground area under the home, except under open porches, decks and recessed entries.
- Seams. Joints must overlap at least 12 inches.
- Placement. It may run under or around footings at grade. Tears and voids must be repaired according to the manufacturer’s installation instructions.
- The exception. It is not required in arid regions with dry soil conditions.
Read that first line again, because it is the part people get backwards. Skirting is what creates the requirement. An open, unskirted home does not need one; the moment you close the perimeter in, you do.
The number nobody knows: 1:150 versus 1:1,500
24 CFR 3285.505 sets crawlspace ventilation, and it ties the two things together:
| Condition | Minimum net ventilation opening |
|---|---|
| Without a vapor retarder | 1 sq ft per 150 sq ft of floor area |
| With a vapor retarder | 1 sq ft per 1,500 sq ft of floor area |
A tenfold reduction. For a 1,000 square foot home that is the difference between roughly 6.7 square feet of vent opening and 0.67.
This is why the barrier is worth doing even where you might argue it is optional: it is what lets the skirting be a skirt rather than a grille, and it is the reason a properly installed home does not need to be ventilated like a barn.
Note that some installer sources quote a different ratio — “one square inch of vent per square foot of floor area” — and do not mention the vapor retarder reduction at all. That figure does not match 3285.505 and is probably a local rule or trade shorthand. Go with the federal standard unless your state program says otherwise.
A mobile home vapor barrier is not the belly wrap
Two different membranes, two different places, two different jobs.
| Ground vapor retarder | Bottom board / belly wrap | |
|---|---|---|
| Where | On the soil, under the home | Stapled to the underside of the floor joists |
| What it does | Stops moisture rising out of the ground | Encloses the floor insulation; rodent and weather barrier |
| Which standard | Installation standard, 24 CFR 3285 | Construction standard, 24 CFR 3280 — factory-fitted |
| Who installs it | The installer, on site | The factory |
Your home has both, or should. When someone says “the vapor barrier under my mobile home is torn,” they almost always mean the belly wrap. The repair, the cost and the consequences are different for each.
What happens without one
Ground moisture evaporates into an enclosed crawl space and has nowhere to go. It condenses on the coldest surface it finds, which is the underside of your floor.
From there the sequence is predictable: the belly insulation takes on water and stops insulating, the bottom board sags under the weight, the floor decking stays damp, and rot and mould follow. Mould under the floor can migrate into the living space through duct penetrations. The musty smell people describe in older manufactured homes very often starts here.
Soft floors are the symptom that finally gets attention, and by then the repair is not a sheet of plastic.
What it costs
Indicative 2026 US figures, from a single cost aggregator — treat them as a range rather than a quote:
- DIY materials for about 1,000 square feet: $450 to $650, roughly $0.45 to $0.65 per square foot for sheeting and tape, labour excluded.
- Contractor installed, 1,000 square feet: around $1,215 to $1,805 for economy 6-mil, $1,704 to $2,606 for 10-mil, and $3,108 to $4,010 for premium 15 to 20 mil.
- Add roughly $1.25 to $2.00 per square foot if moisture or mould remediation is needed first.
The code minimum is 6-mil. Thicker material is more durable under foot traffic when someone is down there working, which is the real argument for it rather than vapor performance.
Two cases the code does not settle
Dry climates. Section 3285.204’s own exception covers arid regions with dry soil, so it is explicitly handled.
A concrete pad. The section does not address a home sitting over a full concrete slab. The reasonable reading is that the slab already does the job, but that is an inference rather than a citation. If you need a defensible answer for an inspection, ask your state’s manufactured housing program.
Common questions
Can I use a regular builder’s plastic?
If it is at least 6-mil polyethylene, yes. What matters is thickness, full coverage and 12-inch overlaps.
Do I tape the seams?
The standard requires a 12-inch overlap and repair of tears; it does not require taping. Taping helps hold it in place while you work and is cheap.
Does it go under the piers or around them?
Either. The standard permits it to run under or around footings at grade.
Will a mobile home vapor barrier fix a damp smell on its own?
Only if the source is ground moisture and nothing has rotted yet. If the belly is already wet, that has to come out first.