How to Choose a Base Layer

Brrr it’s getting cold! If you find yourself relating to the chap above at this time of year, consider some thermal clothing – an insulated jacket or a base layer to keep you warm this winter. But what makes a base layer so important? How do you choose one? Why not just wear an extra cotton t-shirt? Base layers are essential kit for outdoor activities in both hot and cold conditions, helping to keep your temperature at a comfortable level while managing moisture – drawing sweat away from your body. Two thin layers can be warmer yet lighter than one thick layer, because the air trapped between layers serves as thermal insulation. Before you take the plunge, have a read of our base layer buying top tips.

What is a Base Layer?

Base layers (or first/inner layers) form the foundation of the layered clothing system, where the user wears layers of clothing, each with a specific function in order to keep yourself warm, deal with moisture, and offer versatility in varying conditions. Usually made of merino wool (natural), polyester or polyester blends (synthetic) or even silk, they serve to move moisture from the body to the next layer, making the wearing feel warm and comfortable.

Icebreaker Women's Vertex

Icebreaker Women’s Vertex Merino Base Layer

Layered clothing system:

  • Base Layer
  • Mid Layer
  • Shell Layer

The traditional setup is a thermal base layer (generally synthetic or merino) with a ‘mid-layer’ on top, usually in the form of a fleece jumper/jacket or an insulated down/synthetic jacket/vest for the colder conditions. Over that is a shell layer (aka ‘hard shell’) such as a Gore-Tex Jacket which serves to protect against wind and rain abuse whilst remaining breathable enough to allow moisture to pass through.

Synthetic or Merino?

These two differing types of insulation fabric offer different properties, advantages and disadvantages. As a general rule synthetic base layers, such as those from Helly Hansen and Arc’teryx, are strong on performance in aerobic situations of varying intensity because they are quick to dry, quicker than natural fibers and are extremely light weight. I know people who’ve used them for winter running when it’s really cold out, and not needed anything else over the top.

The Merino Sheep - Majestic fella, isn't he?

The Merino Sheep – Majestic fella, isn’t he?

Merino wool is generally regarded as the warmer of the two fabrics (dependent up on fabric weight, of course) and best for cool and cold conditions. After all, what adapts better to cold weather better than the evolution of a species to its environment? Icebreaker Merino products use wool from the Merino sheep of the Southern Alps, New Zealand. The Merino has to stay cool in summer temperatures of up to 35ºC (95ºF) with its light breathable coat which is then grown to be used as insulation in the winter to stay warm in -20ºC (-4ºF) winter.

Base Layer Pro’s & Cons

Icebreaker Men's - Tech Long Sleeve

Icebreaker Men’s – Tech Long Sleeve

Merino Pros

  • Very warm. A fantastic natural insulation and ideal for the coldest weather.
  • Excellent all around performance & temperature regulation in a variety of climates & conditions 
  • Soft & incredibly comfortable – non itchy thanks to ultra-fine merino fiber
  • Odour resistant & naturally anti-bacterial – Because of wool’s natural anti bacterial properties Merino can be worn for multiple days without getting smelly!
  • Excellent moisture management. Uses absorption process to transfer moisture away from the body. Absorbs around a third of its total weight and evaporates it gradually.
  • Good Stretch resistance – Above-average elasticity. Retains shape after being stressed.
  • Breathable – moisture vapour escapes without making you feel clammy
  • Machine washable and wrinkle resistant.
  • Firesafe – least flammable of fibres


  • Doesn’t dry as quickly as synthetic layers.
  • More expensive than synthetic base layers
  • Can be vulnerable to shrinkage in the wash

Synthetic base layer Pros

Helly Hansen Women's Dry Synthetic Base Layer

Helly Hansen Women’s Dry Synthetic Base Layer

  • Excellent moisture wicking – Great when performing ‘start stop’ activities
  • Quicker drying than merino
  • Hard wearing, robust fabric – especially when combined with nylon
  • Easy to wash and wear – no need to iron
  • Ideal for use in wet and humid conditions
  • Good for high intensity aerobic activity
  • Very soft against skin
  • Cheaper to buy than the more than more expensive merino


  • Petroleum-based. Relies on oil for its production
  • Can become odorous after wearing for multiple days in a row

Things to consider:

What is your intended use?

What are you going to be using this product for? Where will you be using it? Yes, you’re wearing it to stay warm, but are you going to be performing high intensity activity? Are you going to be exercising or completing a tough walk, or is it for more sedate activities, watching the kids play football on a crisp Sunday morning or out Christmas shopping around town? These are all things to consider – Synthetic base layers are generally regarded to transport or ‘wick’ moisture away from the body to the outside of the garment quicker than merino. That’s not to say that merino is slow, but synthetic generally has the edge for those who need a garment to dry quickly.

Fabric Weight:

Base layers come in a variety of fabric weights for differing conditions. You’ll need to consider what you’ll be doing and where you’ll be doing it to get the most appropriate base layer Depending on what activity you’ll be performing and the conditions you’ll be encountering. We’ve categorised our Icebreaker base layers into three simple fabric weight divisions, lightweight, midweight and heavyweight. Lightweight: For cool to quite cold conditions. Midweight: Quite cold to cold conditions. Heavyweight: Cold, very cold or windy conditions. For more flexibility you could try two thinner base layers instead of one heavier layer. This way you can add/remove one of the layers to suit the conditions, ideal for hiking or cycling.


Base layers should fit snugly to the body and look neat. Any distinct gaps or distance between the fabric and skin is just room for extra air which needs to be warmed and can be inefficient. You will find there are specific products for male and female users in order to suit the different body shapes men and women have. Helly-Hansen-Men's-Warm-Fre Be sure to ensure there is no exposed areas of the body when wearing your base layer, as these are parts of the body that will not be protected from the cold. Check that arm/sleeve lengths as well as back lengths are long enough to ensure no skin is exposed when reaching or stretching. A neck zip can be useful to control temperature, to keep you warmer when it’s really cold, or to allow for cooling by unzipping when you start to feel the heat. Higher collars (such as this) keep your neck warm. If you’re sensitive to chafing, select a product with less seams. Consider Icebreaker Bodyfit products – These are jersey knit, which allows for a finer fibre which is longer lasting, more durable, softer and more comfortable.


Merino is a premium fabric and is expensive to produce than a synthetic layer – Icebreaker go to great lengths to ensure that their products are ethically sourced and produced. If price is an issue then consider the synthetic option.

Why not just wear an extra t-shirt to stay warm?

This is a pretty common question, and while wearing an extra t-shirt would keep you a bit warmer, as soon as you sweat you’ll find that cotton is a poor insulation. It soaks up sweat, and takes a long time to dry which can result in swings in body temperature. In very cold, windy and hostile outdoor conditions this could be potentially life threatening. We hope this guide has helped you in making your base – layer purchasing decision. Keep an eye on this blog for more information and buying guides coming soon!

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