I built a keezer collar using 2x6 redwood a couple fo weeks ago. I was originally going to insulate with 1" foam, but haven't done so yet. With it set at 48F it rarely kicks the compressor on, and I'm not really sure if I'm going to insulate or not.
I still need to caulk it though, and that will likely be all I do as far as increasing the insulation.