The Smartwool Intraknit Thermal Merino wins top honors for its weight class thanks to its innovative seamless knitting technology that blends swaths of warm fabric with panels that add durability and ventilation — all without requiring additional seams, which saves on weight and bulk. We think this is a pretty cool innovation, and it seems to work. The slim, athletic fit excels as a base layer, allowing you to top it off with additional layers for a complete system to prepare you for the harshest of conditions. Extra long sleeves, torso, and a drop tail hem ensure that the Intraknit Thermal covers you no matter how you move. The midweight Merino wool blend fabric is a great "just right" weight to use for most cool to cold days outdoors. Combining strict ethical production standards and technical functionality is a hard task, but the Smartwool Women’s Merino Sport Long Sleeve Top delivers on both fronts. Proudly boasting ZQ certification, not only does it feel good from a consumer perspective but the top also performance perfectly. One of the best midweight base layers for when you need warmth and freedom of movement thanks to its fabric blend and seamless ventilation pattern Thin and lightweight Merino blend, 1/4 zip enhances thermal regulation, extra long sleeves and torso

An uber comfortable, warm, and breathable 100% Merino wool base layer for the coldest of conditions Cornish brand Finisterre has long offered a range of merino underwear, but they’ve always been an environmentally conscious company, and last year they added bamboo base layers to their range. A natural product, bamboo is both sustainable to produce and the end product will eventually biodegrade, unlike synthetic poly tops, which will outlast us all and release micro fibres into the water system and food chain every time they’re washed. Weak durability keeps this top a dedicated base layer, seams found converging in pinch points, pricey As always, the most sustainable clothes are the ones you’ve already got, but if you do need to invest in a new base layer, look for garments woven from natural, biodegradable materials (wool, bamboo) or recycled synthetics, made with natural dyes by brands with good environmental principles and robust manufacturing ethics. Lightweight base layers are less likely to have them, but in a more substantial base layer, a partial-length zips is an excellent feature, allowing you to dump excess heat during activities. We like thumb loops too, which stop sleeves from riding up when you put on another layer, and prevent gaps appearing between your top and your gloves. Moisture managementThese types of questions will help guide you in the direction of what holds more importance for you as a buyer. Decide which metrics are most important for your sport or activity—whether that's ice climbing or shoveling snow in your driveway—and base your decisions on those factors first.

Trail runs were incorporated into our test plans to assess breathability and versatility. Though silk offers a higher warmth-to-weight ratio than other fabrics, it is less breathable than other natural fibers. For a decade, our team of experts has bought and tested dozens of the best base layers on the market. For this update, we compare 17 of the most popular options side-by-side. From the ski slopes to alpine climbs to sleeping out under the stars, our in-depth analysis derives from real-world adventures undertaken while wearing each model. We score all base layers across key areas of performance, like warmth, breathability, and drying speed. Whether it's for the warm or cold season, a quality base layer is sure to keep you dry and comfortable, no matter where your next outdoor escapade takes you. While there are myriad styles to choose from, the best base layers are there to do one job well: keep the wearer warm and safe. Additional features will add comfort and functionality, but the thermal properties are the most important factor. The following are some things to consider. Thermal insulationUnique features like thumb loops and a hood makes this Merino wool blend base layer super versatile Slim athletic fit requires a slim athletic body type, expensive, not as durable as cheaper synthetics Whichever pair of women's running shorts you choose, you’ll benefit from a unique combination of technical detailing and clean-cut designs to make sure you can train to the best of your ability. New Balance: Tech First Natural fabrics – especially merino wool, but also bamboo – work very well in the best base layers, but you’ll often see manmade materials like polyester and Lycra employed in the mix, and these can help make the garment closefitting (as a base layer should be), maintain its shape, add to its durability and improve the all-important thermal performance, especially if it gets wet. As mentioned, the Cotopaxi Liso isn't the warmest option, as it's thinner and less warm than many of its rivals. It's also less warm than wool shirts of the same weight. If gone unwashed, the synthetic fabric will develop a funk if you after a few days of wear, despite the Polygiene application applied to the fabric. Be mindful when washing the Liso: all that stretch is accomplished by using a hefty amount of spandex in the fabric, and this will lose its stretchiness after enduring too many dryer cycles. If you want a great budget wool top, have a look at the Meriwool Merino 250.