A guide to merino wool base layers, an essential item for thru-hiking.
Courtesy of Woolx
A base layer (aka long underwear or thermals) is the shirt and pants that you wear as your first layer of clothing. It often is close fitting and meant to keep you warm when the temperatures start to plummet. It's essential for cold weather hiking and useful in the swing season when temperatures are fluctuating. In the warmer weather, a base layer is ideal as a dry layer for sleeping.
Merino wool is a natural fiber harvested from Merino sheep. Though Merino sheep originated from Spain, almost 80 percent of merino wool now is obtained from sheep raised in Australia and New Zealand. Merino wool is the gold standard for base layers because of its has many desirable qualities.
WARMTH: Like most wool fabrics, merino wool is extremely warm. The individual fibers are slightly crimped, creating air pockets that trap warm air. It makes an excellent base layer for winter and swing season outings.
BREATHABILITY: Merino wool fibers are among the most breathable and moisture-wicking fabrics available. Each strand can absorb up to 30 percent of its weight in moisture; wicking it away from your skin and out towards your surrounding environment. As a result, the fabrics feel cool and dry against your skin even when you are sweating up a storm.
ODOR RESISTANCE: Merino wool is naturally resistant to odor-causing bacteria thanks to its ability to manage moisture. Without moisture, odor-causing bacteria cannot multiply and cause your wool shirt to stink. Because of its odor-control properties, merino wool clothing doesn't need to be washed as frequently as cotton or synthetic fabrics. When hiking, I can wear merino wool socks for up to a week and t-shirts for three days before washing.
UV PROTECTION: Merino wool naturally protects your skin from the sun's damaging rays. Most wool clothing has a UPF rating of 30+ or higher.
DURABILITY: Merino wool is naturally crimped, giving it some elasticity that allows it to bend and stretch instead of tearing when under stress. Each fiber also is made with keratin, the same tough fibrous protein molecules that make up our hair, nails, and outer layer of skin. Each wool fiber can be bent and flexed more than 30, 000 times before it is damaged.
COMFORT: Merino wool is known for its soft fibers that don't itch and feel great against your skin. Each strand is very thin compared to regular wool. It moves smoothly across your skin and doesn’t snag or poke you. As a result, you don't get the scratchy, itchy feeling you experience from the large fibers of regular wool. Most wool garments also are blends and mixed with stretchable fabrics like elastane, so that the clothing can move with you as you hike or climb.
QUICK DRY: Merino wool's thin fibers allow each garment to be finely spun and woven into a lightweight fabric that dries as quickly as most synthetic fabrics of equal weight.
Merino wool sheep source in New Zealand.
Merino wool clothing is pricey and there are several factors you should consider before handing over your hard earned cash. We outline the most common features you will encounter and provide some buying advice so you can purchase the correct base layer for your needs.
Most merino wool garments are categorized by fabric density. You'll often see it classified as a number such as 150 or 250; it refers to the number of grams of merino wool in every square meter of fabric. These densities are then used to classify the clothing as lightweight, midweight, or heavyweight. The weight you choose depends on the type of activities you do and when you do them.
Ultralight (less than 150 g/m2): Ultralight merino is a great option when you're hiking in the heat. It will help you keep both sweat and odors to a strict minimum while also shielding your skin from the sun.
Lightweight (between 160 and 190 g/m2): Lightweight merino wool base layers can be worn all year round. They're not too hot and not too cold. You can wear them alone in warm temperatures or underneath a jacket when the weather gets cool. Also great for sleeping.
Midweight (between 195 and 250 g/m2): You’ll want a midweight merino layer for the cold winter months. It's a perfect base layer for snowshoeing and skiing. As temperatures fluctuate from below freezing to above freezing, you can easily add or remove outer layers and stay comfortable with your merino 250 base layer at the core.
Heavyweight (greater than 250 g/m2): You'll want to wear a heavyweight merino base layer when you are standing or sitting outside in cold temperatures for any amount of time. These dense layers are great for ice fishing, hunting or other similar low-key activities. A heavyweight merino wool base layer may be too warm for strenuous activities such as hiking or climbing.
