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Specs are compiled from manufacturer listings and verified buyer reviews and can change over time — please confirm the key details on the product page before buying.
If you pick a fabric that traps sweat against your skin, you will feel cold and clammy the moment you step outside. The fix is a natural fiber that wicks moisture, and the two best contenders are merino wool and mulberry silk.
I’m Mohammad Maruf — the founder and writer behind WellFizz. This guide is built by comparing the manufacturers’ published specifications and the patterns across verified customer reviews, so you get each pick’s real strengths and trade-offs instead of marketing spin.
Here is the honest answer on the best fabric for long underwear: merino wool wins for all-day warmth and odor control, while silk wins for next-to-skin luxury and lightweight comfort.
Our Picks at a Glance
How To Choose The Best Fabric For Long Underwear
Long underwear works only when the fabric traps body heat while letting sweat evaporate. A fiber that fails at either job leaves you shivering despite extra layers.
Natural fibers beat synthetics for comfort
Merino wool and mulberry silk are both natural protein fibers that feel gentle on sensitive skin, unlike coarse wools or clammy polyester blends. Merino is prized for regulating temperature across a wide range of activity levels, while silk is celebrated for its smooth, almost liquid feel that moves easily under multiple layers.
The weight number tells you the warmth
The “175” you see on many merino base layers refers to the fabric weight in grams per square meter — a mid-weight option meant for cool to cold weather. Lighter weights suit milder days or high-exertion activities, while heavier weights are built for deep winter. Match the weight to your climate and activity level for comfort.
Quick Comparison
| Model | Best For | Fabric | Weight | Fit | Amazon |
|---|---|---|---|---|---|
| icebreaker 175 Leggings★ Best Overall | All-day warmth & odor control | 100% merino wool | 175 gsm | Snug, true | Amazon |
| METWAY Silk Set | Ultra-soft luxury & layering | 100% mulberry silk | Lightweight | True to size | Amazon |
| Icebreaker 175 Leggings | Cold-weather warmth | 100% merino wool | 175 gsm | Comfortable | Amazon |
In‑Depth Reviews
1. icebreaker Men’s Standard 175 Everyday Cold Weather Wool Base Layer Leggings
Our pick — over 4.5★ from 500+ verified ratings; the strongest balance of quality and price.
The merino layer that handles a full farm day without sliding or smelling.
If you wear your base layer for hours, not just a quick commute, this is the one. The 100% merino wool rib knit fabric is soft and breathable, and buyers report it stays comfortable from a cold morning start through an active day of chores — “Immediately comfortable, warm and ready for long days of all day farm choring” is how one owner put it, adding that it never slid down the back like cheaper options.
At 175 grams, this fabric weight hits the mid-weight balance for cool-to-cold weather, and buyers describe superb sweat-wicking and odor control even when you work up a sweat. It is noticeably thin but still warm, so you get enough insulation without the bulk you would feel under jeans or hiking pants.
Pure merino’s trade-off is durability. While the warmth and comfort earn the most praise, a handful of owners mention seams ripping and stitches splitting after repeated wear, so this is a layer to treat with reasonable care in the wash. Rated 4.6 stars across 500+ ratings, it leads this list on overall satisfaction, and it beats the other Icebreaker leggings here on score — 4.6 versus 4.1 — making it the stronger merino buy of the two.
What wins people over
- Warmth is the top praise — over a dozen owners single it out for keeping them cozy in freezing conditions
- Comfort runs a close second, with the soft, non-itchy feel against the skin getting consistent applause
- Lightweight build means it layers easily under jeans or trekking pants without bulk
The catch to know
- Durability is the recurring gripe — treat the seams gently and wash on delicate cycles
- Premium pricing sits above synthetic alternatives, which some buyers balk at
Reach for this if: you need a breathable, odor-resistant layer for long active days in cool weather and you value comfort over cost.
Look elsewhere if: you are hard on your gear and need something that shrugs off rough washing without worry.
2. METWAY Silk Long Underwear Men’s Mulberry Silk Long Johns Top and Bottom Set
The silky base layer that disappears under your clothes like a second skin.
If your skin is picky about textures, this is the fabric to beat. The METWAY set is made from 100% knit mulberry silk, a material buyers describe as soft, lightweight, and smooth enough to ease movement under multiple layers of clothing. For anyone with skin sensitivity to texture, the comfort is the whole point — “good for adding warmth but especially for the comfort” is how one owner summed it up.
Silk’s superpower is temperature versatility. Customers note it keeps you warm outdoors but prevents overheating when you step back inside, because the lightweight fabric breathes instead of trapping heat. It works as both a cozy layer for sleeping and a smooth base layer under daily clothing, and reviewers point out it fits true to size and keeps the chill off during walks between the car and the store.
