Maintain some perspective, however, as a quality base layer needs to be more than just comfortable when you're active in the outdoors. An itchy fabric might be annoying, but a product that collects moisture and doesn't keep you warm when wet can potentially be life-threatening. It is for this reason that cotton has almost disappeared from technical outdoor apparel. Sure, cotton is comfortable, but once this fabric becomes saturated, it cannot provide any insulation. A wet and cool situation can quickly lead to hypothermia, or worse. From this you should see that the selection of appropriate long underwear is more important than it may initially seem; both for your happiness and health.
This is our Buying Advice article covering the theory behind base layer selection. For a more in-depth discussion on how to properly layer, be sure to read our three part layering systems article. For the main review and comparisons between eleven of the most popular products on the market today, see The Best Long Underwear and Base Layer Review. Thicker, insulating layers are found in The Best Fleece Jacket for Men Review.
The principles of modern layering theory rest on the idea that the best way to stay comfortable in an ever-changing environment is to use a system of garments that can be easily layered over one another as conditions or exertion levels change. Practically, this usually consists of three pieces of clothing: a foundational base layer to interact comfortably with the skin, a middle layer to provide insulation, and an outer shell to protect everything else from wind and precipitation. Like a Russian doll, each subsequent piece is sized a little larger than the last to allow for freedom of movement and the ability to add or shed a layer at will. Other variations exist, such as extra insulating layers for extreme cold, but that is the most basic system.
Arguably the most impact-laden layer on your comfort in this system is the base layer. It is there, against your skin, to soothe you or irritate you. Unless you try to get a tan, it will be on you at all times and sometimes for days at a stretch, from sleeping bag to trail or big wall ascent. Therefore, careful selection based on your own preferences and intended use is important. Some shoppers are just looking for a nice looking long sleeve. If that's the case, it's ok, and we can help by offering our opinions. However, style is subjective and your own fashion sense should weigh heavily in the selection.
While it isn't a standard set in stone, base layers are usually marketed using several different thicknesses, or weights, which can be categorized into three basic groups; lightweight, midweight, and expedition weight. Merino wool products almost always list their fabric weight (measured in grams per squared meter). Some manufacturers of blended and synthetic fabrics have also began publishing the fabric weight, perhaps to avoid not having a spec that another company has, but this system is not yet ubiquitous throughout the market.
Thicker products did tend to provide more warmth and durability overall. Thinner products were generally more breathable and dried fast. Opportunity cost!
These are the thinnest and are generally reserved for the shoulder seasons, as well as some summer days. They also perform well during intense physical exertion in cold weather. They usually feel great and can cost a little less than the thicker stuff. For merino wool, and synthetic and blend products that care to include it, the weights range from around 100 to 195 g/m². The Rab Merino+ 160 was our favorite lightweight model amongst the eleven contestants in our tests. Our Best Buy award winner, The North Face Warm, also falls into this category. The upper end of this spectrum, Silk weight, is another type that is occasionally offered by companies as an even thinner weight than light.
This is the most multi-functional weight, and tops in this class are ideal for spring and fall or combined with insulating layers in the winter. Fabric weights between 200 and 295 g/m² are usually considered midweight. Seven of the eleven tops reviewed fell into this category, the SmartWool Merino 250 Base Layer being the best example, coming in at 250 g/m². We also liked many features on the 231 g/m² Arc'teryx Rho AR, which also functions quite well as an insulating mid-layer.
The thickest long underwear, checking in at 300+ g/m², is called expedition weight for the arduous adventures it is usually associated with. These are shirts intended for truly cold conditions, but many can also work as insulating layers at other times to expand their uses and justify their high price tags. We didn't include any expedition weight models in our review this year.
Base layers come in three primary constructions: wool, synthetic, and blended material. Choosing between these materials is perhaps the most important choice when deciding on which long underwear to buy.
Merino wool is the fabric of choice for today's premium long underwear. It is composed of fine wool fibers shorn from a fancy breed of sheep. The result is a fabric that is much softer, and significantly less itchy, than the average wool sweater. Tops made from this stuff are going to be more expensive, but they should also be warmer for the weight, breathe better, and dry quickly.
One of the major pros of merino wool is that it does not absorb body odor the way synthetic materials do, and stays fairly stench-free. This leads to less trips to the laundromat, and confidence in removing your other layers when taking a break in the ski resorts after a day of exercise. Durability is a concern with this delicate material. For those without student debt or wool allergies, this is the best material you can get. The SmartWool 250 is our favorite merino wool, with the REI Merino Midweight, Icebreaker Oasis, and Minus33 Isolation Midweight Wool falling behind the Editors' Choice winner.
