Best Long Underwear of 2026

When the mercury drops, and staying warm isn't just about comfort but about safety, Aclima's WarmWool Crew and Pants are what you reach for. This isn't your average merino base layer; it’s a substantial 400 gsm double-face knit that redefines 'warm.' Hailing from Norway, Aclima understands cold, and they've engineered this set for serious performance in frigid conditions. It's the kind of gear you trust on an overnight winter expedition or during extended periods of low activity in extreme cold. The dense knit traps an impressive amount of body heat, creating a microclimate that insulates even when damp. While heavier than most base layers, the warmth-to-weight ratio is still exceptional for its category, making it a viable primary insulating layer.

This set isn't trying to be an ultralight marvel for shoulder season hikes. It's built for demanding winter environments where warmth is paramount and breathability, while still good, takes a slight backseat to thermal retention. The construction is robust, promising longevity through seasons of hard use. It’s a significant investment, but for those who consistently face truly cold weather, the peace of mind and performance it delivers are well worth the cost. Consider this your cold-weather insurance policy, designed for anyone who refuses to let winter dictate their adventures.

Construction & Materials

The WarmWool set is crafted from 100% merino wool, but it's the 400 gsm double-face knit that sets it apart. This dense, looped construction creates a remarkably effective thermal barrier, trapping warm air close to the body. Unlike lighter merino weaves, this fabric has a more substantial handfeel, almost like a thin fleece, yet it retains all the natural properties of wool: odor resistance, breathability, and the ability to insulate when wet. The stitching is flatlocked throughout, preventing chafing during prolonged wear, and the cuffs and waistband are well-finished for comfort and durability. This isn't a delicate garment; it's built to withstand the rigors of serious outdoor use.

Real-World Performance

In genuinely cold conditions—think single digits or below zero Fahrenheit—this base layer performs. It's our top pick for a reason: unparalleled warmth as a true next-to-skin layer. It excels during periods of low to moderate activity, such as ice fishing, winter camping, or downhill skiing. Even during higher output activities, the wool's inherent breathability and wicking properties manage moisture effectively, preventing that clammy feeling. Crucially, if you do get sweaty, it dries surprisingly quickly for its weight, maintaining much of its insulating power. This characteristic is vital in environments where a chill can quickly turn dangerous.

Weight & Packability

At 400 gsm, the WarmWool set is undeniably heavier and bulkier than most merino base layers. It won't pack down to the size of a water bottle like some ultralight options. However, for the warmth it provides, it's still remarkably efficient. When you consider that this single layer often replaces a lighter base layer plus an additional mid-layer fleece, its overall impact on your pack's weight and volume can be a net positive. It's an item you choose for its specific, high-performance function, not for its minimalist packed size. The trade-off is well worth it when deep cold is a factor.

Durability & Longevity

Durability is a strong suit here. The dense, double-face knit is naturally more robust and less prone to snags or pilling than finer merino weaves. We've put these through multiple seasons of hard use, and they hold up exceptionally well. The stitching remains solid, and the fabric retains its loft and shape. While all wool can eventually show wear, especially with abrasive activities, the Aclima WarmWool demonstrates a higher resistance to degradation than many of its lighter counterparts. Proper care—washing on a wool cycle and air drying—will significantly extend its lifespan, ensuring it remains a staple in your cold-weather kit for years.

Who This Is For

This set is for the serious winter enthusiast. It’s ideal for ice climbers, mountaineers, winter campers, arctic explorers, or anyone spending extended periods in extreme cold where heat retention is non-negotiable. It's also an excellent choice for individuals who are naturally cold or have low activity levels in freezing temperatures. If you often find yourself layering multiple lighter base layers, this might be the single solution you've been looking for. It's overkill for mild winter days or high-output activities in merely cool weather, but when the conditions demand maximum thermal protection, the WarmWool delivers without compromise.