The building-insulation kind comes in various thicknesses (1/2", 1" 1 1/2", 2", 4"), sizes (2'x 8', 4'x 4', etc) and insulating R-values. The denser and thicker the product, the more expensive it is.
Since the inside of my modules are 2'x 4' (and sometimes 30"x 48"), I'm interested in the Styrofoam that is 2'x 8' and which is readily available at local building supply stores. When I build 30"-wide modules, I simply glue two pieces together.
There are usually two types of 2'x 8' Styrofoam boards - butt edge and "shiplap" edge. As the name implies, two sheets of butt-edged Styrofoam simply butt up against each other. The edges of two sheets of shiplap edges will overlap each. While I prefer the butt-edge sheets as I have to cut off the overlap-edge on shiplap, you'll have to use what is readily available to you.
As Styrofoam is an oil-based product, the price will vary greatly. You can save quite a bit of money if you buy your Styrofoam at your local contractor's building supply store.
Styrofoam cuts very easily. You can score it deeply with a utility knife and snap it in half. You can cut it with a hacksaw blade or hand saw. It cuts really nice on a table saw if one is available to you. I'm going to cut my Styrofoam using a hand saw. Styrofoam can be very messy when cut with a saw so I'm cutting it outside on my picnic table. Measuring the inside of the module frame, I need a piece that is 22 1/2" wide and 46 1/2" long. Love that pink! (Or is it blue? Maybe it's green.)
I stored my Styrofoam outside in my tent garage and the chipmunks decided to make a home in it. So I'll have to patch some holes for the second module.
Rub the edges of the cuts with your hand to make sure the crumbs stay outside and aren't brought into the house.
Dry-fit the Styrofoam into the of the module frame to make sure that it fits snugly in place. I like to put the lettering topside on the module as I'm going to paint the module deck. This will leave a nice"clean" look on the underside. Where there is any "friction fit", trim the sides of the Styrofoam.
Don't be concerned if there are gaps between the Styrofoam and the module frame. In our next blog, we're going to use some expandable polyurethane glue (you might know it as "Gorilla Glue") to glue the Styrofoam in place. Later on, we'll smooth out any other gaps with some spackling compound.
Check and make sure the Styrofoam doesn't poke above the edges of the module frame. If they do, score the Styrofoam underneath the gusset and shave a slice or two out of the Styrofoam. This won't weaken the Styrofoam in any way as the Styrofoam isn't glued to the gussets.
Up next, things start to get kind of messy as we get ready to apply the polyurethane glue.