Find Your Perfect Base Layer
Discover Now
How to choose the right base layer warmth for winter
When winter hits, your base layer becomes more than just your first layer. It becomes your foundation for comfort, performance, and protection from the cold. But not every day in winter is the same. Some days call for extra insulation. Others demand breathability and ease of movement.
At icebreaker® we design Merino wool base layers using our Heat Index system, which reflects how warm a garment performs during thermal regulation testing.
The numbers you see in product names, such as 175, 200, 260, or 300, represent this Heat Index scale. As the number increases, the level of warmth increases.
Whether you're chasing alpine summits, layering for a chilly morning commute, or warming up for early trail runs, choosing the right warmth level helps you stay comfortable in changing conditions.
Understanding base layer warmth levels
In the outdoor industry, base layers are often described using lightweight, midweight, and heavyweight categories to help explain how warm a layer will feel.
You may also see these categories compared with fabric weight ranges, which many outdoor brands describe using GSM (grams per square meter).
Rather than labeling garments by fabric weight alone, icebreaker uses the Heat Index system to organize base layers based on their overall warmth and performance in cold conditions.
This approach helps you choose the right layer based on activity level, temperature, and layering needs, instead of relying on fabric weight alone.
Heat index guide: choosing the right base layer
|
Heat Index |
Weight Class |
Best For |
Featured Styles |
|
75 |
Ultralight |
High-output training, warm climates |
75 Cool-Lite™ Featherlight™ |
|
125 |
Lightweight |
Fast-paced winter hikes, aerobic activity |
ZoneKnit™ (125) |
|
175 |
Lightweight |
Mild winters, indoor/outdoor movement |
175 Everyday |
|
200 |
Midweight |
All-season layering, cool temps |
Oasis, ZoneKnit™, MerinoFine™ Ace |
|
260 |
Heavyweight |
Cold winter conditions, low-movement moments |
Tech, ZoneKnit™ Seamless, Tech Vertex |
|
300 |
Extra Heavyweight |
Extreme cold, static pursuits |
300 MerinoFine™ Polar |
Base layers by condition
cold conditions (260-300 Heat Index)
When temperatures drop below freezing, you need reliable warmth that doesn’t rely on synthetics. These heavyweight base layers are built for maximum insulation, made from 100% merino wool for natural softness, warmth, and breathability.
Best for: Alpine starts, winter camping, polar climates, ice climbing
Top picks:
• 300 MerinoFine™ Polar Long Sleeve Roll Neck Thermal Top Premium warmth, crafted from 15.5-micron superfine Merino.
• 260 ZoneKnit™ Seamless Long Sleeve Crew Thermal Top Breathable zones and a second-skin fit for dynamic cold-weather activity.
Mild conditions (200 Heat Index)
When conditions vary or activity levels shift, a midweight base layer is your most adaptable option. These 200 layers deliver the right balance of warmth and breathability, ideal for winter sports, travel, or everyday layering.
Best for: Resort skiing, winter hiking, layering under outerwear, mixed conditions
Top picks:
• 200 Oasis Our all-season original. Reliable comfort, performance-ready.
• 200 Oasis+ Suit One-piece base warmth, tested by elite explorers.
• 200 ZoneKnit™ Designed for movement with breathable mesh zones.
• 200 MerinoFine™ Ace Lightweight, ultrafine Merino for sleek performance.
Warm conditions (75-175 Heat Index)
Not every winter day is freezing. For mild climates or high-exertion activity, go light. These ultralight and lightweight base layers are made to manage sweat vapor, regulate body temperature, and support movement.
Best for: Trail running, warm-weather travel, indoor workouts, layering flexibility
Top picks:
• 175 Everyday Classic comfort and year-round versatility.
Body-mapped breathability for uphill efforts and indoor training.
• 75 Cool-Lite™ Featherlight™ The lightest base layer we offer. Moves with you, disappears on skin.
How to choose the right base layer
Ask yourself:
• How cold will it be? Colder conditions call for 260 or 300, for freezing temps.
• How much will you move? More movement means lighter weight. Choose 75 to 175 for high-output activity.
• How often will you wear it? Daily wear calls for versatility. 175 Everyday and 200 Oasis are your all-rounders.
• Want to avoid synthetics? All icebreaker® base layers are made with natural Merino wool, not petrochemical fabrics.
Built for performance. grounded in nature.
Every icebreaker® base layer is made with Merino wool, a renewable, biodegradable fiber that:
• Naturally thermoregulates to adapt to your environment
• Manages moisture vapor to help you stay dry-feeling
• Resists odor buildup, so you can wear more and wash less
• Feels soft against skin, with no synthetic itch or plastic feel
This is natural performance, engineered by nature and refined for your pursuits. Explore the full range of merino wool base layers and stay warm naturally.
