You can use any kind of insulation that you can cut and fit into the Energy House. One inch is a good thickness. We like using the pink, white or blue styrofoam boards or the kind with foil on both sides.
An easy way to mark the size of the windows is to cut the wall insulation to size. Then slide it in the Energy House and use the window cutouts to mark the insulation.
Remember to check the measurements for the insulation, especially if your Energy House is made out of a cardboard box that's a different size than a beer case "shell". Also, some 'shells' are a little different size. Make adjustments as needed.
The insulation should fit snugly, but you should be able to slide it in and out without breaking it.
If you break the insulation board, just tape it back together.