R-value is the single number most often printed on insulation bags and rebate paperwork — and the most often misunderstood. Here's what it measures, what it doesn't, and what targets make sense for a Maine mobile home.
R-value is a measure of thermal resistance: how well a material resists the conductive flow of heat through it. The higher the number, the more slowly heat moves through that material when there's a temperature difference across it. It's the inverse of U-factor (U = 1 ÷ R), which building codes sometimes use instead to describe how much heat a whole assembly — a wall, window, or roof — lets through.
R-value is additive across layers. A wall built from siding, sheathing, cavity insulation, and drywall has a total R-value roughly equal to the sum of each layer's R-value, minus losses where the insulation is interrupted by framing — a stud, a header, a window jamb. That interruption is called thermal bridging, and it matters more in a mobile home than a site-built house because the framing members (2x2s and 2x3s in older wall cavities) sit closer together and take up a larger share of the wall area.
| Material | R per inch | Notes |
|---|---|---|
| Fiberglass batt | R-3.1–3.4 | Loses effective R-value when compressed to fit a narrow or irregular cavity |
| Blown fiberglass (loose fill) | R-2.2–2.7 | Attic application; lower density than dense-pack |
| Blown cellulose (loose fill, attic) | R-3.2–3.8 | Settles over time in open attic applications |
| Dense-pack cellulose (wall cavity) | R-3.6–3.8 | Packed to 3.5–4.5 lb/ft³ so it doesn't settle; also resists air movement |
| Open-cell spray foam | R-3.6–3.8 | Air seals as it insulates; vapor-permeable |
| Closed-cell spray foam | R-6.0–7.0 | Highest R per inch; also a vapor barrier and adds rigidity |
| Rigid foam board (XPS/polyiso) | R-4.0–6.5 | Used as continuous exterior insulation over framing |
Homes built to older HUD Manufactured Home Construction and Safety Standards were insulated to whatever the code required at the time and place of manufacture — often a milder climate zone than Maine's. It's common to find walls insulated to only R-11, floors around R-14–22, and roof cavities in a similar range. Maine's own construction standard, the Maine Uniform Building and Energy Code (MUBEC), sets out exactly what a new home in the state's climate is expected to achieve today — and it's the most useful available benchmark for how far an older mobile home's insulation typically falls short.
| Assembly | Typical existing | MUBEC target | Common upgrade material |
|---|---|---|---|
| Roof / attic cavity | R-14–22 | R-60 | Blown cellulose or fiberglass, full cavity depth |
| Exterior wall cavity | R-7–11 | R-20 + R-5 ci, or R-13 + R-10 ci | Dense-pack cellulose, plus rigid foam sheathing on re-side |
| Floor / belly cavity | R-11–22 | R-30 (R-38 in Aroostook County) | Dense-pack cellulose or spray foam against a repaired belly board |
| Marriage line (double-wide) | Often unsealed | Continuous, air-sealed | Spray foam and dense-pack at the mate wall |
“ci” = continuous insulation. Figures are general reference ranges for planning purposes, not a substitute for a site-specific energy assessment. See the MUBEC codes page for the full prescriptive table, and note that MUBEC governs new construction rather than existing manufactured homes directly — it's used here as a climate-appropriate target, not a legal retrofit requirement.
For a given assembly, conductive heat loss is proportional to the area of that assembly and the temperature difference across it, and inversely proportional to its R-value. In plain terms: double the R-value of a wall and, all else equal, you roughly halve the rate of heat that conducts straight through it for a given indoor–outdoor temperature gap. That's why the first few inches of insulation in an empty cavity make a dramatic difference, while each additional increment on top of an already well-insulated assembly saves progressively less. Going from R-0 to R-11 matters enormously. Going from R-49 to R-60 in an attic still helps, but by a smaller margin per dollar spent.
This is also why the biggest, most poorly insulated surfaces in a mobile home — a thin belly cavity and thin walls, both starting from a low baseline R-value — are usually where a dollar of insulation buys the most heat-loss reduction.
R-value is a steady-state measurement of conductive heat flow through a material. It says nothing about how much air is moving through gaps, seams, and penetrations in the building envelope — and in an older mobile home, air leakage is frequently a larger source of heat loss than conduction through under-insulated cavities. A wall insulated to a respectable R-value can still lose heat rapidly if cold outside air is infiltrating through an unsealed rim, a torn belly board, or an unsealed marriage line.
That's the gap a blower door test is built to measure. It doesn't test insulation — it tests airtightness, which is the other half of the heat-loss equation.