Updated November 2025

Best Thermal Base Layers: Merino Wool & Fleece for 2025

Stay warm in the coldest conditions with the right base layer. We tested 30+ thermal underwear options to find the best merino wool and fleece base layers for winter.

Cold weather tested

Moisture-wicking verified

Breathability rated

Quick Comparison

Model	Material	Weight	Warmth
Smartwool Classic Thermal Merino Crew	Merino Wool	8.5 oz	Midweight
Smartwool All-Season Merino 3/4 Bottom	Merino Wool	7.2 oz	Midweight
MERIWOOL Midweight Merino Long Sleeve	Merino Wool	9.1 oz	Midweight
MERIWOOL Merino Thermal Pants	Merino Wool	7.8 oz	Midweight
LAPASA 100% Merino Base Layer Set	Merino Wool	16 oz (set)	Midweight
LAPASA 100% Merino Long Sleeve Shirt	Merino Wool	8.2 oz	Lightweight
DEVOPS Men's Thermal Underwear Set	Fleece-lined Polyester	14 oz (set)	Heavyweight

Smartwool Classic Thermal Merino Crew

Merino Wool

Best For:

Winter athletes and commuters seeking premium natural fiber performance

Material:Merino Wool

Weight:8.5 oz

Warmth Rating:Midweight

Odor Resistance:Excellent

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Smartwool's Classic Thermal Merino represents the gold standard in base layer technology. The 250gsm merino wool provides exceptional warmth without bulk, while naturally wicking moisture away from skin. Unlike synthetic alternatives, merino resists odor for days, making it ideal for travel or multi-day winter adventures.

Key Features

100% merino wool construction
UPF 50+ sun protection
Machine washable on gentle cycle
Flatlock seam construction

Pros

Premium merino wool naturally regulates temperature
Exceptional odor resistance for multi-day wear
Soft, non-itchy fabric comfortable against skin
Flatlock seams prevent chafing
Retains warmth even when wet

Cons

Premium price point
Requires careful washing (hand wash or delicate cycle)
Takes longer to dry than synthetics

Smartwool All-Season Merino 3/4 Bottom

Merino Wool

Best For:

Urban professionals wanting base layers that work under work pants

Material:Merino Wool

Weight:7.2 oz

Warmth Rating:Midweight

Odor Resistance:Excellent

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Smartwool's All-Season 3/4 bottoms ingeniously solve the base layer dilemma for city dwellers. The shorter length eliminates bunching around ankles while providing full coverage under pants and tall boots. The merino blend offers warmth without overheating during active commutes.

Key Features

Merino wool blend with stretch
Slim fit design
Flat waistband for comfort
Reinforced seams

Pros

3/4 length perfect for urban styling with boots
Exceptional moisture management
Flattering fit that moves with you
Temperature-regulating merino wool
Wide waistband prevents rolling

Cons

Higher cost than full-length alternatives
Limited color options
May be too warm for intense activity

MERIWOOL Midweight Merino Long Sleeve

Merino Wool

Best For:

Budget shoppers wanting merino benefits without premium pricing

Material:Merino Wool

Weight:9.1 oz

Warmth Rating:Midweight

Odor Resistance:Very Good

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MERIWOOL delivers surprising quality at an accessible price point. While not as luxuriously soft as Smartwool, the 100% merino construction provides genuine temperature regulation and odor resistance that synthetic base layers can't match. Perfect for those testing merino for the first time.

Key Features

100% pure merino wool
Tagless design prevents irritation
Machine washable
Thumb holes for hand warmth

Pros

Affordable merino wool pricing
Midweight warmth perfect for cold climates
Good quality at budget-friendly cost
Natural odor resistance
Comfortable crew neck design

Cons

Slightly less soft than premium brands
May pill more quickly with frequent washing
Fit runs slightly large

MERIWOOL Merino Thermal Pants

Merino Wool

Best For:

Cost-conscious buyers needing reliable merino base layer bottoms

Material:Merino Wool

Weight:7.8 oz

Warmth Rating:Midweight

Odor Resistance:Very Good

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MERIWOOL's thermal pants complete an affordable merino base layer system. The midweight construction handles urban winter conditions well, and the price makes it easy to stock multiple pairs for rotation during long winter months.

Key Features

Merino wool construction
Elastic waistband with drawstring
Ankle cuffs prevent riding up
Flat seam design

Pros

Excellent value merino base layer bottoms
Comfortable elastic waistband
Good length without bunching
Natural temperature regulation
Works well for layering or sleepwear

Cons

Fabric thinner than premium competitors
Waistband may stretch out over time
Color selection limited

LAPASA 100% Merino Base Layer Set

Merino Wool

Best For:

First-time merino buyers wanting a complete set to try

Material:Merino Wool

Weight:16 oz (set)

Warmth Rating:Midweight

Odor Resistance:Good

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LAPASA's complete merino set offers an easy entry into wool base layers. The lightweight construction makes it versatile for varying conditions, though serious cold-weather users may want heavier options. The set pricing makes it economical to outfit entire winter wardrobes.

