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Mobile home belly wrap repair is the job everybody puts off, because it means getting on your back under the house and because a torn membrane does not leak into the living room. It leaks into the floor, quietly, for years.
It has half a dozen names — bottom board, belly board, underbelly, black bottom, and in some regions Cemplex — and what it does is more than hold the insulation up.
What the material is
On a modern home, layers of woven high-density polyethylene tape, extrusion-coated with black low-density polyethylene, sometimes with fibreglass insulation sealed inside the middle ply. Replacement stock is typically 12 to 16 mil and rated for something in the order of 15 to 20 years.
On an older home you are more likely to find asphalt-impregnated fibreboard or a heavy tar paper. That difference matters for repair, because the tape sold for this does not stick to the old rigid board.
What the federal standard actually says
This is where the subject gets more interesting than it looks.
24 CFR 3280.305(g)(6) requires that bottom board material, with or without patches, “must meet or exceed the level of 48 inch-pounds of puncture resistance as tested by the Beach Puncture Test” per ASTM D781. It goes on: the material must be suitable for patches, the patch life must be equivalent to the material life, patching instructions must be included in the manufacturer’s instructions, and the bottom board “must be tight fitted against all penetrations.”
Read that carefully. The standard anticipates patches and sets a durability bar for them. A strip of duct tape does not clear 48 inch-pounds of puncture resistance.
On the installation side, 24 CFR 3285.804 covers bottom board repair directly: inspect for loosening and damage, replace any missing insulation before closing the repair, reseal splits and tears with tape or patches per the manufacturer’s instructions, verify that P-traps are insulated and covered, and tape or seal all edges of the repaired area.
That is a checklist written into federal regulation, and almost nobody doing this work has read it.
What it does
Four jobs at once: it holds the floor insulation against the underside of the deck, it keeps rodents out, it works as a moisture barrier, and it protected the floor cavity from road spray when the home was towed to the site.
The fourth one is done. The first three are ongoing, and the belly cavity is also where your crossover duct and a good deal of your plumbing live. A torn belly is not just an insulation problem; it is an open door to the duct.
Why it fails
No source ranks these by frequency, so take the order as the consensus of people who repair them rather than as data:
- Rodents. Cited first almost everywhere.
- Previous repairs. A plumber or HVAC technician cut it open, did the work, and did not reseal it.
- Water from above. A supply leak, a drain leak, or condensation off an uninsulated duct.
- Sagging. Old fasteners let go, the membrane drops, the weight of wet insulation tears it.
- Transport and age.
Mobile home belly wrap repair, three levels
Tape a small tear. The named product is Flex-Mend belly repair tape — a woven polyethylene film with a high-tack adhesive, sold in 4-inch by 25, 50 and 180 foot rolls and a 28-inch by 25 foot roll for larger areas. The method: clean the area, lay the tape, then heat-activate the adhesive with a heat gun or hair dryer. It will not stick to the old rigid fibreboard underbelly.
Patch a hole. The Department of Energy’s weatherization job aid specifies a patch at least 3 inches larger than the hole all round, a waterproof permanent adhesive, stapled or stitched, with all seams sealed weather-tight.
Re-support and replace. Sagging insulation goes back up with wood lath tacked across the bottom of the joists at roughly 24-inch centres before you close the membrane. A full replacement is a five-step job: strip the old material, inspect and reseal the ductwork while you can see it, check the plumbing, fit new insulation, fit new bottom board.
If the belly is coming down anyway, that middle step is the bonus. It is the only time you will ever have a clear view of the crossover duct.
The wet insulation question
Sources split on this, so here is both positions.
One says insulation that is damp but not mouldy or contaminated can be dried in place with fans. The other says that once the barrier has been breached and the insulation is wet, mouldy or rodent-infested, it comes out, goes in a bag and is replaced.
The second is the conservative answer and the one 3285.804 leans toward, since it requires missing insulation to be replaced before closing. If there is any doubt — smell, staining, droppings — take it out. You are not going back under there for another five years.
What it costs
Assembled from several 2026 cost guides rather than one, so treat as ranges:
| Job | Cost |
|---|---|
| DIY tape or small patch materials | $150–$400 |
| Professional small tear patch | $150–$300 |
| Vapor barrier repair | $150–$400 |
| Insulation replacement | $200–$1,000 |
| Rodent damage repair | $200–$1,200 |
| Full underbelly replacement | $2,000–$6,000 |
Labour generally runs $250 to $500 a day. No source gives separate single-wide and double-wide figures; a double-wide has roughly twice the area, which is the sensible way to scale it.
Common questions
Is an intact belly required for FHA or VA financing?
We could not find any HUD, FHA or VA text naming the bottom board as a specific requirement. The foundation, anchoring and skirting are what those inspections are built around. An appraiser can still call visible damage as a required repair.
Can I use house wrap instead?
It is not the same material and does not meet the puncture requirement the standard sets. Use bottom board material.
Is belly wrap the same as the vapor barrier?
No, and the confusion is universal. The vapor barrier is polyethylene on the ground under the home. The belly wrap is fastened to the underside of your floor.