Recently we received an inquiry from a return customer from Texas. He wrote at length, and along the way he described when he doesn't wear any tallit katan:

I don't even wear even the one pair of mine when it's hot out in Texas heat.I don't think it's not derech lavush to wear clothing to make you sweat, and I have a heart condition where my tachycardia (rapid heart rate) can send my heart into v-fig (irregular heart rate) if my heart rate gets up to about 80 to 90 if I forget one of my medications. This is not the case with the talis gadol which is for appearances in shul, like when you get an aliyah. But just as we don't wear tzitzis when we go swimming, I don't see the point in wearing tzitzis when it's going to make you sweat even more. It's like wearing a shatnetz parka coat in the heat: that's not derech lavush.

Just to make sure all of our readers are on the same page, let me run through that last sentence. What this Jewish Texan means is that since theoretically you might be able to put on a parka coat in the heat if it's made from a fabric that is prohibited to wear, since it might not be considered wearing per se, likewise you can't fulfill the mitzvah of tzitzit by wearing a garment that does not provide you with any benefit. Clothes are meant to keep you warm in the winter and cool in the summer, right? So if adding another layer on a blazing hot day, i.e. putting on your tallit katan, does not help you in any way, maybe that wouldn't be considered wearing, so you're not doing a mitzvah.

He certainly has a valid point. Citing Beit Yosef O.C. 10, Rabbi Ari Enkin put it this way:

One who is uncomfortable wearing a wool Tallit Katan in the summer due to the heat should certainly consider wearing a cotton one. This is because it is vital that the Tzitzit garment be one that serves a clothing-type function and is a garment that one enjoys wearing. Indeed, there have been eminent rabbis throughout the ages who dismissed the view of the Shulchan Aruch in favor of the Rema and only wore Tzitzit made from cotton.

Still, let me toss out a thought that might apply in certain cases -- despite the heat. In a word, what if a garment serves a function other than protecting you from the heat and cold? Allow me to illustrate...

Last week we went to Tiberius for a mini vacation. There was a heatwave, and it just so happens that Tiberius is typically one of the hottest places in the country. While packing, it occurred to me that I might do well to carefully consider which tallit katan to take along.

As you might imagine, as a long-time tzitzit seller, my personal collection of tallit katans is varied. Usually I wear wool (though I'm Ashkenazi, and many Ashkenazim stick with cotton), but I also have a few traditional-style (not undershirt-style) cotton tallit katans. I always wear an undershirt, regardless of the weather, and I maintain that if you're wearing three layers of cotton -- sleeved undershirt, tallit katan and shirt -- cotton is not going to keep you cool, but rather hot and clammy, primarily since three layers of cotton don't do a good job of wicking perspiration. However, if you opt for a tank top undershirt, that does make a difference, since you're getting a lot more ventilation on your shoulders and along your sides.

I also took along a DryFit tallit katan, which I wear for hiking, biking, etc. 

So to make a short story long, when we headed down to the Kinneret (a.k.a. Sea of Galilee) I started out with short-sleeve undershirt + traditional cotton tallit katan + button-down shirt. It was baking. You step out of the water, and even without a towel you're dry in a few moments, and then start heating up fast. You're first thought is: 'How long will it take to get to the car, and how long will it take til I start feeling the air conditioner?' You try to make a dash for the parking lot before the boys notice there's a stand selling popsicles with a hechsher that no parent could refuse. But of course they spot it, and when they lodge their request, you immediately imagine a bleak scene trying to explain your rush to the car to your wife ("What? Why on earth didn't you buy them a popsicle??"), so with no great excuse, you get waylaid for a few minutes.

When I left the beach and headed up toward the car, putting on my button-down shirt seemed out of the question. On the other hand, once I was a bit away from the beach, I felt a bit uncomfortable wearing just pants and an undershirt. So I decided to put on my tallit katan, though the last thing I wanted was to wear another layer. Yet with a tallis katan on, just hanging loosely, I felt a bit more dressed. So perhaps you could argue that the tallit katan does provide you with benefit, namely kavod. 

Yeah, but then the moment you put on another layer, is that tallit katan still providing you with some benefit? I don't have an answer to that.

Also, this brings up another question. Some poskim hold that wearing a tzitzit garment, namely an undershirt tzitzit, is not the proper way to fulfill the mitzvah of tzitzit, since an undershirt is typically worn to absorb sweat. But if it's a stand-alone garment, i.e. a t-shirt tzitzit or a DryFit, the main purpose is not to absorb sweat, but to cover you up. (Note that the manufacturers almost always make these products with sleeves, whereas an undershirt tzitzit garment is sleeveless.) So shouldn't wearing a t-shirt tzitzit garment on your skin be perfectly okay when worn alone, even according to those poskim who frown on undershirt tzitzit?