How to Choose a Base Layer


December 15, 2023


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merino base layer

The key to staying warm and dry during an outdoor activity is simple: you need to wear a good base layer. The question is why use it and, above all, how to choose it. 

A true essential for all your outdoor adventures (cross-country skiing, hiking, running…), a base layer should be adapted to your level of effort and the weather conditions. And contrary to what some might think, it’s not only useful in winter underneath a thick sweater and a hard shell. Spring and autumn can also bring fickle weather and cool temperatures.  

If you’re looking for the best base layer for an active winter, we recommend you turn to Icebreaker, a brand known for its technical, durable and versatile products, most often made from merino wool. Your future favourite base layer is close at hand! 


In this article, you will discover how to choose the perfect base layer with Icebreaker:

  1. What is a base layer?
  2. Choosing a base layer: 3 key elements
  3. Icebreaker and its merino wool density ratings
  4. Base layers: which materials?
  5. Icebreaker's innovations and technologies

What is a base layer?

Before we discuss base layers, let’s recap on the multi-layer system, which is the best method for maintaining and regulating body temperature in cold weather. The idea is simple: before going skiing or hiking in winter, you pile up a base layer, a mid layer for insulation, and an outer layer (shell or jacket) to protect you from the wind, snow, etc. Each layer has its own role, and the advantage is that it’s very easy to remove one to adapt to changing weather conditions. That is the whole point of the multi-layer system.

The first layer is quite possibly the most important. Even with the best ski jacket in the world, you can still get cold if you’re not wearing a good base layer, which often consists of a long-sleeved sweater for the upper body and a pair of leggings for the lower body. This next-to-skin layer helps regulate body temperature and wick away perspiration to keep you dry.

In addition to its thermal function, it also helps release moisture to the outside. That said, you should avoid layering two base layers on top of each other, as this will slow down the evacuation process. If the temperature is very low, you can switch to a thicker base layer. The good news is that Icebreaker has an extensive range of products! The brand offers a variety of merino wool base layers for women and men, providing everything you need to stay warm during physical activities.

Choosing the right size and fit for your base layer

Here’s a simple rule worth knowing: the closer your base layer is to your skin, the better it can wick away moisture. For high-intensity activities (cross-country skiing, fat biking, snowshoeing…), choose close-fitting or body-hugging layers. Be careful, though: a close or body-hugging fit doesn’t mean that your clothing should wrinkle or stretch excessively with the slightest movement.

For lower levels of effort (a leisurely winter hike, for example), you can opt for a slightly looser base layer. It’s also worth noting that men’s and women’s underwear is part of the equation, too. These pieces are in direct contact with the skin, and they can provide different levels of warmth and breathability. Icebreaker makes them out of merino wool, and they only require some minimal maintenance!

As for its base layers, the New Zealand brand offers three different fits, so there’s something for everyone:

  • Slim fit, close to the body,
  • Regular fit, versatile, to wear on its own or in a multi-layer system,
  • Relaxed fit.

Choosing a base layer: 3 key elements

There are three main criteria for choosing a base layer: intensity of activity, outdoor temperature, and level of exertion. Of course, your sensitivity to the cold has an impact as well. These factors will also help you select the right fit and material for your base layer.
Icebreaker has always preferred using natural materials, especially merino wool, in its base layers for women and men.

Intensity of activity

In winter, the clothing you wear for an active cross-country ski outing is different from what you wear for a leisurely afternoon of ice fishing. The more intense the activity, the more heat and moisture your body releases. Cross-country skiing, alpine skiing, trail running… these are just some of the sports that make you break a sweat! No other choice here than to opt for a lightweight base layer that breathes well and dries quickly.

If your activity is less intense or involves long periods of immobility, the main priority will be to keep your body heat as close to your skin as possible. Get your hands on a slightly thicker Icebreaker base layer.

A base layer is also an essential part of your camping or backpacking kit. No matter the time of year, it’s always wise to have at least one in your bag to keep you warm and dry whenever you need it!


Naturally, the weather will have an impact on your choice of base layer. Before heading out, it’s important to check not only the forecast but also the current temperature, as well as the wind chill and humidex factor. As conditions start to feel more like polar temperatures (cold weather, wind, humidity…), it becomes increasingly important to maintain your body heat.
For maximum comfort, opt for a long-sleeved base layer paired with long leggings to protect your thighs from the cold. Remember not to wear two base layers on top of each other, especially in very cold weather. It’s better to wear a thicker one instead.

