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Temp loss during mash

Hey guys, I’ve been trying to get back into the swing of things but have hit a bit of a snag during mash. I have a 15gal orange (sorry Denny) Igloo bev cooler as my mash tun and the last three/four batches I’ve been able to get up to mash temp (152) using BeerSmith2’s calculations, I screw on the lid and let it go, but I’m losing lots (~10-15 deg) of heat during a standard 60 min mash.

I don’t remember having this much of a problem in the past, might the insulation material in the cooler have been compromised due to the heat of the mash? I’m only pulling ~55% efficiency right now and it’s really frustrating.

I don’t have experience with round MT’s but I have been placing a comforter over my MT during the 60 minutes and might lose 1 or 2 degrees.

I did see one round cooler user on here that cut a circular piece of styrofoam insulation and placed that on top of his grain during the mash and remember the poster saying that he rarely lost any temp.

How close are you coming to filling your MT when you mash?

You should be able to insulate the lid by drilling a few small holes in the top layer and filling with spray in foam. You can use snap in nylon hole plugs to fill the holes.

I also mash in a round (red) cooler and have had no issues in almost 100 gallons. are you covering your tun? I don’t know if it helps but during the mash I have mine covered with 2 sleeping bags and insulate the bottom with a closed cell foam pad. my temp loss is about 2 degrees.

I use a round 5 gallon cooler. I filled the lid with low expansion foam and then cut an inner lid from 2 inch styrofoam. On a bad day, I’ll lose 1deg F over 90 minutes.


I would feel for warm parts of the cooler during the mash. If it’s warm, you’re losing heat. The cover is definitely the worst culprit (I put a winter coat over the cover), but your tap being metal transfers heat as well. I took a block of styrofoam and hollowed it out to fit over the tap during the mash. This cuts down on heat loss as well. Also, the less full it is, the less thermal mass to hold its temperature as well.

+1. Although I use a rectangular cooler, the same premise still applies: lids on any type of cooler aren’t built for keeping heat in. I used 1 1/2" styrofoam with radiant / reflective coating. I had some left over from insulating my garage door, and it work wonderfully. I, too, only lose 1-2 degrees per hour. I added craft (self-stick, strip) magnets to it for easy removal and cleaning.

Are you losing that much heat steadily throughout the mash? Or is it dropping during mash-in? It sounds like you might just not be accounting for the heat lost to the cooler itself. I’d guess that BeerSmith has the ability to do that, or you could just heat the strike water more than you have been.

It must be the cover, I haven’t done any modifications to the cooler to compensate for heat loss. I just assumed that since it’s insulated, it’d be fine. Of course, now that I think about the whole heat rises aspect, I kind of did a facepalm. I’ll try the 2" foam the next time and see how it goes.

It does happen gradually, though, and I’m hitting my target mash temps, so I don’t think heat loss during mash in is the culprit. It’s just odd that I didn’t run into the issue when I first made the Tun.

You may just be taking your initial reading before the cooler, grain, and water all come to an equilibrium temp. Start with higher temperature water and stir for a few minutes before putting the lid on. I really doubt a cooler would lose that much heat in an hour.

My thoughts exactly.

Idris_Arslanian, can you post your numbers for one of these problem mashes? Grist weight and temperature, strike water temperature and volume, initial and final mash temperatures?

+1. If your cooler is warm to the touch, you’re losing heat that way and need to insulate the cooler. If not, then you’re losing heat at the beginning either by striking too cool, not pre heating the cooler or both. You could also be losing heat through hving no lid. Coolers are food safe plastic and should withstand VERY hot water, so pre heat it with as hot a water temp as you can, leave it 20 mins, adjust for strike then mash in.

FWIW I use a rectangular cooler inside a home made polystyrene insulation box.


My round cooler only loses maybe 5F at most over a long 90min mash.

You can preheat your MLT by adding your strike water at 10F over target and letting it transfer heat as it cools.

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