Van Insulation: What Actually Works

Insulation is one of the least glamorous parts of a van build. There are no good photos of it once it's done, and most people rush through it to get to the cabinetry. That's a mistake that can take years to show up and cost you the entire build when it does.

I've done three van builds and one garage conversion. The insulation method I landed on by the third build is what I'd use if I started again tomorrow.

A van is a steel box. When warm, moist air from breathing, cooking, and showering meets a cold metal surface, it condenses into water. That water sits behind your walls, soaks into any organic material (wood, earthwool, MDF), and starts rotting your build from the inside out.

You won't see it for 12 to 18 months. Then you'll smell it. Then you'll pull a wall panel off and find black mould and rotted timber. This is one of the most common long term failures in van builds, and a lot of it comes from bad insulation choices.

Foil backed rigid insulation boards (2.5 R value)

This was my primary insulation for walls and ceiling. I used 2.5 R-value solid double foil-backed insulation sheets, cut to fit each panel.

The foil facing is the key. It acts as a vapour control layer, so warm moist air is less likely to reach the cold metal behind it. That matters a lot more than just chasing a high R value.

The air gap part gets oversimplified online. If you can fit the board neatly and keep things well sealed, I would rather avoid a big hidden cavity against the outer metal. If you want a service gap, it makes more sense on the interior side between the insulation and the wall lining. That also lets the foil face work better as a radiant barrier.

Sound deadening mats (first)

Before any insulation goes in, every bare metal panel gets sound deadening. These are butyl rubber mats, similar to what car audio installers use. They stick directly to the metal and stop the van sounding like a tin drum on the highway.

Clean every panel with white spirits before applying. Grease or dust stops the adhesive bonding, and once the van gets hot on a summer day the mats peel off. You don't want to find this out after everything is sealed up.

Sound deadening goes on first. Every metal panel before any insulation.

Adhesive method

For the roof, I used a thin layer of expanding foam to hold the insulation boards up against the ceiling because it is one of the few options that holds reliably overhead in heat. For the walls, I combined Sikaflex construction adhesive with expanding foam. Any gaps I couldn't fill with boards I packed with earthwool.

Van floors are never flat and always complicated by cross members, wheel arches, and whatever is bolted underneath. My method:

1. Sound deadening on the floor metal first.
2. Insulation boards cut to fit between floor ribs.
3. 18mm marine ply sheets on top, cut to fit, bolted through to the floor.
4. Nylon washers under each nut on the outside to prevent corrosion.
5. Countersunk bolt heads so the finished floor is completely flat.

Check underneath before you drill anything. There are fuel lines and brake lines under there. One wrong hole ruins your day.

The roof is the hardest area to insulate well because you're working overhead and gravity works against you. Expanding foam as adhesive is the right call here because it grips the panel up while it cures and fills any gaps in the process.

Plan your roof cutouts (Maxx Air fan, skylight, solar cable entry) before insulation goes in. Also see the Diesel Heater Guide — good insulation directly reduces how hard your heater has to work. Cutting through insulated panels is messier and harder, and you risk disturbing the bond.

Foil backed boards going into the walls. The foil face points inward toward the living space.

This is one most people skip, and it makes a massive difference. The amount of heat that transfers through from the cab is significant. The engine bay, the dashboard, and the windscreen all collect and radiate heat into the cab area, which then bleeds into your living space.

I built a solid insulated wall between the cab and the living area before I did any other interior work. I designed it with a hatch that doubled as a window and a flat surface. The heat difference is noticeable immediately, especially in summer.

For an Australian climate (hot summers, mild winters), 2.5 R value is sufficient. For colder builds in places like the UK, Europe, or Canada, you want to push for roughly R3.5 to R5 where space allows, especially on the ceiling and floor where heat loss is greatest.

More insulation also means more noise reduction, which improves daily quality of life significantly. The difference between a well insulated and a poorly insulated van on a highway is dramatic.

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