Short answer
Both are breathable, and neither is simply more breathable in every case. Wool tends to manage moisture and temperature better in changing or cold conditions. Cotton tends to let air through freely in hot, dry weather. The weight and construction of the fabric often matter as much as the fiber.
What “breathable” actually means
People use the word for two related things:
- Airflow: how easily air passes through the fabric.
- Moisture handling: how well the fabric deals with sweat vapor and liquid so you do not feel clammy.
A fabric can do well at one and poorly at the other, which is why wool and cotton are often compared with mixed results.
How wool handles air and moisture
Wool fibers can absorb moisture vapor into the fiber itself, before it turns into liquid sweat. This is often cited as the reason wool can feel less clammy against the skin. Wool also tends to keep insulating when it is damp. The trade-off is that a dense or heavy wool knit allows less airflow than a light, loose fabric, and it can feel too warm in hot weather.
How cotton handles air and moisture
Cotton absorbs liquid moisture easily and feels cool and airy in dry heat, especially in loose or open weaves. Once it is soaked with sweat or rain, though, it tends to hold on to the moisture and dry slowly, which can feel damp and cool. That is the main reason cotton is often not recommended for strenuous activity or cold, wet conditions.
Construction matters as much as fiber
Fabric weight and how tightly it is knit or woven strongly affect airflow. A loose cotton shirt generally lets more air move than a heavy, dense wool sweater, while a lightweight merino knit can be quite airy. When comparing, look at the weight and structure, not just the fiber name. Finishes, blends, and fit also change how a garment feels.
When each tends to feel better
- Wool: variable weather, active use, travel, cool to mild conditions, and wearing the same garment for longer.
- Cotton: hot, dry weather, casual everyday wear, and light, loose clothing.
Is merino warmer than cotton?
Generally yes. Merino traps air and keeps insulating when damp, while cotton provides less insulation and feels cooler once wet. Lightweight merino is still worn in warm weather by many people, so “warmer” does not mean unsuitable for summer. How warm either fabric is depends on thickness and construction.
Wool vs linen breathability
Linen is usually considered very airy in hot weather because of its open, crisp structure (see linen fabric for hot weather), and it tends to be a popular warm-weather fabric. Wool is not usually the first pick for the hottest days, but it handles moisture and temperature swings differently. If you want maximum airflow in heat, lightweight linen or cotton are the usual choices.
Disadvantages of 100% cotton for breathability
- Slow to dry once damp.
- Can feel clammy or cool when wet.
- Little insulation when damp.
- Dense weaves, such as heavy denim, allow less airflow than lightweight ones. Different cottons also vary; see types of cotton for T-shirts.
Quick picks
- Hot, dry day, casual wear: lightweight cotton or linen (see the best shirt fabric for summer).
- Hiking, travel, or changing temperatures: lightweight merino.
- Cold weather: wool.
- Not sure: choose by the weight and weave of the specific garment.
For a broader view, see our guide on which is better, wool or cotton, or compare a premium cotton with fine wool in pima cotton vs merino wool.