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Maintaining Mash Temperatures

My mash tun is a picnic cooler.

I thought it was well insulated.

I have mashed with it a few times.

I have lost a lot of heat out of it.

The mash temp may be 152 degrees when I close the top.

When I open it, it’s much lower…15 degrees lower.


What gives?

Mash Tun

Just my opinion, but I am not sure picnic coolers were made to maintain high temperatures for a hour or so.  On thing Mike pointed out about picnic coolers is that their covers are not well insulated.  Where does heat go?  It rises…and it rises right out of my mash tun.

One thing I am going to try next time to maintain my mash temperature is to put a heating pad around it and put a thick blanket on top of it to try to keep the heat from escaping.

Do you have any techniques that you would like to share about maintaining mash temperatures?


Galena Hops


Brew Prep – Brown Porter


  1. Aaron

    I’ve never had that happen and I’ve been mashing in a 10 gallon cooler for about three years. What I usually do though is preheat the mash tun by pouring 5 gallons or so of hot water in it. I roll it around and get the whole thing warm and the pour the water back into my kettle and continue heating it until it’s at the strike point. If your mash tun is super cold then it may bounce back fast.

    Another thought, my cooler is a hard-core igloo cooler — not a cheap one. It may be better insulated then yours… just a thought.

  2. AaronS

    And heat don’t rise. Colder air falls.
    Preheating the tun will help, and a blanket around the tun will help tons.

  3. wayno

    Pre-heating is so very important.

    I was loosing about 6 deg in my 12 gal square cooler. I got some foil backed bubble pack insulation from my local hardware and made a bonnet to cover the top and half way down the sides of my cooler. I now loose about 2 deg in 90 min. A real cheat and easy fix.

  4. Poop Davidson

    I have both a 5 gallon rubbermaid and a 10 gallon igloo. both lose about 2 degrees in 60 minutes. I don’t preheat the mashtun either. I overshoot the strike water temperature by 4 degrees to compensate.

  5. I use a rubbermaid igloo cooler and I only loose 1 degree, everytime. I always preheat with a quart of hot water before mashing. Heat will dissipate into the grain and cooler and you must account for that before hand.

  6. Thanks guys. I think the cheapness of the cooler is affecting the performance, but it appears that some preheating and some more insulation will help. Both of these things are easy fixes.

    Thanks again for sharing your insight.

  7. Griz

    We have both a 5 gallon (Rubbermaid) and a 10 gal (Igloo) cooler that we use as mash tuns. We usually loose about 8 degrees when we dough in so if we’re shooting for a 152 mash we heat the strike water to 160. We always fill the mash tun with the strike water first then stir in the grain so we get all the grain wet. Then we float the thermometer and check the mash temp. It takes a couple of minutes for the temp to settle but once it does I’ve never seen it drop more than a couple degrees over a 60 minute mash.

  8. I had the same problem with a 10 gallon cooler that has a top that is essentially uninsulated (thin, hollow plastic). My fix was to cut a circle of styrofoam that was ~2″ thick and rubber-band it to the bottom of the lid. Now I only lose ~5 degrees over 1 h.

  9. Mumsbrew

    I also found out that keeping your grain warm is a big help. If the grain is cool or cold your going to lose alot of heat trying to warm it up to temp. I found this out on my first all-grain and lost about 8*. Needless to say my starting gravity was off. I like the idea of preheating the cooler. I’ll give it a shot next time.

  10. Scott

    The hollow lids can be easily filled with exspanding foam. Drill a 3/8″ hole and fill with a about a 1/4 can of foam, some will come out the hole, let it dry and cut off the excess.
    Works good.

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