What is hydrostatic head material?

Answer 1

The European Standard a fabric must meet to be classified as waterproof is a hydrostatic head rating of 1,500mm. Hydrostatic head is a fabric's resistance to water pressure equivalent to a column height of water. Whilst a hydrostatic head of 1,500mm will keep out heavy rain, it will not stand up to the increased pressure put on a fabric in general use and by rucksacks etc. On average, outerwear fabrics tend to have a hydrostatic head of 10,000mm and above. Classic Gore-Tex fabric rates 28,000mm and 3 layer Gore-Tex XCR fabric rates 45,000mm! For a garment to be classified as fully waterproof, all seams must be factory sealed or taped on the inside of the fabric (or water penetrates stitch holes). Note that pockets on waterproof jackets aren't necessarily waterproof, check specific details to be sure. In addition, fabrics may have a durable water repellent (DWR) outer coating. Water repellency is a fabric's ability to make water "bead" and roll-off. Effective DWR is desirable in a fabric as water soaking into the outer fibres will inhibit breathability. DWR coatings can and do wear or wash-off (they can be renewed or re-activated) whereas waterproof inner coatings or membranes will to a greater or lesser degree last the lifetime of the garment. A completely waterproof fabric that does not allow passage of the moisture-laden vapour generated by the body during exercise will quickly saturate the wearer as moisture condenses inside. High levels of shell fabric breathability are key to all day comfort and the best outerwear fabrics have both high water resistance and high breathability. Breathability is measured in two ways. Firstly as a rating in grammes of how much vapour a square metre of fabric will allow to pass through in 24 hours (gm/m2/24). The second method (used by Gore-Tex fabrics amongst others) is RET (evaporative resistance of a textile). Lower RET = better breathability. The more active a user's aspirations, the higher the breathability required. For general recreational use, 10,000gm/m2/24 and above is desirable. More active use and backcountry situations required far higher breathability. New outerwear solutions have levels of breathability not achievable in conventional hard shells.