One-Size CatWarm High-Waisted Thermal Underwear — 3-Pack, Mix Your Colors

Three pairs. Any three colors. Repeats welcome. Pick three of the same, two-and-one, or one of each — your call.

Thermal underwear usually makes you choose: warm, or comfortable enough to forget you're wearing it. CatWarm is built so you don't have to — 6.4°C (11.5°F) warmer in independent lab testing, in a knit thin enough to disappear under jeans.

Soft first, warm second

The knit has a faint brushed nap — a velvet feel against the skin instead of the slick, clingy surface most thermal fabric has. The fibers are less than a tenth the width of a human hair: fine enough to feel like nothing, dense enough to hold warm air where you need it.

• 6.4°C warmer, measured. The fabric turns the water vapor your body gives off into heat — the same principle behind Japanese thermal base layers. Peak rise 6.4°C (11.5°F) in independent testing, 3.0°C (5.4°F) averaged across half an hour.
• Stays where you put it. A longer, wider gusset means no shifting, bunching or riding up through a day of sitting, walking and stairs. This is the thing most thermal underwear never gets right.
• High-waisted without the squeeze. The waistband sits above the navel and is wide enough to lie flat instead of cutting a line. Snug, not gripping.
• Silk-lined where it matters. The gusset is lined with a modal blend containing real mulberry silk — softer and more breathable than the main fabric, and it stays dry rather than clinging.
• No winter static. Tested and rated excellent for static resistance, so it won't cling to your tights or shock you when a layer comes off.
• Still itself after 100 washes. Laundered a hundred times in a lab and it held its shape, with barely any twist and no pilling to speak of.
• Tells you when to replace it. A logo inside the waistband slowly changes color as washes add up. When it turns pink, that pair has had its life.
• One size fits most. Four-way stretch, no size guessing — full fit guide below.

✓ Every claim on this page is backed by an independent lab — twenty measurements across three reports, published in full. See the reports

It tells you when to replace it

The color-changing logo inside the waistband is the replacement reminder. It starts blue and shifts a little further toward pink with every wash. Once it's fully pink, that pair has reached the end of its recommended life — time for a fresh one. No app, no battery, nothing to set up. The garment keeps track for you.

Where the warmth comes from

Your body gives off water vapor all day, even when you don't feel like you're sweating. This fabric is built from fibers that grab that vapor and turn it into heat — the same principle behind Japanese thermal base layers like Heattech. Ultra-fine acrylic fibers then hold that heat in pockets of still air close to your skin.

In independent lab testing, the fabric measured 6.4°C (11.5°F) warmer at its peak and 3.0°C (5.4°F) warmer averaged across half an hour — comfortably above the threshold required for certified moisture-heating knitwear.

The usual catch with warm fabric is that it also traps sweat, and damp is how you end up cold. This one was tested for that separately and scored the top grade for pulling moisture away from the skin — and scored higher after washing than before it.

These are fabric measurements taken under controlled lab conditions, not a guaranteed change in your body temperature. How warm you feel depends on you, your activity and the weather. Here's the full explanation of how it works, and the report these figures come from.

Fresh, and it stays that way

The gusset lining carries an antibacterial finish — and unlike most, this one was tested after the equivalent of 150 home washes rather than straight out of the packet. It still cut the test bacteria by more than 99%. It's there to keep the fabric itself fresh through repeated wear, not as any kind of medical treatment.

The fabric also passed the checks that matter for anything worn against skin all day: no formaldehyde detected, no banned dyes detected, and a pH close to neutral.

Three colors, one wash

Buying a mixed pack raises a question nobody thinks of until it's too late: will the dark pair tint the pale one? Color bleeding was tested four different ways — water, sweat, rubbing and washing — and scored near the top of the scale every time. Wash your brown and white pairs together without worrying.

Sealed, so it's yours first

Every pair arrives individually sealed and has never been opened. That's also why we can't take them back once the seal is broken — the same rule that limits returns is what guarantees nothing in your pack has been anywhere else first. Unopened packs can be returned within 14 days.

Fit guide

Measurements are taken flat and unstretched; the four-way knit expands well past these numbers. If you sit above the recommended weight range, the fit will be tighter than intended — worth checking before you order, since opened packs can't be returned.

The invisible first layer for winter commutes, cold offices and weekend trails. Pick your three colors.

Looking for a lighter everyday pair? The Souffle seamless 3-pack is built to disappear under leggings.