The embarrassing history of long underwear is behind us. Gone are the days of dimpled, sweat-soaked cotton, knee-length crotches, and saggy butts that bunch up under your pants — welcome to the era of thoughtful designs and specialized fabrics to match the performance of all of your other technical outerwear. Base layer bottoms are a fundamental ingredient in staying warm and comfortable while outside playing in a range of temperatures and environments, and, as such a critical piece of gear, they deserve the same level of respect. From climbing into your sleeping bag after a long day of backpacking to ice-climbing in a freezer box, your long underwear is rightfully the unsung hero of comfort when getting out to play in colder weather. Long underwear bottoms are generally not stacked in a quiver, so it's important to take some time discussing some key considerations before setting out to choose the perfect pair.
It's smart to consider what conditions you will be using your long underwear in and for what activities. Are you an avid backpacker, an ice climber, an Arctic explorer, a road worker that stands outside all winter, or just a dude that wants to keep warm while you shovel snow through the bitter cold of a Minnesota winter? The more active you are, the more breathability and durability play a role; if you are just looking to fight off cold, then think about opting for a warmer pair.
These factors play a huge role in how a layer will perform and are the more objective ways to judge a pair of long underwear — comfort and fit tend to be much more on the side of personal preference and specifics based on your own body. When thinking about your body's specific needs, consider your normal operating body temperature. Are you someone who always rocks foot-warmer pouches in your boots, or are you the human furnace that will save a less thermally-inclined friend in a survival situation? Another important individual trait related to breathability is how much you sweat — if the answer is a lot, then you are likely also prone to some serious stink after a few days in the field and may want to opt for the natural odor resistance of Merino wool over a pair of synthetic bottoms. Use these considerations as a baseline when considering what type of long underwear fits your needs. From here, we provide some more nitty-gritty information regarding fabrics and weights to help tease out the specifics of what pair of bottoms will aid in your noble quest to send.
The big question is, what's the difference between all-natural and synthetic fabrics? There are tons of options out there — from silk, cotton, and wool on the natural side, to polypropylene and polyester builds on the synthetic side — with additional synthetic materials, like elastane and spandex, used to support base materials and in design features. Polyester and Merino wool are the most commonly used materials, chosen specifically for their reliable comfort and performance. For the sake of our simple discussion — and because the majority of the pairs included in this review are designed with either one or the other or a blended combination of the two — we are going to break down the benefits and disadvantages of these two fabrics.
Merino wool has a long history of comfort and quality and deservingly takes the crown in base layer fabrics. Merino sheep, originally hailing from high altitudes in New Zealand, are the sustainable source for this ultra-material. The wool is incredibly fine, feeling soft on your skin without the typical itch you remember from the wool holiday sweaters of childhood. There are some special characteristics of this wool that help make it so wonderful as a base layer — the all-natural, anti-bacterial properties and the high loft-to-weight ratio that makes for a warm layer, even if the layer itself is fairly thin. Combine that with its superb ability to breathe and wick sweat away from the body, an uncanny ability to regulate body temperature, and the plush next-to-skin feel, and you can see why it's the go-to for elite athletes and average-Joes alike.
Pros of Merino
- Insulates (even when wet)
- Regulates body temperature
- Sustainable, all-natural material
- Cozy and soft feeling next-to-skin
- Breathes well
- Wicks away moisture
- Resists odor build-up
- No need to wash after just one use
Cons of Merino
- Lacks in durability
- Long drying time
- Often not lightweight
- Shouldn't be machine dried
Synthetic long underwear has human-made fabrics from synthesized chemical polymers. The most common types are polyester and nylon, although spandex and elastane are also often found in the mix — albeit in smaller quantities, usually to aid in stretch and mobility. Synthetic base layers tend to cost less and last longer than their Merino compatriots. However, they can be less breathable at a heavier weight and do not regulate body temperature quite as efficiently as Merino. While synthetic fabric has the upper hand in evaporating sweat, nothing beats all-natural fibers to wick that sweat up off your body. Synthetics also tend to hold onto odor more readily. However, they are also usually much more durable and are easily laundered in a normal wash and dry cycle — no need to carefully flat-dry these layers.
Pros of Synthetics
- Durable (resistant to stretching out or pilling)
- Dries fast
- Lower cost
- Easier to wash and dry
- Less absorbent
Cons of Synthetics
- Less warm
- Less odor resistant (notorious for stench build-up)
- Less breathable
- Less efficient temperature regulation
- Petroleum-based fiber is less sustainable unless made from recycled polyester
It is becoming more common to have a garment made out of Merino wool and a blend of synthetic fibers in an attempt to create one layer to rule them all. By doing this, we can have the benefits of Merino wool: great insulation, fantastic temperature regulation, anti-stink, and superior breathability with an added element of durability and longevity, thanks to the synthetic fibers worked into the mix.
Long underwear bottoms are offered across a range of fabric weights: lightweight, midweight, and heavyweight (also commonly referred to as expedition weight). Depending on the type of activities and in what temperatures you intend to use your bottoms, a specific weight may be more beneficial. Most bottoms have an associated fabric weight — see the product rating's chart at the top of our Best in Class Review — and while there are certainly exceptions, we use this number to help guide our definitions:
- Lightweight options exist at around 150 g/m²
- Midweight options exist at around 200 g/m², with most in the 175 - 250 g/m² range
- Expedition Weight exists above 250 g/m², with some fabric weights as high as 400 g/m²
Lightweight bottoms will be great for versatility across seasons that don't include the bitter cold. Lighter fabric bottoms are typically more breathable and not as warm as heavier-weight options, making them ideal for spring, summer, and fall activities.
Midweight bottoms are going to be your most versatile choice. They offer breathability, wicking, and temperature regulation in cool weather, as well as an added layer of insulation when the mercury plummets. Midweight options are a great all-around option — from fall backpacking trips to winter backcountry ski missions — these bottoms can do it all.
Expedition Weight bottoms are ideal for extremely cold environments. These are the bottoms to choose when warmth is at the top of the priority list, and being cold is just not an option. Since these bottoms are beyond the average user's needs — and really only pertain to a niche group of extreme cold adventurers — we didn't test any traditional expedition weight long underwear. That said, the warmest bottoms we tested fall somewhere between midweight and expedition weight.