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How to Choose Long Underwear Bottoms for Women

Photo: Amber King
By Amber King ⋅ Senior Review Editor
Wednesday November 20, 2019
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Perhaps one of the most important pieces of gear to consider is the one that sits next to your skin. A great piece of long underwear (or a base layer) does its job by wicking away moisture and transporting it away from your skin to ensure that you stay dry. This is monumental to ensure that you stay warm during cold spells after moving for long periods of time. In this article, we discuss a few critical considerations before buying your next base layer top. Specifically, we'll talk about budget, fabrics, and fabric weight. We will also discuss the importance of seeing your outdoor outfit as a layered system, and not just a bottom that's supposed to do it all.

Whether you're ice climbing, hiking, skiing, or just sitting around...
Whether you're ice climbing, hiking, skiing, or just sitting around a campfire, an appropriate base layer system is key to keeping you warm. Make sure your insulative layers provide good ventilation while your base layers wick away moisture.
Photo: Amber King

Budget


Most base layer bottoms might seem relatively expensive, but like any important piece of outdoor gear, it's an investment. Base layers constructed of synthetic fabrics aren't as comfortable or high performing but are typically cheaper with better durability. If you can swing paying a little bit more, we would generally encourage you to purchase merino wool options instead because of its ample warmth. To learn more, take a look at the section below.

Long Underwear Fabrics


A great pair of long underwear bottoms are made from either merino wool, synthetics, silk, or a blend. When looking at a great base layer, take a look at the fabrics that your bottom is built from. Each has its own unique set of pros and cons.

While searching the internet, you'll probably find many base layers constructed with cotton that are much cheaper. Avoid cotton at all costs. It will absorb moisture when you sweat. When it cools, it'll keep you cold, dropping your body temperature. The phrase "cotton kills" isn't too far off, especially when you're in sub-zero temperatures.

Merino Wool


You can't really go wrong with a merino wool base layer bottom. This is the superior fabric. Like all great base layer fabrics, it insulates when wet and is extremely cozy and comfortable. Merino wool is not itchy (even when wet) and can provide all-day wear comfort. It also has an exceptional range of temperature regulation and will keep you dry. The materials have an awesome ability to breathe, transporting moisture off your skin and out of the fabric. An added bonus is it doesn't stink…even after being worn for days on end without a wash.

The Smartwool Merino 250 utilizes super plush fabrics that can...
The Smartwool Merino 250 utilizes super plush fabrics that can thermoregulate across a wide range of temperatures. Look for this fabric if you want the best performance out there.
Photo: Amber King

While merino wool is typically the best quality when it comes to performance and thermoregulation, it does have its caveats. Given the soft and supple design of the fabrics, it is typically not very durable. A simple fall or a brush against a sharp branch can cause a small hole. As a result, this type of fabric is best worn underneath another layer. The material also can absorb more water than a synthetic. Not to mention that merino wool is typically a lot more expensive than synthetics. Those caveats aside though, it simply does provide the best performance on the trail when compared to synthetics. If you want a layer that'll keep you super warm even if you get wet or sweat, this fabric choice will keep you warmer longer.

Synthetics


Synthetic long underwear is typically constructed polyester or some form of blend that might integrate elastane, rayon, or other materials to help promote durability, wicking ability, and elasticity. Synthetics are great as they are typically less expensive and more durable than merino wool. The material performs well by wicking away moisture from the while keeping your relatively warm. It is also easier to layer and typically looks better to wear on its own. The fibers don't stretch as easily or lose shape when wet.

The Arc'teryx Rho Lt is a synthetic base layer bottom with a fleecy...
The Arc'teryx Rho Lt is a synthetic base layer bottom with a fleecy lining for ultimate comfort. While most synthetics aren't this cozy, this one is special.
Photo: Amber King

Synthetics are great for many purposes, but when compared to merino wool, they usually fall into second place. The fabrics aren't as soft or as cozy, nor are they as warm after being sweat in. The fibers lose heat more quickly, which can lead to a soggy, cold bottom if you don't have proper ventilation. Overall, thermoregulation ranges aren't as wide…but the fabric still performs. In addition, synthetics typically start to smell if being worn after a few days of not washing. While many brands offer "odor-resistant" finishes, they typically don't work if you've begun to reek and haven't been able to shower for a few days. Choose a synthetic fiber if durability, price, and drying speed is a top priority. They work just fine, but simply aren't the creme de la creme of the fabric world.

Merino-Synthetic Blends


These bottoms offer the warmth and coziness of merino wool with the durability and wicking power of synthetic fabric. This fabric offers the best of both worlds, and if it comes in a thinner weight, it's going to do a kick-butt job at breathing. Look for this blend if breathability is your top priority, and you want the comfort of a merino wool bottom.

The Smartwool 150 features a lightweight construction that is built...
The Smartwool 150 features a lightweight construction that is built with both synthetic and merino wool fibers. It's more durable than other merino wool contenders with impeccable breathability.
Photo: Edward Kemper

Silk


While we didn't review any silk options, you might find a variety of silk-based long john bottoms out there. Silk is renowned for how soft it feels against the skin. Its slippery face fabric fits nicely underneath other layers and is super thin. It doesn't breathe exceptionally well, but the material will wick just fine. It's a great option to add to an already useful base layer set-up if you're looking for a little extra wicking power. The main issue is that its fabric will absorb your trail stink. So wash it every time you wear it.

Fabric Weight


Base layers come in a variety of different weights, depending on how much insulation you need. When determining the weight you need, consider whether you run hot or cold, and what kind of activity you'll be doing. If you're planning on sprinting through the snow on your next pair of snow skates, then opt for a thinner layer. But if you plan on sitting around a ski lift all winter, go for something with a little more insulation. Below we discuss the different weights that you'll see in amoungst most products in this review.

