Insulating your van is going to be a very important step because you want to be comfortable living in your. There are few decisions you are going to have to make during this step and I will cover them all.
First, let's understand a bit about insulation. The purpose of the insulation is to slow the transfer of heat to the inside of the van. The goal is to have a living space that stays cool in the summer and warm in the winter. When looking at your insulation you will notice they all have an R-value. This is the measure of the insulations resistance to heat transfer. The higher the R-value, the better it is going to be at resisting the heat transfer to the inside of the van. This means that the higher the R-value the "better" it is at insulating.
Some people choose not to insulate their van because it can seem like an easy way to save some money on the build. Keep in mind, that some insulation is better than no insulation. I chose to insulate the sides, floor and ceiling of my van.
After a lot of research I decided to go with polyisocyanurate (polyiso) foam board for a few different reasons. This type of insulation is non toxic, has a very high R-value per inch, it is easy to work with and is relatively cost effective. If I had the budget I would have went with sheeps wool insulation simply from a sustainability aspect. You can learn more about sheeps wool here.
I purchased my polyiso foam board from Home Depot but it should be available at any big box retailer.
I found this to be one of the easiest steps during my van conversion. The way my van is, the foam board fit perfectly into the walls and ceiling. I simply measured (twice!) and cut the foam board using a box knife to size.
Once you have the correct size, use this Loctite PL300 Foamboard Adhesive to adhere it to the surfaces of your van. In order to get a firm hold, I used 2x3 wood with some scrap pieces to hold them in place for about 24 hours. Check out the video below for more on this.
After about 24 hours remove the 2x3's holding the insulation in place and you are all set!
I noticed that there were still some areas surrounding the insulation that needed to be sealed off so I decided to use this spray foam insulation to fill in the gaps. Super easy to use! Just spray in the cracks and the foam expands on its own. After it has dried, cut away the excess. The worst part about this step is cleaning up all the access foam afterwards!