The base layer should fit snugly against the body to absorb and wick away moisture as effectively as possible. However, it should also feel comfortable and not hinder your freedom of movement. Base layer tops are slightly longer to prevent gapping at the back when you bend down, which can allow cold air to seep in and cool you down. Therefore, it is also common for base layer pants or long johns to have a higher waist. This helps them stay in place properly and prevents them from sliding down during activities.
How well thermals insulate depends on the choice of material and thickness. Wool usually has better thermal properties than polyester, and a thicker fabric often provides better insulation. The design also plays a crucial role in retaining heat. For example, a brushed inside or a waffle structure can help trap warm air. Other details, such as a high collar or integrated balaclava or hood, can also protect against cold and keep the skin warm.
Base layers made with synthetic materials dry quickly, are stretchy for a more comfortable fit and are very durable. Due to their moisture-wicking properties, synthetic materials are excellent for high-intensity activities.
Thanks to their temperature-regulating properties, wool base layers are suitable for both warm and cold conditions. Therefore, they are also good for activities like downhill skiing, where the intensity varies between high on the slopes and low on the lift. Merino wool is often used in base layers as it is much softer and doesn't itch.
Base layers made with a mix of wool and synthetic materials combine the best of both worlds. Wool's natural antibacterial properties keep bad odors at bay, while synthetic materials make the garment durable and easier to care for and wash.
