Gram for gram, Merino wool insulates better than polyester fleece and manages moisture far more effectively, absorbing up to 35% of its own weight while still feeling dry against the skin. Polyester fleece was invented in 1981 as a cheaper alternative to wool - not a warmer one - and the warmth reputation it's built since is largely down to marketing, not physics.
Part of our series on wool performance - start with The New Performance Standard: Why Wool Wins.
Why do people assume synthetic fleece Is warmer?
Polyester fleece, developed by Malden Mills in 1981 under the Polartec name, was built to solve cost and weight, not warmth. It's cheap to produce, dries quickly, and became the default midlayer for a generation of outdoor brands - so it's simply what most people have worn. Familiarity gets mistaken for superiority.
How does Merino actually trap heat?
Wool fibre has a natural crimp - a wave-like structure - that traps pockets of air between fibres, and it's trapped air that provides insulation, not the fibre itself. Merino's fine 20-micron fibres pack more of these air pockets per gram than the straighter, smoother strands of synthetic fleece, giving it a genuine warmth advantage at equivalent weight.
What happens when either fabric gets damp?
This is where the gap widens. Merino wool absorbs up to 35% of its own weight in moisture while still feeling dry to the touch, because the moisture is drawn into the fibre's core rather than sitting on the surface. Polyester absorbs almost none - any moisture, whether sweat or rain, sits on the surface, and once that surface is wet, the garment's insulating ability drops sharply.
Does synthetic fleece dry faster?
Yes, since it barely absorbs moisture in the first place. But drying speed isn't the same as staying warm while wet, which is the condition that actually matters outdoors. A polyester fleece that dries in twenty minutes is still cold and clammy for those twenty minutes; a Merino layer stays functionally warm throughout, because the fibre itself is doing the insulating, not just the air around it.
Which wins for genuinely cold, wet conditions?
Merino, consistently. It's also naturally odour-resistant, since the fibre's structure doesn't give bacteria the same surface to multiply on that synthetic fibres do - which is why a Merino layer can be worn for days without the fleece funk synthetic garments develop after a single hard session.
Related reading: The New Performance Standard: Why Wool Wins · Is Merino Wool Itchy? The 20-Micron Difference Explained · Woven vs Knitted: Why It Matters for Durability and Warmth