The Tun Cosy: Removable Mash Tun Insulation for Homebrew




About: Engineering nerd, photographer, drummer, homebrewer.

If, like me, you're a homebrewer who uses the same pot for mashing and boiling, then you may have experienced heat loss during the mash that you'd rather didn't happen. This tends to be a problem for smaller batches, or when brewing outdoors in the colder months.

My solution was to make this 'tun cosy' (or I suppose tun cozy if you're in the US), an insulating jacket for my brew pot, which I can slide on and off. I now lose only 1-2 deg C per hour while mashing batches as small as ~2 gallons, and that includes lifting the lid to check on things.

The design uses canned expanding foam, commonly used to fill large gaps in walls for DIY etc. This stuff has excellent thermal insulation properties, and as such it's often used in large quantities to insulate cavity walls in houses. Actually it's similar to what you find around hot water tanks (so it must be good!). Best of all, it's readily available, easy to use and cheap. This whole project cost me £10 (approx. $15), and most of that went on buying a huge plant pot.

In a nutshell, you're going to wrap your brew pot/kettle in aluminium foil, suspend it in a larger container, and then fill in the gap with foam. When the foam goes hard you'll have a tailor-made, removable insulating jacket for your pot.

I tend to brew ~8 litre batches so my pot is quite modest (15 litres), but there's no reason this can't be done for much larger pots, providing they have straight sides and you can find a big enough bin to contain the foam.

UPDATE 2014-03-19: I just had an idea for people with large pots, or who have pots with spigots, or who don't want to lift a heavy pot full of mash: once the foam has set then you could simply slice the whole thing in two. Then, instead of lowering the pot into the tight space, you can bring the two halves of the foam in from opposite sides and hold the whole thing together with a strap. See new step 6...

Step 1: The Gear

So the general plan is to suspend your brewing pot/kettle in a big container and surround it with expanding foam. I found the main challenge to be getting a suitable container. For my 15 litre stock pot I found a big plant pot that would do the job (in the UK you can get one of these from B&Q or Wilkinsons, for about £7). This allowed about three inches dead space around the whole pot, except at the bottom edge of the pot where the taper reduced this to two inches or so.

Requirements for the container

  • You're looking for something a few inches (say ~3") larger than your pot on every 'side' (including underneath)
  • The smaller the space, the less expanding foam you will need
  • The larger the space, the better the insulation, but the returns diminish since mash times are only an hour or so
  • You could make your own container, but be careful about materials - cardboard might go soggy
  • Something like a garden dustbin might be a good choice for large pots/kettles

So, here's everything you'll need

  • Your brewing pot/kettle (duh)
  • A large container as discussed above
  • A can or two of expanding foam (one 750 ml can was enough for me - I got it from Screwfix)*
  • Something to raise the pot off the bottom of the container (see next step)
  • Aluminium foil to wrap around the pot
* apparently you should steer clear of 'easy cleanup' or 'water soluble' varieties of this foam because they're too weak - thanks sharpstick for the tip

Step 2: Suspending Your Pot

We want insulation under the pot as well as around it, so it needs to be lifted up off the bottom of the big container. This is how I did it: just a piece of plywood with three nylon screws.

Advantages of doing it like this:

  • Leaves a nice air gap under the pot for filling with foam
  • You can easily adjust the height of the screws if the pot doesn't sit level
  • Three-point contact means the pot won't rock (kinematic mount)
  • Nylon screws rather than metal to avoid scratching the pot

Whatever you use, it doesn't need to be fixed to the bottom of the big container because the foam will bond everything together. You might want to stick it down with something light duty though, to avoid it shifting for the time being.

Step 3: IMPORTANT! Covering Your Pot in Foil

VERY IMPORTANT! This is what makes your insulation a removable sleeve rather than a permanent addition to your brew pot!

The idea here is to wrap the pot in foil so that, once the foam has set, the pot can be removed. Hence you can mash inside the insulation, then pull out the pot and get it straight on the heat for boiling.

This is quite an easy task if you take your time and do things in a sensible order:

  1. Start with the part of the pot below the handles, where's it's a constant diameter, and work towards the base from there. The foil doesn't have to be pulled tight, but you don't want it baggy either. Try to not crinkle the foil too much. Secure it where necessary with some tape (I used electrical tape).
  2. Gently fold the foil around the base of the pot and tape it again. The foam won't want to invade between tight folds of foil, so there's no need to make all 'airtight'.
  3. Continue the foil wrap past the top of the pot and fold it inside. Carefully scrunch around the handles, avoiding any tears. If the foil tears then patch it up with tape.
  4. Tape over any edges where one sheet of foil ends and another begins.

The thing to remember is that this doesn't have to be pretty, but the foil does need to cover all of the outside of the pot. Any gaps and the foam will make direct contact with your pot and stick to it, and this will only make your life harder when you eventually try to get the pot out.

