Greetings & Welcome!
There are only 2 proper ways to insulate a vehicle. Either all metal needs to be ventilated so that no moisture can get trapped, or your insulation needs to be the wicking type, so that it will wick the moisture to the interior where it can then be evaporated out.
99.9% of the garbage you see online will trap moisture which will lead to rust and mold.
Factory vehicles use the wicking scheme for the floors with carpets, and the venting scheme for the inside of all body panels, plus a third method of adding drain holes all along the bottom of the body and doors to drain away any extra moisture. These drain holes, often referred to as weep holes, need to be kept clear and functioning at all times.
Secondarily, insulate for sound, not temperature. Insulating for temperature control in a vehicle is a waste of time and money. Under the best of conditions it might gain you a whole 15-20 minutes of comfort, and in many conditions it is totally counterproductive. Only proper heating and cooling equipment is going to keep you truly comfortable.
For reference, I’ve been living in vehicles full time since 1973, often in extremely hot, extremely cold, and often very humid conditions. I have tried every conceivable incarnation of insulation imaginable, and none of them are nearly as effective as proper heating & cooling. A $20 kerosene heater, and a ~$40 swamp cooler keep my van comfortable year round, with or without insulation. With a few other goodies like heating/cooling blankets & seat cushions, living in a vehicle can actually be quite comfortable.
"If you prepare for the worst, every thing else is easier." ~ Off Grid