Technology · Insulation
PIR vs XPS vs EPS Insulation for Camper Bodies
Updated
PIR, XPS, and EPS are the three rigid foams most often used to insulate camper and RV walls. PIR (polyisocyanurate) has the highest R-value per inch, is closed-cell, and is more fire-retardant than the polystyrene foams; XPS (extruded polystyrene) is a moderate-R, water-resistant board; EPS (expanded polystyrene) is the lightest and cheapest but the lowest R-value and most absorbent. In a structural sandwich panel, the core also has to carry load — which further separates them.
What each material is
EPS is expanded polystyrene — the white “beadboard” foam, made of fused beads with tiny voids between them. XPS is extruded polystyrene — a denser, closed-cell board (often pink, blue, or green). PIR is polyisocyanurate — a closed-cell foam board, usually foil- or glass-faced, widely used on commercial building roofs and walls for its high thermal performance.
R-value per inch
R-value measures resistance to heat flow — higher is better, and “per inch” lets you compare materials at equal thickness. Typical published values are roughly R-6 per inch for PIR, about R-5 per inch for XPS, and around R-3.6 to R-4.2 per inch for EPS. These are general industry figures — always check a specific product’s data sheet — but the ranking is consistent: PIR gives the most insulation in the least thickness, which matters when wall space is precious.
Moisture behavior
Water in a wall destroys insulation value and invites mold. EPS is the most absorbent of the three because of the voids between its beads. XPS is closed-cell and quite water-resistant. PIR is closed-cell and absorbs little water. In a vehicle that sees temperature swings, condensation, and the occasional leak, a closed-cell core that doesn’t soak up water holds its R-value over the years.
Temperature and fire behavior
Polystyrene foams (EPS and XPS) soften at relatively modest temperatures and melt and burn readily. PIR holds its performance across a wider temperature range and is more fire-retardant, forming a protective char rather than melting away — one reason it’s the standard on commercial roofs. For a body that bakes in desert sun and sits near heaters and cooking, that stability is worth a lot.
Structural use in a sandwich panel
In a structural insulated panel the core isn’t just insulation — it carries shear load and holds the two skins apart so the panel acts as one stiff piece. That rewards a rigid, dimensionally stable core. PIR and XPS are rigid and structural; EPS is softer and less capable in this role. A core that’s both a good insulator and genuinely structural lets the panel be thinner, lighter, and stiffer at once.
Why Foundation uses a closed-cell PIR core
Foundation builds its panels around a closed-cell PIR core because it wins on the measures that matter for a four-season, off-grid, hard-used body all at once: the highest R-value per inch (a continuous R11 in the 1.5-inch van panel, R17 in the 2.5-inch truck panel), very low water absorption, stable performance from desert heat to alpine cold, better fire resistance than the polystyrene foams, and enough rigidity to work structurally in the panel. The trade is cost — PIR is more expensive than the polystyrene foams most campers use — which is a deliberate choice not to insulate down to a price.
A quick comparison of the three rigid foams — typical general properties, not specific product data:
| Property | PIR | XPS | EPS |
|---|---|---|---|
| R-value per inch (typical) | ≈ R-6 | ≈ R-5 | ≈ R-3.6–4.2 |
| Cell structure | Closed-cell | Closed-cell | Open voids between beads |
| Water absorption | Very low | Low | Higher |
| High-temp stability | Wide range, stable | Softens sooner | Softens sooner |
| Fire behavior | Chars rather than melting | Melts / burns | Melts / burns |
| Structural rigidity | High | Moderate | Low |
| Relative cost | Highest | Moderate | Lowest |
R-values are typical published industry figures and vary by product and temperature — check the manufacturer’s data sheet for a specific material. This is general material science, not a Foundation test result.
Common questions
Questions people ask
Which insulation is best for a camper — PIR, XPS, or EPS?
For a four-season, off-grid build, PIR is generally the best performer: the highest R-value per inch, closed-cell so it absorbs very little water, stable across a wide temperature range, and more fire-retardant than the polystyrene foams. XPS is a solid mid-tier board; EPS is the cheapest and lightest but the lowest R-value and most absorbent.
What R-value does a Foundation body have?
Every Foundation body has a continuous R11 insulation layer with no thermal bridging, built around a closed-cell PIR core in a 1.5-inch composite panel — and R17 in the thicker 2.5-inch panel on medium- and heavy-duty trucks.
Why not just use cheaper foam?
Cheaper polystyrene foams save money up front but give up R-value, water resistance, high-temperature stability, and fire performance — and they’re less capable as a structural core in a sandwich panel. Foundation uses PIR because those properties matter over the life of a hard-used body.
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