Step 4: The Fun Bit: Filling With Foam

Adding the foam has to be done in layers, so you'll need to allow enough time. My experience was that each layer added three or four inches of height to the overall level of foam, with at least five minutes for each layer to finish most of its expansion. It took me about half an hour to add all the foam.

Lessons learned

You'll see in the photos that initially I thought a bag of rice would be heavy enough to hold the pot in place. WRONG! In the end I threw in as many heavy things as I could find, so seek out some ballast before you start. The base layer of foam can really push up!

Also it's easy to miss just how much the foam expands. If you watch it it barely seems to move, but in reality it will puff up to four or five times its initial volume over five or 10 minutes. Observe carefully and you'll get a feel for the expansion.


WEAR GLOVES AND SCRAPPY CLOTHES. If the foam dries on your clothes it will never come out. Get it on your skin and you'll end up removing it with sandpaper, not soap :|


Gloves as already mentioned. Plastic sheet on the floor. The foam will drip out the end of the can between uses, and it's best to just let this fall onto your plastic sheet and let it dry there. Wiping this stuff is almost always a bad idea.

The foam sets in the presence of moisture, so I slightly wet the sides of my pot and container to help it along. Probably not essential though.


Ok, here goes:

  1. First the base layer needs to be added, so remove the pot from the container (not the spacer from step 2 though) and squirt an even layer of foam across the entire base of the container. Avoid the temptation to fill it all - you only need a layer around a quarter of the height of the gap and magic will do the rest.
  2. Replace the pot, fill it with as much mass as you can find* and ensure that the pot is centred in the container. We don't want to move it from this point onwards if we can help it.
  3. Wait for the base foam layer to expand and creep around the edge of the pot. Only about five minutes in my case.
  4. Add another layer of foam directly on top of the last layer, working evenly around the pot. Aim for an initial layer height of between half an inch and an inch - it will expand four or five times this.
  5. Wait again for expansion to slow down, but not so long that it starts to harden. Five to 10 minutes again, perhaps.
  6. Repeat steps 4 and 5 until your latest (unexpanded) layer sits just below the rim of the pot.
  7. Now relax and have a homebrew! The foam will take longer to completely set than the can advises, because it's such a deep gap. Leave it at least overnight to fully harden.

* you could fill the pot with water to weigh it down - thanks woolleyy for that idea

Step 5: Trimming and Finishing

The beauty of this foam is how easy it is to shape once it's dry. Using a sharp knife, cut around the rim of the pot so the foam is flush. Also cut or tear the protruding foil and remove the excess to expose your pot.

Something unexpected

At this point I expected the pot to simply lift out of the foam-filled container, but not so. Actually there was barely any adhesion of the foam to the big plant pot, so I actually removed the brew pot + insulation as one! See photo.

In fact, the fit was so snug that it took a little while to get the pot out of its new jacket - I now realise it's because there was no way for air to enter the cavity beneath the pot. You could solve this by poking a hole trough the insulation at the bottom, but after I removed the pot the first time I found the foam had flexed enough to allow air to enter/exit down the sides of the pot. Something to think about though if you have this problem.

UPDATE 2014-03-19: See new step 6 for, possibly, an even better solution to this.


Anyway, you should end up with a perfectly-fitting, custom made cozy jacket for your pot during mashing. You could even make a foam lid, although I just use towels.


My temperature drops during a mash are now consistently less than 1 deg C per hour, and that's including removal of the lid to do pH checks etc.  Also I tend to do ~8 litre mashes, so a "true" BIAB (full volume) mash would hold even better.

TIP: If you're doing BIAB, then you're probably adding the grain to the water rather than the other way around. If this is the case then consider heating your mash water a few degrees higher than usual, then insert the pot in the insulation with the lid off and wait for it to drop to strike temp. This way you pre-heat the insulated jacket a little and avoid any initial losses when inserting the pot into a cold jacket.

Step 6: (Optional) Cut the Thing in Two for an Easy Life!

This step is a later addition to the instructable, because it suddenly hit me that you can save yourself the hassle of lifting (especially big pots) if you cut the finished jacket in two.

The idea is simple: slice the set foam in half, then you can bring the halves together around the pot and secure them with a strap, tape etc. Lifting isn't eliminated entirely, since you'll probably be lifting the pot off a burner, but life is certainly made much easier, especially for removal after the mash has finished.

I recently did this for a 32 litre pot with great results (see photo).


If your pot has a spigot (tap) then there's hope for you yet... assuming you can remove the spigot for the 'moulding' phase when the foam is setting, then you can cut the foam in two, as above, but also take a little extra off so there's a gap for the spigot to poke through. This foam is so easy to shape that I can't see this being a problem. It could even work for some electric boilers.



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    33 Discussions


    1 year ago

    I tried this and it seemed to all go according to plan until I removed the cosy from the mold. The foam that I could visibly see that puffed up through the openings firmed up. But after a few inches the rest never setup and was a liquid goo. What went wrong?! I was so excited to have a perfectly formed cosy. I left it overnight, so it sat for about 12 hours. Am I missing a step? Any help would be greatly appreciated. I would like to try this again if I can figure out what I did wrong. Thanks!

