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If one end of your double-wide is freezing while the other is fine, the mobile home crossover duct is the first thing to look at and the last thing anybody checks.
It is a single length of flexible duct running under the home, joining the two floor sections. It is the only part of your heating system that was assembled outdoors, on the ground, by whoever set the home up. And it is the one most likely to be lying in the dirt right now.
What a mobile home crossover duct is, and why only multi-section homes have one
Your furnace sits in one section. The main trunk duct runs down the middle of that section’s floor. The other half of the house has its own trunk duct and no heat source of its own.
The crossover is what bridges them, passing under the marriage line. A single-wide has no second section, so it has no crossover and none of the problems that follow.
HUD’s own field study makes the structural point about why this component is different: the crossover connection “is done in the field by contractors operating in a far less regulated environment” than factory assembly. Everything else in your duct system was built indoors under inspection. This piece was not.
What the code requires of it
24 CFR 3285.606, on ductwork connections, is specific enough to use as an inspection checklist:
- All connections and collars must be sealed to prevent air leakage, using UL 181A or 181B listed materials.
- Metal ducts fasten to the collar with at least three galvanized sheet-metal screws, equally spaced around it.
- Ducts must be suspended or supported above the ground by straps, at a maximum spacing of 4 feet.
- Straps on flexible duct must be at least ½ inch wider than the spacing of the metal spirals, so the strap cannot cut into the duct.
- Crossover ducts outside the thermal envelope must be insulated.
The construction standard, 24 CFR 3280.715, adds the rest: no portion may contact the ground, under-chassis ducts need a minimum of R-8 insulation with a vapor retarder of one perm or less, the duct must be listed for exterior use, and fasteners must not penetrate the duct wall.
Every one of those is a thing that is commonly wrong on a home that has been in place ten years.
What it costs you when it fails
HUD’s study Air of Importance: A Study of Air Distribution Systems in Manufactured Homes measured it. Average heating-mode delivery efficiency loss across the homes tested was 40 percent — only 60 of every 100 BTUs entering the duct system reached the living space. Cooling-mode efficiency degraded around 18 percent on average.
That is the whole duct system, not the crossover alone. But the crossover is the component sitting in the crawl space with the worst installation conditions, and it is where a disconnection dumps conditioned air straight into the ground.
Worth being honest about one thing: we could not find a published figure for how often crossover ducts are found actually disconnected. Weatherization crews talk about it constantly; nobody appears to have published the rate.
What you will notice
- One section or one end of the home never gets comfortable.
- Weak or no airflow from the vents in the far section.
- Energy bills that do not match the weather.
- The furnace running almost continuously without satisfying the thermostat.
- Condensation, damp or a musty smell under the floor.
How to inspect it
You are going under the home. Bring a torch, wear something you do not care about, and check the skirting access panel before you commit.
Trace the duct from the trunk collar on the furnace side, across the marriage line, to the collar on the far side. What you are looking for:
- Is it off the ground? It should hang from straps, never rest on soil or on the vapor barrier. Straps no more than 4 feet apart.
- Are both ends actually connected? Fully seated over the metal collar, screwed and sealed, with no slip-fit gap.
- Is the strap cutting it? A narrow strap over a spiral-wound duct saws through it slowly.
- Holes, tears, crush damage, low spots holding water.
- Nesting. Rodents in a warm insulated duct is exactly as common as it sounds.
Repair, and the tape question
Replacement duct is insulated flexible duct, R-8 for anything under the chassis, in a diameter matching your trunk collars — commonly 12 to 16 inches, but it has to match your home rather than a number from an article.
Fastening: metal strapping screwed to the floor joists every few feet to hold it clear of the ground, clamped connections at each collar, then mastic over the joint.
Duct tape is not acceptable and never was. It dries out, peels and fails in the heat and humidity under a home, and it is not a UL 181A or 181B listed sealing system, which is what the code requires. Use HVAC mastic, or foil tape that carries the UL 181 listing.
What it costs
- DIY: a 25-foot run of R-8 insulated flexible duct runs roughly $220 to $310, plus strapping, clamps and mastic.
- Professional: a crossover-only replacement sits well below a full duct system job. For reference, full ductwork replacement is quoted at roughly $1,000 to $1,800 for a single-wide and $1,800 to $3,300 for a double-wide.
Against a 40 percent delivery loss, the payback on getting this right is measured in months, not years.
Common questions
My home is a single-wide. Do I have one?
No. There is no second section to cross to.
Can I just tape the hole?
Only with UL 181 listed foil tape or mastic, and only if the duct is otherwise sound and properly supported. Tape over a crushed or sagging duct fixes nothing.
Is a bigger duct better?
No. It has to match the collar and the airflow the furnace was designed for. Oversizing drops velocity and creates its own problems.
How often should I look at it?
Once a year, and after anyone has been working under the home.