Value Village ／ Clothing
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 It stays unpublished until legal review and source verification are complete.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.
| 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 |
[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.
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.
They soak up heavy sweat, fail to dry and cause sweat chill. Quick-dry fabrics for sport, merino wool for hiking.
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.
One extra mid-layer (fleece or knit) adds more trapped air, and more warmth, than a second base layer.
Washing stretches the fabric; once the fit loosens, the effect drops. Replace after 1–2 seasons, and go easy on fabric softener.
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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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