What to Wear Under Merino Base Layers: Bras and Underwear

Quick answer

Wear a thin, smooth, moisture-wicking bra or bralette and merino or synthetic underwear directly under your merino base layer. Skip cotton, thick molded cups and bulky seams because they trap sweat, bunch under a close-fitting top and slow drying. If you need extra warmth, add a thin merino liner next to skin, then your base layer, then insulation.

Why your first layer matters more than your midlayer

Merino base layers work by moving moisture away from skin and regulating temperature, but they can only do that if the layer touching your skin cooperates. Cotton holds water, so a cotton bra or briefs keep you damp and cold once you stop moving. Smooth, thin synthetics or merino next to skin let the base layer sit flat and dry faster.

This matters most for winter sports and cold-weather travel, where you may wear the same system for hours. A bulky bra or high-waisted briefs that roll down create pressure points under a snug thermal top and can leave you colder at the waist and shoulders.

Choose underwear that disappears under leggings

Under merino leggings or a base layer bottom, the goal is a flat, low-profile fit with no visible edges. Look for thin waistbands, laser-cut or bonded edges, and fabric that dries quickly. Merino underwear is a strong choice because it resists odour naturally, which matters on multi-day trips when you are not washing every night.

If you prefer synthetic underwear, pick a smooth, lightweight pair rather than a thick seamless style with heavy elastic, which can still show through and trap heat. Test the pairing at home: bend, squat and twist. If the waistband rolls or the leg edge digs in, it will only get worse on the mountain.

Pick a bra that layers cleanly

For a sports bra under a thermal top, choose a thin, wire-free design with smooth cups and minimal seams. Molded or padded cups add bulk and hold sweat, and they can create a gap between the bra and the base layer that feels cold. A simple bralette or compression-style sports bra in a wicking fabric usually layers best.

If you need more support, look for a bra with a wide, flat underband and adjustable straps that sit flush under the base layer. Avoid hooks or adjusters on the back that can press into your spine when you lean against a pack. For very cold conditions, a merino bra or a thin merino camisole under the base layer adds warmth without much bulk.

When to add a merino liner layer

A liner is a very thin next-to-skin layer worn under your main base layer. It is useful when you run cold, when temperatures drop well below freezing, or when you want to extend a lighter base layer into winter. Merino liner layers explained simply: they add warmth and moisture management without the weight of a midlayer.

Do not add a liner just because you can. Each extra layer adds bulk under a close-fitting top and can restrict movement. If your base layer already fits snugly and you are warm enough, stop there. If you are layering merino for winter sports, start with a liner only on the coldest days and remove it if you start to overheat.

How to layer under a merino base layer, step by step

1. Start with clean, dry skin: Apply any creams or sunscreen well before dressing so they absorb. Moisturiser residue can make a base layer cling and feel clammy. Checkpoint: skin should feel dry to the touch before you add the first layer.
2. Put on a thin, wicking bra or bralette: Choose wire-free, smooth cups and flat seams. Adjust straps so they sit flush, then smooth the band so it does not roll. Common mistake: wearing a padded or cotton bra, which traps sweat and creates cold gaps under a snug thermal top.
3. Add merino or synthetic underwear: Pick a low-profile pair with a thin waistband that sits below the base layer waistband. Smooth the edges flat against your skin. Checkpoint: bend and squat. If the waistband rolls or digs in, switch to a thinner pair before you head out.
4. Add a merino liner only if you run cold: A very thin liner goes directly over your bra and underwear, under the main base layer. Keep it smooth and untwisted. Common mistake: adding a liner on mild days, which causes overheating and extra bulk under a close-fitting top.
5. Pull on the merino base layer and smooth it flat: Work from the hem upward, pressing out folds at the waist, underarms and back. Make sure the bra band and underwear waistband are covered and flat. Checkpoint: raise your arms and twist. No bunching or exposed edges means the system is ready for a midlayer.
6. Test the full system before you commit: Wear the complete setup for 15 to 20 minutes indoors, then step outside briefly. Note any pressure points, cold spots or damp areas. Next step: adjust one layer at a time, starting with the bra or underwear, before changing your base layer or midlayer.

Products that work well in this layering system

• Women's Merino 175 Everyday Long Sleeve Crewe Thermal Top: A classic, all-purpose base layer T-shirt made with soft and breathable 100% merino wool fabric. Its simple, smooth construction makes it a reliable outer layer over a thin bra or liner, and the breathable merino helps regulate temperature during stop-start activity.
• Women's Merino 150 Siren Thong: Soft and simple underwear that offers minimalist comfort, made with Merino Corespun fabric that is naturally odour-resistant and soft next to skin. A low-profile merino option that resists odour on multi-day trips and sits flat under leggings or a base layer bottom.