I just got an advert from Rapha claiming that base layers keep you cool and dry in the heat:
“The high-performance foundation for racing in hot and humid conditions…this layer accelerates evaporative cooling. By spreading sweat across a wide surface area, it converts liquid sweat into vapour faster”
Wouldn’t your jersey, if of a similar material, do exactly the same thing?
In my experience it keeps you dry but not cool. I have a gore base layer and use it mostly on cold days because in that case it keeps me warm.
Purely anecdotally, I find I feel much hotter/sweatier on the indoor trainer on the odd occasions I don’t have a base layer to wear. I don’t know if its just because the sweat runs more because there’s nothing to soak it up.
I stopped wearing base layers above ~72*F a few years ago. There is no neeed, IMO. Today’s jerseys wick moisture just as well as baselayers, so wearing a baselayer just adds moisture and heat.
10-15 years ago (and longer), it made sense as jerseys didn’t perform as well as they do now.
I always wear a base layer, short or long sleeved in the Autumn/Winter and a vest in the Spring/Summer, as I find that the straps on my bib shorts rub otherwise.
I almost always wear a base layer, all temp ranges. Base layer is a generic term, and certain products are designed to be used in cold temps, others in hot. A well designed hot weather base layer will generally improve wicking and ventilation which has the effect of making me feel cooler. A jersey alone can’t really do that, because there’s no other layer to transfer moisture to.
My personal favorites for hot weather base are Q36.5 (pricey, but top performer and comfortable), Castelli and Pactimo (the ones that use 37c material).
One other factor to consider-I sweat a lot and despite diligently laundering, my tops eventually get a bit smelly or stained. I’ve retired a few base layers, have never retired a jersey for sweat, and my jerseys and much pricier than a base.
I find that warm weather base layers work well in dry heat but make me feel warmer when in a humid and hot environment. Living in the eastern US (Appalachian mountains in VA) I personally only wear light wicking base layers in the shoulder seasons when temperatures are variable. Once we hit May and high humidity I go back to just jerseys.
While traveling the Colorado for riding last summer I wore base layers every day and did stay dryer and felt cooler because of that. I just don’t find the same performance in high humidity.
All my riding in the Southeast US. Think hot summers with 175% humidity. I had been wearing base layers 100% of the time, but last summer on a particularly hot day, I skipped and it was a revelation! I don’t wear them above 80F or so anymore. Feel kind of stupid that I had bought into it the need to wear them in very hot weather.
TheProsCloset assembled some relevant research in this blog piece: Do You Need To A Cycling Base Layer In The Summer? We Examine The Science | The Pro's Closet
Personally, riding in the hot, humid mid-Atlantic region I skip summer base layers for the reasons others cite above.
Started wearing a base layer in summer has been a game changer for me. Cannot say that I “feel” cooler, but sweat is wicked away much better and I don’t have to deal with damp jerseys anymore. Mostly using the Maap mesh base layer, but I suspect any similar design would work just as well.
Thanks Patrick, this is perfect!
And while I appreciate all of the anecdotal response, I’m going to rely on the science, as noted in the article:
“Air velocity accounts for the vast majority (5-46%) of cooling on the bike… More layers complicate cooling by creating multiple microenvironments for air to pass through. So wearing more than a single layer will have anywhere from no effect to a negative effect on cooling.”
“The distance from skin to the evaporation point (the outside of the fabric) matters, and more distance equals less cooling… The thinner the base layer fabric, the better it will be for cooling, but bare skin provides the best evaporative cooling. “
I used to ride with a base layer because I’d also heard that it helps somehow. Then one hot summer day, I ended up riding without one and felt noticeably cooler. Never wore one in summer since.
I’ve always worn a base layer, even on the hottest days here in California, but this thread may be the tipping point for me to go without this summer. I am slightly worried though because my base layers (Rapha exclusively) all have a vertical wear mark from the inside of the zipper (also Rapha exclusively). Hopefully that won’t be a source of discomfort on my soon-to-be bare torso.
I wear a base layer all the time. I don’t wear second skin-tight jerseys, so there’s a little room in there. I hate the feeling of sweat dripping inside my jersey, so having a layer right up next to the skin helps to keep the sweat from beading and dripping. It also helps because I’m pretty hairy. I’m in the UK so it never gets so hot that I’m overheating with it. I also am a fan of the Pactimo ones, as they’re nice and thin and not super expensive.
I have not found baselayers to help in hot weather.
The one situation where it might make sense is low humidity, and a rider who sweats heavily from a few small patches. Wicking the sweat over a wider area might help cooling.
If you tend to sweat all over, it probably won’t help. In higher humidity, evaporation is limited by the relative humidity and you need all the airflow you can get.
On the other hand, I can believe that one thin tight wicking layer could be cooler than bare skin. Sweat cools best when it evaporates on the skin, and does far less cooling if it drips off. That’s why I wear a baselayer and no jersey indoors on the trainer.
Ditto - i am persuaded by the science, and i have noted at the TDU each year v.few pros wearing baselayers on the hottest days! (but they do use ice vests and ice down jersey!)
When I started riding, I wore looser jerseys, and my inexpensive bibs had straps that would chafe, so I always wore a baselayer. I stopped when I got a Rapha Pro Team Aero jersey that convinced me a baselayer wasn’t needed in hot weather because it was so good at keeping me cool. Agree that it makes more of a difference if one doesn’t wear a skintight jersey with modern fabrics, but I now save my warm weather baselayers (sleeveless and/or mesh) for the indoor trainer rides where I don’t wear a jersey.
I wear base layer mostly to keep my outer layer dry. It’s useful for longer days out with changing climate conditions. I find it more comfortable in general, but don’t really feel a cooling effect vs no base layer.
Unavoidable if you have a hairy chest. 
My summer is always in the 40ºC range and a proper base layer for summer (usually a tank top) is a must for me, keeps the my torso dry-ish, yes, jerseys get wet.
I’m reminded of the arguments about how a styrofoam helmet with holes in it is cooler than no styrofoam insulation at all… or a motorcycle jacket with vents is cooler than no jacket at all… 