Good base layers will wick moisture away from your body. In warm weather a wicking base will help keep you cool by provide significantly more surface area than your skin to promote evaporation of your sweat. A base layer can provide addition surface area only if it retains it’s structure when wet. Many fabrics (cotton for example) collapse when saturated making it feel hotter and sticky.
In cold weather convection cooling tends to be the bigger issue than evaporation… so a wicking base layer which pulls water away from your skin helps you stay warm. When the temperature is always below freezing, you might want to consider vapor barrier clothing which provides a moist micro climate rather than wicking moisture away.
You want your base layer to minimize water retention. In general I would recommend getting a long sleeve base layer with a deep front zipper. This provides maximum flexibility. In warm weather you can push up the sleeves and open the neck zipper. While not a “critical” issue, odor retention is also a consideration for many people.
Besides assisting in managing thermal conditions base layers are often selected to provide protection from the sun and biting bugs.
I normally wear a Icebreaker 150wt Anatomica tee-shirt. I find this shirt very comfortable, look nice enough that I can wear them to work, and can be worn several days in a row without starting to stink. When it gets colder I layer on a merino wool hoody. If I am planning to engage in very high energy activities such as running I switch to synthetics shirts which absorb less moisture and dries more quickly. When exercising (>55F) I use a Columbia Titan Ultra Short Sleeve Trail Running Shirt. This shirt breaths well, dries quickly, the Omni-Freeze seem to keep my skin a bit cooler, and it doesn’t get too stinky on a multiple day trip. If I am going to be in the sun continuously for several days I use a Voormi River Run Shirt which in a synthetic/wool blend that resists stink for multiple days and lets me avoid having to use sunscreen. I treat it with permethrin for some protection from bug bites. In cooler conditions I switch to a Patagonia Capilene 4 hoody (now called Patagonia Thermal Weight Hoody) which is the very best base layer I have used when it’s <55F. I have been reasonably comfort doing a big uphill push when it was 55F and ok with sleeves pushed up and zipper fully open up to 70F. With it zipped up, hood up, and a windshirt worn I have been comfort running to around 10F, hiking hard to 25F, walking/active to 40F, stand around to 50F. Cap4 with a windshirt to control how much air is allowed to pass covers a huge range of conditions because the Cap4 fabric is so air permeable. It dries quickly, and because of the grid which holds much of the fabric off the skin it is quite comfortable even when wet.
Synthetic or Wool?
One of the more contentious issues is whether wool or synthetics are the best material for base layers. The folks from backpackinglight.com created custom shirts which had wool on one side and synthetic on the other for real-time, side-by-side comparison. Their results were reported in comfort and moisture transport in merino wool and capiline. The reviewers concluded that wool vastly superior when it comes to odor control and feels less clammy when wet, while capilene dries more quickly (~50%), and is warmer for the weight. My more casual experiments had similar results, though I think that if the BPL folks had been using a heavier base layer, the synthetic would have dries more than 50% faster and a grid pattern synthetic might have been less clammy. The comparison failed to note that wool is not as durable as most synthetics of comparable weight. I would also note that for periods up to a week, I have found synthetic with decent treatments keeps stink down to an “acceptable” level. There was a post of reddit about the science of odor control.
Wool has started to make a come-back last few years. There have been a number of articles such as psychovertical’s The Wonders of Wool encouraging people to reconsider a wool base layer. Many people have discovered high quality Merino wool can be comfortable against the skin and to resist odor when worn days (if not weeks) in a row. Fun story of shrek the merino sheep. I have good experience with wool clothing from Smartwool, Icebreaker, and Patagonia. My biggest complaint with wool is that featherweight (<=150wt) material shows wear fairly quickly. I would estimate I typically get the first hole after 80 days of wear, though I get several hundred days of use before I need to retire the item. Many manufacturers have been adding a bit of nylon which greatly increases the durability of wool.
Malden Mills Power Dry is my favorite polyester base material because it’s soft against the skin, has a reasonable amount of stretch so it moves with me, and it moves moisture effectively. Better yet, the moisture handling is based on the bipolar fabric that that uses the physical prosperities of a combination of materials rather than a chemical treatment which can wear out. Besides moving moisture more effectively than most of the other synthetic wicking technologies, bipolar materials tend to feel less clammy when wet. A nice refinement to Power Dry is the x-static treatment which embeds silver to reduce odor from microbes. Different companies use different amounts of silver. Looks like you want >8% silver for effective performance. The Polygiene treatment seems pretty good. Power Stretch is not as soft as Power Dry, but more body hugging and durable with a fuzzy inside and a smooth outside… great for cool-cold weather climbing and other abrasion prone activities. Marmot DriClime might be better than anything else when it comes to moving moisture away from the skin, but it tends to grab anything that isn’t baby smooth and is not as stretchy or soft as PowerDry. There are a number of “in-house” base layer brands which are good. I found VisaEndurance seems seems like a pretty good material at a reasonable price. Some people rave about Norwegian Brynje mesh base layers for cold weather because they dry quickly and give a lot of warmth for weight provided the next layer restricts air flow. In the US you can find Brynje sold a few places such as reliableracing. Personally, I think Patagonia Cap4 with it’s high void spacing provides most of the advantages of Brynje in a much nicer package.
Tightly Woven Synthetics
Shirts made from tightly woven (not knit) nylon or polyester such as Supplex, SolarWeave, Solumbra, or Talsan can be useful in moderate to hot weather. These shirts provide protection from sunburn and biting bugs, dry very quickly, and are very durable. The four downsides of tight weave shirts are that in hot weather they blocks more airflow than many other bases (so feels warmer), they tend not to wick so sweat sticks to your skin, the fabric is a bit rough (my elbows get irritated after a few days of wear), and they will get stinky after a day of heavy sweating (though unlike polypro the smell washed out easily). If I am expected really bad mosquitos I use a now discontinued Outlier Air Forged Nylon Oxford button up shirt. I used to love the Rail Riders Adventure Shirt thanks to it’s side venting and high quality fabric but it’s torso is cut too generously for me now. Some female friends seem to like Lightheart Gear’s Hiking Dress.
Chemicals For Bugs
Tight woven shirt provides protection from biting bugs but are very uncomfortable to wear in hot weather. Columbia and Ex Officio make woven shirts with a more open weave for more air permeability combined with a long lasting permethrin treatment which I have found effective, especially the Dryflylite which seems to be discontinued. Another approach which I have been experimenting with is to treat more traditional base with with permethrin at home, or to to sent to InsectShield for a longer lasting treatment.
CoolMax base layers can often be found for around for slight more money than a cotton tee-shirt. Dupont has licensed CoolMax to multiple suppliers which has resulted in competition that keeps the price down.These shirts aren’t as well finished as more expensive base layers but they are much less expensive and are very usable. For warmer weather you might already have some shirts that would work well: a biking jersey, running shirt, a soccer jersey, or light weight woven synthetic button-down.