Measured: Fabric is judged by the label's fiber content percentages, listed heaviest fiber first, and by cloth weight in grams per square meter or ounces per square yard.
The basics
What to know about thermal underwear & base layers
Against the skin, and close
Thermal underwear sits directly against skin and is cut close enough to touch nearly all of it. Outdoor labels call the same garment a base layer, and the two names cover one product. What separates it from an ordinary long-sleeve tee is the job rather than the shape. This piece exists to move sweat off your skin and hold a thin blanket of still air in place. It is graded by fabric weight so you can buy warmth in measured amounts. Sets sell as a matching top and bottom, or as a union suit that joins both halves with no opening at the waist.
Three layers, three jobs
Three layers do three separate jobs, and confusing them is why people shiver inside expensive coats. Your base layer handles moisture, a mid layer of fleece or knit traps air, and a shell blocks wind and rain. Warmth itself is nothing but still air held close to you, because air carries heat away roughly 25 times slower than water does. How much air a base layer holds is printed as grams per square meter, written gsm, the weight of one square meter of cloth. 120 to 160 gsm is light, 190 to 260 gsm middle, and 260 to 400 gsm heavy. Knit structure adds to that: flat jersey lies smooth under a shirt. A waffle or grid knit builds a honeycomb of air pockets with channels the vapor can follow out.
Why cotton fails in the cold
Cotton fails in the cold for measurable reasons, not superstition. The fiber takes up about 8 percent of its weight in water in ordinary room air and keeps drinking past that, storing it inside the fiber. Water replaces the still air the cloth was holding, and your heat leaves 25 times faster than it did dry. Wool behaves the opposite way, absorbing up to about 30 percent of its weight as vapor before the surface feels wet, and insulating the whole time. Polyester holds under 1 percent because it cannot absorb at all, so sweat travels along the outside of the fibers and evaporates from there. Merino stays warm damp and holds off smell for three or four wears while drying slowly. Polyester and polypropylene dry in roughly half that time, cost a third to a half as much, and soak up the body oil that feeds odor bacteria.
Pick the weight by the work
Pick the weight from how hard you will work and how long you will then stand still. Walking the dog at 45 F or climbing a hill generates real heat, so a Lightweight Base Layer near 150 gsm is enough and anything heavier soaks. Ski days, cold offices and commutes that mix walking with waiting suit a Midweight Base Layer at 200 to 250 gsm, the weight most people wear most of the season. Standing at a touchline or working outdoors below 20 F calls for a Heavyweight Base Layer, since a body that is not moving makes almost no heat of its own. Under tailoring, a Silk Base Layer is the only one that vanishes inside a sleeve. Waffle Knit Thermals earn their place around the house rather than under a sweating day, and a Union Suit closes the cold strip that keeps opening at your waistband.
Warmth you cannot take off
Warmth you add is warmth you cannot shed without undressing. This makes overdressing self-punishing: you sweat into the one layer meant to stay dry, then chill the moment you stop. Merino buys odor resistance and heat while damp, then charges in money, drying time and fragility, since fine wool wears through at straps and hips inside two seasons unless the yarn carries nylon. Synthetics buy speed, strength and a low price, and pay in smell and in static through dry cold weather. Fit is no comfort preference either. Buy a base layer roomy and it stops touching your skin, and a layer not touching you moves no sweat at all.
Look for a stated gsm
Look for a stated gsm figure before anything else, because a maker who prints the number has told you more than one calling the fabric warm. Turn the garment inside out and run a thumb along the seams: flatlock seams lie flat on both faces, and a ridged seam under a pack strap raises a sore spot by the fourth mile. Placement counts as much as seam type, so favor a shoulder seam moved off the top of the shoulder, and bottoms with a gusset rather than four seams meeting at the crotch. In merino, a micron figure of 19.5 or finer feels smooth instead of prickly. 12 to 20 percent nylon keeps a 150 gsm wool top alive past one winter.
Never use fabric softener
Fabric softener is the worst thing you can put on any of it, because the waxy film it leaves blocks the surface travel synthetics rely on and clogs the vapor uptake wool works by. Wash cool, at 30 C or cold, inside out, in a mild detergent. Keep wool away from the dryer and from bleach, since heat plus tumbling felts the fibers and can take a full size out of a merino top in one cycle. Synthetics that have turned sour respond to a soak in half a cup of white vinegar, and never to hot water, which bakes the odor in. Store wool clean and sealed over summer, because moth larvae feed on protein fiber and are drawn to the oil left in a garment put away worn.
Buyer checklist
Before you buy thermal underwear & base layers
Comfort depends on accurate measurements, soft contact points and care that suits how often the item will be worn.
01
Measure regularly
Band, cup, waist and hip measurements can change. Use the maker's current chart rather than relying on an old label.
02
Check every contact point
Bands, wires, elastic, seams, straps and closures should sit without digging, rolling or rubbing.
03
Read the fiber blend
Cotton, modal, lace, microfiber and elastane differ in breathability, stretch and drying time.
