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How to Choose the Perfect Pair of Men's Long Underwear Bottoms

All in a line  our recruits for testing.
By Roland Mott ⋅ Review Editor
Thursday December 20, 2018

As we've mentioned before, the embarrassing history of long underwear is behind us. Gone are the days of your dimpled cotton under slacks that slouched like a mopey teenager at the Winter Dance and had a crotch the same length as the legs. Welcome to the era of thoughtful designs and technical fabrics. Base layer bottoms are a fundamental ingredient in staying warm and comfortable while we're outside playing in a range of temps and environments. From climbing into your sleeping bag after a long day of backpacking to ice climbing in a freezer box, bottoms are the unsung heroes of comfort in chilly weather. As we usually only own one pair of longies for years it's important to choose the perfect pair for how you're going to use them.

Key Considerations


It's smart to consider what conditions and activities you are planning on using your long johns for. 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 thru the bitter cold of a Minnesota winter? The big things to think about outside of comfort and fit, which are specific to each person, are warmth, breathability, and durability. How important are each of these factors in how and what you will be using your bottoms for? Using these considerations as a baseline, we give you more specific information below to aid in your noble quest to send.

A pile of the finest. Synthetics and merino's playing nice.
A pile of the finest. Synthetics and merino's playing nice.

Fabric Materials


The most common and efficient materials that base layers are made from are polyester or nylon (synthetic) and merino wool (natural). Each one is efficient and offers its own set of benefits and downsides. We lay out these differences for you below.

Merino Wool


Merino has earned its place on the throne and dons the crown as king of base layer fabrics. Merino sheep, originally hailing from high altitudes in New Zealand, produce the merino wool naturally. The material is incredibly fine, feeling soft on your skin without the typical itch you remember from the wool holiday sweaters of childhood. The natural characteristics of this wool are what help make it so wonderful as a base layer, such as the anti-bacterial properties that keep your garment from holding onto any stench your body produces. Combine that with its superb ability to breathe and wick sweat away from the body, its uncanny ability to regulate body temperature, and the soft and cozy next to skin feel, you can see why it's the go-to for elite athletes and average joes alike.

100% merino. The stamp of a quality fiber that will perform well
100% merino. The stamp of a quality fiber that will perform well

Pros of Merino

  • Ability to insulate (even when wet)
  • Regulation of body temp
  • Sustainable, natural material
  • Cozy and soft feeling next-to-skin
  • Breathes well
  • Wicks away moisture
  • Resists odor build-up
  • No need to wash after a single use
  • Versatility

Cons of Merino

  • Lacking in durability
  • Long time to dry
  • Not lightweight
  • Pricey
  • Best to avoid drying machine

Synthetics


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). Synthetic base layers tend to cost less and last longer than their merino compatriots. They are usually less breathable and do not regulate body temperature as efficiently as merino, though, and also tend to hold onto odor more readily.

The Patagonia Capilene Lightweight  a stellar synthetic base layer.
The Patagonia Capilene Lightweight, a stellar synthetic base layer.

Pros of Synthetics

  • Durability (resist stretching out or pilling)
  • Dries fast
  • Lower cost
  • Easier to wash and dry
  • Less absorbent

Cons of Synthetics

  • Less warm
  • Less odor-resistance (notorious for stench build-up)
  • Less breathable
  • Less efficient temperature regulation
  • Petroleum-based fiber (less sustainable, unless made from recycled polyester)

Blended Fabrics


It is becoming more common to have a merino wool base and to blend a synthetic fabric into the garment. 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. The SmartWool Merino 150, for example, wraps merino wool around a nylon core to allow the thin and super breathable merino to have a little more durability.

80% merino with a polyester and elastene blended in for durability and stretch.
80% merino with a polyester and elastene blended in for durability and stretch.

Fabric Weight


Long underwear bottoms are offered across a range of fabric weights: lightweight, midweight, and expedition weight. Depending on what type of activities and in what temperatures you intend on using your bottoms, a specific weight may be more beneficial for you.

Lightweight bottoms are going to be great for versatility across seasons that don't include bitter cold. Lighter fabric bottoms are typically going to be more breathable and not as warm making them ideal for activities across spring, summer, and fall. The REI Co-op Lightweight Bottoms are a great option for summer backpacking, and the Patagonia Capilene Lightweight Bottoms are a favorite for fall river trips. Lightweight is a great choice if you are operating outside of the super cold seasons or you pursue high output activities in cooler weather.

Midweight bottoms are going to be your most versatile choice. Offering breathability, wicking, and temp regulation in warm to cool weather as well as great insulation in colder temps. Midweight options are a great all-around, do it all baselayer from fall backpacking trips to winter backcountry ski missions. Our favorite midweight bottoms that we tested are the Outdoor Research Alpine Onset.

Expedition Weight bottoms go to bat in 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 only pertain to a niche group of extreme cold adventurers), we didn't test any traditional expedition weight long underwear. That said, the two warmest bottoms we tested fall somewhere between midweight and expedition weight. The SmartWool Merino 250 and the Arc'teryx RHO LT consist of thick fabrics that are quite breathable and incredibly warm. These two bottoms offer the warmth close to that of an expedition weight with the versatility of a midweight, like Jerome Bettis or your "plump" friend that is surprisingly athletic.


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