Insulating our Ram Promaster

Insulation is a highly debated topic in the DIY van conversion world. We’re not really looking to get into that here, our main goal is to show you the process we took and help you along the way for the type of insulation we chose.

After a lot of research, we ended up insulating our RAM Promaster with 1″ polyiso rigid foam. Polyiso has a high R-value and doesn’t absorb moisture, both important qualities when thinking about van insulation. We looked through a LOT of van conversion forums and this method of insulation seemed to have good results.

First order of business was to measure and cut for the larger panels inside the van. I had read about how messy and difficult cutting polyiso could be, but I had no problems using a standard box knife. I kept things running smoothly by making sure I had a fresh blade in the knife. Once the foil wrapping started to bunch, I put in a new blade.

Running the knife along a straight edge a couple times and then folding the panel (effectively breaking the rigid insulation) proved to be the most effective method.  Once folded, I could easily cut the rest of the way through. Boom, done!

After fine tuning the fit of the boards, it was time to adhere the polyiso panel to the inside of our Promaster. I used 3M 90 spray adhesive for its quick cure time and then followed up with a bead of Great Stuff Pro Gaps & Cracks Sealant to make sure it wasn’t going anywhere. I’ve seen others exclusively use Great Stuff Pro, but by using the 3M, I eliminated the need to prop the polyiso up with boards for an extended period of time. Great Stuff foam has a much longer cure time.

You’ll make a lot of progress pretty quickly with the larger side panels. Once you start getting to the smaller and awkwardly shaped ones, your production takes a nose dive. For example, I had to get a little creative to accommodate a crease in the lower side panel.

After cutting out the ceiling pieces, we marked where the ribs of the van would hit the polyiso. This saved us a lot of adhesive spray and time. We used the same technique as before: sprayed adhesive on both surfaces, made a picture frame of Great Stuff Pro on the polyiso board, held the panel in place for a couple minutes, and then foamed around the perimeter.

We continued this process throughout the rest of the Promaster. When it got too tedious cutting out tiny polyiso pieces, we decided to just use the spray foam to fill in the remaining gaps.


We still need to insulate the floor, but in order to do so we first need to remove the factory flooring. Check out Part II here!

Pro Tips:

  1. Keep a fresh blade in your box knife to prevent the foil backing from ripping
  2. Don’t be cheap (I was at first). Buy the Great Stuff Pro Gun and Gun Cleaner!
  3. Start with the easy big panels first to gain experience
  4. Be patient and don’t rush the job. It starts to get tedious once you get to the smaller sections
  5. Be proactive about keeping your Pro Gun clean

Materials/tools used:

  1. Four Polyiso rigid foam boards
  2. Three cans of Great Stuff Pro Gaps & Cracks Sealant
  3. One Great Stuff Pro Gun
  4. One can of Great Stuff Pro Gun Cleaner
  5. Four cans of 3M 90 spray adhesive
  6. Two box knifes and extra blades (I think this style would work better than what I had on hand)
  7. straight edge (I REALLY wished I would have bought one of these. I just used a level I already had)
  8. Tape measure
  9. Three cases of Le Criox (I hate this stuff, but I keep drinking it!)

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