Thermal underwear must balance warmth, moisture movement, drying and comfort beneath other layers. Recorded testing covered 37 thermal tops and bottoms, mostly in matching sets, across merino, synthetic and blended fabrics. Four recommendations emerged for different priorities.
What follows is recorded testing, translated. Nobody at Truthful Reviews has worn or used any of these garments.
What matters in thermal underwear
Merino balanced warmth, moisture management and odour control, but pure wool was more vulnerable to abrasion, pulled threads and moth damage. Synthetics generally dried faster and proved tougher, although some felt artificial. The strongest blends paired natural-fibre comfort with synthetic drying speed, stretch and durability. Cotton absorbed too much moisture and then felt cool and wet.
Fit mattered almost as much. A close cut held warmth near the body but needed enough stretch for free movement. Short sleeves and hems rode up when reaching, while three-quarter bottoms avoided excess fabric inside a ski boot. Flat seams, raglan sleeves, gussets and mapped mesh improved comfort or moisture movement. A stand collar added coverage; a half zip vented excess heat.
How the garments were assessed
All garments were assessed in size M on a tester measuring 181 cm and 70 kg, then washed repeatedly at 30°C to check shape and dimensions. Wear trials combined mountain routes with changing pace and stops, running and everyday use. The same midlayer and softshell kept outdoor comparisons consistent.
Material feel, workmanship, drying, warmth, moisture and movement were observed. Fit checks covered closeness, length, cuffs and whether tops stayed tucked in; collars, zips, thumb loops, body mapping, mesh and pockets were judged by function.
The recommendations
Best overall — Ortovox 230 Competition Zip Neck
This narrowly led by combining substantial warmth with excellent comfort and climate control. Its merino blend uses 16.5-micron wool. Seamless circular knitting creates a close, flexible fit, while body mapping moves from 190 g/m² to 240 g/m² where more insulation is useful. Mesh-like underarm zones helped moisture escape.
The high collar, half zip, wide cuffs and raglan sleeves made the design unusually complete. It stayed comfortable during pauses after hard climbs and in everyday wear, although milder days or fast ascents could make it too warm. A women’s version uses the same material with a different fit.
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Best blended set — Löffler Shirt Transtex Hybrid and Long Underpants Transtex Hybrid
Two-layer Transtex Hybrid fabric blends cotton, Lyocell and synthetics. Moisture moved outwards and spread across the surface; both pieces stayed comfortable through sweaty climbs and summit stops. The range is intended for about -5°C, with harder activity possible in colder conditions.
Body mapping, close cuts and flat seams worked well, while the bottoms’ elastic waist supported without digging in. The shirt lacked a stand collar, and its sleeves and hem rode up during overhead reaches. The Austrian-knitted fabric carries named Oeko-Tex certifications. The Löffler W Shirt L/S Transtex Hybrid and Löffler W Long Underpants Transtex Hybrid produced similarly strong comfort, mapping and workmanship.
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Best light all-rounder — Salewa Zebru Responsive L/S
This thin merino-blend top suited active use without becoming needlessly hot. A seamless body map alternated warmer and airier zones, and the close cut moved freely. It worked across broad conditions, including stop-start outings, and as an everyday layer.
The trade-off is that deep cold may call for a faster pace or a warmer midlayer. Equipment is deliberately sparse — there is no collar or zip — in service of low weight. The shirt is made in Italy, and a women’s version is available.
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Best value set — Snocks Thermounterwäsche-Set
The synthetic Snocks set was the surprise value pick. Both pieces felt comfortable, dried quickly, moved moisture capably and supplied good warmth. Their close, stretchy cut stayed stable without restricting the recorded wearer, although some may find it too tight. Light body mapping and sparse flat seams worked well.
Equipment is basic: the top lacks a stand or zip collar, while the bottoms have a gusset and the sleeves have thumb loops. Its recycled polyester is Global Recycled Standard certified; the set also carries Oeko-Tex Standard 100 and Green Button certification. A women’s set uses the same material with a different fit.
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Also tested
Smartwool Intraknit Thermal Merino Baselayer came very close to the winner, with a comfortable merino blend and useful ventilation zones, but its shoulders, sleeves and slightly forgiving cut could be sharper. Search Amazon UK · Search Amazon US
Haglöfs Latnja Wool Crew Neck Men felt surprisingly warm for its light merino blend and fitted neatly, though moisture movement lagged the leaders and hard use in deep cold may expose its limits. Search Amazon UK · Search Amazon US
Haglöfs Latnja Wool Long John’s Men shared the top’s soft, close and unrestricted fit, but its merely adequate moisture management made it better for steady winter outings than the fastest sessions. Search Amazon UK · Search Amazon US
Haglöfs Latnja Wool Crew Neck Women and Haglöfs Latnja Wool Long John’s Women delivered the same high level of wearing comfort in their recorded trials. Search Amazon UK · Search Amazon US Search Amazon UK · Search Amazon US
Odlo Active Warm Eco Shirt was a very warm, soft-feeling synthetic top with quick drying and good moisture transfer, though it felt sweatier than the merino blends and its sleeves shifted when reaching. Search Amazon UK · Search Amazon US
Dynafit Tour Light Merino L/S was thin and probably too cool for many wearers, but its warmth-to-weight balance, moisture movement and close raglan cut suited strenuous ski touring or cold-weather trail running. Search Amazon UK · Search Amazon US
