Best Thermal Base Layers for Travel 2026: Merino Picks

Best thermal base layers for travel: buy for standing still, not for sweating

Most base-layer reviews are written for people climbing mountains. A winter trip is different: you stand at a market stall, wait on a cold platform, ride a chairlift, then walk into an overheated museum. That asks for a layer that is warm when you are still, does not smell after two days, and dries overnight on a hotel towel rail.

That is why merino wins this category and why weight matters more than brand. Skip this page if you are going to winter sun in the Canaries: you need none of this. Everyone heading to the Christmas markets, Iceland, Hokkaido or a ski resort, read on.

The eight picks at a glance

VoyageHacks compared the published specifications of 8 base layers and 6 independent reviews from Wirecutter, OutdoorGearLab, Treeline Review, GearJunkie and CleverHiker on 4 October 2026. Fabric weight, fibre content and micron count below are each manufacturer’s published specification, read on its own site that day. Nothing here was tested by VoyageHacks, and no price is shown because Amazon’s live price is the one that counts.

Fibre diameter matters because finer wool feels softer: the Woolmark Company puts merino between under 17.5 and about 24 microns, and all three brands that publish a figure sit at the fine end.

Smartwool Classic Thermal Merino crew: the one to buy first

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The Classic Thermal is Smartwool’s midweight line (formerly called Merino 250): 250 g/m² of 100% merino with shoulder panels and flatlock seams so a backpack strap does not rub. It is warm enough to stand through a December evening at a market and fine on its own under a jacket in a café. GearJunkie named the women’s crew and bottom its Best Overall women’s base layer on 2 September 2026, after a tester wore them five days running on ski trips in the Dolomites with no odour.

The men’s version has the same fabric. OutdoorGearLab gave the Classic Thermal quarter-zip its Editors’ Choice and highest overall score in the April 2026 men’s test, and Treeline Review lists the Classic Thermal as its pick for extreme cold. CleverHiker rated it 4.5 out of 5 in December 2024, with price, snug fit and missing thumb holes as the drawbacks.

Buy it if your trip means hours outdoors at low effort: markets, sightseeing in sub-zero cities, lift-served skiing. Skip it if you will be hiking uphill or running hot; the All-Season below breathes better. The Minus33 Midweight is the looser-fitting alternative.

Smartwool Classic Thermal bottom: the leggings worth packing

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The bottom uses the same 250 g/m² merino and was reviewed by GearJunkie together with the crew. Bottoms earn their space when you stand still for hours: a northern lights tour, a snowy temple visit in Japan, a ski day. Buy them if you are going to Iceland, Hokkaido or the Alps. Skip them if your trip is mostly indoor museums and short walks; a top alone covers that.

Smartwool Classic All-Season: the one to fly in

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The Classic All-Season is a 150 g/m² lightweight layer with merino next to the skin spun around a recycled nylon core (88% merino, 12% nylon). The nylon makes it more durable than thin 100% merino. Treeline Review named it its Best Overall base layer in February 2026, and OutdoorGearLab gave the women’s version a Best Buy. Wirecutter lists it as worth considering, noting the women’s cut goes up to 4XL, and marks it down only for slight itch and no thumb loops.

Buy it if you want one layer for a long-haul flight, a city break and active days on the slopes under a shell. Skip it if you get cold easily: at 150 g/m² it is a layering piece, not a thermal on its own. Smartwool’s care label allows a warm gentle wash and tumble dry on low.

Minus33 Midweight crew: Wirecutter’s warmest pick

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The Minus33 Midweight is 245 g/m² of 100% merino at 18.5 microns with a regular, slightly roomy fit. Wirecutter’s updated guide (30 September 2026) names it the best base layer for people who need more warmth, for men and women, and says it performed best on cold low-effort days such as resort skiing and neighbourhood walks. Minus33 rates it for 30 to 60°F when you are inactive.

It is also the most size-inclusive pick: men’s sizes run to 6XL with a 61-inch chest, women’s to 3XL. Buy it if you run cold or want a base layer that looks like an ordinary long-sleeve shirt. Skip it if you sweat a lot: Wirecutter found the looser fit moves moisture less effectively and that 100% merino is less durable than a blend. The Smartwool Classic Thermal is the snugger alternative.

Meriwool Merino 250: the value midweight

Meriwool’s 250 is a midweight merino long sleeve at 250 g/m² and 18.5 microns, the same weight class as the Smartwool Classic Thermal. OutdoorGearLab made the men’s Meriwool Merino 250 a Top Pick in its April 2026 test, describing it as merino performance without the premium cost, and noted it felt slightly itchy at first and softened after washing. The linked listing is the women’s version; the men’s is sold under the same name.

