Base Layer 101

 It’s base layer season, and I’ve seen a lot of questions about brand and fabric recommendations. Most answers differentiate wool and synthetic as if that’s the only point of differentiation in the market. There are lots more ways to add value and comfort to a garment and offer something different to the customer.

First, let’s get on the same page about what your active layers are supposed to do because that’s where a lot of confusion starts:

  • Base layer: wick sweat and dry quickly
  • Insulating layer: trap and hold air to be warmed by your body heat.
  • Waterproof layer: keep snow and rain off you, let water vapor escape from the inside of the jacket through the membrane

Note that the base layer’s job description doesn’t say anything about keeping you toasty. It’s a common misconception that a heavier base layer is a warmer base layer. The heavier your fabric is, the more water it holds, the slower it dries, and the colder it feels. Midlayers, on the other hand, don’t have to wick, so most are made out of completely hydrophobic materials like fleece or puffies with polyester fill It(like Primaloft). They stay really warm because they don’t get very wet. So if you’re struggling to stay warm during winter, consider bulking up your midlayer, but also look at going lighter on your base layer.

Why do brands make “heavyweight” or “expedition weight” base layers?

  • Inactive base layers don’t get sweaty and don’t need to wick and dry. If you wear a base layer hunting and fishing, around the house, or out walking the dog, dry time isn’t a consideration. Likewise, if you’re snow camping, you can change out of your sweaty layer once you’ve made it to your campsite and slip into something thick, warm, and dry for around camp or to wear to bed.
  • Sometimes it’s necessary to ask your layers to do double duty. I have the Dakine Callahan Fleece base layer top for things like mountain biking or skimo races where it’s really annoying to adjust layers during quick and frequent transitions between uphill and down. If the fabric is a more expensive, highly engineered textile, it can provide adequate performance when weight, space, and time are of utmost importance.
  • Customer confusion generates demand. In my own experience, I’ve seen that some brands are more motivated to capture a sale, even if it doesn’t solve a problem for their consumer. Most heavyweight base layer manufacturers have a hunt & fish business, or a “cold weather lifestyle” side of their brand (like LL Bean or Lands End). But there are also some active brands going after a cash grab.

Now that we’ve covered fabric weight, the next decision is the fabric content and the classic wool vs. synthetic trade off discussion. This conversation usually goes like this: one person solicits base layer recommendations. Another person responds with “TRY WOOL. WOOL IS THE BEST. IF IT’S NOT A $120 WOOL SET, THEN IT’S TRASH.” And I get how this happens. Most people start with budget polyester pieces because it’s the only option for entry-price-point pieces. When they want to upgrade, wool typically catches their eye because wool occupies most of the top price tiers. But that’s not necessarily a reflection of quality. So let’s dive in to both fabrics:


Wool production starts with a breed of “Merino” sheep from a handful of countries, but Australia and New Zealand are most known for their wool supply. The supply and demand curves for wool are heavily influenced by drought conditions in the Oceanic region. Sheep are sheared, creating staples (little hairs or snippets that get spun together into thread). The staples are graded for their diameter. “Merino” can cover anything from 25 microns down to 11 microns. The smaller the diameter, the softer and less itchy the wool feels. Here’s a snapshot of the brands that disclose the gauge of their merino:

  • WoolX: 17.5 – 18.5
  • Ortovox: 18 – 19
  • Mons Royale: 20.5, with a few styles at 18.5
  • Kari Traa: 19.5
  • Patagonia: 18.5
  • Ibex: 18.5 – 19.5
  • Norrona: 17.5
  • Meriwool: 18.5
  • Minus33: 18.5
  • Terramar: 18.5
  • Voormi: 21.5

Once wool is sheared and graded, it gets sent to processing. Most wool processing is in China, which also adds to the cost given the tariff increases from the trade wars in 2018. All wool is cleaned to remove the greasy lanolin from wool, as well as any dirt and impurities. Wool goes through several hot water tanks, alternating between detergents (either a solvent or alkali solution) and rinsing tanks. It’s important to get wool extra clean, otherwise it won’t dye evenly. Different brands use different methods (same for dying processes) that can impact the handfeel of the garment.

Once clean, the staples are spun into threads. Some brands combine wool with other fibers, like a nylon core. This helps with durability and makes it possible for wool pieces to hold up well when washed with normal laundry. Blended fibers also helps brands make thinner layers than they could with wool alone.

A lot of people gravitate towards wool for ski season, since it’s one of few materials that provides warmth when wet, since wool generates heat as it dries. Skiing is a “start & stop” sport where you generate a lot of heat (and sometimes nervous sweat) on the downhill, and then barely move while you’re in lift lines, and wool does an exceptional job regulating those large temperature variations. Wool also is an inhospitable environment for the bacteria that emits body odor. In simpler terms, it’s less likely to stink.