Smartwool Classic Thermal Merino Baselayer-Hoodie used 100% merino for soft, medium-weight warmth; the relaxed cut also worked alone, although its hood could clash with hooded outer layers. Search Amazon UK · Search Amazon US
Schöffel Merino Sport Shirt mixed merino, synthetics and Lyocell for a smooth feel, but started cool, dried less quickly and fitted too loosely to excel as a base layer. Search Amazon UK · Search Amazon US
Smartwool Classic All-Season Merino 3/4-Tights felt soft and ended above a ski-boot cuff, but the cut was loose enough to flap around the legs during the day. Search Amazon UK · Search Amazon US
Icebreaker Merino 200 Oasis Thermo-Langarmshirt was a versatile 100% merino layer for walking or everyday wear, but it alone developed a small sleeve hole after only a few weeks. Search Amazon UK · Search Amazon US
Decathlon Skiunterwäsche BL 500 Funktionsshirt dried quickly and used a brushed inner face plus back mesh, but its synthetic feel and short, constantly untucking cut counted against it. Search Amazon UK · Search Amazon US
Under Armour UA CG Armour Twist Mock supplied warmth and acceptable moisture movement, but felt synthetic, cooled quickly after hard effort and had a tight neck opening. Search Amazon UK · Search Amazon US
Mammut Trift Long Tights Men combined merino and synthetics with mapped warmer and airier zones, making a comfortable winter-sport bottom, albeit one of the pricier options. Search Amazon UK · Search Amazon US
Under Armour UA CG Armour Twist Legs were exceptionally tight, difficult to pull on and dug into the backs of the knees, though the inner-leg mesh and thigh pocket were useful. Search Amazon UK · Search Amazon US
Decathlon Wedze Skiunterwäsche BL 500 Thermohose offered plenty of warmth, mesh behind the knees and at the gusset, but its close cut was only moderately successful. Search Amazon UK · Search Amazon US
Salewa Cristallo Warm Alpine Merino Responsive brought more warmth than the Zebru with excellent comfort and climate control, but lacked the collar, thumb loops, raglan cut and body mapping needed for a recommendation. Search Amazon UK · Search Amazon US
Salewa Cristallo Warm Alpine Merino Responsive Langarm Baselayer Damen produced the same findings as the men’s version in its recorded trial. Search Amazon UK · Search Amazon US
Columbia Midweight Langarmoberteil felt more synthetic and its reflective lining showed no significant advantage over rivals, though the cut and warmth were sound. Search Amazon UK · Search Amazon US
Columbia Midweight Stretch Tight had the same lining, a comfortable stable waistband and four-way stretch, but its cuffs made worrying cracking sounds whenever it was removed. Search Amazon UK · Search Amazon US
The women’s Columbia Midweight Langarmoberteil and Columbia Midweight Stretchhose returned similar results: initially unfamiliar material feel followed by good warmth. Search Amazon UK · Search Amazon US
Under Armour ColdGear Compression Mock für Herren was a serviceable sports top, but neither as warm nor as compressive as claimed; synthetic feel and prominent arm seams further limited it. Search Amazon UK · Search Amazon US
Under Armour Herren ColdGear Leggings fitted genuinely close, stayed stable on a comfortable waistband and worked well alone for cold-weather exercise despite their synthetic feel. Search Amazon UK · Search Amazon US
Under Armour Women’s ColdGear Leggings and Under Armour Women’s ColdGear Mock Neck Long Sleeve produced similar findings during snow walks and made most sense for wearers who would also use them on their own. Search Amazon UK · Search Amazon US Search Amazon UK · Search Amazon US
Craft Active Extreme X CN LS M paired a thin waffle knit with good warmth-to-weight and moisture transfer, but numerous insufficiently flat seams rubbed during wear. Search Amazon UK · Search Amazon US
Craft Active Extreme X CN LS W avoided the seam problem, although other tops at its level managed moisture more effectively. Search Amazon UK · Search Amazon US
Woolpower Crewneck 200 was warm, comfortable and nearly seamless enough to double as a midlayer, but pulled threads and early pilling kept it from the recommendations. Search Amazon UK · Search Amazon US
Woolpower Long Johns 200 shared the warm unisex fabric and clean, unrestricted cut, together with the same tendency to pull threads and pill. Search Amazon UK · Search Amazon US
Calida Langarmshirt aus Wolle und Seide balanced warmth with a soft merino-and-silk fabric, but its seam thread initially scratched and its equipment was sparse. Search Amazon UK · Search Amazon US
Calida Longpants aus Wolle und Seide added a useful gusset and close cut, though the same scratchy seams and a rather narrow waistband weakened the result. Search Amazon UK · Search Amazon US
Uniqlo Heattech Extra Warm Kaschmirmix felt exceptionally soft and fitted well, but was among the least warm pieces and worked better as a light standalone top. Search Amazon UK · Search Amazon US
Uniqlo Heattech Thermo Langarmshirt was comfortable, quick-drying and versatile for mild days, but less warm than promised and too simply equipped to stand out as functional underwear. Search Amazon UK · Search Amazon US
Uniqlo Heattech Thermo Unterhose was warmer than its matching shirt, with an effective stretch fit, unobtrusive seams, a practical fly and tidy cuffs. Search Amazon UK · Search Amazon US
Bennies Thermounterwäsche Set provided plenty of warmth from its brushed synthetic inner face, but felt basic and artificial, while the top’s short body and sleeves rode up. Search Amazon UK · Search Amazon US
Verdict
Choose by effort level as well as temperature. The Ortovox 230 Competition Zip Neck is the strongest all-round winter top when warmth and comfort come first. The Löffler Transtex Hybrid pair balances active moisture management with a notably pleasant blended fabric, while the lighter Salewa Zebru Responsive L/S is better suited to high-output days. For a close-fitting top-and-bottom set with capable fundamentals and fewer extras, the Snocks Thermounterwäsche-Set is the sensible saving.
For adjacent kit, see our hiking backpacks guide and men’s running shirts guide.
