I've tried it in an oven at 200F. It worked fine. It actually gained a few degrees in the hour. So, I might try what wilserbrewer said and just turn the oven off after placing the kettle inside. I found that the better method is to heat the mash water in the kettle to the target temp, place the kettle on a folded towel on the ground, add grains, place lid on kettle, and cover with several other towels. I never lose a degree like this.