To attach the layers of foam insulation board to the wood parts of the module and to each other I use the Liquid Nails product.
It does not take a whole lot of this adhesive to get the job done. This photo shows how much I use to stick one layer on top of another. Notice that I also try to get some on the profile boards where they meet the edges of the foam board. I used 1 tube of Liquid Nails for this project.
When the section of foam board is wide and flat enough, I will place something on it to keep some weight on it while it sets. I normally allow it to set for 24 hours before continuing to the next step. Since these three layers shown in this photo were all the same size and right on top of each other, they were all done at the same time.
The top layers are generally not going to be so flat on the top so to hold those in place while the adhesive sets, I will pin it to the lower layer with some large nails as shown here.