6 Best Breathable Merino Base Layer Bottoms for Hot Weather That Beat Shorts
Why wear pants in the heat? Lightweight merino bottoms offer superior breathability, moisture control, and sun protection over shorts. We review the 6 best.
You’re three miles into a hike, the sun is beating down, and the horseflies are staging a full-scale assault on your legs. You thought shorts were the obvious choice for a 90-degree day, but now you’re regretting it. This is where the experienced trail wisdom comes in: for hot, exposed, or buggy conditions, the right pair of long bottoms can be vastly superior to shorts. And counterintuitively, the best tool for the job is often a pair of ultralight merino wool base layer bottoms.
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Why Merino Wool Bottoms Excel in Summer Heat
Let’s get the big question out of the way: wool in the summer? Absolutely. We’re not talking about your grandfather’s itchy winter sweater. Modern, ultralight merino wool (typically around 150 gsm, or grams per square meter) is a performance fabric that breathes incredibly well, wicks sweat away from your skin to keep you cool, and has a near-magical ability to resist odor after days of use.
Unlike shorts, which leave your skin exposed, a lightweight merino bottom offers a full-coverage shield. This provides crucial protection from three of summer’s biggest annoyances. First, it’s a physical barrier against biting insects like mosquitoes and flies. Second, it guards your skin against scrapes from overgrown trailside brush or rocky scrambles. Most importantly, it delivers all-day sun protection without the need to reapply sticky sunscreen.
When you weigh the benefits—thermoregulation, bug defense, and sun protection—against the minimal weight penalty, the case for merino bottoms becomes clear. They provide a more comfortable and protected experience on the trail, especially on long days above treeline, in dense forests, or across sun-scorched desert landscapes. It’s a simple gear choice that solves multiple problems at once.
Smartwool Merino 150: The Ultralight Standard
When you’re starting your search, the Smartwool Merino 150 is the benchmark against which all others are measured. It’s a classic for a reason. Made from 100% merino wool (or sometimes a blend with a nylon core for durability), this piece is exceptionally light and breathable.
Think of it as your go-to for a wide range of summer adventures. On a high-country backpacking trip, it’s the perfect layer for sleeping comfortably on a mild night and then wearing through a cool, misty morning. For a day hike that starts in the shade and ends on a sun-blasted ridge, its temperature-regulating properties are second to none. The main tradeoff here is durability; its ultralight nature means it’s not the best choice for heavy bushwhacking. But for established trails and all-around use, it’s the standard.
Icebreaker Cool-Lite: TENCEL Blend for Hot Days
What happens when you hit truly sweltering heat and humidity, the kind you find in the Smoky Mountains in July or a coastal rainforest trail? That’s where a specialized blend like Icebreaker’s Cool-Lite fabric shines. This isn’t pure merino; it’s an engineered mix of merino wool and TENCEL™, a fiber derived from eucalyptus.
This blend creates a fabric that feels cooler to the touch and wicks moisture even more efficiently than 100% merino. The TENCEL fibers actively pull heat and moisture away from the body, enhancing the natural properties of the wool. These bottoms are built for those days when you know you’ll be sweating from the moment you leave the car. They dry remarkably fast, making them ideal for trips with creek crossings or unexpected downpours. The addition of a nylon core in the yarn also boosts durability without adding significant weight.
Patagonia Capilene Cool Merino for All-Day Comfort
Patagonia takes a different approach to the merino blend with their Capilene Cool Merino line. They combine merino wool with recycled polyester, creating a fabric that excels in long-term comfort and moisture management. It feels incredibly soft against the skin, making it a great choice for multi-day trips where you’ll be living in your layers.
This blend is a true workhorse for varied conditions. The polyester content helps the fabric dry quickly and withstand more abrasion than pure merino, while the wool provides its signature odor control and thermoregulation. This is the bottom you choose for a week-long backpacking trip where you need one piece to function as sleepwear, a hiking layer under rain pants, and a comfortable camp lounge pant. It strikes a fantastic balance between performance and pure, next-to-skin comfort.
