[james_cornell]

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

[klickitat jim]

It will be more full, so probably it will hold heat longer. All you can do is try it. Im betting it will work fine. Try preheating the cooler so you dont lose temp too. Heat retention is related to mass too, so a 2 gallon cooler wont hold the heat like the same cooler twice as big. In otyer words its easier to cool off 2 gallons than it is to cool 4. Also some of your heat loss is from opening it to ckeck. After you insulate your lid, try your test again. Fill it with x degree water and check it again in an hour.

[Slowbrew]

I'm not going try to prove this mathematically but in my experience a mash will hold temp longer than plain water.  The only way to really know is try it.

I would definitely put foam in the lid.  It made a difference in my first round 5 gallon mash tun.  I also drilled a small hole in the lid, dead center, to I could put a long thermometer in during the mash so I wouldn't have to open the cooler.

YMMV

Paul

[WDE97]

I use a 10 gal Igloo water cooler for my mash tun. I always preheat it by putting two quarts or so of hot water into it for a few minutes and sloshing it around to heat the tun up. I then dump that water and add my mash water. I also put several thick beach towels on top of the lid, put a towel under the tun (granite countertop is cold) and wrap a down sleeping bag around it. I never lose more than 1-2 degrees during a 60 minute mash, even with the house at 66F. 

[joe_feist]

I have the 10 gallon igloo. I filled the hollow lid with thinking, "it can't hurt." Seemed to make sense at the time and it was easy. I would agree that it also makes sense that the greater mass of a full mash would help retain heat. Never did a side by side and I don't have any experience with a small mash. Good luck

[james_cornell]

Since I wanted to do small batch all grain brews and not deal with constantly maintaining mash temp in a pot, I figured a 2 gallon cooler would be better since the mash and grain for 1 gallon brew in a 5 gallon cooler would leave a lot of dead space, which I thought would lead to significant heat loss, but I also would leave a significant portion of the wort in the cooler since more would sit below the spigot.

Im gonna fill the lid with foam tonight and give it another try.

[brewinhard]

Yeah, you shouldn't be losing that much heat in an hour.  +1 to pre-heating your mash tun with boiling water (or very hot water) before doughing in.  If you don't do this you will lose at least a handful of degrees when mashing in.  After I get my mash temps where I want them, I then wrap my entire cooler with a thick beach towel around the sides and lid to keep my temps stable. I rarely lose a degree with this method.