Clothing made with 100 percent merino wool is costly. As a result, some manufacturers use a wool blend that pairs merino wool with another fabric such as nylon, polyester, or polyamide. With 100 percent merino clothing, you get all of the benefits of merino wool, but you'll pay dearly for it. A single base layer bottom of 100 percent merino wool will set you back at least $100. One hundred percent wool clothing also tends to break down faster than wool blends. You have to be a bit more gentle in how you wear and wash your 100 percent wool base layers.
Merino wool blends retain most of the benefits of merino wool with a less expensive price tag. You'll want at least 80 percent wool to get the full benefits. Any less than 80 percent and you start to lose the warmth, moisture wicking, and odor control of the wool fabric. The best blends are corespun which spins the wool fiber around a nylon, polyester or polyamide core fabric. The inner core fabric provides extra durability over the long run, while the outer wool layer provides warmth and moisture control.
Closeup of fabric from Minus33's Chocorua Top
Most merino wool clothing, especially base layers, are designed with a slim fit that feels comfortable under clothing. There is a wide range of clothing styles that you can mix and match depending on the prevailing weather conditions. For the harsh temperatures of winter, long-sleeve tops or zippered shirts can be paired with long underwear style bottoms. In milder conditions, you can pair a zippered long-sleeve shirt with lightweight leggings or even ¾ length Capri-style pants. Once summer hikes, you don't have to ditch your beloved merino wool. You can wear a pair of merino wool underwear under your shorts and match it with a lightweight short-sleeve crew for a breathable, wicking, odor-free outfit.
Seams: Since it will be up against your skin, look for flat seams in any base layer to avoid chafing. Ideally, there should be no zippers or buttons. Opt for elastic waistbands and flap-style openings instead.
Extra Features: Most merino wool clothing is designed for active lifestyles and will include convenience features such as a hood, thumb holes, and pockets. A flap-style fly is helpful for the men to use the restroom.
Material: 100% Australian merino wool, interlock knit
Insulation Weight: Midweight, 230 g/m2
Layer weight: Size Large = 9.6 oz
Price: $65.99 on Amazon
Warm and comfortable is how we describe the Minus33 Chocorua baselayer top. Clothing from Minus33 is made from superfine Merino wool that is softer than most merino wool clothing. The Chocorua has a roomy fit that isn't too baggy nor too skin tight. It’s perfect for wearing as a base layer in the winter or night-time camp clothing during the spring and fall. It has a drop-tail hem to keep your back covered and a regular fit that provides plenty of room.
Combine it with a pair of Kancamagus midweight bottoms and you've got yourself an awesome midweight base layer.
Material: 100% merino wool
Insulation Weight: Lightweight 170 g/m2
Price: $60 on Amazon
Minus33's base layers never disappoint, and the Ticonderoga is no exception. At 170 g/m2, the Minus33 Ticonderoga is slightly warmer than traditional lightweight base layers. It is great for spring and fall hiking or to keep warm on chilly summer nights.
The Ticonderoga is a regular fit crew-neck that pairs nicely a pair of Saratoga lightweight bottoms. Note that if you prefer tighter fits, then consider getting one size down.
Material: 100% merino wool
Insulation Weight: Light- to Midweight, 150 g/m² to 250 g/m²
Price: $75 - $225 on Moosejaw
Smartwool's Merino 150 fabric is the perfect weight - not too heavy and not too light. It is ideal for three-season usage and works great as an underlayer for active winter sports. The company also sells a Merino 205 line, which is made with thicker wool that performs best as an underlayer in the winter.
Smartwool is known for its colorful and creative clothing. No matter your taste in clothing Smartwool has a color, pattern or style to suit you. For tops, you can choose between tank tops, short-sleeve shirts, long-sleeve shirts or a 1/4 zip crew. Smartwool makes matching bottoms and even a one-piece top and bottom combo.
Material: 83% merino wool, 12% nylon, 5% spandex
Insulation Weight: Midweight
Price: $50-$100 on Amazon
Outdoor Research Alpine Onset series takes the best of Merino wool and mixes it with nylon and spandex for flexibility and durability. The layer fits comfortably - it's not too tight, not too loose and moves when you move. The nylon and spandex also help to hold the shape of the garment through repeated washings. The Alpine Onset clothing gets high marks for its breathability. It is a midweight layer suitable for active sports in the winter, short walks in the fall, and sleeping in a wide range of temperatures.
