I built a 2-gallon mash tun for 1.25 gallon batches. It's your standard 2 gallon igloo cooler but not that cheap crap they sell at target, thicker insulated walls with a hollow approx 1.25" thick lid. I tested it by dumping about 1.25 gallon hot water, letting it cool down to 158 degrees (took about 20 min), then putting the lid on for an hour. I checked it twice during that hour (20 min intervals) and lost approx 10 degrees over the hour.
So here's my question: first, I know I can drill small holes and fill the hollow lid with insulating foam and I will probably do this, but when approx 3lbs of grain is added how will this help heat retention?
Thanks