I'm planning on camping at the Grand Canyon...
I'm planning on camping at the Grand Canyon in December. Would this pad be enough insulation along with a winter sleeping bag?
depends on the type of bag and how you sleep that will determine if you stay warm, but this pad should do it's job fine. The foam will keep you warm as the temps should be right around freezing which should not be too bad. Also, depends on where you are camping, as the temperature varies fairly significantly depending on where you are. If you are in the inner gorge, this will more than do the job.
for me, i would take a short inflatable to put on top just to increase the cushion and to put mor distance between you and the ground, but in all honesty, Ian is right, this should od you just fine...
If you are cold weather camping, I suggest you get a closed cell pad.