Buy it if you want midweight merino warmth for a single winter trip and do not want to pay for a premium brand. Skip it if you have sensitive skin and cannot wait out a couple of washes. One honest note: Meriwool’s own page title says “merino blend” while its copy and Amazon listing say 100% merino, so check the label on arrival.

Merino.tech 165: the cheapest 100% merino

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Merino.tech’s 165 is a lightweight 165 g/m² top in 100% merino at 17.5 microns, UPF 25, and this listing bundles a pair of merino socks. OutdoorGearLab gave the Merino.tech crew a Best Buy in both its overall and men’s 2026 tests for breathability at its price, and scored it lower on durability because the fabric is thin. Wirecutter tested the heavier Merino.tech 250 and found its neck seams itchy; that is a different weight from this one.

Buy it if you want a merino layer for flights and city days on the smallest budget. Skip it if you want one shirt for years: the 150 g/m² Smartwool All-Season with its nylon core is the longer-lasting alternative. Merino.tech says to wash it cold, lay it flat to dry and skip fabric softener.

32 Degrees Heat Lightweight: the cheapest layer, with a catch

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The 32 Degrees Heat Lightweight crew is a fitted synthetic in 40% polyester, 34% acrylic, 22% rayon and 4% spandex, sold in men’s and women’s cuts. No major publication has reviewed it. It is here because it is the cheapest way to add a thin warm layer under jeans and a sweater for a weekend city break.

The catch is sweat. Wirecutter tested Uniqlo’s Heattech, a similar polyester, acrylic and rayon blend, and found it soaked up sweat and clung when the wearer was active. Buy it if you will mostly walk slowly and stand around, and can wash it every night. Skip it if you are skiing, hiking or wearing the same top two days running; any merino pick above is the alternative.

What independent reviewers found

Merino, synthetic and fabric weight: what the numbers mean

A base layer is the garment worn next to the skin to move sweat away so it can evaporate. Thermal underwear is the older name for the same thing, though Wirecutter draws a line: base layers manage moisture while you move, thermal underwear just holds heat while you sit.

Fabric weight is written in grams per square metre (g/m², also GSM): the mass of one square metre of the fabric, so a higher number means a thicker, warmer layer. The brands on this page use roughly these bands:

• 150 to 165 g/m², lightweight: flights, city days, active skiing under a shell. Smartwool All-Season, Merino.tech 165.
• 245 to 250 g/m², midweight: standing still in the cold. Smartwool Classic Thermal, Minus33 Midweight, Meriwool 250.
• 320 to 400 g/m², heavyweight: Arctic or very cold stationary use. Merino.tech and Meriwool both sell one; few travelers need it.

Wirecutter found a 185 g/m² fabric the most versatile for walks to moderate runs, and heavier fabrics too warm once you sweat. For a trip, that means pairing a light layer with a fleece beats one thick layer you cannot take off on a heated train.

Merino versus synthetic comes down to odour and drying time. Merino is wool from merino sheep, finer than ordinary wool. In a peer-reviewed University of Alberta study published in the International Journal of Clothing Science and Technology in 2014 (funded by Cotton Incorporated), polyester T-shirts worn for 20 exercise sessions smelled stronger than cotton both before and after washing, and the authors cite their earlier work rating polyester more odorous than wool. Synthetics dry faster and survive a hot dryer. Wirecutter summed up its synthetic budget pick bluntly: “so it’s not a great option for travel”.

Cotton is the one to avoid if you will sweat. REI’s Peter Bradley told Wirecutter that cotton holds the most water of the three fibre types, and the CDC’s hypothermia guidance tells you to remove wet clothing. A cotton waffle thermal is fine for lounging in a chalet, nothing more.

Edge cases and what goes wrong

• Overheating indoors. Christmas-market cities mean ten minutes outside, then a hot shop or tram. A midweight crew you can wear on its own, or a quarter-zip, beats a heavy layer you have to strip.
• The hot dryer. Hotel laundry dryers run hot. Wirecutter warns that heat shortens 100% merino’s life; hand wash and lay flat instead.
• Holes in thin merino. 165 g/m² 100% merino wears through at the elbows and under backpack straps sooner. A merino and nylon blend such as the All-Season lasts longer.
• Itch in the first week. OutdoorGearLab found the Meriwool slightly itchy until washed, and Wirecutter flagged the Merino.tech 250’s neck seams. Wash once before the trip.
• Sizing too loose. A base layer only moves sweat if it sits close to the skin without squeezing, per Wirecutter’s testers. Size by the chest chart, not your T-shirt size.
• Synthetic smell on day two. Pack one top per day or a merino layer; a synthetic worn twice becomes the smell in your packing cube.

How VoyageHacks picked these eight

VoyageHacks compared the published fabric weights, fibre content and care instructions of the base layers recommended by Wirecutter, OutdoorGearLab, Treeline Review, GearJunkie and CleverHiker, then kept only models sold in stock on Amazon in a current version on 4 October 2026, covering both men’s and women’s cuts. Ridge Merino and REI Co-op, Wirecutter’s top and budget picks, are not sold on Amazon, so they are named here but not linked.

