LAYERING FOR HIKING CLOTHING EXPLAINED
Hiking and backpacking provide people with an opportunity to escape the stressful daily city life and explore natural landmarks – mountains, hills, canyons. Most amateur hikers need only a pack, sturdy and comfortable hiking shoes or boots, and appropriate hiking outfit to keep them warm and comfortable in low temperatures and cool and comfortable in high temperatures. (For more info about basic hiking equipment check out: Packing for a Day Hike or Packing for a Multi-day Hike)
Hiking clothing encompasses a wide range of products. At the core of this market are clothes that may be considered for hiking but also cross into other outdoor activities such as mountaineering, trail running, mountain biking, and winter sports.
It is an accepted practice to use a layered clothing system for safe and enjoyable participation in these sports. This allows the wearer to adapt his hiking clothing in response to changing comfort or weather demands. The three layers are the following:
- Base layers that are worn next to the skin and designed to rapidly wick sweat away from the body. They have moisture management, cooling effect, anti-UV, odor control, quick dry properties. They’re used in sportswear as a first layer or base layer.
- Mid layers, which trap air and create insulation. Mid layer clothing is used in order to prevent heat loss from the body.
- Outer layers consist of windproof and water resistant shells that protect the underlying layers from wind and rain.
All layers are engineered to move moisture through and out of the clothing system.
Sweating is the main way of body cooling during outdoor activities. Sweat must be evaporated to create a cooling effect, and the latent heat required to achieve this is taken from the body surface. The process is most efficient when the humidity is low. Therefore hiking clothing must be engineered for the efficient transfer of water vapor and liquid sweat, which might otherwise accumulate in fabric layers and lead to discomfort. This is achieved through wicking and breathability.
Water collected in fabric layers decreases garment insulation by displacing trapped still air and conducting heat away from the skin. The more water accumulated, the greater the reduction in insulation. That’s why cold temperatures combined with windy weather can lead to hypothermia. It is important that the outer shell of the layered clothing system must also provide a buffer from wind and rain, i.e. it should be windproof and waterproof or water-resistant.
Creating a modern hiking outfit presents a complex problem for the manufacturers. Hiking clothing must address constantly shifting comfort needs, which change according to energy demands and weather conditions over a long period of time.
From left to right: Base layer (Top), Mid layer (Fleece), Outer layer (Insulation Jacket)
Features of base layers
The base layer has two main properties: it has to be skin-friendly (comfortable) and it has to be moisture-wicking and quick dry, as the insulation of wet fabrics is reduced.
A base layer is used to manage moisture next to the skin, rapidly dispersing sweat to prevent chilling and encouraging efficient evaporation at the garment outer surface. Ideally, the liquid is diffused over a wide area in the outer layer, where it can dry quickly, while the body stays dry. The movement of moisture along a fiber surface is referred to as “wicking”. Garments are elastic, allowing the fabric to remain in contact with the skin while maintaining freedom of movement.
The wicking properties of the base layer mainly depend on the capillary structure of the fabric, as well as the moisture absorption capacity and the surface properties of the fibers used. This allows moisture to diffuse until all areas of the garment are equally wet. However, the transport is influenced by the amount of sweat present.
The amount of sweat absorbed by the base layer is mainly dependent on the fiber type and fabric thickness. The absorption of moisture in hygroscopic fibers (especially wool) generates heat, which may have a positive effect on thermal comfort. Some scientists favor a wool base layer to keep athletes warm and dry in sports in cold conditions.
Base layers fabrics
Human skin can probably distinguish different amounts of water in a fabric, but this seems to be dependent on the material used. Differences in humidity levels could only be discerned in cotton, but not in polyester or other synthetics.
Polyester is the most commonly used fiber because it’s relatively cheap, easy to care for, and readily modified. It’s hydrophobic which effects moisture movement, without absorption, resulting in a quick drying fabric. It can also be modified in different ways. COOLMAX is one example of a modified polyester fiber allowing moisture to disperse and evaporate quickly while being highly air permeable to improve drying time.
Polypropylene has very low moisture absorption and good wicking properties. The fibers are soft, however, it tends to absorb oil, which leads to odor retention.
Merino wool is highly absorbent. Each fiber has a structure that allows it to stretch, recover, and insulate effectively. It is hygroscopic, which allows it to absorb moisture into the central structure of the fiber, without feeling damp or clammy. As moisture is absorbed, the hydrogen bonds in the water molecules are broken and chemically react with the molecules in the wool fiber to generate heat. This provides warmth in cold conditions while absorbing perspiration and facilitating thermoregulation in warm conditions.
