If you're planning to spend any quality time in the mountains then you're going to need some good quality waterproofs. You should have at least a jacket (and ideally overtrousers) manufactured from one of the range of modern synthetic breathable waterproof fabrics.
These fabrics do make a huge difference in terms of comfort when compared to cheaper non-breathable waterproof fabrics. Non-breathable fabrics will tend get wet on the inside due to condensation build up, regardless of how effectively they keep the rain out. These are usually constructed using Polyurethane (PU) coated nylon fabrics and can provide a budget alternative to breathable waterproof trousers where sweat is less of an issue. They are also perfectly adequate for children as they sweat less and grow out of their clothing quickly.
For upperwear, particularly if you are going to be active, it pays to get the most breathable waterproof shell you can afford. There is a huge selection of breathable waterproof fabrics around, some better than others. As a rule of thumb when it comes to waterproofs, 'you get what you pay for!'
The Market Leaders
The two most breathable fabrics available are the very well known 'Gore-tex' and 'eVent', which is quickly gaining a devoted following. Both fabrics rely on a membrane to keep the rain out and both fabrics rely on membranes manufactured from the same material, expanded PTFE (Polytetrafluoroethylene). This material has excellent hydrophobic (water hating) properties, but also allows moisture vapour to pass through it via millions of microscopic holes.
The dirt, oils and grime associated with outdoor activities, can easily clog these microscopic vents. To maintain breathability the ePTFE needs to be protected from becoming contaminated. This is where Gore-tex and eVent differ. Gore-tex relies on a PU moisture permeable, oleophobic, or oil-hating coating integrated with ePTFE. This does reduce the breathability slightly, but greatly increases the durability of the waterproof and breathable properties of the membrane.
The primary difference with eVent is that there's no PU layer. The PTFE protection is integrated into each individual pore. This means moisture vapour is vented much more efficiently and quickly, which means you stay drier and more comfortable. This is what eVent call 'Direct Venting'.
In practice, there is no doubt that eVent is the more breathable fabric. However Gore-tex and particular Gore-tex Proshell garments are still highly breathable and offer fantastic, proven durability for the most demanding mountain activities such as climbing and mountaineering. Also due to eVent's efficiency in venting, eVent can have a chilling effect in very cold weather. This is due to the fact that air, warmed by the body, isn't allowed to build up inside the jacket.
So for high energy activities such as climbing, running, cycling or if you just get really sweaty when out in the hills, then eVent is the fabric to go for. Its lightweight packability and great breathability also make it great for trekking and Alpine Climbing. It is first-rate for general hill walking and it excels in keeping you really comfortable and dry when you're working hard.
Gore-tex garments really shine when it comes to durability (perhaps with the exception of Gore-tex Paclite, which is primarily designed with packability and lightness in mind). Gore-tex garments have a long and proven track record of being used in some of the harshest climates around the globe. Gore-tex is considered to be not as breathable as the more recent eVent and it can begin to feel clammy when you work up a sweat. This is particularly noticeable in warm, wet weather (i.e a typical British Summer day). However, as mentioned before, this slower rate of breathability is a plus point in really cold weather.
Recent Gore-tex developments have attempted to rectify this by creating a new 'weave' style scrim layer. This allows the water vapour to pass through as a greater rate than previously. Whether the improved breathability is as efficient as eVent remains to be seen.
With both eVent and Gore-tex, 3-layer construction (where the PTFE membrane is sandwiched between layers of tough outer fabric) is the toughest and most durable method of construction. The other method is 2-layer construction where the PTFE membrane is bonded to the outer fabric, but is protected on the inside of the garment with a loose mesh or fabric liner. If the garment has a loose liner then it is most likely a 2-layer garment. These tend to not as long lasting as 3-layer garments and are generally heavier and warmer, making them more suitable for less active pursuits. A good example of a 2-layer jacket is the Berghaus Cornice, which is designed primarily with low-level walking and general country pursuits in mind.
3-layer garments are really the best choice for the more serious user. Mountaineers, climbers, winter mountain walkers or anyone who's going to be hard on their jackets wants to look seriously at 3-layer. A good example of a 3-layer Gore-tex jacket is the ArcTeryx Theta AR which is a great all-purpose jacket, designed for year-round hill walking, mountaineering, climbing and skiing. A good eVent alternative would be the Montane Superfly XT which, of course, has all the advantages of a Gore-tex jacket (tough, lightweight and 100% waterproof) as well as having far better breathability.
New Gore Branding
As a consequence of developing the new Scrim Layer, Gore have renamed their fabrics as follows :
Previous Gore-tex Name
New Gore-tex Name