During the summer months, one thing many homebrewers struggle with is keeping fermentations cool. One common method of cooling is the wet T-shirt method. This is a simple, but effective, method of cooling carboys. As someone who has used this method extensively (even after getting a chest freezer), I thought I’d share a few tips.
The basic idea of the wet T-shirt method is to put a wet T-shirt over a carboy, and let the heat loss from evaporation cool the carboy and it’s contents. For continual cooling, the carboy can be placed in a container with water at the bottom, so water wicks up the T-shirt as it evaporates. Those large, 100-qt. (95-L) picnic coolers will hold two 6.5-gallon (26-L) carboys. Brewers also use trashcans for this.
Depending on a variety of things, the wet T-shirt method can cool a 5-gallon (19-L) volume of beer between 5 and 10 °F (~3–5 °C). The main variables here are humidity, airflow and surface area. Air conditioning takes moisture out of the air as it cools, so the wet T-shirt method works well in air conditioned spaces. If you keep your carboy somewhere humid, the method will work less well. If you place a fan to blow across the carboy, you will move humid air away and increase the effectiveness of the cooling. You only need a slight breeze for this. I sometimes put my carboys under a ceiling fan on low, and that works fine.
One mistake homebrewers sometimes make is to add a lot of water to the cooler, trash can or tub holding the carboy. The more surface area that water is evaporating from, the more effective the cooling. As such, adding water cuts down on surface area (on the side of the carboy) that could be shedding heat. Add only enough water to keep the T-shirt wet and not dry out overnight. (If the carboy is submerged to any extent, you’ll have to keep that water at or below your fermentation temperature as it will heat or cool the carboy by convection.)
Experience has taught me to change the T-shirt every couple days. If not, it will begin to get moldy, and you don’t want mold growing near your beer. If you use old T-shirts that are not destined to be worn again (and I recommend this), you can add a small amount of bleach or Star San to the water reservoir to retard the growth of microorganisms. Every four or five days, change the water.
Another consideration is lifting the carboy out of the water. Wet glass is very slick. If you have a carboy hauler, use this to lift the carboy out of the water. If not, be careful when lifting. I usually take the T-shirt off the carboy, then lift it out and straight onto a dry towel. I dry the glass (and my hands) thoroughly before lifting it to where it will sit when I rack to keg or secondary.
It’s worth your time to dip a paper towel or washcloth in a dilute bleach solution or working strength Star San solution and wipe off the carboy before taking off the airlock. It’s been sitting in a damp environment and could harbor all sorts of bacteria or wild yeasts.
If your overnight temperature is going to dip significantly, you can roll the T-shirt up the sides of the carboy and then roll it back down into the water when you wake up the next day.
Finally, if possible, use a carboy without a stick-on strip thermometer or cover the thermometer with clear packing tape as the water will foul it in a matter of days.
As low-tech as it seems, the wet T-shirt method can be a reliable method of keeping your fermentation temperatures in check.