The addition of a 3-ounce breathable windshirt extends the comfort range of your clothing in high winds, cool temperatures, and light rain like no other layer. A poncho, windshirt and your base layers provide you with enough warmth and protection to cover 90 percent of the inclement weather you’ll encounter during summers in the Continental US mountain ranges.
—Ryan Jordan, “SuperUltralight,” Lightweight
Backpacking & Camping
Wind Shell Rationale and Functions
In the above quote, Ryan Jordan sums up the case for wind shells (jackets, parkas, shirts) quite well. Here is my statement using this same rationale: A separate wind and water-resistant layer is essential in a high performance, layered clothing system for hiking and backpacking; it can provide so much benefit for so little weight.
Conceptually, wind shell layers have at least three features or functions (besides style and aesthetics) that serve to move them into the “essential” gear category. First, they are tightly woven in order to minimize the amount of moisture getting in from the outside (as well as keeping sun and insects out when hot out). Second, because they are breathable they will transfer moisture from perspiration to the outside (i.e., it slows down evaporative cooling). Third, they are wind resistant enough to prevent convective heat loss.
Regarding this third function, if your base layer gets saturated (“wets out”), the wind shell layer will restrict heat loss so that the saturated layer is kept close enough to skin temperature to remain comfortable (i.e., a wet suit effect). This third feature essentially functions as insulation because of the dead air space created (the higher the level of wind resistance, the greater the insulation factor). This third insulating feature of windshells can be summarized as follows: a simple, unlined wind shell can provide, ounce-for-ounce, more warmth than any other clothing item in your layering system.
Three criteria are important for the three functions detailed above to work well:
Windshell Options and Configurations
Windshell layers, on my definition, are unlined and not waterproof. They are usually made from tightly woven nylon or polyester fabrics and constructed in one of four configurations (for the torso): button-up long sleeve shirt, sleeveless vest, full or partial zip wind jacket; full or partial zip parka with hood. Ultralight wind shell jackets and parkas can be found weighing as little as three ounces.
One interesting option for those into making-your-own-gear (MYOG) is to construct a roll-up nylon wind shield covering only your front (assuming your pack will cover your back). Another option using this same line of thinking is to wear an unzipped wind jacket on backwards (keeping the backside open to avoid sweat buildup).
If possible, compare the breathability of competing garments by blowing through them. Some are so tightly woven as to have poor breathability and some so loosely woven that they provide no protection from precipitation. The trick here is to reach that balance point between breathability on one hand and wind/water resistance on the other.
Wind shell garments are often treated with a durable water-resistant (DWR) outer finish that needs to be renewed regularly to maximize water resistance. If so treated, it will function well for light rain and snow protection.
Since this wind shell garment can be used as an only-clothing-layer, especially if it is hot and sun or bug protection is desired, it should be soft enough to wear against your skin.
What about cost and quality? On the high end, wind shell garments can cost well over a hundred dollars. The cheapest solution is a nylon windbreak garment found at stores like Target and Wal-Mart. Low cost garments should usually be treated with a DWR after-market product.
Waterproof and Breathable Garments Generally Poor Substitute
General Principle: Even though waterproof shell fabrics (e.g., Goretex and eVent) are becoming more breathable, they are generally not breathable enough to transfer all of the moisture from perspiration to the outside air. Also, because of this lessened breathability, they offer too much dead air space insulation in your garment layering system (i.e., easy to get overheated).
There are exceptions to this general principle. Exceptions occur when two or more of the following conditions are present:
Most hikers and backpackers will find that waterproof and breathable garments, no matter how advertised, are unsatisfactory for this wind shell layer most of the time, especially when involved in vigorous hiking with a substantial backpack.
Wind resistant layers are essential if one is committed to treating layering as a the fine art. If so committed, click on this link for an in-depth discussion of all layering system options: The Fine Art of Layering.