Why do baselayers need to fit closely?
A baselayer needs to sit close to your skin so it can efficiently move sweat and moisture away from your body. This process, known as wicking, draws moisture into the fabric and transfers it towards the outer surface, where it can evaporate.
A close-fitting baselayer also helps prevent cold air from getting between your skin and the fabric. It should feel comfortable without being restrictive, with no excessive creasing or rucking.
What is wicking?
Wicking is the ability of a fabric to draw moisture away from your skin and spread it across a larger surface area so it can dry more quickly. Effective moisture management is particularly important during strenuous activities, when you can quickly become hot and sweaty.
A baselayer won't actually make you sweat less – sweating is your body's natural way of regulating temperature. Instead, a good baselayer helps manage the moisture produced, keeping you more comfortable as you exercise.
Remember that if you're overheating, simply removing a layer or reducing your pace can sometimes be just as important as the clothing you're wearing.
Baselayers vs thermal underwear
Although the terms are sometimes used interchangeably, technical baselayers and thermal underwear have different priorities.
Technical baselayers are designed primarily to manage moisture while providing an appropriate level of warmth. They are available in different weights, from lightweight options for warmer conditions to heavier garments for cold-weather activities.
Traditional thermal underwear is generally designed primarily for warmth and is particularly useful when you're less active. It may not offer the same level of moisture management as a technical baselayer.
Synthetic or natural fibres?
Synthetic baselayers, usually made from polyester or similar fibres, are lightweight, highly breathable and fast-drying. They are particularly effective at moving moisture away from the skin, making them a popular choice for high-intensity activities.
The main drawback is odour retention, although many modern synthetic baselayers feature anti-odour treatments. Look for garments containing recycled synthetic fibres if sustainability is a priority.
Merino wool is naturally odour-resistant and absorbs moisture vapour while helping regulate body temperature. It is an excellent choice for multi-day trips when you may not be able to wash your clothing regularly. The downside is that merino generally takes longer to dry than synthetic fabrics.
Many wool baselayers also contain a percentage of synthetic fibre to improve durability and help the garment retain its shape.
Avoid cheaper activity clothing with a high cotton content. Cotton absorbs moisture but dries slowly, making it less suitable for strenuous outdoor activities in cool or changeable conditions.
Don't forget hands and feet
Moisture management isn't just important for your torso. Liner gloves and technical socks can help manage sweat and provide an additional layer of insulation for your hands and feet, particularly in cold conditions.
Looking after your baselayer
Always follow the manufacturer's care instructions. Avoid fabric conditioner unless specifically recommended, as it can interfere with the moisture-wicking properties of technical fabrics.
If odour becomes a problem, specialist outdoor clothing washes from brands such as Nikwax and Grangers can help freshen technical garments and extend their useful life.
Choosing the right baselayer
The best baselayer is one that fits comfortably, works with your activity level and provides the right balance of warmth and moisture management.
Consider lightweight synthetic fabrics for high-output activities and warmer conditions, merino wool for natural odour resistance and temperature regulation, or a blend if you want a balance of performance, durability and comfort.
Above all, choose a baselayer that fits your body shape properly, without creasing or riding up. Try it on with the other layers you intend to wear to make sure everything works comfortably together.