The builds compared
| Build and lid | Indoors | Outdoors, July | Ice per day, indoors |
|---|---|---|---|
| Converted chest freezer · Insulated lid | 15 W · 60 BTU/h | 25 W · 80 BTU/h | 11 lb |
| Insulated plunge tub · Insulated lid | 25 W · 90 BTU/h | 35 W · 120 BTU/h | 15 lb |
| Wooden barrel · Basic cover | 95 W · 320 BTU/h | 150 W · 510 BTU/h | 52 lb |
| Inflatable tub · Basic cover | 125 W · 430 BTU/h | 190 W · 660 BTU/h | 67 lb |
| Metal stock tank · Basic cover | 235 W · 800 BTU/h | 620 W · 2,110 BTU/h | 124 lb |
Model estimate with the lid on: indoors at 72 °F, outdoors at the July average in Chicago (77 °F), in the shade. The ice column holds the temperature indoors with two 3-minute plunges a day. Typical insulation for each build; a better lid or extra insulation lowers every figure.
The table runs our plunge model with the same water in each build. The differences are large: a chest freezer's foam walls let in the least heat, around 17 W for 400 litres held at 8 °C in a 22 °C room, while a metal stock tank with a basic cover lets in about 230 W, roughly fourteen times as much. Outdoors in summer the gap widens, and direct sun adds more on top.
Converted chest freezer
The most efficient container you can buy cheaply, and it brings its own cooling. A 15 cu ft freezer holds about 425 litres, enough to sit in with your knees bent.
- It is not built to hold water: people seal the inside seams with a water-safe sealant so water cannot reach the insulation or the metal.
- It will freeze the water if left on its own thermostat: a plug-in temperature controller with a probe in the water keeps it at your target.
- Electricity and water: plug it into an outlet protected by a GFCI/RCD, keep the cable and plug dry, and unplug it before you get in.
Stock tank
Roomy, sturdy and cheap, in galvanised steel or plastic, but uninsulated. Indoors it can work with ice for occasional plunges; for daily use it needs a chiller and, ideally, insulation around the sides and a proper lid. Outdoors, shade it.
Inflatable tub
Cheap, quick to set up and light enough to move. The thin walls insulate better than steel but far worse than foam, so a daily inflatable plunge usually ends up on a chiller. Pick one with an insulated lid.
Wooden barrel
Wood insulates moderately and looks good outdoors. Barrels need to stay filled so the staves stay swollen and tight, and they usually pair with a chiller.
Ice or a chiller?
Ice is fine for a plunge now and then: the ice bath calculator tells you how much for one bath. For daily plunges, the ice to hold the temperature adds up fast, from about 5 kg a day for a chest freezer to more than 50 kg for a covered stock tank in our indoor example. A chiller then pays for itself; the cold plunge running cost calculator compares the two with your prices.
Sizing a chiller
A chiller has to remove the heat leaking in, the heat you bring in with every plunge, and still have spare capacity to cool fresh tap water down. Small chillers also lose some capacity as the water gets colder. Insulating the tub and using a good lid shrink the chiller you need more than anything else. The chiller calculator checks a chiller against your tub, room and climate, including how long it takes to cool a fresh fill.
Clean water
Cold water grows algae and bacteria more slowly, but it still does. Plan for a filter and a sanitiser, or change the water often, and keep a lid on to stop leaves, dust and insects. Shower before you get in.
Safety
- Never plunge alone and keep the first sessions short; get out if you shiver hard, feel numb or dizzy.
- Electrics: chillers, pumps and freezers belong on GFCI/RCD-protected outlets; have an electrician check outdoor sockets.
- Weight: 400 litres of water weighs 400 kg; check the floor or deck before you fill.
- Health: ask a doctor first if you have a heart condition or high blood pressure. See cold plunge benefits and risks.