I use a 10 gal igloo preheat for 20 min the add grain and a 2" styrofoam lid the the cooler lid and still lose 3-4 degrees. I also add the grain 2 degrees hotter and stir the piss out of it till my arm is dead and usually close it up 1 degree hotter than the target. What am I doing wrong. My grain to water ratio is 1.30
my guess would be that you are not preheating adequately, or you are not wating long enough for the grain water temp to stabilize...the mash temp will drop for about 5 minutes till it becomes constant...checking it too soon will indicate false highs IMO.