But since wool is a natural, staple fiber, it’s less durable, and some blends shrink easily. Check the care labels to see if they need to be washed and dried differently than your usual laundry. The FTC labeling requirements require that care directions are more “liberal” (aka they give the customers the most options for reasonable care over 1 method that maximizes the life of the garment), so I put all my wool pieces on delicate or handwash them, regardless of the care instructions. I also only use Woolite since it’s gentler on delicate materials compared to everyday enzyme detergents.

Wool also doesn’t wick as well or dry as fast as synthetics, so if you get hot and stay hot, synthetics will likely be a better fit. I also prefer them from spring to fall when temps are warmer.

Wool is a great fiber, but a lot of the expense isn’t attributed to premium performance. Wool has a complex supply chain that has a lot of dependencies. You can’t ramp production up or down when it comes to living animals, and wool only makes up 3% of the global textile market, so plants that process, spin, and knit or weave wool are relatively niche. Part of the expense goes to feeding and caring for animals, as well as a supply chain that spans 3 continents to get here to North America.


Polyester is a made of PET plastic that’s piped through “spinnerets” into long strings like dental floss, called “filaments.” Filaments are twisted together into thread, and thread is woven into fabric.

Polyester yarns themselves don’t wick. Instead, textile engineers create a “wicking structure” through in the knitting structure that creates capillary action. They also use wicking agents that chemically modify polyester with a hydrodynamic finish. For the longest time, I thought polyester was polyester was polyester. In truth, cheaper polyesters are less sophisticated from a textile engineering standpoint, while high end knits that are licensed from textile pros like Polartec are much more advanced.

The vast spectrum of polyester offerings contributes to some people’s belief that wool is far superior. And yes, if you move from an $18 top from Cuddl Duds to a Smartwool 150 base layer, the change in performance will be stark. But that would also be true if they had traded into a Patagonia Capilene piece, one by Arcteryx with Polartec Power Stretch, or a Dakine one with NanoRed fibers by Libolon.

Polyester is pretty indestructible when it comes to basic washing and wearing. (Climbing’s a different story; rough rock faces will put holes in any knitted fabric). Low cost items might have less durable seam work, but overall, it’s a fabric you can toss in with any kind of laundry without compromising durability.

Polyester is more affordable because animals don’t have to be kept alive to yield fibers. Polyester is 60% of the textile industry, so there’s more flexibility in the supply chain and it’s easy to find fabric mills (where the fabric is knitted) in close proximity to factories (where the fabric becomes a garment). It’s also easier to shift production if tariffs increase or if COVID temporarily shuts down production in a given region. And it’s technically plastic, and plastic is cheap. The only reason some prices approach 3 figures is due to the R&D in the engineering process.

Polyester is a hospitable environment for the stink-emitting bacteria that causes BO, but there are a few fixes. But first, quick reminder: bacteria lives on most surfaces, including our skin. Most strains are benign, some are essential to our health, and some strains are infectious. Bacteria on your armpit and in the underarm of your base layer isn’t a major threat to hygiene and sanitation. But if odor becomes an nuisance, soak the garments in white vinegar, a baking soda solution, or enzyme cleaner (like pet stain remover). When we have big odor issues at our house (like a diesel spill a few weeks ago), we rotate through all 3 options til the stench is gone. Some manufacturers also add a treatment like Polygiene to their garments, which is a silver-based agent that kills bacteria. I have a lot of thoughts on those. But long story short, anti-microbial resistance is a growing issue and liberal use of antimicrobials is what drives mutations.

Polyester also doesn’t warm when wet unless it is a premium wicking fleece, but it wicks better and dries faster. I tend to prefer wool for resort days when I have huge variations in activity, but I like and advanced polyester for touring and 3-season use since my back will always sweat under my backpack regardless of how cold the rest of me is.

Which one’s more sustainable?

That doesn’t have an easy answer. Eco-fashion brands would like to lead you to believe that wool is sustainable because it’s natural, and that polyester is plastic that will go on to kill fish in the form of microplastics. I roll my eyes because anyone who believes wool is part of their “plastic free lifestyle” hasn’t stepped foot on a farm. In other words, the environmental impact extends well beyond the fabric content.

Is the PET used in polyester virgin or recycled from plastic waste? (PET is the plastic used in soda bottles and many food containers). What about the wool – virgin or recycled? What processing and topical treatments were used? Are the chemicals toxic to humans or animals? Is the treatment process closed loop or is there waste, and how is that disposed? How far apart are the different components of the supply chain and what methods do brands use to get product to you? Is the fabric blended? Your polyester needs to be pure in order to be recycled at end-of-life. Same for wool – a nylon blend can’t go in your compost.

The most sustainable base layer is one that gets worn heavily and works for a broad range of conditions, minimizing your need for more options. It’s a piece that’s durable and holds value in the resale market, keeping that piece in production if it no longer fits you or serves you. Sustainability means examining apparel overconsumption, not finding greenwashed items to absolve consumer guilt.

What about other fiber types?