    1 reply

    Reply 1 year ago

    I think just leave it longer. This foam requires moisture to cure, so in a confined gap I've found some brands of foam can take a long time..

    Try wetting all the surfaces before applying the foam, then leave it at least 24 h, or longer if you can. Maybe stand it somewhere humid (laundry?). That should be enough to remove it, then even if it's still not quite hardened it'll finish up more quickly with all the surfaces exposed..

    Good luck!


    3 years ago

    Any issues with the foam turning brown after mashing a few batches? I've seen some people exposing canned foam insulation, namely the Great Stuff brand in the states, to heats of 150 degrees and up and the insulation would start discoloring. Great article! I'm going to make one this weekend!

    1 reply

    Reply 3 years ago

    Hi Jeremy,

    Sure, my foam has turned brown.. from all the wort I've splashed on it ;) I've never heard of the heat effect before, but it wouldn't bother me if the colour changed to be honest. Besides, the foil coating covers the foam on the inside.

    I'd love to hear how your build goes!

    Great article !! and it got me thinking as i brew just 1 gallon batches at the moment, do you think the same principle would work as insulation for my glass demijohn to maintain a heat for fermentation?? Also what size pot are you using in your example?

    3 replies

    Reply 4 years ago on Introduction

    Thanks. Yes I don't see why this wouldn't work for fermentors too, if you find you need to keep things warm in a cold environment. The yeast generate heat, so you may have to do a couple of tests to make sure things are in the right range..

    The pot in my example is 15 litres and I use it to make 2 gallon batches (2x 1 gallon demijohns).


    Reply 4 years ago on Introduction

    thanks for your reply, Yes yeast generated heat could prove to be an interesting factor. could be that i need to core an area around the demijohn so it isn't flush to give it some head room.

    Can I ask how you are set up with BIAB (what / where can i get the bags ) what do you use to get 2 gallon recipes?

    Many thanks


    Reply 4 years ago on Introduction

    I use a large nylon grain bag with a fine grade mesh, available from many homebrew suppliers.

    I usually get a 'standard' 5 gallon recipe and simply scale it for 2 gallons. Your efficiency and mash strike temps are trickier - you can only go so far to calculate them, then it's just a case of trying a few brews. Brewing software such as BeerSmith can take a lot of the guesswork away though.

    Good luck!


    5 years ago on Introduction

    I found this project while trying to find DIY for a cozy for a Heat retention Cooker;

    This is perfect!!! I will be making several of these for different sizes and for much sought after gifts!! Thank you!


    5 years ago on Introduction

    I'm not interested in homebrewing, but this would be a perfect setup for making yogurt! It's just another substance that needs to be kept warm and at a constant temperature for several hours.


    5 years ago on Introduction

    Very nice work. Love the set up and ingenuity. One thing to keep in mind about the foam. Almost all single component spray foams degrade with uv light exposure. There are some made for outdoor use and are typically black in color. I would suggest you paint or seal (latex or water based paints work well) to keep the foam from becoming brittle or having bits flake off over time. Keep calm and have a home brew.

    1 reply

    Reply 5 years ago on Introduction

    Can't edit post, oh well- "paint to keep the foam from becoming brittle and flakey" is the gist.


    5 years ago on Introduction

    I can't wait to try this! I've just been wrapping my pot in layers of blankets and towels. Something I expect will help release the pot from the foam... use compressed air! Fire up your air compressor and shoot a blast of air along the joint between the foam and pot. This has worked wonders on other molds I've done that are sticky... I don't see why it wouldn't work here too. Cheers!


    5 years ago on Introduction

    Awesome! As someone who is moving away from extract kits and onto partial mash this is a fantastic idea!

    A project for next weekend methinks!

    1 reply

    5 years ago on Introduction

    Great Idea and write-up. I hoped to get some ideas on insulating a keggle mash tun but with the ribs kegs have I dont think the insulation would come back off. I guess I could make it in two halves and tie it around the keg.

    1 reply

    Reply 5 years ago on Introduction

    Ha! Oh! I just added that idea to the instructions. Cross my heart I only just saw your comment. Anyway yes I think this would work (can't believe I didn't think of this in the first place tbh). If you do it then I'd be interested to hear how it goes...


    5 years ago on Step 5

    Great idea! I BIAB as well, though I use a keggle. For those not familiar, it's a normal size keg (approx 15.5 US gallons), so I'll have to use more foam and maybe a small garbage bin rather than a flower pot. Excellent write-up!

    1 reply

    Reply 5 years ago on Introduction

    Yeah, a garbage bin should be a good one. Take a look at step six which I just added - you might be better cutting the whole thing in half when you're done.

    Depending on whether the foam sticks to your garbage bin, you may have to saw through the bin too >_<


    5 years ago on Introduction

    Just added a new step 6, after the realisation that cutting the finished article into two pieces can not only save you some hassle, but also could even make this viable for pots with spigots or electric boilers :-)