THE KNOWLEDGE

NUMBER CRUNCHING

Waterproof fabrics, whether own-brand or made by a third party like Gore-Tex,

usually undergo extreme lab and field tests. You will often see they have a hydrostatic head or water column rating. Usually expressed in mm, this is determined by applying pressurised water to the fabric until it leaks. To be classed as waterproof (as defined by the British Standard) it must be 1500mm minimum. Most fabrics far exceed this. It’s one very geeky way to measure the relative effectiveness of waterproof fabrics, but it doesn’t simulate dynamic, real-world conditions like wind-driven rain or the jiggling of a heavy rucksack. Neither does it reflect durability.

Another figure often quoted is the moisture vapour transmission rate,

sometimes referred to as breathability.

Obviously, a fabric doesn’t actually breathe. It’s not alive. What this number describes is the rate at which water vapour (from sweating) passes through one square metre of the fabric. It’s measured in grams over 24 hours. Sometimes the denier of the face fabric is listed too, denoting the fibre thickness. Depending on the material, a higher denier fabric tends to be stronger.

CARE AND REPAIR

Over time you may notice your jacket begins to feel muggy and wet inside. Water doesn’t ‘bead’ and sluice down the face fabric as well as it did when new. It’s time to wash and re-proof.

Always consult the care label first. Washing a jacket using a dedicated cleaner removes built-up grime which impedes vapour transfer (aka ‘breathability’). A low-heat tumble dry can revitalise the face fabric’s durable water repellent. A spray-on or wash-in re-proofer renews this rain-shedding coating. Look for products made by Nikwax, Grangers or Storm.

A small rip or frayed seam tape is easily fixed. Seam sealers, repair tapes and patches made by the likes of GearAid and Stormsure can give your jacket a new lease of life.