The Alpine Onset bottoms come separately and feature a functional fly.
Material: 100% merino wool
Insulation Weight: Midweight, 250g/m2
Price: $50 on Amazon
Meriwool stands out for its affordable pricing. All of its base layer clothing costs well under $100. Meriwool tops have a comfortable fit that you can wear either as a standalone shirt or a base layer. The bottoms are equally as comfortable for use during any cold weather activity or sleeping.
Material: 100% merino wool
Insulation Weight: Lightweight, 200gm
Price: $80 - $100 on Moosejaw
Icebreaker is known for its quality clothing and its commitment to sustainability both in the source of its materials and how their clothing is made. A slim-fitting base layer, the Icebreaker 200 Oasis series is ideal for hiking and sleeping in colder temperatures. Icebreaker offers the Oasis in a variety of styles including crew tops, zippered tops, and bottoms.
Icebreaker spends some time on the design of their clothing The Oasis shirt has offset shoulder seams that reduce chafing when you are wearing a backpack. It also has a drop tail hem that prevents the shirt from riding up and exposing your back to the elements. Another favorite is the legless bottom that stops above the boot and won't add any uncomfortable seams around your ankle.
Material: 100% merino wool
Insulation Weight: Midweight, 230 g/m²
Price: $70-100 on Amazon
WoolX is known for its super soft merino wool that never itches and feels as soft as cashmere. The company sticks to the basics with solid colors and the standard styles you’d expect for an underlayer. It looks and feels like long underwear and isn't a layer you could wear outside of camp. Though simple, the clothing is quality made with non-chafing flatlock seams, cuffs to keep pants and sleeves from riding up and slim, athletic fit. Its line of midweight base layers (including their base layer pants) is warm enough for spring, fall, and winter.
Material: 55% merino wool / 45% nylon
Insulation Weight: Midweight, 200g/m2
Price: $89 on KUHL
Kuhl's Kondor baselayer is known for its comfort and durability. It has a slim fit and breathable design. The Kondor is seamless and offers more stretch and durability than traditional flatlock seams. Best of all, it a wool blend that mixes nylon with merino wool creating a soft layer that wicks away moisture and lasts longer than traditional wool. It pairs nicely with the Valiant merino pants.
Material: Nucliex (81% merino wool, 12% nylon, 7% elastane)
Insulation Weight: Midweight
Price: $120 on Moosejaw
Unlike most wool baselayers which use 100 percent wool, the Arc'teryx Satoro AR Crew Neck Shirt is a nylon blend. The long-sleeved midweight merino base layer offers the best of merino wool with the enhanced durability of nylon and elastane. Each piece of wool is wrapped around a core nylon fiber. This nylon gives the clothing some extra stretch and can handle the rigors of long sweaty days on the trail coupled with regular washings.
Shop for women's top
Do you wear underwear under your base layer?
Wearing underwear under a base layer comes down to personal preference and your hiking conditions. Some people always prefer to have an underwear layer, while others don't like that extra bulk. We recommend wearing an underwear layer when you anticipate removing layers because of exertion or rising temperatures. If you do need to remove your base layer, then you'll want underwear underneath, or else you'll have to go commando. If it is cold, and you know you won't be shedding layers, then feel free to leave the underwear at home.
How to wash a merino wool base layer?
Merino wool should be washed on a gentle cycle with cold or lukewarm water. Avoid hot water because it causes the wool to shrink. Use a mild soap and do not add bleach or fabric softener as they both degrade the wool fiber. Air dry the wool garment by laying it flat as hanging it may cause it to stretch. If you must use a dryer, then use a low heat setting.
By Kelly Hodgkins: Kelly is a full-time backpacking guru. She can be found on New Hampshire and Maine trails, leading group backpacking trips, trail running or alpine skiing.
About Greenbelly: After thru-hiking the Appalachian Trail, Chris Cage created Greenbelly to provide fast, filling and balanced meals to backpackers. Chris also wrote How to Hike the Appalachian Trail.
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