What is a Base Layer? Thermal Underwear Explained

Last Updated on 14/02/2022

What is a Base Layer

What is a Base Layer?

Layering clothes to stay warm is standard practice, but what is a base layer? Base layers are thermal clothing worn next to the skin to provide insulation and comfort under your regular clothes. Base layers can include underwear, pants, tops, hats, gloves, socks, and neck buffs typically used in cold weather.

The clothing you wear closest to your body should be warm and wick away moisture to keep you dry. Likewise, base layers should have a close fit to wick away moisture without restricting movement.

Whether you’re skiing, running, or walking in the winter, base layers can make the difference between being warm and comfortable or cold. These base layers come in various weights and materials, each of which has benefits and drawbacks.

Base layers are more effective in keeping you warm than one thick layer. By trapping air between the layers and building them on top of one another, you insulate yourself more efficiently. Not only that, but your base layers will wick away sweat much more than thicker layers.

Sweat can cool down and make you cold. In warmer conditions, wicking away sweat helps your body regulate temperature better. These base layers can also stop you from smelling too offensive on longer trips.

What is Base Layer Clothing?

Base layer clothing is like your underwear for the outdoors. These thin layers sit close to your skin and are the foundation of warm, comfortable days outside.

What is a Base Layer Shirt?

A base layer shirt is a close-fitting thermal top worn next to the skin as the first insulation layer. Base layer shirts can be either long sleeve or short sleeve and wick moisture away from your skin so that you can stay comfortable. Base layer shirts vary in thicknesses to be worn as part of a layering system or on their own. You can get short sleeve versions however it is the long sleeve thermals that provide the most value. Even better if your baselayer has a hood.

What are Base Layer Pants?

Base layer pants come in the full leg, half leg, or underwear options. Like the shirt, these come in a choice of thicknesses. It’s not quite common to wear full-length base layers in the summer, but some people like to walk or run in leggings. Also, base layer underwear wicks moisture away from sensitive areas and helps to avoid chafe where you don’t want any.

How Tight Should a Base Layer be?

It’s best to think of base layers as requiring an active fit. Meaning they should sit snug against your body without inhibiting your movement. Keeping them snug lets you layer base layers more, too. We have a guide on how tight a base layer should be if you want to learn more.

It can be accessible in warm weather to think that a loose-fitting base layer will help to keep you cooler on a hot day. Base layers keep you cool by wicking away moisture, which loose-fitting clothing does less effectively. It might feel backward, but a close fit will keep you cooler than baggy layers.

What is the Warmest Base Layer?

Merino wool layers offer the most outstanding warmth to weight ratio, as well as excellent moisture-wicking and anti-odor qualities. We look at this more in the next section.

If you are looking for warmth, layering base layers can help track warm air and insulate you more effectively than one heavyweight layer. It’s essential to make sure that all of your layers wick. Otherwise, you will trap moisture and remain damp and cold.

Different Base Layer Materials

For the most part you will only see synthetic base layers and merino wool base layers in your average shop but you can also get blends with silk and bamboo. Here is a brief explanation of the different base layer materials:


The most popular synthetic fabric is polyester, but there are also nylon or other similar fabrics. Artificial materials are the most effective at wicking moisture and are popular in the summer as cooling layers.

Synthetic layers usually last the longest. They are also often far cheaper than merino or alternatives. Synthetic fabrics do repel odors for a time but need washing more regularly than wool alternatives.

Merino Wool

Merino wool is the best baselayer fabric. Merino wool is a complete overhaul of those lumpy jumpers you used to get at Christmas. These ultra-fine fibers are comfortable and soft to the touch.

Merino wool offers the best warmth to weight balance and is particularly popular among winter walkers and skiers. Yet, these layers often lack the stretch and elasticity of synthetic materials, so they might not be as good for running.

The wicking and odor resistance gives merino wool a special place in the kit store of multi-day adventurers. You can wear these for days at a time, and people will still sit next to you around the campfire.

Wool Blend

Wool blends balance the effectiveness of merino with the fit of a synthetic layer. These often fit closer than merino layers and balance comfort, wicking, and warmth. They are also a more cost-efficient choice than pure merino.


