Good base layers will wick moisture away from your body. In warm weather a wicking base will help keep you cool by provide more surface area than your skin to promote evaporation of your sweat. This only works if if the base layer 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.
Generally you want your base layer to minimize water retention. A long sleeve base layer with a deep front zipper 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 an Icebreaker 150wt Anatomica tee-shirt and briefs. I find the Anatomica shirt very comfortable, look nice enough that I can wear them to work, and can be worn many days in a row without starting to stink. If I am planning to engage in very high energy activities such as running (>50F) I use a Xenith “Under the Lights” shirt made from Polartec Delta. This shirt breaths well, dries more quickly than cotton or wool, but slower than the typical fast dry synthetic shirts. It has a 3-dimensional texture which allows it to be comfortably worn when damp, and it doesn’t get too stinky on a multiple day trip. If I am going to be in the sun for more than a couple of hours I use a UPF 50 shirt. In cooler conditions sometimes 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. Cap4 with a windshirt to control how much air is allowed to pass covers a huge range of conditions because the Cap4 fabric very air permeable. In colder conditions I manage vents to minimize / eliminate sweating. 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. Because the the grid 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 (such as the Patagonia Thermal Weight, Polartec Power Grid), the synthetic would have dries more than 50% faster and a grid pattern would 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 backpacking. Less days in the city around other people. There was a post of reddit about the science of odor control. My experience is that in moderate to hot conditions while I am engaging in “light” work wool is superior. In cold condition, or if I am working hard and sweating a lot, high performance synthetics are better.
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. I have good experience with wool clothing from Icebreaker and to a lesser extent Smartwool and Patagonia. Note: I don’t “baby” my wool clothing. Wear them for activities that sometimes are abrasive, machine wash my wool with normal detergent, and use a drier on “low”. My biggest complaint is the featherweight (<=150wt) 100% merino wool shows wear out fairly quickly. I typically found these garments would develop a hole after 80 days. Many manufacturers have been adding a bit of nylon which greatly increases the durability of wool. For example the same “model” of garment which was changed to 85% merino with 15% from nylon started to look “tired” after around 500 days, and got the first small hole after 600 days of wear. The shirt was still wearable (around the house) for many more “wears”. Some people talk about how wool is “too expensive”. I get around 600 days out of a $60 shirt. That’s 10 cents / day for the most comfortable shirt. The best performing synthetic shirts got at least 1200 days of wear and cost around $60 which was around 3 cents / day. Budget synthetic is around $20 which works out to 1.6 cents / day. The synthetic is 8.5 cents / day cheaper… but I think that’s a reasonable cost to pay in moderate to hot conditions for significantly better odor control, thermal regulation, and over all comfort. For a bit more about how wool works, follow the links on reddit pos lets talk about wool and enjoy the story of shrek the merino sheep.
Malden Mills Power Dry and Polartec Power Grid 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. Related to Brynje is fine track elemental layering. Personally, I think Patagonia Cap4 with it’s high void spacing provides most of the advantages of Brynje in a much nicer package.
In moderate to hot weather when I am going to be outdoors all day I will sometimes use a sun hoody. There are a number of sun shirts that are beloved by others like the OR Echo which feels cooler than most hoodies I have tried. I have also burned through it because I have very sun sensitive skin. Of the UPF 50 hoodies I have tried, the Arcteryx Cormac and the MH Crater Lake Hoody are my favorite.
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 sorts of shirts work best if they are somewhat loose with some ventilation. That way the sweat on your skin evaporates near your skin maximizing the cooling, 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 if the shirts are primarily Nylon 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 or a Solbari Weekend Shirt. I loved my (discontinued version) of the Adventure Shirt from Rail Riders but had to retire it when I lose weight and it was way to baggy through the torso. 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. I will note that when facing heavy bug pressure, say bug force >6 permethrin only slows down the bites, doesn’t prevent them.
Background / Science
Check out Stephen’s By the Numbers Analysis of the Wool, Alpaca & Polyester
Finetrack Elementals, Svala, Brynje, Castelli, Montbell zeo-line cool mesh baselayers
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. Uniqo AIRsm base layers seem well regarded but I haven’t personally used them.