The Science of How Certain Fabrics Keep You Warm

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.

Protecting Yourself From the Elements

If you need to be out in wet weather for an extended period of time, look for a waterproof, breathable shell. Gray explains that “breathable” waterproof clothing does not actually allow air to pass back and forth. “What it’s letting out is water vapor.” he says. “Most waterproof breathables have no air permeability at all, so there is no real air exchange between the inside and the outside.” Instead, he explains, it’s a chemical exchange—liquid moisture inside your clothing can escape in a vaporous state.

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Faced with windy rather than wet weather? There’s a reason why windy days can leave you feeling chilled to the bone. “Wind scrubs the warm boundary layer from around our bodies, making it colder,” says Gray. It causes the moisture on your skin to evaporate and that evaporation cools you down.Fortunately, Gray adds that almost all breathable waterproof fabrics are also windproof. Athletic wind shell jackets are a great choice for gusty days. Just be sure your clothing forms a tight seal around your neck, wrists and pants hem since these are areas where wind can enter, forcing warmer air out.Finding outdoor clothing that fits you properly has other benefits. “Tighter clothes that are closer to the skin will allow for better wicking away from your body,” explains Beechinor. That means they’ll allow excess sweat to transfer from you to the garment and then evaporate. But if your clothes are too tight, they can block blood from circulating through your body, a quick path to frostbite.

In general, it’s about the layers. “Most people find the traditional layering system works,” Gray describes, “An outer rain shell, a mid layer that’s insulative and a base layer that’s wicking.”Particularly prone to the cold? Slip on a lightweight, breathable windshirt after your base layer and before the outer layers. This will prevent cold moisture from building up beneath your clothes—and keep you nice and toasty.

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  • Prince Valiant

    I find it hard to understand when it comes to base layer that have a blend of fibers such as Uniqlo heat tech with a series of blends include acrylic,rayon,ployeaster,spadex… Does such formula effectively works well in arctic condition?

  • Tanya Love

    It is very simple. You want an open weave up against your skin that makes least contact to conduct heat away from you. This is conductive heat loss. I find that a 50/50 polyester/cotton thermal underwear is good for that. The cotton actually wicks away the moisture from your skin, the polyester does not absorb water so air spaces in that keep you warm and the big squares all air trap the air and do not conduct heat away from your body. Wool would probably do that even better but I personally found it too itchy.
    If you have a synthetic right up against your skin it seems to get clammy and then you start to smell then you can get itchy. Anyway that is what I like to wear next to my skin. Then again comes the next layer. I find a snug but not tight soft fuzzy type synthetic like polyester or acrylic is the best. Then on top of that a nice loose weave sweater. I personally like acrylic ones because the moths don’t eat holes in them. The beauty with a loose weave but thick yarn is that it allows the air to circulate so the moisture can be removed. More on that with the outer wear. The beauty with the open weave is if you take off your jacket the air blows through when it is warm or indoors and you do not overheat. With your jacket on I have a snow boarding jacket. It has a thing around the waist that you can clip closed that is elastic and does not let the air go up your jacket. This is good and bad. This is good when you are sitting or it is very very windy or you are atv ing or snowmobiling. The air blowing up your jacket can really chill you. Also the snowboard jacket is water proof so it really blocks out the wind and keeps you warm. Now if you are walking and start building up a sweat and getting warm you just undo the waist thing and open your jacket at the top and a little air circulating around will remove the moisture and you will still stay warm. So you can adjust this jacket for all sorts of conditions. Also if you are far up north I wear polyester gloves inside my snowmobile mitts. If you have to do any adjusting you can take off the mitts and use your fingers without getting frost bite. In really windy conditions it can happen real fast. In the far north you can also get a full face ski mask like the burglars wear. You can get frost bite on any exposed skin. Or you can get a huge a parka and the good ones have a huge hood that sticks out so the wind blows around it and your face does not get frost bite.
    The hood can stick out 4 to 6 inches. Great for the north but you can get run over in the city because it can block your view but it does protect you from the wind.

  • Ashley Dais

    Thank you! I was looking at getting a trench coat from NY&Co and was looking to see of the expense was worth it (keeping me warm vs. just fashion). This helped me learn that its definitely worth it!