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The Best Thermal Underwear

Updated Dec 11, 2022
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Do you like exercising, hiking, or playing outdoors in the cooler months? How about working in the yard or going for walks? Do you do fieldwork, fish, or hunt? Or maybe you enjoy engaging in outdoor sports or playing in the snow? Chances are good there’s some outdoor activity you do in the winter months. And if so, you’ll probably find a set of comfortable, functional base layers—lightweight pieces of clothing to wear under sweaters or jackets or pants—to be a worthwhile investment.

Ridge Merino Men’s Aspect Midweight Wool Base Layer Long Sleeve Shirt, a versatile men’s long-sleeve top

Ridge Merino Men’s Aspect Midweight Wool Base Layer Long Sleeve Shirt

A versatile men’s long-sleeve top
This thin, merino-blend shirt comes in basic colors for wearing alone or layering under clothing.
We recommend
Ridge Merino Women’s Aspect Midweight Wool Base Layer Long Sleeve Shirt, a versatile long-sleeve top for women

Ridge Merino Women’s Aspect Midweight Wool Base Layer Long Sleeve Shirt

A versatile long-sleeve top for women
This is essentially the same shirt as the men’s version, in a more-tapered cut.
We recommend
REI Co-op Midweight Base Layer Crew Top - Men's, a synthetic long-sleeve top for men

REI Co-op Midweight Base Layer Crew Top - Men's

A synthetic long-sleeve top for men
If you’re a first-time hiker, backpacker, or camper, this is a decent top to get you out there.
We recommend
REI Co-op Midweight Base Layer Crew Top - Women's, a synthetic long-sleeve top for women

REI Co-op Midweight Base Layer Crew Top - Women's

A synthetic long-sleeve top for women
If you’re trying an activity for the first time and aren’t sure yet which layers will work best for you, this is a good starter shirt.
We recommend
REI Co-op Midweight Base Layer Bottoms - Men's, synthetic leggings for men

REI Co-op Midweight Base Layer Bottoms - Men's

Synthetic leggings for men
These stretchy, form-fitting leggings come with a fly—something that leggings twice the price often don’t have.
We recommend
REI Co-op Midweight Base Layer Tights - Women's, synthetic leggings for women

REI Co-op Midweight Base Layer Tights - Women's

Synthetic leggings for women
These leggings are stretchy, soft, and very comfortable on curvy bodies—one of the advantages of synthetic fabrics.
We recommend
Smartwool Merino 150 Base Layer Long Sleeve - Men’s, a merino wool top for long, active days outside

Smartwool Merino 150 Base Layer Long Sleeve - Men’s

A merino wool top for long, active days outside
This merino wool shirt is fitted but not tight, and it wears well under backpack straps.
We recommend
Smartwool Merino 150 Base Layer Long Sleeve - Women’s, a merino wool top for long, active days outside

Smartwool Merino 150 Base Layer Long Sleeve - Women’s

A merino wool top for long, active days outside
This one is similar to the men’s shirt, but in a more-tapered cut.
We recommend
Minus33 Chocorua Men’s Midweight Wool Crew, a thicker wool top for men

Minus33 Chocorua Men’s Midweight Wool Crew

A thicker wool top for men
This soft, well-made merino wool top is thicker than our other picks, so it’s a great shirt to wear under looser clothing, like waders or rain gear.
We recommend
Minus33 Ossipee Women’s Midweight Wool Crew, a thicker wool top for women

Minus33 Ossipee Women’s Midweight Wool Crew

A thicker wool top for women
Differently named but the same top as the men’s version, this merino wool top is thick enough to wear by itself or under a jacket or sweatshirt.
We recommend
Minus33 Kancamagus Men’s Midweight Wool Bottom, thicker wool leggings for men

Minus33 Kancamagus Men’s Midweight Wool Bottom

Thicker wool leggings for men
These merino wool leggings have some room in the crotch, so they’re best worn as an undergarment beneath rain pants or snow pants.
We recommend
Minus33 Franconia Women’s Midweight Wool Bottom, thicker wool leggings for women

Minus33 Franconia Women’s Midweight Wool Bottom

Thicker wool leggings for women
These merino wool leggings are thick, soft, and comfortable, and they work best as a layer under waders or snow bibs.
We recommend
Under Armour Men’s ColdGear Base 3.0 Crew, if you’re going to get dirty

Under Armour Men’s ColdGear Base 3.0 Crew

If you’re going to get dirty
Because this crewneck stands up to a lot of washing, it’s a good option for sports practice, obstacle races, or any activity where you might get really dirty.
We recommend
Under Armour Women’s ColdGear Base 3.0 Crew, if you’re going to get dirty

Under Armour Women’s ColdGear Base 3.0 Crew

If you’re going to get dirty
A long-sleeve crewneck top for women, this version has sleeves with thumbholes (the men’s version does not).
We recommend
Icebreaker Men’s BodyfitZone Merino 260 Zone Long Sleeve Crewe, a techy wool top for high-output activities

Icebreaker Men’s BodyfitZone Merino 260 Zone Long Sleeve Crewe

A techy wool top for high-output activities
This top was especially loved by our snow sports testers because it kept them from overheating. It’s pieced together with different fabric panels, so it’s form-fitting yet doesn’t restrict movement, a rare combination for a wool garment.
We recommend
Icebreaker Women’s BodyfitZone Merino 260 Zone Long Sleeve Crewe, a techy wool top for high-output activities

Icebreaker Women’s BodyfitZone Merino 260 Zone Long Sleeve Crewe

A techy wool top for high-output activities
Especially loved by our snow sports testers because it kept them from overheating, the women’s version of this top is pieced together slightly differently to accomodate a different body shape.
We recommend
Icebreaker Men's BodyfitZone Merino 260 Zone Leggings, the best wool leggings we’ve tried

Icebreaker Men's BodyfitZone Merino 260 Zone Leggings

The best wool leggings we’ve tried
This pair ranks among our all-time favorite merino leggings. However, these leggings don’t have a fly—a dealbreaker with all the other leggings our testers tried. But testers liked the fit so much they were willing to make an exception.
We recommend
Icebreaker Women’s BodyfitZone Merino 260 Zone Leggings, the best wool leggings we’ve tried

Icebreaker Women’s BodyfitZone Merino 260 Zone Leggings

The best wool leggings we’ve tried
After four years of testing, these are our favorite merino leggings because they prevent overheating. We particularly love them because they are great at stretching with your every move—more so than most wool leggings we've tried.
We recommend

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