Ridge Merino Aspect: UPF 30+ Sun Protection
For adventures in the desert, at high altitude, or on any trail with zero shade, sun exposure is a serious concern. The Ridge Merino Aspect bottoms are designed with this challenge in mind. While all long pants offer some sun protection, the Aspect line is specifically rated to UPF 30+, giving you certified protection you can count on.
Ridge Merino uses a blend that incorporates their (m)Force™ Technology, which wraps merino fibers around a nylon filament. This construction dramatically increases the fabric’s strength and durability, making it a great option for more rugged terrain where you might be scraping against rock or tough vegetation. If your primary goal is to find a durable, sun-protective layer for relentless exposure, the Aspect is a top contender. It’s a practical choice that prioritizes protection without sacrificing the breathability you need in the heat.
Black Diamond Solution 150 for High-Output Pursuits
If your outdoor days involve moving fast—think trail running, fastpacking, or alpine climbing—you need gear that can keep up. The Black Diamond Solution 150 Merino Bottoms are engineered for exactly these high-output scenarios. They utilize NuYarn technology, a unique spinning process that drafts merino fibers around a nylon core, creating a fabric that is stronger, stretchier, and faster-drying than conventional merino.
The result is a base layer that feels incredibly light but is surprisingly tough. It moves with you during dynamic activity and manages sweat with extreme efficiency, preventing that clammy feeling after a hard effort. This is the piece for the mountain athlete who demands the highest performance-to-weight ratio. It’s not just for keeping you warm; it’s a technical tool for managing your microclimate when you’re pushing your physical limits.
Minus33 Chocorua: Lightweight Year-Round Value
Not every piece of gear needs to be the most technical or expensive. Sometimes you just need something that works reliably without breaking the bank. The Minus33 Chocorua is that piece. Made from 100% 170 gsm merino wool, it’s a straightforward, no-frills microweight bottom that delivers all the core benefits of merino.
This is an excellent starting point for someone building their first layering system or a seasoned hiker looking for a solid, all-purpose piece. It’s fantastic as a summer sleeping layer, providing just enough warmth for a cool mountain night. It’s also a capable hiking layer on its own in mild weather. While it might not have the advanced blends or specialized fit of other models, its simplicity and value are its greatest strengths. It’s a testament to the idea that good gear doesn’t have to be complicated.
Choosing Your Merino: Fabric Weight and Fit Guide
Navigating the options can feel overwhelming, but it boils down to a few key factors. By matching the gear to your specific needs, you can make a confident choice. Focus on fabric weight, fit, and material composition to find the right bottom for your adventures.
First, consider the fabric weight, measured in grams per square meter (gsm). For hot weather, you should be looking almost exclusively in the 120-170 gsm range, often called "ultralight" or "microweight." A 150 gsm bottom is the versatile sweet spot, offering a great balance of breathability and durability for three-season use.
Next, think about fit. A "next-to-skin" or "athletic" fit is best for maximizing moisture-wicking, as the fabric needs to be in contact with your skin to pull sweat away. A "regular" or "relaxed" fit can be more comfortable for wearing as a standalone pant or for sleeping. Your intended primary use—as a true base layer or as an outer pant—should guide your fit decision.
Finally, evaluate the material: 100% merino vs. a blend.
- 100% Merino: Offers the best odor resistance and a very soft natural feel. It’s the purist’s choice for comfort and multi-day funk prevention.
- Merino/Nylon Blends: Significantly increases durability and longevity. This is the best choice if you’re hard on your gear or hike in rugged, abrasive environments.
- Merino/TENCEL or Polyester Blends: Enhances drying speed and can add a "cool-to-the-touch" feel. Ideal for extremely hot, humid conditions or high-sweat activities.
Ultimately, the best choice is the one that aligns with where and how you hike. Don’t overthink it—any of these options will serve you far better than shorts on a hot, buggy, sun-drenched day.
The perfect piece of gear is the one that gets you outside and keeps you comfortable enough to enjoy the experience. Whether you choose a high-tech blend for a fast-and-light objective or a simple, reliable classic for a weekend camping trip, the goal is the same: to spend more time on the trail. So pick a pair, pack your bag, and go make some memories.
