What are Base Layers? A Guide to Thermal Wear (2024)

Whenever you’re planning your outdoor pursuits, you need to make sure you have the correct clothing to match both the activity and a potential change in weather. In the UK, the weather can change multiple times daily – and quickly.

As the weather warms or cools, so do you, and you will need to add or remove layers. A base layer, or thermal underwear, should be your first layer no matter what you do or where you go.

Read on for Winfields Outdoors complete guide to base layers and thermal clothing…

A base layer is a layer of clothing that is closest to your skin, almost acting as ‘second skin’ that can keep you warm or cool.

Base layers and thermal underwear provide a layer of warmth while, at the same time, absorbing and removing your sweat to keep you comfortable while on the move – this is called ‘wicking’.

Thermal clothing is versatile clothing and comes in different fabrics and styles that will benefit various climates, conditions and activities. In most cases, they will be made from a synthetic material and can be called base layers, thermals or compressions.

Base Layer Tops

No matter what you do outdoors – hiking, walking or climbing – a thermal top is always a must-have in summer or winter.

For summer or warm temperatures, short-sleeves and vests are the best options for activities. Vests are the coolest base layer choice, followed by short-sleeves and long-sleeves.

In winter, long-sleeves are often the best choice. However, a short-sleeve can work depending on what you’re doing. Longer sleeves will keep you warm and protect you from any weather or wind chill by keeping your blood flowing.

Reminder: Consider what you will be doing and wearing on top of your base layer before buying.

Base Layer Bottoms

Base layer bottoms are great for adding extra warmth under your trousers while enjoying the outdoors or as a standalone layer when running.

Designed to retain body heat and wick away moisture, bottoms, or leggings, are an ideal all-rounder for everything from snow sports and cycling to hiking and running. You can even wear them under jeans or workwear if you’re really cold.

In summer, you can wear base layers on their own or underneath shorts if you start a hike or climb when it’s very warm. Bottoms also include compression shorts, which add heat retention and prevent muscle soreness during sport.

Synthetic Base Layers

Synthetic base layers are generally the cheapest and most commonly found among stores and brands. Synthetic thermal layers are lightweight, quick-drying and provide excellent moisture wicking. This makes them perfect for high-energy activities and sport, which is why they are now a common sight among athletes where their bodies are always moving.

Reminder: Synthetic fabrics are best for muscle compression. This is why compression shorts, compression sleeves and vests are a common sight in football, rugby, basketball, tennis and other sports.

A drawback of synthetic fabrics is that they are not naturally antibacterial. This means odour from your sweat will be retained, making them rather smelly unless washed immediately. Unless you buy a warm synthetic base layer, they are not as naturally warming as merino wool fabrics. So, always check when buying.

Merino Wool Base Layers

Merino wool is the most naturally warm of all base layer fabrics – which is why it is often the chosen material of thermal clothing. Merino wool regulates body temperature, keeping you nice and warm while preventing overheating – perfect when hiking. Small air pockets draw excess heat away from your body while retaining heat within the fibres.

Reminder: Merino wool is also naturally antibacterial, so your body odour won’t stick to the fabric. Unlike some synthetics, wool fibres are non-abrasive, making them suitable for sensitive skin.

The main drawback of merino wool is that it takes much longer to dry than synthetics and they can be more expensive. As a more natural fibre, wool can be less effective at moisture than a synthetic base layer, which can make them better for less frequent use, such as hiking and walking.

This often depends on you and what you prefer, but there are general rules for each type of base layer fabric.

Synthetics should feel tight against your skin while providing you with plenty of flexibility to move. Don’t think that because it’s a stretchy compression layer or base layer that you should buy a size smaller than usual.

Merino wool can be looser than a synthetic fabric and should be comfortable next to the skin rather than tight on it. Think of them as more of an ‘athletic’ or ‘fitted’ body fit than being skin-tight.

Base layers are traditionally designed to retain your body heat and wick away sweat and moisture. This is why base layers are not waterproof.

Thermals and base layers are a second skin – the first stage in a layering system. They are lightweight, so if it’s waterproofing you’re after, we’d recommend investing in true waterproof clothing – like jackets, trousers and socks. Base layers and thermal clothing can be worn all-year-round thanks to their technology to build or reduce heat, meaning you can wear them for a hike, whatever the weather.

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