Simply put, in a perfectly waterproof-breathable environment, you would not shiver or sweat. The fabric is smart enough to stay dry. The hitch: The more waterproof a fabric is, the more it sacrifices breathability, and vice versa. The optimal balance between the two is the Holy Grail of technical performance fabrics.
"Waterproof" and "breathable" are relative terms. "You're waterproof and so is a submarine," says Kurt Gray, a fabric-technology expert. "The difference is by degrees." For instance, a fabric might keep you dry in a downpour but get soaked the second you kneel on a wet surface. But it's still considered waterproof.
Waterproof-breathable is achieved by either laminating or coating a thin membrane onto the inside of a fabric. "Coatings are like painting a wall," Gray explains, "and laminates are like wallpaper." Laminates (like Gore-Tex) are generally more uniform but can make fabrics stiff and heavy. Coatings are less uniform, but can achieve a higher level of breathability and make for softer fabrics.
Waterproof-breathables have a Durable Water Repellent (DWR) finish. The DWR causes moisture outside the fabric to bead up and roll off. While the DWR can impair breathability, it also, ironically, is key to helping the garment maintain its breathability in wet conditions: Without a DWR, even a waterproof fabric can become saturated, trapping moisture inside. Since a DWR keeps the fabric dry, it helps moisture escape, too.¿b.c.
Dressing smart: Even the newest, techiest fabrics have their limitations. To overcome them, consider:
Fit: With waterproof-breathable technologies, the closer to the body, the better. It has to do with physics¿the pressure gradient¿and how far the moisture has to travel before it reaches the surface of the fabric, where it can evaporate. If your clothing is too loose, your sweat will condense inside.
Layering: What you wear under your shell has a huge impact on the shell's performance. You want undergarments that wick moisture from your skin and an insulating layer that can transport moisture. If you wear a cotton T-shirt and a sweatshirt, they'll soak up your perspiration, making your high-tech shell as effective as the nylon windbreaker you wore in eighth grade.
Features: You can still sweat in a breathable garment, even with wicking layers underneath. And when you overload a garment's vapor-transmission capability, venting is key. Look for pit zips and mesh-lined venting pockets, and make sure they're placed where your pack straps won't cover them.