Some nomads choose to live in completely uninsulated vehicles to save money, save space, and reduce the complexity of their build. For those who need or want to go the removable build route, forgoing insulation altogether may be a requirement. So how does the uninsulated option work? If you are willing and able to follow good weather, it's possible to skip insulation altogether.
Without insulation in your build, you'll work toward keeping cool in hot weather via passive cooling and ventilation, and keeping warm in cold weather with personal insulation (clothing, sleeping bags) and additional heating expense.
Financial considerations
If your budget is very low, that might mean you can't afford to insulate. Instead, you'll treat your vehicle as essentially a metal tent. But without insulation, your costs later may be higher later due to increased heating needs. You may also need to spend considerably more on fuel to chase comfortable weather or change elevation regularly. Even driving to find a good spot with shade for your vehicle may be a challenge in some regions.
If you choose the uninsulated option when building out your van, changing your mind later is generally going to require the complete removal and replacement of your build. As such, it's generally a bad idea to have an expensive build without insulation.
Temperature risks
“ Your van is a box made out of sheet metal. It will heat up like an oven in the sun. It will lose heat like a flagpole in the cold.[1]”
As you likely know, temperatures inside a closed uninsulated vehicle can get dangerously hot when the sun is shining, even when it's not particularly hot outside. If you can spend your days living out of your van instead of in it, this may be acceptable. If you are traveling part time and won't use your vehicle in the winter, the uninsulated option is much more palatable.
Your water tanks and plumbing lines (if present) need to be protected from freezing at all times. If the weather takes a particularly cold dip when you aren't expecting it, you need to heat or drain your water first. See winterizing your vehicle. In hot weather, warm water tanks will grow bacteria more quickly, meaning more frequent cleaning and water purification is necessary.
If you have pets, temperature control is much more important since you may need to leave them in your vehicle for shopping and other errands; the uninsulated van option will likely not work for you. If your pet is visible or audible from outside the vehicle, well-meaning strangers may break a window in order to rescue your pet, even if the temperature inside is safe for your pet.
Items in your van will also be subject to temperature extremes, which could damage them. This can be mitigated somewhat by placing your sensitive electronics, cameras, and other items inside a dedicated dry cooler (no ice!); this slows the temperature gain during the day and should keep your items close to whatever the ambient temperature was when you placed the items inside.
Food stored in your vehicle may be damaged due to high or low temperatures, and spoilage rate increases in high temperatures. See keeping your food cold.
Space and weight
Insulation isn't very heavy, but it is often rather bulky due to its primary function of stopping air movement, convection, and conduction. Going without insulation in your van can easily mean a cubic meter or yard of additional living space. No insulation on your floor or ceiling can mean an effective increase in ceiling height, which may be a big benefit if you would otherwise be too tall to stand comfortably. The height difference could mean you can buy a lower-roof van, which improves fuel economy
Removable insulation
To some extent, you can utilize removable insulation. This usually takes the form of window coverings that will block a considerable amount of heat or cold. You could also hang blankets, thick curtains, or Mylar sheets around you to reduce heat transfer. You can subdivide your space with these to reduce the amount of space in your van that needs to be heated or cooled.
Further discussion of uninsulated builds.