When using a cooler this worked fine. When using a SS keg as MLT my temps drop, I usually direct fire to raise temps twice during the mash. I use 2 layers of foil bubble wrap and blue sleeping pad to insulate. Temps still drop. Best results when I also use a sleeping bag over keg AND previous mentioned insulation.
When I add the grains the temps appear to stabilize at the desired temp but w/in 15 min they begin their downward path. Add heat, appears stable then they drop again within another 15 or so minutes. After the next direct fire temps finally stabilize. I never preheat anything, I think I need to bring my strike temps way higher than beersmith says to do, let it stabilize,~20-30 minutes then mash in. I believe the volume of steel continures to absorb water heat.
Drinking Oatmeal stout, Nut Brown ale, Arrogant bastard clone, Belgian Dubbel
Conditioning: Belgian Dubbel, Arrogant Bastard clone, Firestone Wookey Black IPA clone
Secondary (lagering @35F) Oktoberfest, Bock
On Deck Red IPA