While base layers are often associated with winter, they are just as essential in spring and autumn, when outside temperatures vary significantly. You can wear yours alone or layer it with an insulating mid-layer or soft shell.

Perspiration and sensitivity to the cold

Are you the type of person who starts to shiver and gets cold as soon as you stand still for more than two minutes? Do you break a sweat at the slightest incline? In either case, a base layer is essential for regulating your body temperature (heat retention and breathability). You can then adjust by adding or removing other layers of clothing.

Icebreaker and its merino wool density ratings

Merino wool base layers are definitely the best choice for winter (and beyond). This natural fabric regulates body temperature wonderfully, dries quickly, is comfortable and breathable… Naturally antimicrobial, merino wool doesn’t retain odours, so it doesn’t need to be washed after each use. In short, it’s practically flawless! That’s why New Zealand brand Icebreaker uses it so much.
Choose your base layer according to the warmth/breathability ratio you need for your outdoor adventures.

The weight of Icebreaker base layers

Simply put, the thicker the fabric of your base layer, the warmer you’ll feel (although warmth is not the main purpose of this next-to-skin layer). Icebreaker uses simple number ratings to indicate the weight of each base layer. If the number is low, the garment is lightweight or ultralight; the higher the rating, the thicker the fabric. The numbers range from 120 (lightest) to 320 (warmest). There’s something for every outdoor enthusiast, whatever the activity and the season!

With a rating of 150, for example, you’re all set for a moderate to high-intensity activity (like hiking) on a mild spring day. At 250, the fabric provides more warmth, making it preferable for low or moderate-intensity activities like ice fishing.

Here are the various weight ratings for Icebreaker base layers, so you can choose according to your specific needs:
● 120+: Featherweight; for warm to hot temperatures
● 150+: Ultralight; for all seasons
● 200+: Lightweight; for cool to moderate temperatures
● 240+: Midweight; for cold weather
● 320+: Heavyweight; for the coldest temperatures

Base layers: which materials?

Women’s and men’s base layers can be made of merino wool, cotton, synthetic fibres… there are many possibilities. If there’s one thing to remember, it’s that you should avoid cotton. A base layer is worn close to the skin and should be able to wick away moisture, which cotton cannot do (it also takes a long time to dry).

Merino wool

Icebreaker is a big fan of merino wool, and it’s no surprise, as it is arguably the best material for a base layer. This natural fibre has insulating properties and is resistant to odours and humidity. It’s also very comfortable, and it doesn’t irritate the skin, which is exactly what you want for a long hike or a day on the groomed trails. Its antimicrobial properties make it an ideal choice for multi-day adventures with few clothing changes. In addition to drying quickly, it can absorb up to 30% of its weight in water before feeling damp.
Sourced from the soft woolly coats of New Zealand sheep, this natural, biodegradable material has proven thermoregulatory qualities, keeping you warm in winter and cool in summer. Properly maintained, it will last for years!

Synthetic fibres

Synthetic fibres (polyester, nylon, polypropylene, etc.) are very good at wicking away sweat, and they dry pretty quickly. They are also generally less expensive than merino wool. Of course, there are many technologies, product qualities and brands to choose from. Overall, synthetic fibres are a good option if you’re seeking a cost-effective alternative or if you tend to sweat a lot during physical activity. However, they provide less insulation than natural fibres and may feel less comfortable when wet.

Merino wool and synthetic fibre blends

Some base layers are made from a blend of merino wool and synthetic materials to combine their qualities. This is a good compromise for outdoor enthusiasts who are looking for versatility.

Icebreaker’s innovations and technologies

From the outset, Icebreaker has focused on natural materials, particularly merino wool. The brand has also developed its own technologies and solutions to maximize your comfort when you’re out exploring in cold, rainy, or windy conditions.

Icebreaker’s major innovations

Here are some of the solutions developed by the New Zealand brand over the years:
● BodyFitZone™: breathable zones for the most intense activities.
● Cool-Lite™: lightweight blend of breathable materials, designed to wick perspiration and moisture away.
● ZoneKnit™: strategically placed zones to help regulate body temperature and allow the skin to breathe during high-intensity activities (cross-country skiing, winter running…), letting excess heat escape freely.
● Icebreaker Seamless: seamless construction that stretches without friction, with knit zones for added support and breathability.
● RealFleece™: authentic brushed merino wool, naturally warm and very soft to the touch.

How to choose the perfect winter running gear

Layering, base layers, mid layers… you now know all about it. Get ready to enjoy your autumn hikes or winter trail runs with peace of mind, knowing that your body temperature is under control! Choose your best base layer for winter and gear up for your next adventure!


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