Thermal underwear sometimes referred to as long underwear or long johns, is a base layer that you wear under your outer layers.

Thermals essentially trap your body heat (due to the tight fit) instead of letting it escape. During colder months, this is the first thing you put on. Then you layer everything else on top of it. This is the key to staying warm, especially on those bone-chillingly cold days or during winter activities like snowshoeing, skiing or winter camping.

Made of different warm materials, long underwear is a saving grace in harsh winter winds and freezing temperatures.

Why You Need Thermals

You need thermals to stay warm! Besides being super comfortable, this is the ultimate key to keeping yourself toasty when the cold outside is extra daunting.

Thermals are awesome because you get to layer everything else on top of it, and can shed those layers as it warms up throughout the day.

When participating in winter activities, thermals are especially important. Besides trapping your body heat, they also wick the sweat and moisture off of your body instead of absorbing it. This keeps you warmer than you would be just wearing a cotton long-sleeve under your coat.

I can’t recommend having a dependable base layer enough. They’ll come in handy more than you know, whether you’re braving the arctic tundra for an expedition or just going to grab a coffee in the snow.

What to Look for in Thermal Underwear

Choosing the right thermal underwear is super important, as they’re often quite pricey. You wouldn’t want to spend a lot of money on some thermals, only to find that they weren’t exactly what you need or want.

Here are four things to look for when deciding between different pairs.

Material

The first thing to think about when it comes to thermals is the material.

With so many different options, it can get a little overwhelming, so here are the main fabrics you’ll run into, as well as the benefits (and possible downsides) of each.

Merino Wool

Merino wool is a top pick for long underwear, especially for activewear or taking on long trips.

Merino wool is pretty versatile and won’t be bulky under your other layers, making it a great choice. The only downside is it tends to be more expensive than other materials.

Polyester Blends

Another common material for long johns is a polyester blend.

Mixing polyester with other fabrics like merino wool or synthetics makes it a less expensive option while still properly locking in heat. It may not wick moisture as well, but it will keep you cozy, that’s for sure.

Silk

Okay ladies, they say that diamonds are a girl’s best friend, but have you met silk thermal underwear?

Silk thermals feel super smooth against your skin. They are also super lightweight, making them great for packing or days that aren’t as frigid. Silk wicks away moisture but does require more frequent washing than merino wool.

The best thermal underwear for women often features popular and recommended fabrics, such as silk and merino wool, because they are lightweight, ultra-warm, and wick away moisture. Outdoor brands specifically choose certain fabrics that are ideal for traveling in the winter.

Weight

Besides the type of fabric, you need to decide how heavy you want your thermal to be.

If you’re looking for everyday long underwear, you can probably opt for something lightweight and thin that you can slip on under your jeans or long-sleeve shirt. If you plan on doing more cold-weather activities, you’re going to want something heavy to keep you warm in extreme cold.

Here are the main weights you’ll run into on your hunt for the perfect pair of thermals.

• Ultralight – This is good for mild cold, and for daily wear when it starts getting chilly out.
• Lightweight – A step up from ultralight, these thermals will be a little heavier without feeling bulky. You can wear them for daily use, but also for physical activity like skiing, winter jogging or even exploring a new city that has a bit of a chill to it. As long as you get that blood pumping, lightweight thermal underwear should suit you just fine.
• Midweight– If you tend to get cold easily or are experiencing harsher conditions, you’ll want a pair of midweight long underwear. I personally go for this weight because I find them to be the most versatile. They keep me warmest, without overheating. Midweight thermals are good for skiing on colder days, snowshoeing, or going sledding!
• Heavyweight – These thermals are for extreme cold. If you’re big into winter camping or backpacking, you’ll definitely want a pair of heavyweight thermal underwear to keep you warm when spending time outside in below-freezing temperatures.

Style

You can find thermals in a couple of different styles these days.

Typically, you can find a two-piece set that comes with a long-sleeve top and tight-fitting bottoms. That said, sometimes you’ll find tops and bottoms sold separately, so be conscious of that when shopping around.

You may also come across a full one-piece thermal suit. While pretty rare, a full suit is super useful for the more extreme cold when you want to have continuous thermal protection instead of a gap between top and bottom.

Some long underwear comes in unique fits like extra long (for all you tall gals), petite, or plus size to make sure that you have a pair that really fits you and your body type.

Whatever style you choose, make sure that you pick form-fitting, stretchy thermals. You can look for the term “four-way stretch” which indicates that the thermals are indeed as stretchy as you want them to be. These will be the most comfortable, provide you with the widest range of mobility, and keep you warmest.

Features

Besides all of the basics, some thermal underwear will come with added features that are worth looking out for.

You may find some thermals that have thumb holes in the sleeves (great for skiing or backpacking), small pockets or compartments on the side or in the waistband for your phone or earbuds, high neck, v-neck, hoods, or UV protection!

One of my favorite features in heavyweight thermals is a fleece lining. This makes them unbelievably cozy! They aren’t the best for high amounts of activity because fleece absorbs sweat, but for the coldest days out and about, fleece-lined thermals are my go-to.

Best Thermal Underwear for Women 2021

Here’s a roundup of some of the best thermal underwear for women out there, hopefully you’ll find a base layer that’s best for you.

Silk Thermals Women's Turtleneck Knitted Style

This silk bottoming shirt is more than a little more comfortable. It is made of fine-selected mulberry silk fabrics. Soft and light, born for elegant women who pursue a natural and comfortable life.

Silk Tank Tops Cool And Breathable Style

The camisole that is made of real silk, not only has a good wearing effect but has the function to build distinctive and healthy skin and disease of prevention and cure. The silk fabric is smooth and soft, exerting a subtle massage effect on the skin, absorbing and helping to remove sweat and secretions from the skin, keeping the skin clean.

Silk Thermal Underwear Set Solid Style

Silk feels naturally soft and smooth. It is close to the skin and cannot be matched by any fiber. Known as the "fiber queen", the second skin of the human body, it can truly make people's minds and bodies get the best care and enjoyment.

Silk Camisole Lacey Ribbed Style

Mulberry silk ribbed camisole is very soft and skin-friendly. The shoulder strap is a common bra shoulder strap, which has elasticity and can be adjusted. The surface of the shoulder strap is as shiny as silk, showing high grade, and the details are well done. The vest is very practical, you can wear it alone when you are on vacation by the sea, it is very dazzling and sexy. The tube top design is very eye-catching, and it is also excellent with a short jacket. In short, it will be a vest that will surprise you!