There are 3 main types of fabric you can choose for a baselayer, https://www.gooutdoors.co.uk/walking/clothing/baselayers Merino wool, bamboo or synthetic fabrics.
Whatever you choose- make sure you avoid cotton layers. Cotton soaks up sweat like a sponge and when it's wet; it draws heat away from the body, which is why you can feel cold while sweating in cotton, and in the worst case scenario can cause hypothermia. (Hypothermia can be contracted even in 'mild' weather!)
Merino wool is a fantastic natural baselayer fabric that is warm, lightweight and breathable. Although 'fluffy' in appearance, the fibres of a merino wool product are lighter than any other sheep's wool, making them soft and comfortable next to the skin. It is highly pliable, moving with you and stretching back into shape after activity or washing
Merino wool is also naturally antimicrobial, which means it deters bacteria and any subsequent odour.Merino baselayers are also particularly effective as thermal underwear in the cool or cold conditions found when skiing or mountaineering, while thinner weights are perfect for summer activities.
A relatively new addition to the world of layering, bamboo baselayers are made from bamboo extracts sometimes combined with fibres such as cotton, Lycra and even merino wool.
Bamboo baselayers are a tighter fit than a 100% merino wool baselayers and are great at holding their shape.
The super soft baselayers are also ideal for those with sensitive skin who find wool irritates them.
Bamboo is an eco-friendly, sustainable high-performance fabric; able to dry quickly and to wick away odours and bacteria. Ideal for mountain activity in milder conditions, world travel and expeditions.
Synthetic fibres make excellent fabrics for use in baselayers. The hard-wearing materials wick moisture and disperse it out over a large surface area. This accelerates evaporation of the moisture you produce through sweat; therefore keeping you dry, your temperature regular and your skin comfortable. They are also stretchy and quick drying.
Many modern synthetic fabrics will have anti-bacterial treatment to prevent odours.
Choosing your synthetic baselayer is simple - the thinner baselayer will wick moisture and be cooler; the thicker will still perform but will have more warmth.
Synthetic baselayers can also be more friendly on the wallet than natural fibres.