Vital S1 Base Layers – Built for the Backcountry

By: KUIU Staff

You’ve earned the right to be picky. When every ounce counts, every degree of temperature swing matters, and every stalk or glassing session can stretch from dawn until last light, your base layer isn’t just underwear—it’s the foundation of the entire system.

That’s why we built the new Vital S1 Base Layers with one non-negotiable goal: the most advanced, lightest, highest-stretch synthetic next-to-skin system we’ve ever put into the mountains.

The secret? XLANCE® — a next-generation polyolefin stretch fiber that finally kills the three sins that have haunted nearly every synthetic base layer for decades: persistent odor, extra weight, and premature failure.

Here’s what actually happens:

Spandex filaments are oleophilic—they love body oils and fatty acids from sweat.
Those microscopic droplets cling to the fiber and create a perfect breeding ground for odor-causing bacteria.
Once the odor is baked in, regular washing can’t touch it.
Spandex also adds weight, slows drying (even small percentages tighten the knit structure and trap surface moisture), and loses recovery over time.

Hardcore mountain hunters have known this for years. That’s why the same guys who wouldn’t touch cotton in the backcountry cringe when they see spandex or elastane on a materials label. And yes, spandex has the same pitfalls when blended with merino wool base layers.

Enter XLANCE^® – The Stretch Fiber That Changes Everything

Vital S1 is the first hunting base layer to replace spandex entirely with XLANCE®, an advanced polyolefin-based stretch yarn engineered specifically for the abuse of mountain hunting.

What XLANCE delivers that spandex never could:

Built-in resistance to sweat, body oils, chemicals, heat, ozone, and UVA/UVB radiation
Long-lasting elasticity and shape retention—season after season
Incredibly low weight (the entire fabric is only 105 grams per square meter)
Gentle, low-force stretch—noticeably more comfortable and fatigue-free than the aggressive snap-back of traditional elastane
Faster drying because it doesn’t compact the fabric structure like spandex blends do

Paired with our proven, low-sheen 72% polyester base and permanent IONIC+ antimicrobial (extruded into the yarn, not a topical wash that fades), the result is a base layer that stays light, dry, and odor-free even after days of high-output effort.

Features that actually matter in the field:

Hybrid Raglan sleeve – zero lift, full draw or ice-axe swing mobility
Flatlock seams – chafe-free even after 12+ hours of hiking.
Layered thumbholes –easy on or off layering
Drop-back hem – stays tucked when you’re bent over or glassing
UPF 30 – built-in sun protection for high-alpine exposure
Odor resistant + moisture wicking + quick dry – the holy trinity for activity/temperature swings