“Breathable water” in materials refers to a fabric’s ability to block liquid water while allowing water vapor, such as sweat, to pass through. This combination of waterproofing and breathability is achieved through engineered textile structures and membranes. It enhances comfort and performance in various products and environments.
How Breathable Waterproof Materials Work
Breathable waterproof materials use a selective barrier that differentiates between liquid water and water vapor molecules. They primarily utilize two types of membranes: microporous and non-porous.
Microporous membranes feature billions of tiny pores, too small for liquid water droplets to pass through. These pores are large enough for individual water vapor molecules to escape from the inside. This allows sweat vapor to diffuse outwards while preventing rain or external moisture from entering.
Non-porous hydrophilic membranes operate differently, as they do not contain pores. These membranes absorb water vapor from the high-humidity environment inside the garment. Water molecules then diffuse across the membrane to the lower-humidity exterior, where they are released as vapor. This process relies on a water vapor pressure differential, where higher moisture inside drives vapor movement to the outside.
Common Breathable Waterproof Technologies
A technology for breathability and water resistance involves expanded polytetrafluoroethylene (ePTFE) membranes, famously utilized in products like Gore-Tex. These membranes are incredibly thin and contain billions of microscopic pores. The hydrophobic nature of ePTFE causes liquid water to bead up and roll off, while the tiny pores permit water vapor to escape.
Polyurethane (PU) coatings are another common approach, where a layer of resin is applied to the fabric surface. These coatings can be microporous, blocking liquid water but allowing vapor to pass, or non-porous and hydrophilic, absorbing and transporting water vapor. PU-coated fabrics are known for their flexibility, durability, and waterproof capabilities.
Many breathable waterproof materials are constructed as laminates, involving multiple layers fused together with heat and adhesive. These laminates typically consist of an outer “face” fabric, a membrane layer (e.g., ePTFE or PU), and sometimes a backer fabric for added durability and comfort. Two-layer laminates bond the membrane to the outer fabric, leaving the membrane exposed on the inside, often requiring a separate lining. Three-layer laminates, on the other hand, bond the membrane between the outer fabric and an inner lining, creating a single, more durable composite.
How Breathability and Water Resistance Are Measured
Water resistance in fabrics is quantified using the “Hydrostatic Head” test, which measures the maximum water pressure a material can withstand before water penetrates. A fabric sample is sealed under a tube, and water is gradually added, increasing pressure until leakage occurs. The height of the water column, measured in millimeters (mm), indicates the fabric’s waterproof rating; a higher number signifies greater water resistance, with typical mid-range fabrics having values around 5,000 mm and high-performance ones exceeding 20,000 mm.
Breathability is commonly assessed by two primary metrics: Moisture Vapor Transmission Rate (MVTR) and Resistance to Evaporative Transfer (RET). MVTR measures the amount of water vapor, in grams, that can pass through a square meter of fabric over a 24-hour period (g/m²/24hrs). Higher MVTR values indicate better breathability, with a rating of 10,000 g/m²/24hrs considered very breathable and some reaching up to 40,000 g/m²/24hrs.
The RET value, on the other hand, quantifies a material’s resistance to water vapor evaporation, typically measured using a “Sweating Guarded Hot Plate” test. A lower RET value signifies higher breathability, meaning less resistance to moisture transfer. For example, an RET value between 0-6 indicates extremely breathable fabric, while values above 20 suggest low breathability.
Practical Applications and Evolution of Breathable Materials
Breathable waterproof materials extend beyond outdoor clothing, finding diverse applications across various industries. They are widely used in footwear, such as hiking boots and athletic shoes, providing protection from external moisture while allowing internal perspiration to escape. These materials also appear in medical textiles, including surgical gowns, wound dressings, and hygiene products, where moisture management and barrier properties are important. Industrial uses for these advanced fabrics include protective military wear, firefighting garments, and specialized cleanroom clothing, where maintaining comfort and safety in demanding environments is paramount. Beyond apparel, breathable waterproof membranes are integrated into construction materials like roofing, as well as electronic enclosures and battery packaging to prevent condensation and protect sensitive components.
The development of breathable waterproof materials has seen continuous innovation, with a growing emphasis on sustainability. Historically, many of these materials relied on per- and polyfluoroalkyl substances (PFAS) for their water-repellent properties. However, due to environmental and health concerns regarding their persistence, the industry is increasingly shifting towards PFAS-free alternatives for both membranes and durable water repellent (DWR) finishes.