This is the layer in direct contact with your skin. Its main purpose is to transport or “wick” moisture off your skin and move it toward the surface of the fabric where it can evaporate. If your base layer holds moisture, you’ll quickly start to feel cold when you slow down or stop for a rest. Choose a thickness, or fabric weight, based on how cold it is and how active you expect to be. Look for seamless or flat-seamed garments that won’t rub against your skin when combined with outer layers or with a pack. And aim for a snug fit that isn’t constricting. Learn more about how to choose base layers.
This layer adds insulation, traps body heat to keep you warm, and continues moving moisture outward. Materials that are fuzzy like fleece are a good choice because they insulate without feeling bulky. And they are highly air permeable so warm, moist air can easily pass through them. Gridded fleece and high-loft fleece trap warm air without adding bulk. Other mid-layer options are lightweight, low-profile insulated pieces – they can weigh less and compress nicely in your pack while still being warm. Mid-layers should be roomy enough to accommodate a base layer and allow movement, but should still be somewhat snug.
Your final layer, sometimes called a “shell,” protects you from the elements. Depending on the climate, you might want a layer that blocks wind, sheds precipitation or does both. It’s important that this layer is still breathable and allows the moisture from your inner layers to escape. Your outer layer should fit easily over your base and mid- layers, without being so loose that all your warmth escapes. And it should still allow you to move freely. Want more info? Get additional tips on how to choose a rain jacket.
Natural or synthetic?
The type of fabric you choose will depend on climate, activity level, and desired amount of warmth. Certain fabrics, such as cotton are not ideal for layering as they retain wetness and can quickly make you feel chilled.
Synthetic base layers
Synthetic fabrics such as polyester and recycled polyester absorb very little water, so they are quick to dry. These fabrics have good stretch and are easy to care for. They make great base layers – except that they can retain odours if worn for multi-day trips. To combat that, many synthetic base layers have antimicrobial treatments to cut down on unwanted odours.
Wool base layers
A base layer made of merino wool can absorb up to 35% of its weight in moisture and remain dry to the touch since the moisture gets pulled inside the fibres. It has a soft texture and for the same weight, it will be warmer than synthetics. Wool is also naturally odour-resistant, so it’s a good choice if you’ll be working up a sweat day after day. Plus it’s highly breathable, great for temperature regulation, and is long lasting. The drawbacks with wool? It takes longer dry out, and it’s likely to be more expensive and less durable than synthetics. The bonus is that since it’s naturally antimicrobial, you don’t need to wash it as often.
Polyester fleece is a classic insulating mid-layer. It traps warm air, it’s durable and absorbs very little moisture. Fleece can be a little bulky though. If space and weight are a concern, you might choose a gridded or high-loft fleece, or opt for a synthetic insulated piece that holds warmth while being compressible enough to carry in a very small pack.
Down or synthetic insulated mid-layers are both good options if you’re looking for a combination of lightweight and warm. Down provides incredible warmth for weight, and can last a very long time if properly cared for. When wet, though, it loses much of its insulating value and is slow to dry. Synthetic insulators are typically less expensive than down and are easier to care for, but they are typically heavier and bulkier. They also dry out quickly and keep much of their original insulating value when they’re wet.
Waterproof, windproof fabrics and insulating and materials in an outer layer keep the rest of clothing system functioning as it should.