Thermal underwear: unsung heroes of the outdoors

When the temps start to drop having good thermal underwear (also called long johns or base layers) to keep you toasty is a must. Technical base layers and thermal underwear are the foundation of any outdoor layering system. Whether you’re hitting the slopes, exploring local trails or tackling a first ascent, these key pieces move moisture away from your skin to keep you warm and comfortable all day long.

How to choose the right thermal underwear

There's a ton of different materials to choose from like merino wool, synthetic or blends of these and some have features like sweat wicking, odor control and insulation. To choose thermal underwear, start by thinking about the temperature and your activity level. If you wear sports bras, also consider the amount of support you’ll need for higher-impact activities like running compared to lower-impact pursuits like hiking or stretching.

Merino wool vs. synthetic base layers

Merino wool base layers are soft, breathable and naturally antimicrobial and odor resistant. So they're great for travelling or multi-day camping as well as activities like camping trips, leisurely hikes and downhill skiing. Merino wool can absorb up to 30% of its own weight in water before it starts to feel wet, so it's also a good choice for summer activities to help keep you cool.

Synthetic base layers are made from high-performance polyester or polypropylene. Lightweight and quick-drying, they’re also usually more durable and budget-friendly than merino wool. Because of their sweat wicking properties, synthetic base layers shine during high-output activities like trail running, cross-country skiing and mountain biking. They also dry quickly with your body heat, are highly durable, contain antimicrobial treatments and often have a UPF rating for extra UVA / UVB sun protection.

Merino-blend base layers are like the one size fits all of thermal underwear. They are usually more durable than 100% merino wool, can withstand a lot of wear and are a good option if you're short on space. They offer the best of both worlds, with the durability of synthetic fabrics and the temperature-regulating qualities of wool.

MEC has a great thermal base layering system to help you choose the right thermal underwear that you need for your activity. Check out in our article about how to choose the right base layers.

Top your base layers with an insulating mid-layer and a weather-blocking outer layer to handle whatever your day has in store.