Silk is undoubtedly the softest and most comfortable material against your skin. Silk base layers are not as effective as wool or synthetic material at wicking, though. Meaning that while they might not suit activities with a lot of movement, they are suitable for gentle exercises.

Silk does not have a natural odor repellent, as merino does. The compromise is to wash your silk base layers much more frequently to avoid the smell becoming ingrained. Silk is also not a very durable material.

Bamboo Blend

Bamboo base layers are a relatively new design, but the all-natural materials make them a good eco-choice. They feel similar to merino but slightly softer. Bamboo layers may not be as warm or odor resistant as merino wool, but they are noticeably cheaper and popular among skiers and winter walkers.

base layer under hiking clothes

What is a Base Layer for Running?

The best base layer for running is going to wick moisture away effectively. An activity that raises your heart rate and makes you sweat as much as running requires excellent wicking qualities.

Because of this, synthetic materials are often the most popular choice among runners in warm climates. Synthetic materials have the best wicking qualities, and this helps to keep you cool and comfortable.

For those running in colder conditions, merino or blended layers can keep you warm and dry. Whether these suit you will depend partly on your style of running and how warm you naturally run.

What is a Base Layer for Cycling?

Cycling base layers keep you warm on the cooler days and dry on the warmer days. The best base layers for cycling tend to be synthetic materials that fit close to the skin and wick moisture away. These close-fitting layers also prevent flapping and wind resistance too.

If you’re cycling in cooler conditions, you might need warmer or multiple layers to stay warm. However, it’s vital that these still wick effectively to avoid getting too warm and sweaty.

What is a Base Layer for Skiing?

Skiing and snowboarding base layers usually focus more on warmth than wicking, especially if you’re skiing in a resort. Sometimes, it can be challenging to balance staying warm on the chairlift and not overheating on the groomer. It’s still essential to have wicking layers, though, as any trapped moisture will cool down quickly when you stop.

Merino layers are usually the best for skiing and snowboarding, with their excellent warmth to weight balance. However, these are popular with ski tourers, too, who will often ascend in just their base layers and shell ski pants before adding layers at the summit. This balance means you won’t fill your jacket with sweat and cool down on the descent.

Cross country skiing is often more like running or cycling than skiing. Thus, your base layer needs will depend on how active you are while skiing. Very active cross country skiers may require synthetic base layers alone to wick more effectively. Those on a more leisurely ski might need the warmth of merino wool.

How to Layer Base Layer Thermals

Usually, one base layer is enough. However, the clue is that these are the first layer, and you will wear other clothing over the top. We’ll look at what to wear over the top in the next section.

If you choose to wear many base layers, it can be an effective, lightweight way to stay warm. In a sport where moisture wicking is the most critical attribute of clothing, multiple base layers can be more effective than a single thick insulative layer.

Start with your thinnest layer and add your extra layers over this. Remember, if all your layers are very tight-fitting, you might find that your athletic fit becomes a bit more restrictive. If you know you will be layering base layers, it might be worth trying this before you buy them.

What to Wear Over Base Layer Clothing?

Whatever you choose to wear over a base layer, it needs to be able to wick moisture. There’s no actual use to your base layer wicking away sweat if the next layer traps it.

Usually, for hiking, you will need a lightweight pair of pants over your base layers. On your top half, the base layer alone is often enough. A second base layer or a fleece and a windproof shell should keep you warm and dry in freezing temperatures.

If you wear waterproof or insulated layers over your base layers, make sure they have ample ventilation or wicking qualities. This avoids a build-up of condensation.

Conclusion: Are Base Layers worth it?

Yes. Base layers can be the difference between a warm and comfortable day or a damp and miserable one. In the summer, these layers keep you dry and cool rather than soaking in sweat.

Finding the suitable base layers for your sport is essential. If you take part in many sports, this might mean you need many base layers. Different people may have a preference for layering systems that work for them. But all sound layering systems start with quality base layers.

Gear Assistant
Gear Assistant

This article has been written and/or edited by Andrew N. 20+ years of hiking, mountaineering, and camping experience, with access to all the latest outdoor gear.

Gear Assistant