Blown-in insulation — dense-pack cellulose, blown fiberglass, or spray-applied foam — is generally the right tool for retrofitting an existing mobile home, because it fills irregular, narrow, and hard-to-reach cavities without opening up walls or ceilings.
New construction can use batt insulation cut to fit an open, accessible cavity. A retrofit on an occupied mobile home is a different problem: the cavities are already closed in behind drywall, paneling, or a belly board, they're often irregular in depth because of wiring and plumbing runs, and batts that were installed decades ago have frequently settled, sagged, or been compressed — all of which reduces their real-world R-value well below the material's rated number.
Blown-in insulation solves that problem two ways. In an open attic, loose-fill cellulose or fiberglass is blown directly over the existing insulation to full target depth. In a closed wall or belly cavity, a small hole is drilled and the material is packed in under pressure — dense-packed — until it fills the entire cavity, working around obstructions and reaching corners a batt never could. Done correctly, dense-pack also resists air movement through the cavity, which narrows the gap between a wall's rated R-value and its real performance.
The roof cavity is usually the most cost-effective place to add insulation, because it's typically the easiest to access (through a roof cap, gable vent, or access hatch) and can be brought up to a full R-49–60 target without disturbing the living space below. Blown cellulose or blown fiberglass are both standard choices; Efficiency Maine's program specifications generally require cellulose, blown fiber, or foam to qualify for the attic insulation rebate, since batt insulation on its own does not meet the program's eligibility rules.
Mobile home walls are narrow, which caps how much R-value a cavity fill alone can deliver. Dense-pack cellulose installed via the drill-and-fill method — drilling small access holes from the exterior (or interior, if re-doing wall finishes) and packing each stud bay solid — is the standard retrofit approach, since it doesn't require removing siding or interior wall material. Where the exterior siding is being replaced anyway, adding a layer of continuous rigid foam sheathing over the studs breaks the thermal bridging through the framing and adds R-value that a cavity fill alone can't reach.
This is usually the weakest link in an older mobile home's envelope, and the one most worth prioritizing. The floor structure sits on a thin bottom board (often called the belly board) that's meant to hold factory-installed fiberglass batts in place from below. Over time that board sags, tears, or gets punctured by rodents, and once it fails, the insulation above it falls loose, leaving long uninsulated gaps directly under the living space — which shows up as cold floors and a much higher risk of frozen pipes.
Where the two halves of a double-wide join, the mate wall is assembled on site rather than in the factory-sealed environment the rest of the home is built in — which makes it a common, and frequently overlooked, source of both air leakage and heat loss. A proper retrofit seals the marriage line continuously along its length, typically combining spray foam at the seam with dense-pack cellulose in the adjoining cavity, and should be part of the scope any time significant heat loss or drafts are traced to the center of the home rather than the exterior walls.
Skirting encloses the space under the home but is generally not considered part of the conditioned building envelope, and it is typically not an insulation material itself. It's worth noting because Efficiency Maine's program rules specifically treat skirting as ineligible for the underbelly insulation rebate — the rebate targets insulation and air sealing of the floor cavity itself, not the skirting panels around it. That said, intact, well-sealed skirting still matters: it moderates the temperature of the space beneath the home, which reduces pipe-freeze risk and eases the load on whatever insulation and air sealing has been done in the belly cavity above it. Skirting should be treated as a complement to underbelly insulation, not a substitute for it.
| Material | R / inch | Air sealing | Moisture tolerance | Typical use |
|---|---|---|---|---|
| Blown cellulose (attic, loose fill) | 3.2–3.8 | Low on its own | Hygroscopic; buffers moisture reasonably well | Open attic cavities |
| Dense-pack cellulose (wall / floor) | 3.6–3.8 | Moderate–high when packed to spec density | Hygroscopic; needs a repaired, sealed cavity | Closed wall and belly cavities via drill-and-fill |
| Blown fiberglass | 2.2–2.7 | Low | Does not absorb moisture; dries quickly if it gets wet | Open attic cavities |
| Open-cell spray foam | 3.6–3.8 | High | Vapor-permeable; not a moisture barrier | Belly board, marriage line, rim areas |
| Closed-cell spray foam | 6.0–7.0 | Very high | Acts as a vapor barrier and air barrier in one step | Belly cavity, marriage line, rim joists, high-moisture areas |