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How to Choose the Right Base Layer Top

How to Choose the Right Base Layer Top
At a nice rest belay ledge at last. The SmartWool Merino 250 flaunting its versatility through a full day of climbing in a wide range of temperatures and keeping us comfortable for the entire ride.
Credit: Roland Mott
By Roland Mott ⋅ Review Editor
Wednesday November 3, 2021

Is it time to finally give your Grandpa's hand-me-down, saggy cotton long johns the boot and upgrade into the 21st century? A quality base layer is the kick-off point of having functional clothing set up for getting outside to play. The critical function of this piece of clothing is to regulate your body temperature through a range of temperatures and intensity of activities by providing warmth, wicking sweat away from your body, and keeping you dry while being comfortable and allowing for ease of movement. There's a lot to consider when choosing a base layer that will benefit you in your specific outdoor passions and lifestyle for years of adventures and everyday use.

Key Considerations

It is important to consider what activities, conditions, and intentions for the use you have for a base layer. From that point, you can get into the nitty-gritty of fabrics, weights, features, etc. Are you an athlete that pushes yourself in the harshest conditions on earth, or do you need something for walking your dog and shoveling snow in the middle of winter? The big considerations, outside of largely subjective ones like comfort and fit, are warmth, breathability, and durability. Do you need a layer that will keep you warm through the icy grips of winter, or do you need a more lightweight top that will breathe when you're sucking wind and sweating like a dog pedaling up technical terrain on your mountain bike in the fall? Is durability an important quality for your intended activities? Again, we can't stress enough the importance of understanding how and for what you will be using a base layer. Determine that first. Moving from there, here are a few things that may help in your research process.

base layer - the first hint of sun while skinning up mt. hood. backcountry...
The first hint of sun while skinning up Mt. Hood. Backcountry skiing, from sweating on the ups and cooling down on the descent, is an activity that demands more versatility in a base layer.
Credit: Roland Mott

Natural vs. Synthetic Fabric

There are several materials that these tops can be made out of, including silk, cotton, Merino wool, polypropylene, polyester, etc. For ease of digestion and relevance, we're going to cover only the materials included in our current review selection — merino wool, polyester, and silk — and lay out the pros and cons to each.

base layer - all-natural or synthetic? merino-wool blends may be the way to go if...
All-natural or synthetic? Merino-wool blends may be the way to go if you want to rock the same shirt for a 3-day backpack... or maybe synthetic, if you want that instant evaporation, cool-to-the-touch-feeling of a sun shirt.
Credit: Aaron Rice

Merino Wool

Merino wool has deservingly taken its place as the best fabric to use for a base layer. Merino wool comes from a specific breed of sheep that live at higher elevations in New Zealand. Merino sheep have extremely fine wool, which makes for a delightfully soft feel next to skin with minimal to no itch. Cashmere, coming from sheep in the Himalayan region of Kashmir, would be the only sheep with finer wool than Merino. Merino wool is known for its unmatched ability to regulate body temperature through wide ranges of output and temperatures. Merino's natural anti-bacterial properties combined with its superb ability to wick sweat and dry quickly allow it to be used time after time without holding onto stink.
Incredible body temperature regulation
Natural fiber
Soft feel next to skin
Insulates when wet
Odor resistant
Don't have to wash regularly
Less durable (prone to developing holes, pilling, stretching out)
Longer dry time
Typically heavier
Typically more expensive
Can be finicky to launder
base layer - the 100% merino wool smartwool merino 250 crew sports a super cush...
The 100% Merino wool SmartWool Merino 250 Crew sports a super cush and comfy merino fabric and boasts fantastic warmth but with great wicking and breathability skills.
Credit: Roland Mott


Base layers made of synthetic fabric are usually made with Polyester or incorporate a blend of a natural fiber like Merino Wool with Polyester. Synthetic options are usually less expensive and more durable than their Merino friends, although they are typically less breathable and not as warm. They also tend to build up a significant stench-factor faster.
Durable (doesn't stretch out or pill as much)
Quick to dry
Typically less expensive
Easier to care for
Typically absorb less water
Not as warm
Tend to hold onto sweat/moisture longer
Notorious for being stinky
Not as breathable
Not as efficient temperature regulation
Petro-based fiber (if not made from recycled polyester, not as sustainable as wool)
base layer - the polyester made of partially recycled material in the north face...
The polyester made of partially recycled material in The North Face Warm Crew is thick, comfortable, and very warm but lacks in breathability and durability.
Credit: Roland Mott

Blended Fabrics

Many tops, including a number in our review are made of a Merino wool/polyester blend. For the sake of generalization, consider that these blended knits will have a blend of the positives and negatives related to each of the full-fiber options. Often these Merino/poly-blends shine brightest when it comes to moisture wicking — the wool fibers naturally wick sweat away from the body, and the lightweight polyester does quick work of evaporating it from the outside surface of the layer.

base layer - it's all about moisture-wicking -- pulling sweat away from your skin...
It's all about moisture-wicking -- pulling sweat away from your skin and up to the outer-layer of the garment, where it can easily evaporate -- keeping you comfortable, even when working your hardest.
Credit: Jill Rice


One of the finest fabrics in the world, silk is normally reserved for the likes of fine linen bed sheets and high-fashion. A natural fiber spun from cocoons of silkworms, the origins of silk date back nearly 4,000 years, although it is not often utilized when it comes to technical clothing. Silk is often more expensive and less durable than either wool or synthetic fabric, but cannot be beat when it comes to its ratio of weight-to-warmth-thickness.
Natural fiber
Incredibly soft next-to-skin
Thin fabric is easily layered over
Impressively warm
Quick dry times
Not very breathable
Not naturally odor-resistant
Prefers hand washing
base layer - made with 100% silk, the rei silk crew is soft, lightweight, and...
Made with 100% silk, the REI Silk Crew is soft, lightweight, and impressively warm for how thin it feels.
Credit: Jill Rice

