How to Maintain Mash Temperature with BIAB

In many ways, Brew in a Bag (BIAB) is the easiest, simplest setup for brewing all grain beer. Unlike the traditional three vessel setup, all you need is one pot to serve as your hot liquor tank, mash tun and boil kettle. There are a lot of reasons to utilize the BIAB method, including saving money, reducing the space needed for equipment, and eliminating the issue of stuck sparges. When I started brewing in a small apartment, BIAB was simply the only way I could brew all grain beer. As I have gained more space, the simplicity of BIAB has continued its hold on me and is still the main way that I brew. As much as I love it, using one vessel for the entirety of your brew day can come with its own set of downsides as well.

One major disadvantage to the BIAB method is the lack of an insulated mash tun. In traditional three vessel setups, the mash usually takes place in a cooler. The cooler is insulated, which means that once the water and grain are mixed and the lid of the cooler is put on, the temperature is likely to only drop one or two degrees. Hitting the mash temperature that you want is important; higher mashing temperatures generally produce a heavier-bodied beer, whereas a lower mashing temperature will create a more fermentable wort, leading to a lighter-bodied beer. Maintaining your mash temperature is a critical variable in producing the beer that you want, as well as producing consistent beer batch to batch.

With BIAB, mashing is done in a metal kettle. The problem is that if left alone, the mash temperature can drop drastically in the 45 to 90 minutes it takes to create fermentable sugars. This can lead to the wrong type of sugars being produced for your recipe. Luckily, there are a few different things that we can do to keep our temperatures from dropping too much.

Method #1 – Applying heat

With this method, you are simply keeping your kettle on your heat source and applying a small amount of heat as needed to keep the temperature constant.

Pros Cons
  • No new equipment is needed, so it is easy to get started.
  • A consistent temperature can be achieved fairly easily once you figure out the correct amount of heat to apply for your system.
  • Can be tricky to keep the temperature consistent. Especially at first, constant monitoring is needed to be sure that the right amount of heat is being applied.
  • Since the heat is being applied to the bottom, some stirring is required to disperse the heat throughout the mash tun.

Method #2 – Insulative wrap

For this option, you wrap your mash tun with an insulating material to keep the heat in. This insulating material can be anything from a blanket to specially designed insulative wrap. The most common material that homebrewers use is called Reflectix, which is a reflective insulation used in construction projects. The reason that Reflectix is so popular is because it is easy to wrap around a mash tun and to cut sections out to accommodate valves or thermometers. One downside of Reflectix is that it is a radiant barrier, which means that it works most effectively with an air pocket between it and the mash tun. This is fairly easy to set up with a couple of spacers (you can see a spacer made of Reflectix in the image below), or you could take the efficiency hit and just mount it straight to the mash tun. A word of caution however: Reflectix has been known to melt when exposed to a flame. If you are using a burner or a gas stove to heat your wort, you will want to make the Reflectix removable for when the flame is on.

Reflectix on kettleCreating an air pocket

Pros Cons
  • No need to continually heat or monitor.
  • Can usually maintain your mash temperature within a few degrees.
  • Reflectix works best with an air gap, which can be a little tricky to set up.
  • Insulation needs to be removable if you use a flame to heat your wort.
  • Minimal cost (~$20)

Method #3 – Stick it in the oven

If your setup allows for it, sticking your mash tun into your preheated oven is the easiest way to maintain your mash temperature. Setting the oven temperature to its minimum and leaving it there for the duration of the mash will usually do the trick, however some ovens may run too hot for this to work. For these ovens, a little experimentation may be required; turning the heat off at some point before or during the mash may work best. Don’t be too worried about getting it perfect, the oven doesn’t need to be your exact mash temperature. As long as you are somewhat close (±20ºF) your mash is unlikely to change temperature. Another thing to consider is the physical size of your equipment; if your mash tun is too large, you may not be able to fit it into your oven. This is my personal favorite method; my eight gallon brew pot fits nicely into my oven, and I don’t lose a single degree over the duration of a mash!


Pros Cons
  • Extremely consistent temperature.
  • No need to monitor. Use that time to clean. (Or drink!)
  • It may take you a couple brews to know when/if to turn off the heat.
  • Larger mash tuns will have a hard time fitting into an oven.

Brew in a bag is an easy, low cost way to brew all grain beer. Maintaining your mash temperature doesn’t have to be hard; all of these methods will work to varying degrees. You will find that the best method for you will depend on both your equipment and how you run your brew day.

Looking for an easy way to serve your homebrew? Check out my review of the Intertap Flow Control Faucet!

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