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The water column: wet weather protection from your rain jacket to the tent

How everything stays dry thanks to matching value

© Klättermusen

You probably also know the old truism that there is no such thing as bad weather, only bad clothing. Although this saying is sometimes a bit annoying, it holds a lot of truth, because when it's rainy outside, only the appropriate layer of clothing reliably protects you from wetness and cold.

The resistance of a garment to water penetration is usually measured in terms of water column – but what is that and how does it affect your mood when you come home at the end of a rainy day? Here you can find out what the water column is all about, and what you should look out for when buying a rain jacket, ski pants or camping equipment.

Why the water column is so important

A relaxing walk in the woods through the country rain, the ambitious skiing holiday with powder-like snow, or a stormy camping weekend by the lake: The number of outdoor adventures that bring you into contact with all things water are endless. To ensure that you don't get wet (and cold) feet in any of these situations, the water column is an important purchasing criterion for clothing and equipment.

The water column distinguishes "waterproof in theory" from "waterproof in the wild". Just because a material is labelled as waterproof doesn't mean you're guaranteed to stay dry – because the degree to which a fabric will keep the wet out, and deal with environmental stresses, varies. Information about this is provided by the water column. It should, therefore, be a decisive purchase criterion.

What does "water column" mean, and what does it say?

The water column is very important if you want to stay dry for a long time. In general, the water column describes the pressure that a textile fabric can withstand without water getting on the inside – in other words, it tells you what stresses a fabric can really withstand in terms of its impermeability.

Example: A child will stay dry in a pair of snow pants with a 4,000 mm water column until the last sled run, while your bottom will be wet in an identical model before you've tightened your snowboard boots in the snow. This is because an adult person weighing 80 kg exerts much more pressure on the fabric when sitting than a child weighing 20 kg.

Therefore, the required water column always depends on one's own weight, the load level and the external circumstances. However, in order to assess these factors correctly, it is advantageous to know the measurement methods.

The value of the water column is usually given with the unit of mmWC (column height in millimetres). © Klättermusen

How is the water column determined?

There are several methods to calculate the water column. Generally, the water pressure on a textile is increased step by step in a laboratory test – until the first drops are pressed through to the inside. The decisive factor here is the time that elapses until penetration. The longer the fabric withstands the increasing pressure, the higher the water column.

If you buy products from the USA, you can multiply the given value by a factor of 2-3 before comparing it with European values – because in the USA a substance is artificially aged by 5 years before being tested.

The name water column is, therefore, meant literally: To increase the pressure, an empty column is placed on the fabric and filled to an increasingly high level with water. The water column is indicated with the unit of mmWC (column height in millimetres) or mWC (column height in metres). The alternative designations mH2O or mmH2O are also often found.

1 mWC or 1,000 mmWC corresponds to a pressure of just under 0.1 bar.

However, as there are different standards, conditions and measurement procedures depending on the test laboratory, you should consider the water column more as a rough guide and less as an exact measurement – if the same fabric is measured by different manufacturers, the ratings can differ significantly. Apart from that, conditions such as air pressure, temperature, motion sequences or material wear have a completely different impact in practice than under laboratory conditions.

In Germany, garments with a water column of 1,300 mm or more are already considered waterproof and may be labelled as such.

Your rain jacket should have a water column of at least 10,000mmWC to keep you dry through the day. © Klättermusen

What water column should a rain jacket have?

That depends mainly on the conditions under which you use them. In general, high-quality rain jackets are usually equipped with 10,000 mmWC as standard – this is already a very good water column rating for a rain jacket for hiking. Even for winter sports, a 10 m water column rating for the ski jacket will keep you safe and comfortable on the mountain and the slopes.

If, on the other hand, you are travelling under extreme conditions, for several days or with a lot of luggage, it may well be more: Here, 20,000 mmH2O is recommended. Especially if you are carrying a heavy backpack, the pressure on your shoulders can be very high. In this case, a well-equipped hardshell jacket is the right choice.

When buying a tent, you should pay attention to the water column specifications of the tent wall and floor. © Klättermusen

Which water column for the tent?

When buying a tent, you often come across not one, but two specifications for resistance: on the one hand, the heavily stressed tent floor and, on the other, the tent walls, which are subject to less stress. To ensure that the entire tent fabric remains waterproof, you should choose at least 4,000 mmWC for the floor. For the outer shell, on the other hand, a 3,000 water column is also sufficient.

Field of application and water column – an overview

Here we have summarised the recommended water column for various products as a table for you:

EquipmentRecommended water column
Rain jackets (without backpack load)min. 10,000 mmWC
Rain jackets (with backpack load)min. 15,000-20,000 mmWC
Tents floor / outer shellmin. 4,000 mmWC / min. 3,000 mmWC
Rain trousersmin. 15,000 mmWC
Ski jacketsmin. 10,000 mmWC
Ski trousersmin. 15,000 mmWC
Backpacksmin. 15,000 mmWC

The water column above which equipment is waterproof depends decisively on the exact type of equipment. Especially with tents and backpacks, however, it is not only the fabric itself that is important, but also the design: The best membrane becomes useless if water can get inside through leaking seams, flaps or folds.

