Van Building Insulation: Why We use Polyiso Foam Board
Insulation is probably the most debated topic in van life. Unless you’re living in a place with mild temperatures year-round—like Hawaii—insulating your van is essential for staying comfortable in both cold and hot climates. It's also the single most important step in reducing your reliance on shore power and enabling off-grid camping.
The Problem with Insulation Trends
Many van builders shy away from polyisocyanurate (polyiso) foam board insulation. Part of this is aesthetic—it doesn’t look as impressive in photos and therefore isn’t considered “cool.” But polyiso foam board is one of the most effective insulation options available.
Commercial van conversion companies often use Thinsulate because it is quicker to install and looks nice in marketing materials. Thinsulate has lower R value (R3.3), costs more and will result in condensation. The condensation will hopefully dry out, but is still a negative.
DIY builders tend to favor wool, since it’s a natural product and heavily promoted by its manufacturers. Wool is lovely material to work with, but it has the same negatives as Thinsulate, lower R value (R3.3) and is harder to install. Meanwhile, polyiso foam board manufacturers don’t advertise to van builders — so unless you’ve looked into the science behind it, its value might be easy to overlook.
CampMaker Method: Years of Polyiso Foam Board Experience
We’ve been using polyiso foam board for van insulation for the past six years. Here’s how we use it:
• Walls: Flat surfaces are insulated with spray-foamed polyiso foam boards. The lower half of the walls can fit two layers of 1-inch foam (R12).
• Ceiling: We install a combination of 1-inch and ½-inch boards (R9 total), with pine furring strips over ceiling ribs. These strips add about R3 and make attaching the finished ceiling material easier.
• Doors & Tight Spaces: For areas where foam boards aren’t practical—like inside the doors—we use Thinsulate.
• Floor: We use XPS insulation here because it’s fully waterproof. Undetected water leaks to the floor can be an incredible challenge to repair and XPS has the best water resistance. It sacrifices a bit of R value (R5) compared to Polyiso (R6), but is worth it for the water resistance. However, we don’t recommend it for walls or ceilings—it melts at 160°F, and van sheet metal can easily reach that in direct sun. Polyiso foam board, by comparison, handles up to 265°F.
Why Polyiso Foam Board Wins
• R-Value Efficiency: Polyiso foam board provides the highest R-value per inch and takes up the least space—a big plus in a small van.
• Scientific Test: According to a well-regarded insulation test, fiber insulations like wool and Thinsulate allow water vapor to penetrate and condense on van walls. Polyiso foam board, when properly sealed, does not. As the test concluded: “Both of the fiber insulations (wool and Thinsulate) do allow water vapor to penetrate and condense both in the outer layers of insulation and on the van wall. The Polyiso foam board with sealing around the edges did not let water vapor penetrate and does not condense water on either its outer or inner face.”
Addressing Common Concerns
• “Doesn’t Polyiso foam board Lose R-Value in Cold Weather?” Technically yes, but the drop is minimal enough that it doesn’t significantly impact real-world performance and negate all the other reasons to use polyiso foam board.
• “What about EPS foam insulation?” EPS foam board (expanded polystyrene) is not the best choice due to its susceptibility to moisture damage and potential for disintegration under stress. EPS foam can absorb moisture, which significantly reduces its insulating ability and can compromise its structural integrity. It can disintegrate under vibration, making it unsuitable for the constant movement and bumps experienced in a van. Compared to other options, EPS foam has a shorter lifespan in a van environment.
• “What About Squeaking?” This is avoidable if you glue the board to the van’s metal surface and use spray foam around the edges. This creates a seal that prevents both squeaking and moisture intrusion.
• “Is It Harder to Install?” Compared to Thinsulate or wool, polyiso foam board takes about the same amount of effort but it is helpful to have some instruction in order to do it well. The standard time to insulate a van with Thinsulate is 16 hours and foam board takes about the same amount of time.
• “Is it More Expensive?” Thinsulate is about $850 per van compared to $500 for polyiso foam board and XPS foam board and $700 for wool.
Polyiso foam board and XPS Expenses: 1 inch XPS 4x8 - $60 x 3 = $180 for the floor. 1 inch Polyiso foam board 4x8 - $30 x 4 = $120 for walls and ceilings. ½ inch Polyiso foam board 4x8 - $20 x 3 = $60 for the second layer on the ceiling and upper half of the walls. Gaps and Cracks Spray Foam Pro Series - $15 per can x 5 = $75, Pro Series Spray Gun = $65
In short, polyiso foam board foam isn’t flashy, but it works. It’s effective, durable, and can outperform more popular options.