Thinsulate vs wool vs foam board vs other
You should insulate your van, but don't obsess over it. No matter how good your insulation is, on a hot day your van will reach exterior temperature quickly, and on a cold day insulation can only retain heat - not create it. Good ventilation and air circulation will do more for your comfort than perfect insulation ever will.
You also have a windshield and windows that are single-pane glass with no insulation at all - so don't lose sleep over thermal bridging or tiny gaps in your walls.
Before you start insulating, consider adding sound deadening material to large flat metal panels (floor, wheel wells, doors, large wall sections). This reduces road noise and rattling dramatically, and it needs to go directly on the bare metal - under your insulation.
You don't need to cover every square inch. Focus on large flat panels that resonate when you tap on them. Even 25-30% coverage on those panels makes a noticeable difference.
Same butyl rubber material, fraction of the price. This is one of the easiest savings in a van build.
Without an air gap on both sides, Reflectix does almost nothing for insulation. It can work as a radiant barrier with proper air gaps, or for window covers, but don't use it as your primary wall/ceiling insulation. A lot of people use it because it's cheap and easy, then wonder why their van is still hot or cold.
For most first-time van builders, go with Thinsulate (SM600L). It's the most expensive insulation option, but I think it's worth it for the time savings and peace of mind.
The biggest advantage is that it's just easier to work with. You can cut it with scissors, it conforms to all the curves in your van without leaving gaps, and you press it into place with spray adhesive. No precision measuring, no mixing ratios, no filling gaps with canned spray foam for hours. When you're already learning a ton of new skills on your first build, having one less thing to stress about is valuable.
It's also breathable - if moisture gets behind your walls (from a leak or condensation), it can dry out over time rather than being trapped. With foam board or spray foam, trapped moisture stays trapped. This breathability is honestly one of the biggest reasons I prefer it.
And there's no smell. Spray foam and new foam board off-gas chemical smells that can linger for weeks. With Thinsulate, you can install it and sleep in the van that night.
If your budget is really tight, foam board can absolutely work - but you need to be meticulous about cutting, fitting, and sealing every gap. With Thinsulate, you just install it and move on to the next step.
Unlike most components on this site, Vevor doesn't produce a synthetic insulation comparable to Thinsulate - so I went with the name-brand 3M Thinsulate SM600L for my build.
If you search "RV insulation" on Temu, you'll find a variety of generic alternatives that are cheaper and probably somewhat similar, but I can't vouch for their quality or safety. With something that's going to be hidden inside your walls for the life of the build, I'd rather pay for the known quantity - but that's a personal call.
If you're on a bigger budget, sheep's wool has similar breathable properties and is also a great choice.
Insulation also prevents condensation. Warm, humid air inside your van hitting cold metal = water droplets. Over time, that means rust and mold. Insulation creates a thermal break so the inside surface of your wall stays closer to interior temperature, reducing condensation.
This raises the question of vapor barriers. Some materials (foam board, spray foam) create a vapor barrier that blocks moisture from reaching the metal. Breathable materials (Thinsulate, wool) let moisture pass through but also let it dry out. Both approaches work - each has trade-offs.