04
Follow care instructions
Heat and aggressive washing shorten elastic life. Consider how much special care is realistic for you.
Buy your usual top size rather than one up. With the garment on, gather the fabric at your ribs: more than about an inch of slack lifting away from skin means sweat has no fiber to move into and the layer has stopped working.
Raise both arms overhead and look at the hem. It should still sit two inches below your waistband, since a base layer that untucks opens a cold band of skin across your lower back exactly where a pack sits.
Pull each sleeve down and check that the cuff reaches the base of your thumb. Cuffs finishing at the wrist bone slide up your forearm whenever you reach. A thumbhole is what holds a sleeve down inside a jacket.
Squat all the way down in the bottoms. The waistband should stay where you put it and the ankle cuff should stay below your sock top, because a leg that creeps up bunches behind the knee inside a boot.
Measure your bust and hips and read them against the maker's own chart instead of your dress size, as base layers are cut on a close block and run one size smaller than an everyday knit from the same label.
For a union suit, measure from the hollow of your throat down through the crotch and back up to the same point, then check that figure against the torso length on the chart. A suit an inch short drags at your shoulders every time you sit down.
Care
Caring for thermal underwear & base layers
Leave fabric softener and dryer sheets out permanently. Both coat the fiber in a waxy film that blocks the surface travel synthetics depend on and stops wool taking up vapor. This is the entire reason you bought either one.
Turn everything inside out and wash cool, at 30 C or cold. The inner face carries the body oil and salt, so it is the side that needs the water moving across it.
Keep wool out of the dryer and away from chlorine bleach. Heat with tumbling felts the fibers together and can pull a full size out of a merino top in one cycle, and chlorine dissolves the scale layer on the fiber for good.
Soak a sour synthetic in half a cup of white vinegar for twenty minutes before it goes in the machine, and wash it cool. Hot water sets odor into polyester rather than removing it.
Dry flat or on a rail rather than over a radiator. Direct heat hardens waistband elastic, and slack elastic on bottoms is what lets the leg creep up under a boot.
Seal wool away clean for the summer months. Moth larvae feed on protein fiber and are drawn to the body oil in a garment put away unwashed. A top hung dirty in April comes out holed in October.
Questions
Common questions
What size thermal underwear should I buy?
Take your usual size. Base layers are cut on a close block and are meant to touch skin, so sizing up defeats them. Check your bust and hips against the maker's chart, then judge the fit by lifting the fabric at your ribs. Under an inch of slack is right. Caught between two sizes, size down on tops and up on bottoms if you squat or kneel at work.
Can I wear thermal bottoms under jeans?
Yes, and 150 to 200 gsm is the range that fits. A flat seamed bottom in that weight disappears under straight or relaxed jeans and adds real warmth on a cold commute. Slim and skinny jeans are the limit. Anything at 250 gsm or above reads as visible bulk at the knee and thigh. A waffle knit is too thick for any jean cut close.
Merino or synthetic, which should I buy first?
Merino, if your days are stop-start or you travel, since it stays warm while damp and goes three or four wears between washes. Synthetic, if you sweat hard, wash often or want two sets for the price of one, because it dries in roughly half the time and resists abrasion far better. Blended fabrics with a nylon core give most of both.
What fabric weight do I need for the temperature?
Roughly, 120 to 160 gsm suits active use down to about 40 F, 190 to 260 gsm covers 20 to 40 F for mixed walking and standing. 260 to 400 gsm is for staying still below 20 F. Those bands assume a mid layer and a shell over the top. Move a band lighter if you work hard, since your own output adds more heat than a heavier fabric would.
Why can't I just wear a long-sleeve cotton tee underneath?
Cotton absorbs your sweat and holds it. Water in the cloth carries heat away from you around 25 times faster than the still air it replaced. The shirt then stays wet for hours because cotton stores water inside the fiber. On a mild day that is only clammy. In real cold, or if the weather turns, it is the layer that makes you dangerously cold.
How often do thermals need washing?
Merino goes three or four wears, since odor compounds sit inside the fiber where bacteria cannot reach them. Synthetics need washing every wear or two, because polyester holds body oil on the surface and that oil is what smells. Sweat heavily in either and wash it that night; salt left in the fabric abrades fibers and stiffens the knit.
Can I sleep in thermal underwear when camping?
Yes, and a dry set is warmer in a sleeping bag than nothing at all, since the fabric holds air against you. Change out of the pair you walked in first, because a damp layer keeps pulling heat out all night. Merino is the better sleeping fiber for smell over several nights. For beds at home, the Sleepwear guide covers the trade-offs better.
Will merino base layers itch?
Not at the grades sold for next-to-skin wear. Itch is mechanical, not allergic: fibers coarser than about 30 microns are stiff enough to press nerve endings instead of bending. Merino runs 17 to 19.5 microns and bends, so it reads as smooth. If a wool top prickles, the fiber is coarse or the blend carries recycled wool, and no amount of washing softens it.