Pack the layer with a packable down jacket, winter boots that grip on ice, and a heated vest or hand warmers for the coldest evenings. The Christmas market packing list, the ski gear to buy before you rent and where to go in December cover the rest of a winter trip.

Sources

• Wirecutter (The New York Times), The Best Thermal Underwear, updated 30 September 2026, checked 4 October 2026.
• OutdoorGearLab, 10 Best Base Layers of 2026 and The Best Base Layers for Men, both updated 1 April 2026, checked 4 October 2026.
• Treeline Review, Best Base Layers of 2026 (Long-Term Tested), 21 February 2026, checked 4 October 2026.
• GearJunkie, The Best Base Layers For Women Of 2026, 2 September 2026, checked 4 October 2026.
• CleverHiker, Smartwool Classic Thermal Base Layer Review, 5 December 2024, checked 4 October 2026.
• Smartwool, Women’s Classic Thermal Merino Base Layer Crew and Men’s Classic All-Season Merino Base Layer Crew, checked 4 October 2026.
• Minus33, Midweight 245 Men’s Thermal Base Layer Crew, checked 4 October 2026.
• Meriwool, Women’s Merino 250 Base Layer Long Sleeve, checked 4 October 2026.
• Merino.tech, Men’s Merino Long Sleeve 165, checked 4 October 2026.
• 32 Degrees, Men’s Lightweight Baselayer Crew Top, checked 4 October 2026.
• The Woolmark Company, What is the Wool Fibre?, checked 4 October 2026.
• McQueen, Harynuk, Wismer, Keelan, Xu and de la Mata, Axillary odour build-up in knit fabrics following multiple use cycles, International Journal of Clothing Science and Technology 26(4), 2014, checked 4 October 2026.
• CDC, Preventing Hypothermia, updated 30 January 2024, checked 4 October 2026.

Frequently Asked Questions

What is the best thermal base layer for travel?

For most cold-weather trips, a midweight merino crew of about 250 g/m² is the best thermal base layer, because merino can be worn for several days between washes and stays warm when you stand still. VoyageHacks’ pick is the Smartwool Classic Thermal Merino crew (250 g/m², 100% merino), which GearJunkie named its Best Overall women’s base layer in September 2026. Choose the 150 g/m² Smartwool Classic All-Season for flights and mild city days, and the Minus33 Midweight (245 g/m²) if you run cold.

Is merino or synthetic better for a travel base layer?

Merino, if you will wear the same top more than one day. A University of Alberta study in the International Journal of Clothing Science and Technology (2014) found polyester fabrics smelled stronger than cotton after wear and even after washing, and cited earlier work rating polyester more odorous than wool. Synthetic dries faster and costs less, but Wirecutter wanted to wash its synthetic pick after every use. Pack synthetic only if you can wash every night.

What base layer weight do I need for a winter trip?

Use 150 to 165 g/m² for flights, city sightseeing and mild days, and 245 to 250 g/m² for standing still in the cold: Christmas markets, ski lifts, Iceland or Hokkaido in winter. Wirecutter found heavier fabrics too warm when you are working up a sweat, so a lightweight layer under a fleece suits active days better than a thick one.

How many base layers should I pack?

Two merino tops and one pair of bottoms cover a week for most travelers: wear one while the other dries. Wirecutter’s testers wore each shirt for two to three consecutive days, and found that the more merino a shirt contained, the less it smelled. Add a second pair of bottoms only for a ski trip, where they get sweaty every day.

How do I wash a merino base layer while traveling?

Hand wash it in cold water with a little wool or mild detergent, roll it in a towel to squeeze the water out and lay it flat to dry overnight. Merino.tech’s care instructions say cold water, gentle cycle, wool detergent, no fabric softener and lay flat to dry. Never put 100% merino in a hot dryer: Wirecutter warns it shortens the garment’s life.

Do I need thermal bottoms or just a top?

A top is enough for most city days. Wirecutter found that most people doing active outdoor sports overheat in base-layer bottoms unless it is well below freezing. Pack bottoms when you will stand still in the cold for hours, such as an evening at a Christmas market, a northern lights tour in Iceland or a day of lift-served skiing.

The bottom line

Buy the Smartwool Classic Thermal crew for cold, low-effort days and add the matching bottoms for Iceland, Japan or the slopes. Fly in the Smartwool Classic All-Season, choose the Minus33 Midweight if you run cold, and go Meriwool or Merino.tech to save money. Tap any card above for the current price, then see the travel gifts for women and for men if this is a present, the Black Friday travel deals if you can wait, or the travel gear hub for the rest of the kit.