silly question. but whats the point of a warm weather base layer? wouldnt one layer be better?
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Wicks sweat for better evaporation cooling.
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but dont $150 jerseys do that?
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I wear a long sleeved one on the sunny days to help a bit with blocking the sun.
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No.
It takes a layer of space between the Jersey and skin. Just like how a down jacket works, insulation goes both ways.
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The Cyclist article Chad links is a good read, with input from various clothing manufacturers (who don’t agree!). For what it’s worth, I’ve got good quality jerseys and good quality base layers, and I ditch the base layer when it gets warm. I just find I overheat even with warm-weather specific base layers, but then I tend to run pretty warm.
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I started wearing one a few years ago and I find a noticeable increase in comfort. It keeps my jersey from sticking to me and it really does feel cooler. And on super hot days if I have an underlayer on, I can unzip my jersey without totally embarrassing myself and grossing people out 
That is a significant cooling effect!
Its also great in the cool/cold for sweat control too so I wear one in the winter as well.
Once I started wearing an under vest, I moved to wearing one on every ride.
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+1 on the “Borat underwear” mesh base layers in the summer.
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Buying one now with the hopes that summer will come sooner…
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The type you buy is important. I have some older ones that I wear as a cold weather base layer. I wouldn’t use it on a hot day though that is what it was originally designed for.
The newer ones that look like they belong in the bedroom are what you want. Really helps to wick and keep the air flowing. It’s definitely worth paying a little more here.
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From my experience, the “feature” in these vests is the fact the fabric is dimensional (a waffle weave or outright mesh with holes). This both lifts your jersey off your skin and provides some room for air movement. Besides the basic comfort increase by not having a sweaty jersey clinging to you, this increases the sweat wicking and evaporation and really does keep you cooler and drier in most conditions.
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I tend to wear base layers year round and pick which one based on temperature. With a good summer base layer I don’t find I get any hotter. Having it tight and form fittin is important to maximize contact.
A base layer that is loose and too big isn’t going to function as intended.
Jersey choice is probably most critical though, especially when it is humid. On hot days I prefer to pair with a thin jersey that doesn’t hold on to much sweat. The worst are some of the cheaper jerseys that get wet and heavy when saturated. If you can take it out of the washer and it still feels light, this is going to be a pretty good choice.
In terms of base layers, Assos summer line are very good. Castelli has some thin waffle ones as well that are good. Rapha’s Pro Team base layer is one of my favorites as it is quite durable and hold up really well even with repeated washing. @Jonathan mentioned on one TR podcast he found Rapha base layers hot, but didn’t mention which specific one. The Pro Team’s wick super well and I’ve worn them on the hottest days without issue.
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For me it’s a comfort thing. I sweat a lot, and will sweat as soon as it gets slightly warm regardless of whether I am wearing a baselayer under my jersey or not. This can massively increase issues with irritation from clothing - such as bibstraps and seams rubbing. The baselayer just provides that really consistent feeling layer that minimises irritation - as long as its skintight and well made it doesn’t shift around as I move, so the clothes above it rub against it rather than against my sweaty skin.
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Rab C was ahead of his time 
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Bit of a specific use case but I wear one as it prevents the bib straps tangling on my nipple piercing!
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Oh
I dug up this thread because I’ve been wearing base layers this year on cool and hot days. I felt noticeably hotter with the base layer than without. Thought I would see what the Forum felt about it.
I also experimented with this on the trainer. I have 3 strong fans and I ride in a room that is pretty stable at 72 degrees. I do the same workout every Monday indoors (Glassy -2) and as a general rule I don’t sweat much during this workout. On days where I wear the base layer on top vs. bibs only, I sweat like crazy. I understand it is different that wearing it outside but with the airflow from my fans I would think the difference would be less significant. But to me, I find the base layer measurably hotter indoors and out.
A quote from Chad’s article linked above:
Anyone who tells you to wear a base layer when it’s hot either a) doesn’t ride a bike, b) is just trying to sell you base layers or c) is grossly misinformed. Actually, it’s probably a combination of all three,’ says Steve Smith, brand manager at Sportful.
I like them in milder temps and they do feel nice under the jersey but for me they are just hotter with than without.
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I used to wear a base layer all the time…until maybe +85* or so.
A few years ago, I realized I was getting hotter with the base layer…I now stop wearing a base layer around 75*.
A good jersey wicks just as well as a base layer…seems that having an extra layer will only interfere with any cooling effect since there is limited air flow.
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