Rain jackets with 20,000 mmWC water column

When buying waterproof clothing, also look for breathability © Klättermusen

The relationship between water column and breathability

Waterproof fabric with a high water column is inherently less breathable than airy, permeable natural materials. But that doesn't mean you're doomed to sweat: It is enough to pay special attention to the information on breathability. Two values provide information on this.

The MVTR value is used to determine how vapour permeable a garment is – the higher the value, the better the water vapour can pass from your skin through the fabric to the outside. A garment is considered very breathable if it has a minimum value of 10,000 g/m²/24h. Below 3,000 g/m²/24, on the other hand, little or no vapour is allowed to dissipate.

To avoid excessive sweating, all layers must be breathable – but only the top layer must be truly waterproof.

The RET value, which is also often used, is given in numbers from 20 to 0. Here, you should pay attention to the lowest possible value of 0 to 6, because, in this range, textiles are classified as particularly breathable. Values of 6 to 10 are still sufficient if you are doing normal activities such as walking or moderate hiking.

To maintain optimal breathability, you should also wash outdoor and functional clothing regularly according to the care instructions. In this way, the pores are freed from skin grease, sweat or dust.

Weak points you should look out for

Whether clothing or equipment: Most processed textiles have areas that are more susceptible to water penetration than others – but even leaky seams or a few centimetres of damaged membrane are enough to make you feel uncomfortable and clammy or to get the contents of your rucksack wet, despite the high water column. Therefore, pay attention to these weak points when buying rain and snow clothing:

  • All seams & zippers should be sealed / taped / glued
  • A wind flap on the front zip provides additional protection
  • Rain jackets should be designed for the backpack load
  • Make sure you have extra snow or rain protection on your sleeves if necessary & legs

To keep your equipment and water column in top condition, you can regularly refresh DWR coatings and waterproofings. On the one hand, washing machine waterproofing agents are suitable for this purpose, which are put into the laundry with your gear, but, on the other hand, you can also use waterproofing spray or special wax. Always follow the care instructions carefully to achieve the best possible results and avoid damaging the membrane.

The water column for GORE-TEX products

If you're looking for water-repellent and breathable clothing, the name GORE-TEX is simply unsurpassed: The special membrane has pores that are large enough for water vapour but too small for raindrops. The result: The humid air heated by the body heat gets out almost unhindered – but the water in its pure form does not get in.

You can always recognise waterproof GORE-TEX products by the black brand logo – those with white labels, on the other hand, are not waterproof.

GORE-TEX textiles, therefore, also have a high water column and are particularly breathable despite their windproof function. Different target groups are addressed with different product categories. For example, competitive athletes with little luggage are best served by the GORE-TEX Active range, while Gore-Tex Pro offers particularly high resistance and durability. The brand provides you with several product categories:

  • GORE-TEX: classic, most affordable standard membrane
  • GORE-TEX Active / Shakedry: lightweight & especially breathable
  • GORE-TEX Pro: hard-wearing & robust for high-level demands
  • GORE-TEX Paclite: particularly small & light pack size

The choice of products should, therefore, depend on what you intend to do with your gear, and which functions are particularly important to you. Be honest with yourself, because top gear does not always give you the decisive advantage: On a half-day tour with light rain showers, a standard model keeps you just as dry as the high-end expedition equipment with a 30,000 water column.

Membranes generally lie under the top layer of fabric. To prevent so-called wetting-out (i.e. soaking of the outer layer), DWR waterproofing with a beading effect helps.

Of course, you don't necessarily have to use GORE products for a good combination of breathability and high water column, as some other brands are now competing with Primus: Popular waterproof alternatives are, for example, the non-porous climate membranes from Dermizax or the environmentally friendly PFC-free Sympatex membrane, which is particularly suitable for packing.

At a glance: The following should be observed with regard to the water column

  • Choose the water column to suit the activity, load and weather
  • The higher the load, the higher the water column must be
  • Also look out for weak points such as seams or zips
  • High breathability as with GORE-TEX membranes is particularly important
  • Regular washing and re-waterproofing maintain the wetness protection in the long term

Conclusion: Needs-based selection is the key

The water column is a decisive factor in whether and how long you stay dry in rain or snow. It is specified in mmWC or mWC – the higher the value, the higher the resistance to water. Finding the right rain gear with the optimal water column can be overwhelming, especially for outdoor beginners. However, if you match your equipment precisely to the intended use and your personal needs, you will quickly find the ideal companion through stormy and rainy times – so that no weather will ever get you down in the future.