How to Choose a Base Layer: Effort, Fibre and Weight

Most people choose a base layer by asking how cold it is going to be. The better question is how hard you will be working. A base layer's first job is not to be warm, it is to move sweat off your skin, because dry skin stays warm and damp skin chills the moment you stop. Get that right and the rest of your clothing suddenly works much better.

Here is how to choose a base layer in three decisions: how hard you will be working, which fibre suits you, and then which weight. Along the way we point to the base layers we stock from Patagonia, Icebreaker, Odlo, Smartwool, Dare 2b and Helly Hansen, and to more detailed guides where a topic deserves its own space.

Find your starting point

Start with the kind of day you have in mind, then read on for the reasoning behind each pick.

Why a base layer is about sweat first

Even on a cold day you sweat on the climbs. If that moisture stays against your skin, it cools you rapidly when you stop for the view or a sandwich, which is when most people feel coldest. A good base layer pulls moisture away from the skin and spreads it out so it can dry, which is why technical base layers are built from merino wool or polyester rather than cotton. Cotton soaks up moisture and holds on to it, which is the opposite of what you want next to your skin outdoors.

This is also why the warmest base layer is not automatically the best one. A layer that is too warm for the effort makes you sweat more than it can move, and you end up damp and cold. Matching the layer to the effort is what keeps you comfortable.

Step one: how hard will you be working?

The brands we stock design around effort as much as temperature. Patagonia describes Capilene Thermal Weight as a microfleece whose interior grid traps body heat and moves sweat during high output, so even its warmest base layer is built to breathe. Odlo pitches Active X-Warm for the moment when the temperature drops and your pace picks up. And Icebreaker asks you to think about your activity level alongside the conditions when choosing a weight.

As a rule of thumb, the harder you work, the lighter the base layer you need, because your own body is producing plenty of heat. The slower and colder the day, the more warmth you want from the layer itself.

• High output: fast walking, running, ski touring and cycling. Go light and fast-drying.
• Mixed effort: most hill days in Ireland, with steady climbs and regular stops. Midweight is the sweet spot.
• Low output in the cold: resort skiing with long lifts, winter walks with family, standing at a sideline. Go warmest.

If you only buy one base layer for Irish winter walking, make it a midweight. Patagonia calls Capilene Midweight its most versatile baselayer, with balanced warmth across a wide range of winter conditions.

Step two: merino or synthetic

Both fibres make excellent base layers, and they are good at slightly different things. Merino wool from Icebreaker and Smartwool is soft, regulates temperature and naturally resists odour, so it suits multi-day trips, travel and everyday wear. Synthetic layers such as Patagonia Capilene and Odlo Active Warm are built from polyester that wicks sweat and dries quickly, and they stand up well to hard use and frequent washing.

There is no wrong answer, only a better fit for how you use it. We compare the two in detail, including blends that mix both, in our guide to merino vs synthetic base layers.

Step three: choose the weight

Once you know your effort and your fibre, pick the weight within a range. Each brand names its weights differently, so here is how the ranges we stock line up.

A few things worth knowing about those names. Icebreaker's numbers give you a sense of warmth: the higher the number, the warmer and heavier the fabric, and Icebreaker describes its 200 layers as lightweight for moderate to cool conditions and its 260 layers as midweight for colder days. Icebreaker has also renamed its 260 Tech as the 260 Oasis, so you will see both names on the same style. Odlo's Active X-Warm adds double-layered insulation in key areas on top of the Active Warm recipe.

Thicker does not always mean warmer, either. Patagonia lists Capilene Thermal Weight at 4.5 oz (153 g) against Midweight's 4.1 oz (140 g), so its warmest layer is barely heavier. The extra warmth comes from the grid structure trapping air, which also keeps it low in bulk under a midlayer. We explain the whole Capilene system in our guide to Patagonia Capilene.

Fit: close, not tight

A base layer can only move moisture it is touching, so it should sit close to your skin. Icebreaker cuts its merino in a slim fit and Odlo describes its Active Warm range as fitted and next-to-skin. Close should not mean restrictive, though. You want to be able to reach overhead and bend without the fabric pulling.

Resist the urge to size up. A loose base layer leaves gaps where moisture sits on your skin instead of being drawn away, and it bunches under your other layers. Look for details that reduce rubbing too: flatlock seams lie flat against the skin, and offset shoulder seams, which Icebreaker uses on its Oasis tops, keep seams away from where pack straps sit.

Tops, bottoms and necklines

Most people start with a top, and the neckline matters more than you might think. A crew neck is simple and sits flat under a fleece. A zip neck lets you dump heat on a climb and close it again when you stop, which makes it the more flexible choice for hiking and skiing. A turtleneck or high collar adds warmth around the neck without a separate neck gaiter, which is handy under a ski jacket.

Base layer bottoms are worth adding once the weather turns properly cold or you are skiing. They add warmth under walking or ski trousers with very little bulk. Browse men's baselayer tops, men's thermal leggings, women's thermal tops and women's thermal leggings.

Where a base layer fits in your layering

Your base layer is the first of three layers. Over it goes a midlayer such as a fleece or light insulated jacket for warmth, and over that a shell to keep out wind and rain. Each layer has one job, and you add or remove them as conditions change. Our guide to layering clothing for the Irish outdoors covers the full system, and how to choose a waterproof jacket takes care of the outer layer.

The same thinking applies to your feet. Socks are the base layer for your feet, and the right fibre makes just as much difference there. See our guide to the best hiking and walking socks.

Caring for your base layers

Always follow the care label, because the right wash varies by fibre and construction. Merino generally prefers a cool, gentle wash and drying flat away from direct heat, and it needs washing less often than you might expect: airing it out overnight is often enough between wears. Synthetic base layers are easy to wash and dry quickly, which is handy on a trip.

Odour control treatments help between washes. Patagonia finishes Capilene Lightweight and Thermal Weight with HeiQ Mint odour control, and Odlo uses HeiQ Mint on its Active Warm tops.

Base layers for skiing and for kids

Skiing asks something particular of a base layer, swinging between hard effort on a run and sitting still on a chairlift. Our guide to ski base layers covers what to wear under your ski jacket and trousers, and how many sets to pack. If you are kitting out children, our parent's guide to kids' base layers covers fit, fibre and school ski trips.

Find your base layer

Decide how hard you will be working, choose the fibre that suits how you use it, then pick the weight. Get the fit close, not tight, and the rest of your layers will thank you. You can browse all our base layers, or go straight to men's base layers, women's base layers and merino base layers. Our teams in store are always glad to help you find the right one.