Waterproofing is essentially the process of making a garment waterproof. This is achieved by the construction of the garment and by applying a waterproof finish to the outer layer. In this guide we delve into how waterproofness is measured, how garments are made waterproof, explain the different levels of waterproof and provide advice on how to look after your waterproof items.
How is waterproofness measured?
All waterproof garments are given a waterproof rating which relates to the fabrics ability to resist letting water through when put under pressure. This is most often measured in laboratory conditions but manufacturers may also do their own testing. Waterproof ratings can be expressed in two ways- as mm or pounds per square inch (PSI).
Hydrostatic Head Test- mm
In a hydrostatic head test a section of fabric is pulled tight over a clear 1 inch by 1 inch square tube and sealed. The tube is then slowly filled with water and monitored over 24 hours to see how much water can be added before the material begins to let water through. A rating of 2,000mm means the tube of water was 2,000mm tall when the material started to let droplets through. In other words the fabric can withstand 2,000 mm of rain in a day. In the UK a minimum hydrostatic head rating of 1,500mm is required for a garment to be classed as waterproof. In simple terms the higher the number the more waterproof the material is. See the video below for more details.
Pounds per Square Inch- PSI
Some manufacturers will use the Pounds per Square Inch (PSI) measurement rather than millimetres to determine how waterproof a garment is. To be classed as 100% waterproof the garment has to have a minimum of 3 PSI by British Standard. Most waterproof fabrics including Gore Tex will however have a rating of 40 PSI.
There are a couple of ways to make a jacket or pair of trousers waterproof and breathable. The garment will either have a waterproof membrane bonded to the underside of the outer layer (often called the face fabric) or the interior of the fabric will be coated in a liquid solution. A DWR treatment will be applied to the outside of the fabric in both cases.
Waterproof Laminates (Inside)
Gore Tex and eVent are the most recognised laminates. In these garments a membrane or laminate usually made of ePTFE (expanded Polytetrafluoroethylene, also known as Teflon®) or PU (Polyurethane) will be bonded to the inside of the face fabric.
The membrane will consist of tiny holes too small to let water enter but large enough to allow water vapor to escape. Oil, sweat and chemicals can causes PTFE membranes to lose their ability to keep out water, so the membrane is protected by an ultra-thin layer of Polyurethane or other oleophobic (oil-hating) treatment.
In a 2 layer jacket a separate fabric liner usually made from mesh is stitched to the inside of the jacket but in a 3 layer jacket the membrane is sandwiched between the face fabric and liner.
Waterproof Coating (Inside)
A waterproof coating, using polyurethane (PU), is applied to the inside of the fabric replacing the laminate. HyVent (used by North Face) is probably the most recognised waterproof coating.
Generally a coated waterproof is less durable and less breathable than a membrane waterproof.
Durable water repellency/DWR (Outside)
All waterproof and water resistant garments will be treated with a DWR (durable water repellency) coating on the outermost layer (or the face fabric). If a garment has been treated with a DWR water will bead up and roll off a garment rather than saturating the material.
A ‘wet out’ is when water saturates the outer layer of jacket or trousers rather than being repelled. This is normally a sign that the DWR needs to be restored. Dirt and oil and abrasion can break down the DWR over time so it needs to be reapplied so as not to affect the garments performance.
A DWR will often be used alongside a waterproof membrane to increase the garments waterproof and breathable properties. A jacket that just has a DWR and no waterproof membrane will be cheaper than a membrane jacket but won’t be suitable for conditions where heavy rain is likely.
With wear your waterproof garments will lose their ability to repel water as the DWR is worn away or the membrane is blocked by dirt and oils.
When it comes to the aftercare of your waterproof products there are a few different options. It is important that you wash your waterproofs with a specialist cleaner designed for waterproof wear before reproofing where possible.
For clothing you should wash with a technical cleaner and then either spray your reproofer on use a wash in one. Waterproof sprays tend to work best on clothing with a waterproof coating, whereas wash in proofers are more suited to those with a waterproof membrane. Wash’n’proofs are also available which save you time and hassle by cleaning and reproofing your waterproof garments in one wash.
When choosing a waterproof jacket or trousers it is important to understand the limitations of the garment you choose. For example, a jacket with a hydrostatic head rating of 2,000mm is not going to withstand a heavy, prolonged downpour where a jacket with a rating of 20,000mm is designed for these conditions.
If you do not look after your waterproof garments then their lifespan will be significantly reduced and they will not perform to their highest level. A clean jacket will work much better than one that has not been cleaned in years.
If you will be wearing your jacket for active pursuits you need to consider the breathability. It is common for wearers to complain a jacket has ‘leaked’ when in fact it is their perspiration they are feeling. Remember a breathable jacket will only work if it is teamed with breathable layers underneath.