I entered all-grain homebrewing the way many of us do: I found the simplest and most affordable method that worked. For me, this was a combination of a large pot on the stove and a grain bag for easy infusion mashing and batch sparging. Using a single pot for the mash and the boil reduces your initial equipment costs and removes the need to transfer liquids between vessels, but keeping your mash temperature constant in a bare metal pot can be a challenge. This is especially true when you’re brewing small batches, because you don’t have the thermal mass of a large volume of water.
My solution was to make this “tun cozy,” an insulating jacket for my brew pot, which I can easily remove when it’s time to start the boil. I now lose less than 4 °F (2 °C) per hour when mashing. It’s perfect for single-vessel brew-in-a-bag setups like mine. It can even be used for a standalone mash tun or hot liquor tank.
The design uses canned expanding foam, handy for filling large gaps in all sorts of do-it-yourself jobs around the home. This stuff has excellent thermal insulation properties. Even better is that it’s readily available, easy to use, and inexpensive. This whole project cost me around $25.
In a nutshell, you’re going to wrap your brew pot in aluminum foil, stand it on a spacer within a larger container, and then fill in the gap with expanding foam. When the foam sets you’ll have a tailor-made, removable insulating jacket for your pot.
I’ve made two of these so far. The first was for a rather modest 4-gallon (15-L) pot, which is great for small batches of around 2.5 gallons (9.5 L). More recently I built an 8.5-gallon (32-L) version, which is ideal for 5-gallon (19-L) batches. The great thing about this design is that it works for almost any vessel, providing you can find the right size container to stand it in. This article will guide you through building my 8.5-gallon (32-L) tun cozy, but keep in mind that you can use the same steps to build a tun cozy of any size to fit your needs.
The main challenge will probably be locating a suitable container. I found that a large rubble bucket (Gorilla Tub is a popular brand name) was ideal for my 8.5-gallon (32-L) pot. The foam doesn’t stick well to this type of plastic either, so the bucket can be used again after the project is finished. You’re looking for something between 3–6 inches (7.5–15 cm) larger than your pot on every side (including underneath). The smaller the space, the less expanding foam you will need, but also less insulation it will provide.
In total, this project will take a couple of days from start to finish, however most of that time is spent allowing the foam to harden.
Note: Wear gloves and appropriate (old) clothing, because this expandable foam is very difficult to remove once it has set.
Parts & Tools
2–3 cans of expanding foam
A plywood off-cut and three 3–4 inch (7.5–10 cm) nylon bolts with nuts
Non-serrated blade (a chef’s knife works well)
1. Cover your pot in aluminum foil
To begin, wrap the entire exterior of your pot in foil so that the foam sticks to the foil rather than to your pot. This allows your tun cozy to be a removable jacket rather than a permanent addition to your brew pot!
If your pot has a spigot, then this should be removed and the hole covered with the foil. The pot needs to have straight sides if it’s to be easily removed from the foam later. Secure the foil where necessary with tape (I used PVC electrical tape) and overlap separate sheets by an inch or two. Try to arrive at the base of the pot with surplus foil so you can fold it around the bottom edge. Carefully scrunch around the pot handles, but ensure foil spans the hole where your fingers would normally go. If the foil tears then patch it up with tape. The aim is to have a thin and smooth foil wrap completely covering the outside of the pot, but it doesn’t have to be pretty.
If your outside container is a smooth plastic it likely can be used as is because the foam will not stick well to it. Otherwise, line the inside of the container with foil too.
2. Raise your pot in the container
We want insulation under the pot as well as around it, so it needs to be lifted up off the bottom of the container with a spacing device. I simply used a plywood off-cut with three 3-inch (7.5-cm) nylon bolts evenly spaced by approximately six inches. There are several advantages to this spacer design: It’s lightweight; it leaves a good gap under the center of the pot; you can easily adjust the height of the bolts if the pot doesn’t sit level; a three-point contact means the pot won’t rock; and nylon screws won’t scratch your pot. You might want to fix the spacer down with something light duty to avoid it shifting, however the foam will bond everything together. Ensure everything is centered and level before proceeding.
3. Fill the gap with foam
This is the fun part. The foam needs to be added in layers. My experience was that each layer added three or four inches to the overall height of the foam, and you should give at least five minutes for each layer to finish most of its expansion before applying the next layer.
The first step is to add the base layer, so do this with the pot removed but with the spacer in place. Allow some headroom for the expansion. Next, place the pot on the spacer and center it up in the container. Put a heavy mass inside the pot at this point, as the foam will provide significant upwards force. I simply filled mine with water.
Wait for the base foam layer to expand and creep around the edge of the pot. Methodically build up the layers, waiting a few minutes between each, until your latest layer sits just below the pot’s rim. Now relax and have a homebrew! The foam will take significantly longer to completely set than the can advises, because it’s such a deep gap. Leave it alone for 24–48 hours to fully harden.
4. Trim the foam and remove the pot
The beauty of this foam is how easy it is to shape once it’s dry. Using a long, sharp blade, cut around the rim of the pot so the foam is flush. A chef’s knife works really well for this. Cut or tear any protruding foil to expose the rim of your pot completely and empty the pot.
You can now remove the hardened foam and the pot from the container. If the container is smooth plastic or lined with aluminum foil then the pot and insulation should lift out easily in one unit and the container can be re-used for whatever you like.
Removing the pot from the insulation is the next step. This takes a little wiggling, a little patience, and perhaps some help from a friend. Don’t be scared if it won’t budge; the tight molding produces a formidable vacuum beneath the pot as you try to pull it out. You can always try again after making the first cut in the next step if you can’t get it off. Just be careful not to scratch your pot.
5. Slice and sculpt
Full mash tuns are heavy, so we don’t want to be struggling to lift the pot out of its new insulating jacket on a brew day. A good solution is cutting the jacket in half lengthwise as far as the base, then skirting around the base under one of the halves (see picture). This leaves you with a large piece for placing the pot onto, and a smaller piece that can be brought in from the side.
If your pot has a spigot, then make sure one of your vertical cuts lines up with where the spigot normally exits the pot. Then, once the two pieces are cut as described, you can remove an appropriate amount of extra foam from either side of the vertical cut in order to accommodate the spigot.
6. Finishing touches
You now have a custom-molded insulating jacket for your brew pot that is easy to add before mashing and easy to remove afterwards.
On a brew day I’ll begin by heating water in the pot to a few degrees higher than my strike temperature. Then I’ll put the pot into the tun cozy, add my empty grain bag, and wait for a few minutes with the lid off until I see the strike temperature. During this time the inside of the insulation has been pre-heating a little, which minimizes initial losses during the mash. I use towels to insulate the lid of the pot during the mash, although a molded foam lid would be a great upgrade! After the mash I do a combined mash-out and partial batch sparge by filling the pot with hot liquor and removing my grain bag a few minutes later. I then take off the insulation and get my wort straight on the stove for the boil.