  • Polypropylene: Super similar to polyester. Wicks even better, dries even faster, gets even stinkier. It also has poor UV resistance, so I find polyester to be a better year-round base layer. It also costs less than polyester, which makes me give some side-eye to brands like Helly Hansen, so charge pure wool prices for a wool / polypropylene blend (LIFA Merino).
  • Modal / Viscose / Lyocell / Rayon: All of these are semi-synthetic fibers made from wood pulp. I see modal as an upgraded cotton. Wicks better and dries faster, but not as well as wool or synthetics, and it doesn’t provide very much warmth. Modal is also super soft, drapes elegantly, and rarely wrinkles, so I think of modal as an exceptional travel fabric for pieces that look great straight out of the suitcase and are versatile enough to look at home in lightly active, casual, or slightly dressy circumstances throughout a busy vacation day. It’s also a great fabric for underwear or that one sweatshirt that can pass as gym wear or casual wear, and you’ll never admit that you fall asleep in it from time to time.
  • Acrylic: Acrylic is one of the few fibers I avoid. It tries to be wool with puffy, warm yarns at an insanely low price. But acrylic gets that “fuzzy yarn” look despite gentle wear and care. It’s very warm and very light, and it’s insanely cheap. If you ever see a chunky, wooly knit sweater at Target or Old Navy, there’s high probability that it’s an acrylic blend to hit their price points. But selling a base layer that’s mostly acrylic for $75 and promoting the 10% wool content is both highway robbery and slimy marketing (*cough* Kari Traa *cough*). Most brands stopped using acrylic in base layers in the 80s. The fabric produces more microplastics than polyester. Acrylic production exposes mill and factory workers (usually women) to lots of toxic chemicals proven to lead to poor health outcomes (source). Usually, textiles have tradeoffs and there’s no clear winner in the “best” or “most sustainable” fabrics. But acrylic is a shit fabric through and through that hurts others as it becomes an inferior product. Peak fast fashion shit.
  • Silk: Silk is a great inactive base layer. It’s warm and breathable. It’s slick, so other garments layer smoothly on top of it. But it doesn’t wick and dry all that well. It’s a great option to increase the number of wears you get out of those pesky dry clean or hand wash only garments. I’d put low-cost, non-active polyester pieces like Cuddl Duds Climatesmart or Lands End Thermaskin in this category as well.

Is fabric type the only point of differentiation between garments, or are there other indicators of quality?

Seam work is the other consideration. The type of seam and location of seams can make a garment more comfortable, but that comfort usually comes with an increase in price:

Seam Types

  • Overlock seams sew two pieces of fabric together and there’s a little flap of fabric called the “seam allowance” on the inside of the garment. The larger and more obtrusive these seams are, the more likely they are to cause chaffing. But these seams are cheaper for the garment’s production costs compared to a flatlock seam. Some brands go with an overlock seam, but press the flap to one side and tack it in place to keep the seam allowance out of the way, as a middle ground between the two.
  • Flatlock seams merge each piece of fabric together without any seam allowance. This greatly minimizes the chance of seam chaffing, and it keeps your overlayers from dragging against under layers, so you can move more comfortably.

Seam Placement

  • Outdoor layers typically have different patterning than casual apparel. For outdoor layers, ideally, you can avoid seams in high-chafe areas, like right on top of the shoulder (especially when you have pack straps sitting on top), or directly touching at the thighs. You also want the seams to move with you, so that your garment stays in place when you’re reaching overhead on a climbing route or spreading your legs wide in a kick turn. But more seams mean more money. Same with curved seams. I once had a men’s polo program that I could source cheaply through less experienced sewers because all the lines were straight. But the women’s equivalent had more curved seams and went to a factory with more experienced sewers who charge a higher rate. But higher end brands pony up the extra cost for creative seam placement in order to provide a more comfortable garment. Lower end brands will follow a lot of the same seam patterns as a basic tee shirt.
Arc’teryx and Lands’ End both make polyester base layers, but Arc’teryx is 12 times the price for several reasons, including sophisticated seam work

How often should I wash my base layers?

Max a few uses. Even though wool layers and garments treated with Polygiene resist body odors, it doesn’t mean the pieces are clean. Pieces absorb oily sebum from your skin. Dirt gets embedded in the fabric during off-snow adventures. We spill snacks and après beverages. Soiled fibers break down faster than clean ones, so your wool base layers need more washing than the cashmere sweater that you wear with an undershirt. You’ll definitely notice when they start to feel a little scuzzy. Wool can be aired out to an extent but disregard any advice that says wool is “self-cleaning” due to the antimicrobial properties in lanolin. The entire purpose of the scouring process is to remove lanolin from the wool.

How should my base layers fit?

You want base layers next-to-skin so that they’re able to wick moisture off your body. That contact is essential. But you want them to be loose enough not to impede your circulation. Inactive base layers can be worn looser, since they’re mainly there to trap more warm air and provide insulation.

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