Thermal Underwear: Merino vs Synthetic (Which Keeps You Warmer?)

The short answer

Merino wool thermals are the best all-rounder: warm, breathable, soft (in fine grades) and naturally odour-resistant, so you can wear them more than once. Synthetic thermals (polyester or polypropylene) dry faster, are cheaper and more durable, but start to smell quickly. For skiing, hiking and travel, choose merino; for short, sweaty sessions and tight budgets, synthetic works well.

Searches for thermal underwear in Australia peak from May to July. Here is how to choose, from more than 14 years of working with wool and synthetic fabrics.

How base layers keep you warm

A thermal layer does two jobs: it traps a thin layer of warm air against your skin and moves sweat away, because damp skin loses heat fast. The fibre decides how well it does each job.

Merino vs synthetic

Feature	Merino wool	Synthetic (polyester / polypropylene)
Warmth for weight	Excellent	Good
Warm when damp	Yes, wool stays warm when wet	Less so, but dries faster
Drying speed	Medium	Fast
Odour resistance	Excellent; wear several days	Poor; smells after one use
Softness	Soft if fine (under about 19.5 microns)	Smooth, can feel slippery
Durability	Good; thinner fabrics can hole	Very good
Price	Higher	Lower

Fabric weight (g/m²)

• 150–170 g/m² (lightweight): active use, mild winters, layering under a shirt.
• 200–250 g/m² (midweight): the everyday winter choice for cold mornings, travel and skiing.
• 250 g/m² and up (heavyweight): very cold conditions and low activity.

Micron count: why some wool itches

Merino fibres are measured in microns (thousandths of a millimetre). Under about 19.5 microns, wool feels soft against skin. Coarser wool, above about 25 microns, has fibre ends stiff enough to prickle. Good merino thermals are usually 17–19.5 microns.

Caring for thermals

Merino: wash on a wool cycle at 30 °C with a wool detergent, and dry flat. Many merino thermals are machine washable, but never tumble dry them. Synthetic: wash warm to remove odour build-up, and skip fabric softener, which blocks wicking.

Frequently asked questions

Is merino warmer than synthetic?

Weight for weight, merino is generally warmer, and it stays warm when damp.

Does merino itch?

Fine merino (under about 19.5 microns) rarely itches. Cheaper thermals with coarser wool can.

Can I wear merino thermals for several days?

Yes. Merino resists odour, so many people wear a base layer two or three days in a row when travelling.

For more on wool, see our guides to merino wool clothing and cashmere vs merino.