Are "heat-generating" thermal base layers like Heattech really warm? What if they dry out my skin?
This answer is a draft. It stays unpublished until legal review and source verification are complete.
Every winter the shelves of Japan's big clothing chains fill with "heat-generating" (発熱, hatsunetsu) and "moisture-absorbing heat-generating" (吸湿発熱, kyūshitsu hatsunetsu) base layers — Uniqlo's Heattech is the best known. Most people find them thin and warm, but a steady stream of complaints has also become part of the season: itching, dry skin, and "I sweated and then got cold". Once you know how they work, it is clear who they suit and where they fail.
How moisture-absorbing thermals work, and their limits
| Item | Detail |
|---|---|
| How the heat is made | Heat released when fibres absorb water vapour (heat of adsorption). A blend of rayon, acrylic, polyester and others. The effect is small — generally described as about 1–2°C (2–4°F) in felt warmth. They do not generate heat on their own like a heater or a disposable hand-warmer |
| Why they actually feel warm | ① A thin, dense fabric hugs the skin and holds a layer of air; ② it absorbs sweat (vapour) and keeps the skin dry; ③ stretch leaves no gaps. These three matter more than the "heat" |
| Good for | Commuting, school, the office, walking around town, light housework — activity that does not make you sweat |
| Not for | Hiking, running, shovelling snow, skiing — anything that makes you sweat heavily. The absorbed sweat does not dry and causes sweat chill (汗冷え, ase-bie). For sport, use quick-dry, water-repellent polyester; for hiking, merino wool is the standard |
| Dryness and itching | Rayon and acrylic also draw moisture from the skin, so people with dry or sensitive skin, older people and those with atopic dermatitis may feel itching or dryness. Friction and static between synthetic fibre and skin also play a part |
| What helps | Products with a cotton inner face, cotton blends, silk or merino wool base layers, moisturising before dressing, changing when you have sweated |
| Labelling | The test for "moisture-absorbing heat generation" is standardised in JIS L1927 (established 2020), and claims must be backed by evidence. A figure such as "+X°C" is a value under test conditions |
- Choosing: mainly cotton (gentle on skin, thicker); mainly rayon/acrylic (thin, "heat" claims, drying to the skin); wool blend (excellent temperature regulation, no odour, expensive); silk (gentle, very absorbent, care needed in washing). Covering the "three necks" — neck, wrists and ankles — (high neck, long sleeves, leggings) changes how warm you feel far more than fabric does.
- Layering order: base layer (absorbs) → mid-layer (insulates: fleece, knit) → outer (blocks wind). Adding a mid-layer is warmer than wearing two base layers.
- Washing and lifespan: washing stretches the fibres; once the close fit goes, so does the effect. Expect 1–2 seasons. Fabric softener reduces absorbency, so many makers advise against it. Tumble-drying shrinks and degrades the fabric.
- When you sweat: these base layers dry slowly. On a day you expect to sweat, carry a change or wear a quick-dry fabric.
- Children: they run warm and sweat easily, so thin fabrics that let sweat out suit them. Do not over-dress them.
- Older people: dry skin is common, and the itch → scratch → eczema cycle starts easily. Mainly cotton or silk, and make moisturising a habit.
[Verify] The year and content of JIS L1927 and the guide figure for felt warmth follow explanations from makers and the textile industry. Check with Consumer Affairs Agency and JIS materials.
Misunderstanding 1｜Heat-generating base layers make their own heat, like an electric blanket
They use only the slight heat released when absorbing vapour; the warmth comes mainly from the close-fitting thin fabric and the air it traps. Any "+X°C" figure is a test-condition value.
Misunderstanding 2｜They are warm, so they are ideal for sport and hiking
They soak up heavy sweat, fail to dry and cause sweat chill. Quick-dry fabrics for sport, merino wool for hiking.
Misunderstanding 3｜The itching means my skin is weak, so I should put up with it
Rayon and acrylic draw moisture from the skin, so people with dry skin itch easily in them. Switching to a cotton inner face, silk or wool usually solves it.
Misunderstanding 4｜Two layers are twice as warm
One extra mid-layer (fleece or knit) adds more trapped air, and more warmth, than a second base layer.
Misunderstanding 5｜They work the same for years
Washing stretches the fabric; once the fit loosens, the effect drops. Replace after 1–2 seasons, and go easy on fabric softener.
---
Sources
- Japanese Industrial Standards Committee, "JIS L1927: Test method for moisture-absorbing heat generation of textiles" [JIS L1927：繊維製品の吸湿発熱性試験方法] (2020)
https://www.jisc.go.jp/
- Consumer Affairs Agency, "Functional claims on clothing and the Act against Unjustifiable Premiums and Misleading Representations" [衣料品の機能性表示と景品表示法]
https://www.caa.go.jp/policies/policy/representation/
- Japanese Dermatological Association, "Dermatology Q&A: dry skin and sebum deficiency (winter clothing and itching)" [皮膚科Q&A：乾燥肌・皮脂欠乏症（冬の衣類とかゆみ）]
https://www.dermatol.or.jp/qa/
⚠️ Editorial note: the source URLs above were identified by search. Our working environment cannot reach go.jp domains, so the contents have not been checked against the text. Verify every source before publication and record who checked it and when.
About this article
What we share here are ideas and everyday practices passed down among people living in Japan, together with general information drawn from public sources. They are not official recommendations, and they are not professional instruction. Practices vary between households and regions.
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