Spoiler alert: Always go for fake down when you'll be in wet weather for an extended amount of time

Traditionally, down coats haven’t been great options for wet weather.

“One of down’s downfalls is that when it gets wet, it collapses” and is no longer able to trap air and therefore provide heat, says Laura Akita, the category manager at The North Face.

Today, many down products are water-resistant enough to handle light-to-moderate rain and snow. But for more serious wet weather, you’re going to want a coat with synthetic down.

Synthetic down is made with water-resistant polyester that retains most of its loft when wet, which means it can still trap heat and keep you warm even when it’s soaked. Because synthetic down is made of polyester, most of it isn’t biodegradable.

On the other hand, it’s more vegan-friendly because it isn’t made from the feathers of a goose, duck, or other bird.

Short-Staple Insulation Made of small, polyester fibers that move independently of each other.

Brands include: PrimaLoft Gold, PrimaLoft Bio, 3M Thinsulate, and Columbia’s Omni-Heat. Pros: Less bulky, and has a high warmth-to-weight ratio (meaning you don’t need as much short-staple insulation as others to create a warm coat). The tiny size of its fibers means it’s a great choice for super-thin insulation like 3M Thinsulate, and it also makes it super-compressible, so it’s a great choice for puffer jackets made for traveling or exercise. Cons: It can lose its ability to retain its loft and trap warmth more quickly than other types of synthetic down. So while it’s great insulation, you may need to replace your jacket a bit more often.

Made from long synthetic fibers, typically formed into sheets that curl and coil to create loft.

Brands include: Patagonia’s PlumaFill, PrimaLoft Silver, and Climashield APEX, which you can find in tactical wear like Helikon-Tex jackets. Pros: It’s more durable than short-staple insulation and will retain its loft longer. Cons: The long continuous filament makes it a slightly less compressible option than short-staple insulation for the same weight category.

Made of little balls of polyester insulation that look and feel like down.

Brands include: The North Face’s ThermoBall. Pros: Unlike continuous filament and short-staple insulation, which tend to be stiff and don’t conform as well to the body as natural down, cluster fiber is soft and more easily follows the contours of the body. Cons: Though it’s an excellent synthetic option, it’s still not quite as warm as down.

It can be difficult to gauge online just how warm a synthetic coat will be. You can’t judge it by its puffiness, for example, the way you can with down. But as a general rule, you need more synthetic down than untreated natural down to produce the same warmth. This means a super-warm and cozy synthetic jacket might be a bit heavier than an ultra-warm natural down jacket.

“When it comes to warmth, generally, synthetic insulations are rated in weight—for example, a 40 g rating means that a square meter of insulation weighs 40 grams,” says Ajda Mesic, a lead designer at Target and a graduate of the Minneapolis College of Art and Design’s Master of Arts in Sustainable Design program. “The heavier the weight, the warmer the insulation. Common synthetic insulation weights may range from 40 to 140 grams, but brands may use lighter weight insulation in the sleeves than the body of a coat to offset the final cost and weight of the garment.” In other words, the brand can lower the cost of the coat by decreasing the amount of insulation in the sleeves.

But the jacket’s overall weight is a decent indicator of how warm you’ll be in a synthetic coat. “In general, the higher the weight, the more the insulation,” and the warmer you’ll be, says Haskell Beckham, the senior director of innovation at the Columbia Sportswear Company.

For considerable warmth, consider a jacket in the higher weight range, and make sure the insulation is well distributed throughout. If you’re planning to be very active in your jacket, you can stick with a more lightweight option, including one with thinner sleeves.

While synthetic down is less expensive than natural down, it’s not necessarily a better value: Experts say that coats with synthetic insulation generally need to be replaced more often than those filled with natural down.

“Plastic from petrochemicals is inherently long-lasting, while natural down will biodegrade—but the degradation is not the issue here,” says Alice Payne, an associate professor in fashion at the Queensland University of Technology whose research focuses on sustainable design. Rather, “it is the performance of the down in the coat.” Synthetic down doesn’t retain its loft as long as natural down does; when synthetic insulation compresses, it doesn’t fluff back up as easily as natural down.

“That’s what [apparel design] technology cannot achieve yet—that magnificent initial recovery of the down,” says Nicole Boyer, the technical coordinator of Techno-Espace at LaSalle College in Montreal.

This is because of the complex structure of natural down, which has tiny little ridges, or spines, along each filament. They act like hooks, allowing down to cluster together, fluff up, and trap air, over and over again, she says. “That’s what makes it capable of sticking together, like a grape [on a vine],” she says. “And that’s what we cannot still do with the technology we have.” So even if a synthetic down coat looks like it’s in good condition—no tears in the fabric, the color’s still bright—there’s a chance that it won’t be as warm as it was when it was first purchased.

Photo: George Clerk/Getty Images

If warmth and longevity are your priorities, get a high fill power natural down coat that’s labeled as water-repellent or water-resistant. “There are lots of down products on the market that have been modified and finished,” Beckham says. “There’s quite water-resistant down, for example.” These coats will retain their loft for years in mild to moderate precipitation.

Check the technical specs on the websites (or ask a retail associate) for terms like “water-resistant,” “water-repellent,” or “waterproof” to describe either the down itself or the shell. These designations are typically tested to ensure that the coat will offer protection from moisture. Coats with these features will be pretty safe for wearing in moderately bad weather, say, for example, you’re walking to the subway or running to pick up your kids from soccer practice.

But if you want a jacket that you can wear while riding your bike in a rainstorm, or if you expect to be extremely active (and sweaty) in your jacket, you’re going to want synthetic insulation, and plan to replace your jacket in a year or two.

Because synthetic down quality can vary and it’s so difficult to compare between brands, Tanya Domina, a professor of fashion merchandising and design at Central Michigan University, recommends sticking with well-known retailers.

“If I were purchasing a synthetic winter coat and its primary function was extended warmth, I would purchase from a major [outdoor] brand, such as REI, Columbia, Patagonia, etcetera, who, because of brand reputation, would be more likely to require specific testing from their synthetic insulation suppliers,” she says. “If I were buying a synthetic coat as much for fashion as warmth, or more of an errand car coat, then extended warmth is less likely to be a factor in my purchase decision.”

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