Thanks guys. I can definitely live with losing 8-10 degrees, I think. At what temp do I need to worry? Maybe I will mash a bit higher than I usually do.
ETA: by the way, I will probably test this tonight, if I do I will report the temperature specs back so others can have that data point.
redrocker652002, do you preheat? I typically put ~160 degree water in my cooler first, about half full for 30-60 minutes before I start mashing, to get it to 'warm up'. It drops a bit but stabilizes. After initial mash-in and stir, I usually add a tiny bit more water to adjust, but it never drops more than about 2 degrees over the first hour, and maybe 3 or 4 degrees total when I've mashed out at 90-100 minutes. And that's always without any blankets or towels for additional insulation.
Denny's right about the grains being a stabilizer (thermal mass), you may want to try your hour-long test with a thick mash and see what you get. Is your 10gallon a rectangular cooler, or is it the orange cylinder? Mine is the orange cylinder, maybe there's a difference in the insulation levels/materials.