Lightweight


Made of ultra-thin materials, this base layer is best for high output activities or just cool days where the sun might show its face. Layer it under a warm insulating layer for great wicking capabilities. The thinner construct makes it easier to fit underneath more layers than a thicker option. It is best for aerobic or high output activities on cold days or as a layer on the warmer winter nights.

The REI Midweight Bottoms are super light and breathable, making...
The REI Midweight Bottoms are super light and breathable, making them a top choice for warm runs or other highly aerobic activities.
Photo: Amber King

Midweight


This is the most versatile of all weights. It's warm enough to keep you toasty in sub-zero temperatures and can be worn on its own on warmer days. The fabric is a little thicker and offers great insulation in cold conditions. It still breathes and wicks well, providing warmth for lower output activities like day hiking or snowshoeing.

The Smartwool 250 Merino is a midweight merino wool base layer that...
The Smartwool 250 Merino is a midweight merino wool base layer that is supremely comfortable and cozy.
Photo: Amber King

Heavy or Expedition Weight


The heaviest and thickest of all base layers. We would only recommend this thickness for seriously polar conditions or if you're seeking a base layer that can be worn on its own (without another layer) on cooler days. While the fabric still breaths and wicks well, it's very warm and thick. Layering is possible, but harder then thinner base layer constructs. We did not review any heavyweight options in this review, though some of the bottoms reviewed offer this weight.

The Patagonia Capilene Thermal Weight has a heavier construction for...
The Patagonia Capilene Thermal Weight has a heavier construction for super cold days. Wear it on its own or as part of a layered system.
Photo: Amber King

Fit


In addition to fabric type and weight, you should also consider the fit features of a long underwear bottom. Here we discuss important fit features such as what to look including stretch, different length options, waistband details, seams, and calf width.

A look at the overall fit of the Patagonia Capilene. While base...
A look at the overall fit of the Patagonia Capilene. While base layer bottoms are pretty simple, it's important to look at a few key features.
Photo: Edward Kemper

Stretch


You should buy a pair of long underwear that fits close to the skin with an integrated stretch. Since its main job is to wick away moisture, the layer should sit next to the skin. While merino wool sadly sags after heavy use (when it's not tightly fitted), synthetics offer a closer fit that wicks away moisture well.

These merino wool base layers sit nicely next to the skin, offering...
These merino wool base layers sit nicely next to the skin, offering great warmth on this cold day in Colorado.
Photo: Amber King

Length Options


There are a few different options when it comes to length. Some women prefer a full length fit while others perfect a three-quarter length option. The full length offers full coverage and, ultimately more warmth and breathability. However, some women prefer better ventilation, which is offered in the three-quarter length option. The 3/4-length is also preferred specifically for skiing, reducing the bulk beneath the tall shaft of a ski boot. Another fun option is the full-body onesie! Some manufacturers actually offer long underwear that extends from a top all the way to the bottom. These are incredibly warm and comfortable…but don't offer the best versatility when it comes to ventilation. That said, when considering the kind of long underwear bottom, ask yourself what kind of cut you prefer.

REI Co-op Merino Wool with a full length.
REI Co-op Merino Wool with a full length.
Photo: Edward Kemper

The Waistband


A comfortable waistband that fits well is super important. We noticed that some waistbands are a little itchier than others and don't offer the same level of comfort. When perusing through different options in this review, make sure to take note of which waistbands offer a little more in the ways of cozy fabrics and elastic design. You don't want a waistband that will stretch out over time or doesn't feel good next to the skin.

Flat seams and a wide, comfortable waistband should be sought after...
Flat seams and a wide, comfortable waistband should be sought after if you want a comfortable bottom that'll last for years.
Photo: Amber King

Flat Seams


A quick note here, but always make sure that your long underwear has flat seams. Since the fabrics sit right next to the skin, a seam that isn't flat will cause discomfort and chafing over time. Typically super cheap base layers that you might find at your local department stores will not have flat seam technology. That said, make sure your base layer integrates this. Every piece in this review has flat seams that are comfortable and cozy next to the skin.

Calves and Cuffs


If you spend time crushing hills outdoors, then you've probably got a killer set of calves. Big calves are sexy and powerful, and sometimes base layer bottoms will have a hard time accommodating them. It's important to look for base layers that have "stretchy fabrics," especially around the calf and ankle. When shopping, yank on the cuffs to see if they stretch. If they don't and they look a little too thin (or big), move onto the next option.

The cuffs at the bottom should stretch. This provides a versatile...
The cuffs at the bottom should stretch. This provides a versatile and comfortable fit...especially for ladies with larger calves or ankles.
Photo: Edward Kemper

The Layered Outfit


When choosing a base layer bottom, you need to consider your whole outfit, not just one layer. Since the base layer is meant to keep moisture off the skin, the mid-layer and jacket are intended to be the insulation. Any base layer that you chose will accumulate moisture if the upper layers don't vent properly. So, when figuring out how to stay warm, be sure to choose insulative layers of the right weight, with upper layers that can release moisture when the going gets too hot. Once you've figured all that out, you can get ready for some awesome adventures.

Conclusion


While the market is loaded with different options, but sure you chose a base layer bottom that is made of either merino wool or synthetic materials. Avoid cotton at all costs. After you've decided on the materials and weight you require, try some options on to see what feels good. Once you've bought the base layer that'll lay the foundation for all your outdoor adventures in cool to cold weather, you'll be happily adventuring for years to come.

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