At a lower price point, many outdoor companies offer synthetic long underwear. This stuff is tough and provides a great value, and its performance continues to creep toward that of merino wool. It can, however, be less breathable, especially in the thicker models. They are often slower to dry, too. Unlike wool, the fibers themselves can absorb moisture which overtime can lead to bacteria and unpleasant odors. It can seem as if these shirts absorb stench that comes back time and again after only wearing briefly.
We like these tops for the price conscious or those new to outdoor apparel that may not appreciate the subtle performance benefits wool provides. The top-scoring synthetic material was the Arc'teryx Rho AR, followed by the Best Buy winning The North Face Warm. Other synthetics in our review include the Patagonia Capilene Midweight, Under Armour Base 4.0, and Mountain Hardwear Microchill 2.0.
Two models included in our review this year, the Rab Merino+ 160 and tasc Base Layer, were made of combinations of merino wool and other natural and synthetic materials, such as lycra, viscose from bamboo, and polyester. This was our first time testing blended fabric base layers, and we found one to perform well enough to give it a Top Pick.
Although it would make sense that these hybrids would combine the best characteristics of two more popular material types, we didn't exactly find that to be consistent with our experiences. Manufacturers claim that these products bring together the high warmth to weight ratio, breathability, speedy drying, and odor resistance of wool with the less expensive, more durable synthetic materials. We liked the blended fabric products, but didn't find that they lived up to their claims. We plan to keep our eye on this part of the market, though, and hope to see advancements in the coming years.
Finally, there is cotton. You are probably already familiar with this as it's the fabric of the ubiquitous t-shirt. However, this material is inappropriate for outdoor use because it does not insulate when wet. We like this stuff a lot for activities close to civilization but cannot recommend it for backcountry uses because of the real risks it poses.
Hoods, Zippers, and Thumb Loops
Three products tested include hidden thumb loops, or a more obvious thumb hole, that provide additional warmth without compromising style by pulling the sleeve material further over the hand, and keeping it there no matter what movements your upper body makes. We selected the zip-neck version of most models in this review, because we general like to have more control over our heat release and retention. Many models in this review are also available in hooded versions.
These are features we really like for technical uses because they allow you to adjust your insulation in just a few seconds. They're not great for all uses because they add weight/bulk and also affect the appearance. If these are sacrifices you're willing to make for functionality, most manufacturers have multiple versions to offer.
The cost of long underwear can range from the reasonable to the exorbitant. Generally, high prices coincide with high performance. Our first and second-highest scoring models cost $100 and $110, respectively. We did our best to accurately score each top and think the final numbers effectively show the relative difference between the options. You can use these numbers along with our Price-Value Chart to help you figure out what is actually a good deal when shopping sales.
Fashion vs. Functionality
We understand that many readers are just as interested in functionality as fashion. The focus of this review has largely been from an objective, practical, perspective because that is the fairest way we know of to evaluate tops with specific marketing claims. Although it did not factor into their scores, the three top-scoring products, from SmartWool, Rab, and Arc'teryx, were the best looking models in the collective eyes of our reviewers and friends. We think this has partly to do with their exceptional fit that sets the wearer at ease and brings on confidence, rather than any particular design feature.
Remember that base layer performance relies on a simple, undecorated and untextured design. There is not much to differentiate their styling. To illustrate this, think back to a shirt that didn't fit well and how hard it was to mask that discomfort to the outside world. Constantly adjusting your clothing and showing off your beer belly while you stow your carry-on in the overhead bin are not attractive displays. So, more than brand or bargain, choose the size of your base layer carefully, try it on if possible, and you will reap the cosmetic rewards that comfort brings.
Ultraviolet Protection Factor (UPF)
A UPF rating is a measurement of sun protection, and it can be applied to clothing as well as sunscreen. It seems to be appearing in advertising and packaging all over the place lately. Not all manufacturers choose to advertise this specification, but after contacting each company to find out, we were able to confirm that at three of these models do have a UPF rating. Those models are the top-scoring SmartWool NTS Mid 250, the tasc Base Layer, and the Minus33 Isolation Midweight Wool. It has been our combined experience outside that developing sunburn through clothing is a very rare event. For those to whom this is a concern, the three models listed above could be your best choices for you next long underwear top.
The polyester in synthetic clothing is made from oil and comes with the inherent guilt of fossil fuel consumption. Different tops claim different percentages of recycled polyester that are too varied to list.
Two of the six manufacturers from products with merino wool in this review, Smartwool and Icebreaker claim that they receive their wool from sustainable sources. Both Smartwool and Icebreaker also advertise "No Museling" wool. The manufacturer tasc asserts that one of their reasons for seeking fabrics from bamboo is to avoid unnecessary chemicals. Verification of these claims and the nuances of their individual ethical standards are beyond the scope of this article. You can explore their about pages for additional information at the above links.