Insulation is the most unglamorous part of a van build. You can't Instagram your spray foam, nobody asks about your Thinsulate at a campsite. But get it wrong and your van will be freezing in winter, sweltering in summer, and dripping condensation on everything you own.
We've insulated five vans across five years. Here's what we've learned.
The Condensation Problem
Before picking materials, you need to understand condensation — because it's the thing that ruins poorly-insulated vans. When warm moist air (from your breath, cooking, and body heat) contacts a cold metal surface, it condenses into water. Over time this causes rust, mould, and rot.
The solution is a continuous vapour barrier between the warm interior air and the cold metal shell. Every gap in your insulation is a place where condensation can form hidden inside your walls.
Insulation Materials: What Actually Works
Thinsulate (3M SM600L)
Thinsulate has become the go-to insulation for serious van builders, and for good reason. It's a breathable synthetic fibre insulation that doesn't hold moisture, doesn't compress over time, and can be cut and stuffed into every nook of a van's complex geometry. It won't mould, it won't absorb condensation, and it's genuinely excellent at thermal performance for its thickness.
The downside: it's expensive (around $2–3 per square foot) and needs to be packed tightly to perform well. Leave air gaps and you lose most of the benefit.
Spray Foam (Closed-Cell)
Closed-cell spray foam is the best performing insulation by R-value per inch. It also seals every gap completely, eliminating air movement and condensation risk in the areas it covers. It's the right choice for the floor, wheel arches, and complex curved areas behind the cab.
The problems: it's messy, it's permanent, and over-expanding it can warp your van's body panels. Use it in controlled areas, not as your entire wall solution.
Polyiso Rigid Foam Board
Rigid polyisocyanurate boards (R-6.5 per inch) are excellent for flat areas — floor, ceiling, and the large flat sections of walls. They're affordable, easy to cut, and provide high R-value. They don't conform to curves well, so combine with Thinsulate in the ribs and complex areas.
What Not to Use
Rockwool/mineral wool: Absorbs moisture and is very difficult to work with in a van's curved geometry. Skip it.
Fibreglass batts: Same problem — absorbs moisture, itchy to install, and loses R-value when compressed into van ribs.
Reflectix alone: Not an insulator. Reflectix is a radiant barrier — it reflects radiant heat but provides almost zero resistance to conductive heat loss. Used alone in a van wall, it's essentially worthless for winter. It has a role as a window cover or under the floor, but don't line your walls with it.
A Practical Approach by Zone
- Floor: 1" polyiso rigid foam + plywood subfloor. Simple, effective, walkable.
- Walls (ribs/curves): Closed-cell spray foam in complex areas, then pack Thinsulate into every cavity.
- Walls (flat panels): 1–2" polyiso, secured with adhesive or panel clips.
- Ceiling: Thinsulate throughout — it's easy to work with overhead and conforms to the curved ceiling.
- Doors: Often overlooked. Stuff every cavity with Thinsulate. Foil tape any gaps. Doors are a significant thermal weak point.
- Windows: Reflectix or custom-cut foam covers when parked. This is where most heat escapes overnight.
How Much Insulation Do You Actually Need?
More is generally better, up to a point. For a Transit or Sprinter, aim for R-12 to R-16 in the walls and ceiling. You'll never achieve house-level insulation values in a van — the metal frame creates thermal bridges no matter what — but R-12 makes a genuine difference in both winter and summer comfort.
Don't obsess over achieving perfect R-values. A well-sealed van with modest insulation will outperform a van with high-R insulation and gaps everywhere. Air sealing is as important as R-value.