The trade-off is the fly opening. Several owners point out the opening lacks a button or overlap, which can allow unwanted exposure — the most-discussed fit issue in the reviews. It is also lighter on warmth than merino wool, so for deep-freeze days the icebreaker 175 above offers more insulation per layer. Where silk wins is pure comfort and elegance of feel, which is why this set holds a solid 4.5-star rating across nearly 800 ratings — the most-reviewed product in this lineup.
Luxury that performs: The smooth silk slides easily under work clothes without bunching, and the breathability keeps you from overheating during indoor-outdoor transitions — a genuinely unique advantage over wool.
The fit caveat: Sizing earns mixed feedback overall, so check the measurements carefully, and note the fly closure is a known weak point before you commit.
3. Icebreaker Merino Men’s 175 Everyday Leggings
A warm, quality-feeling merino layer with a durability question mark.
This uses the same 100% merino wool rib knit construction as the top pick, and buyers confirm it delivers on the essentials: warm, soft, and quality-made, with that familiar “you can feel the insulation” certainty against your legs. The fabric stands up well during hikes, doesn’t hold onto smells, and breathes properly so you stay cool as well as warm.
At the same 175-gram weight, this leggings layer gives you the same mid-weight warmth profile as the icebreaker Standard 175 above, which makes the two a fair head-to-head. On score, however, this pair trails its sibling — 4.1 stars versus 4.6 — and the reviews reveal why: the most-cited concern is that pure merino lacks long-term durability. The bluntest verdict came from an owner who watched them “rip after a few uses” and noted that “there’s a reason most brands use a mix of materials rather than pure merino.”
The other complaint that keeps coming up is the missing fly — several buyers were disappointed that a men’s legging arrived with no fly opening at all. If you prioritize warmth and softness and plan to wear these mostly for sleep or under looser layers, the quality feel wins the day. If you need a workhorse that survives rough use, the higher-rated icebreaker Standard 175 is the safer merino bet.
Warmth you can trust: Buyers consistently praise the cozy insulation and breathability, with the wool’s natural odor resistance earning special mention for multi-day use.
Value reality check: The recurring negatives are durability and the lack of a fly — weigh those against the comfort before you buy.
Understanding the Specs
Fiber: Merino Wool vs Mulberry Silk
Merino wool is a natural protein fiber known for regulating temperature and resisting odor, making it ideal for active days and multi-day wear. Mulberry silk is also a natural protein fiber, but it is prized for its ultra-smooth texture and lightweight feel, which makes it perfect for sensitive skin and layering under dress clothes.
Fabric Weight (gsm)
The number in a base layer’s name, like “175,” stands for grams per square meter of fabric. A 175-gram merino layer is a mid-weight option built for cool to cold weather — warm enough for winter treks but thin enough to layer under jeans without bulk. Lighter weights suit milder days, while heavier weights are for deep cold.
FAQ
Is merino wool or silk warmer for long underwear?
Which fabric is better for sensitive skin?
How do I wash 100% merino wool long underwear?
What does the 175 in icebreaker 175 mean?
Do silk long johns have a fly opening?
Will merino wool long underwear itch?
Can I wear silk long underwear for sleeping?
Which is more durable: pure merino or a fabric blend?
Are these base layers true to size?
How does silk compare to merino for odor control?
Final Thoughts: The Verdict
For most buyers, the fabric for long underwear winner is the icebreaker Standard 175 because it blends warmth, breathability, and odor control into one layer that buyers rate 4.6 stars across 500+ ratings. If you want the most luxurious next-to-skin feel and own sensitive skin, the METWAY Mulberry Silk Set holds a 4.5-star rating across nearly 800 ratings — the most-reviewed option here. And for a budget-friendly merino layer that still delivers genuine warmth, the Icebreaker 175 Everyday Leggings is the one to reach for, provided you treat it gently in the wash.
How We Picked
We do not accept paid placement. Every pick is matched to a real buyer and a real use-case; we do not hands-on test units.
Sources & Methodology
Specifications: manufacturer listings and product documentation. Review insights: verified customer reviews, as of August 2026. Pricing: not shown on this page (it changes often); check the current price via the retailer link.
As an Amazon Associate, WellFizz earns from qualifying purchases. This does not affect which products we feature.
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Mo Maruf
I created WellFizz to bridge the gap between vague wellness advice and actionable solutions. My mission is simple: to decode the research and give you practical tools you can actually use.
Beyond the data, I am a passionate traveler. I believe that stepping away from the screen to explore new environments is essential for mental clarity and physical vitality.