Live up in the northeast and most folks who stick around and are full timers use rigid foam board from ground to underside.
The 2 inch thick panel has a R rating of 7.2 or so. typically used for foundations as a thermo break. I guess you can double them up to get a better R value. I think that a SIPs panel(structural insulated panel) could be had from a good lumber company, a lot of times you can get cuts from a mill (if one was in your area) they have nice hi R values and will help keep the snow and cold at bay.
We have some real cold weather at times in upstate NY. Don;t no what the typical temps are over in Oregon, but when it goes below zero you need all Rs you can get and keep the propane filled
Good Luck,