The Science of How Certain Fabrics Keep You Warm

The Science of How Certain Fabrics Keep You Warm

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You shimmy on your wool sweater, slip into your down coat and don a hat fit for a lumber jack. You’re confident that the cruel icy wind doesn’t stand a chance against your layers. But within minutes of leaving your home, you’re sweating and all you feel is the iciness of your cotton T-shirt against your skin.

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When it comes to staying warm, which fabrics you reach for can make all the difference. We’ll help you make smart choices before you step out into the wintery air.

Keeping Heat In—and Moisture Out

Clothing takes on that terrible frosty feeling because of moisture. “Water magnifies the effects of temperature, both hot or cold,” says outdoor industry fabric consultant Kurt Gray, “so that’s why it’s really cold in Chicago or Seattle—because it’s cold and wet. And that’s why 100 degrees feels hotter in Atlanta versus 100 degrees in Santa Fe.”When you want to stay warm, you want to stay dry. And once you break even the slightest sweat, staying warm can be difficult.It also helps to have clothing that holds onto your own natural body heat. “What you want and need from your clothing is an ability to trap air in some manner,” says Brian Beechinor, a product designer with the outdoor company Cascade Designs. “It turns out that air works as a pretty good insulator.”

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How Cold Weather Fabrics Work

Wool and polyester are the two main fibers we count on to keep moisture out and warm air in.“Wool has unique properties that aren’t emulated by any other fiber,” Gray explains. While it’s fairly resistant to water, wool can also absorb moisture vapor from your skin and release it out into the drier air, so you don’t remain wet.Polyester is the dominant fabric in most outdoor gear made from synthetics, such as Nike Dri-Fit and Uniqlo HeatTech. Its tightly woven fibers prevent moisture from getting through the fabric. Polyesters can fight off water in two different ways: either due to a very tight weave or a chemical coating that deflects water.Silk is nature’s version of polyester, Gray notes. While it’s not the warmest base-layer fabric, “silk self-adjusts to the wearer’s heat and humidity as much as wool does, so it is very comfortable,” he says.So what about cotton? Not such a good idea in the cold, according to Beechinor. “The issue with cotton is simple in that it is a highly absorbent material,” he says. “It will soak up moisture, holding it until it gets the right conditions to let it dry.”

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Remember how wetness makes any temperature seem more extreme? When cotton holds onto moisture, Beechinor explains, not only does it make you feel gross and wet, but it can also amplify your own body heat. “If you are wearing cotton as a base layer,” he says, “any strenuous activity will cause perspiration and that sweat will be absorbed by the cotton against your skin.” Your sweat on the cotton then pulls heat away from your body, making you chilly.Nylon may seem like an obvious layering choice, as you might consider wearing tights under your pants to add a slim second layer. But Gray doesn’t recommend that strategy. “Nylon will freeze you,” he says. “It’s high thermal conductivity and it has big moisture regain.” That means it eagerly draws away your body heat and will stay wet for a long time. Rayon has similarly undesirable qualities.

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