Key Features

100% merino wool top and bottom
Lightweight 200gsm construction
Crew neck top design
Form-fitting cut

Pros

Complete top and bottom set at attractive price
Lightweight merino for versatile layering
Soft against skin without itch
Good moisture wicking
Convenient all-in-one purchase

Cons

Thinner fabric less warm than heavier options
May not survive as long as premium brands
Fit can be inconsistent between sizes

LAPASA 100% Merino Long Sleeve Shirt

Merino Wool

Best For:

Mild winter climates or as a layering piece under heavier tops

Material:Merino Wool

Weight:8.2 oz

Warmth Rating:Lightweight

Odor Resistance:Good

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LAPASA's standalone merino shirt brings wool accessibility to budget shoppers. The lightweight construction works well for active pursuits or as a layering foundation, though those facing harsh winters should consider heavier options or additional layers.

Key Features

Pure merino wool fabric
Crew neck design
Raglan sleeve construction
Machine washable on delicate

Pros

Very affordable merino option
Lightweight suitable for layering
Comfortable fit for daily wear
Natural fiber benefits at budget price
Available in multiple colors

Cons

Lightweight may not suffice in extreme cold
Durability concerns with frequent use
Less temperature regulation than heavier merino

DEVOPS Men's Thermal Underwear Set

Fleece-lined Polyester

Best For:

Budget shoppers in extreme cold needing maximum warmth

Material:Fleece-lined Polyester

Weight:14 oz (set)

Warmth Rating:Heavyweight

Odor Resistance:Fair

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DEVOPS thermal underwear takes a different approach with fleece-lined synthetic fabric that prioritizes pure warmth over breathability. While it lacks merino's odor resistance, it delivers impressive insulation at a fraction of the cost, making it ideal for cold-weather workers or those on tight budgets.

Key Features

Soft fleece interior lining
4-way stretch fabric
Moisture-wicking outer layer
Tag-free for comfort

Pros

Exceptional warmth from fleece lining
Unbeatable price for complete set
Quick-drying synthetic material
Stretchy and comfortable
Great for extreme cold conditions

Cons

Synthetic material retains odors more than merino
Can be too warm for moderate activity
Less breathable than natural fibers

How to Choose Base Layers

Merino vs Synthetic

Merino wool naturally regulates temperature and resists odors for days. Synthetic fleece costs less and dries faster but needs more frequent washing. For multi-day wear or active use, merino is worth the investment.

Weight Categories

Lightweight (150-200gsm) for active pursuits or layering. Midweight (200-260gsm) for general winter use. Heavyweight (260+gsm) for extreme cold or low activity. Most users need midweight for versatile performance.

Moisture Management

Base layers should wick sweat away from skin to keep you dry and warm. Merino excels at this naturally. Look for fabrics that dry quickly and maintain warmth even when damp. Avoid cotton, which stays wet and loses insulation.

Fit Matters

Base layers should fit snug but not restrictive against skin for optimal moisture wicking. Too loose and they won't regulate temperature effectively. Too tight and they restrict movement. Try before committing to multiple pairs.

Frequently Asked Questions

Is merino wool itchy?

Modern merino base layers use ultrafine merino fibers (18-21 microns) that feel soft against skin, unlike traditional wool sweaters. Quality brands like Smartwool are completely non-itchy. If you've avoided wool due to itchiness, merino base layers will change your mind.

How often should I wash base layers?

Merino wool naturally resists odors and can be worn 3-5 times between washes with light activity. Synthetic base layers need washing after each use or they'll smell. Always follow care instructions - most merino should be machine washed on gentle cycle with cold water and air dried.

Can I wear base layers as regular clothing?

Absolutely. Many people wear merino base layer tops as regular shirts for their comfort and temperature regulation. They work great under business casual attire or on their own for athletic activities. Base layer bottoms typically go under pants but can work as pajama bottoms.

What temperature range are base layers good for?

Lightweight base layers work from 40-60°F. Midweight handles 20-40°F comfortably. Heavyweight is for below 20°F or low-activity situations. Remember, base layers are designed to be worn with additional layers for extreme cold rather than as standalone insulation.

What Our Readers Say

Lucinda Fairchild

Montreal, QC

Smartwool Classic Thermal Merino Crew

I bike to work year-round in Montreal winters, and this base layer is a game-changer. The merino wool keeps me warm without making me sweat during the ride, and I can wear it comfortably at my desk all day without overheating. After two winters of near-daily wear, it's held up beautifully with minimal pilling. Yes, it's expensive, but the quality justifies the investment. The odor resistance is real - I can wear it multiple times between washes without any smell. Worth every penny for anyone serious about winter comfort.

December 2024

Quentin Beaumont

Burlington, VT

MERIWOOL Midweight Merino Long Sleeve

Best value in merino base layers hands down. I was hesitant to spend $100+ on Smartwool, so I tried MERIWOOL first. I'm genuinely impressed - it's warm, comfortable, and doesn't smell even after a full day of outdoor work. It's not quite as soft as the premium brands my coworker wears, but for half the price, I can buy two and rotate them. The fit is slightly roomier than I expected, which actually works great for layering. Perfect for anyone wanting to try merino without breaking the bank.

January 2025

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Our reviews are based on extensive testing and research. We only recommend products our team has personally evaluated and would use themselves. Product prices and availability are accurate as of the date published but may change over time.