Fabric Weight

Base layers come in a range of fabric weights: lightweight, midweight, and expedition weight. Depending on the season and temperature range as well as the intensity of activity, a specific weight may benefit your intended uses.

base layer - top three in terms of lightweight design, this top provides a light...
Top three in terms of lightweight design, this top provides a light amount of warmth, but really is best suited for warmer weather or high-output activity.
Credit: Jill Rice


Lightweight options are going to be versatile across the three seasons, where snow is not involved and possibly a good choice for energy-intensive snow activities. These base layers are typically going to have high breathability, making them ideal for pursuits where you are working up a sweat in warm to cold conditions. This weight is a fantastic choice and staff favorite for backpacking through the summer, and are awesome for mountain biking and trail running when the warm weather begins to cool off.

base layer - a handsome layering system with the rei co-op merino as the base...
A handsome layering system with the REI Co-op Merino as the base, Patagonia R1 for a mid layer, and the Outdoor Research Transcendent Down Hoody as the outer insulating layer.
Credit: Roland Mott


Midweight options are going to be the most versatile. They are what the majority of people go for, as they offer higher levels of warmth but still maintain versatility with breathability and wicking abilities. These are excellent for the winter season, from shoveling feet of snow to skiing on top of it. Performing comfortably in the cold of winter as well as when the mercury begins to rise again in spring and fall, a midweight is going to offer you a great all-around, do-everything base layer.

base layer - warmth tends to relate directly to weight, and this top is one of...
Warmth tends to relate directly to weight, and this top is one of the top three heaviest in our review, while also one of the three warmest.
Credit: Jill Rice


Expedition-weight tops are going to be your heavy hitters for extreme cold. These are the layers you are going to choose when warmth is a priority, and being cold is not an option. We didn't test expedition weight tops as they are not as commonly used, but a few of the warmest tops that we tested would fall somewhere between a mid- and expedition-weight. The SmartWool Merino 250 Crew, the Patagonia Capilene Air Crew, and Smartwool Intraknit 200 all offer warm, thick fabrics that are extremely warm but boast great breathability allowing them to be versatile in their use across seasons. We would say these models carry close to the warmth of an expedition weight but the versatility of a midweight.


Although this is not part of our reviews of individual products, we believe it is one of or the most important consideration when making a purchase. Both brands and consumers are growing more conscious of the impact of the clothing that we're making and wearing on both the environment and the lives of the people involved in the process. We all hold a responsibility to minimize the unnecessary harm that we either create or support as well as do what we can to improve the lives of the people that work hard so we can have gear that allows us to enjoy the outdoors.

base layer - do you know where your wool comes from? alpaca wool is common in...
Do you know where your wool comes from? Alpaca wool is common in South America, but has yet to make its way into the US outdoor gear market.
Credit: Aaron Rice

The dollars that we spend are one the most important and direct ways that we vote on the kind of environment and community we wish to both create and be a part of. When it comes to the apparel industry, it is important that brands make intentional efforts to move toward producing their clothing more sustainable.

Besides being a leader in sustainable practices when it comes to garment production, Patagonia also does a great job of walking you through what it takes to make these special fabrics — check out their video about the process of making recycled polyester

Patagonia is an aggressive leader in spearheading as well as encouraging other brands to minimize the impact their clothing and business have. Sourcing and manufacturing with sustainable fibers and materials are some of the most significant steps an apparel company can make in reducing their environmental impact. As a consumer, supporting brands and buying products that are made with sustainable fabrics is how we can directly and intentionally vote with our dollars.

base layer - the patagonia capilene lightweight crew is impressive for its...
The Patagonia Capilene Lightweight Crew is impressive for its sustainability, made from 100% recycled polyester and fair-trade certified sewing.
Credit: Roland Mott

Natural fibers are more sustainable than oil-based or synthetic viscose like polyester or nylon. This is because a natural fiber is renewable — sheep will continue growing wool at sustainable rates — whereas the earth does not seem to be replenishing petroleum deposits as rapidly. Recycled content is the most sustainable as the raw product has already been created, and it takes less energy to re-purpose it than it does to source it raw. In the wool world, non-mulesed wool is preferred for animal welfare but is controversial. Mulesing wool is a process where wool-bearing skin is removed to prevent parasitic infection. It is controversial because the process is considered by many as cruel and painful for the sheep. It is a practice that is generally being phased out in the way of more humane alternatives.

Many of us are concerned about where our natural foods come from, why not better understand the supply chain of our natural clothing? Similar to the long-standing Responsible Down Standard for puffy jackets, the Responsible Wool Standard is a fairly new certification. Both are all about promoting animal welfare to support a sustainable method of natural-fiber production, and a standard of traceability — making sure that RWS wool stays consistent, “from the farm to the final product.”

Curious for more information on sustainability in apparel?

We found some great resources for learning more about the sustainability factors for fibers, fabrics, and materials used in apparel and footwear. One is the Higg Index, which is currently one of the most comprehensive rating systems in the industry regarding sustainability, from the extraction of raw materials, processing, manufacturing, shipping, waste, and recycling, taking a holistic approach to assessing this issue. Another is a 2018 study on the climate impact of the apparel and footwear industries, with a link to the full study at the bottom of the linked page.