Features of mid layers
The main purpose of mid layers is to trap air next to the skin and provide additional insulation. Nowadays, these layers are often made of fleece materials. The thermal resistance of such layers is directly correlated with their thickness if no air movement occurs within the fabric.
Several thin layers may be worn or carried depending on the weather conditions. Polyester fleece, down, or synthetic-filled jackets and tops are the most commonly produced clothes of this type.
Mid layers fabrics
Fleece fabric is manufactured from polyester, which is hydrophobic and dries quick. Air is trapped between the fibers in the pile, creating insulation. The fabric is lightweight, breathable, durable, and easy to care for.
Down traps air and provides insulation. However, it is easily damaged when compressed and stored, can absorb a significant amount of water and subsequently collapse.
Features of outer shell layers
The outer shell layer is used to protect the mid and base layers from the outside weather conditions - mainly water (rain and snow) and wind. Water resistant jackets are considered a vital piece of equipment in places with unpredictable weather and are worn or carried for participation in outdoor activities. However, using one of these may rapidly feel hot and clammy during periods of high energy expenditure because they present a barrier to the evaporation of sweat. Therefore, the fabric must be breathable.
In order to avoid excessive sweat accumulation, the outer layer should allow moisture transfer from the body to the environment. The waterproof properties and the water vapor transfer are either achieved with a microporous or a hydrophilic structure, or a combination of both. Waterproofness and breathability are contradictory requirements which means that a compromise has to be made between protection and comfort properties.
Microporous structures contain pores that are permeable to single water vapor molecules, but impermeable to water drops. They can be achieved through tightly woven fabrics, microporous membranes, and coatings or through electrospun nanofibrous structures.
Hydrophilic membranes are non-porous polymer structures and the water vapor transport takes place through the absorption of water molecules and their subsequent transport. These structures swell when they take up moisture. For this reason, a hydrophilic liner is sometimes laminated on the inner side of the membrane. Additionally, it gives the membranes stability.
The waterproof and breathable fabric (WBF) is usually the layer with the highest water vapor resistance. In rainy conditions, the water vapor permeability may be impaired if water is absorbed by the outer shell. Therefore, it is very important that the outer shell layer is treated with a durable water repellent (DWR).
Outer shell layers fabrics
Microporous fabrics are lightweight, densely woven, synthetic fiber structures that are coated or laminated with a polymer. This adds a durable system of microchannels, which are permeable to water vapor molecules, but impermeable to larger liquid water molecules. Laminates are manufactured from expanded polytetrafluoroethylene (ePTFE) and polyurethane (PU), which is cheaper. The first ePTFE laminate was manufactured by Gore-Tex and is still used in a high proportion of hiking clothes.
Hydrophilic coatings and laminates are engineered from polyurethane or polyester polymers. Water is transferred through these materials by a chemical molecular process. They can be added in thin layers to the face of microporous fabrics to improve oil-resistance.
Water vapor transfer is driven down a temperature and humidity grade until a balance is reached on each fabric face. Therefore, diffusion of moisture will be most effective when the ambient temperature and humidity are lower than inside the clothing system. In such conditions, ePTFE laminates have the best breathability performance, followed by hydrophilic and PU microporous laminates and then PU coatings. However, in rainy conditions, micropores can become blocked by condensation and rain, reducing breathability.
The base fabric is usually nylon or polyester, which are both hydrophobic and durable; conversely, the membrane is relatively fragile. The composite structure of a base fabric and membrane described this way is referred to as a 2-layer laminate. This is a classic shell fabric that is used in most hiking and mountaineering jackets. A 2.5-layer laminate has a protective, raised pattern overprinted on to the membrane, which removes the need for a lining. This reduces overall garment weight, making it appropriate for fast and light activities such as trail running and mountain biking. In a 3-layer laminate, the membrane is sandwiched between the base fabric and a softly knitted backing. This results in a heavier but much more durable fabric, which is suitable for professional hiking and mountaineering jackets.
The biggest recent innovation in the shell-fabric market has been the rapid development of perfluorinated chemical (PFC) free membranes and DWR coatings. PFCs ensure better water repellency. However, they are persistent and accumulate in the food chain, causing serious health issues such as hormone disruption, cancer, and immune suppression in children.
Layered clothing systems: a shift towards a fourth layer - the softshell
In the classic three-layered clothing system, each layer offers a specific function. Base layers worn next to the skin are designed to wick away humidity to the outer layers, to stay dry and to offer thermal protection in cold weather. Mid layers focus on thermal insulation and to draw moisture away from the skin to the outer layer. Outer layers protect from the rain, snow, and wind. Combined, these three layers are designed to work together to offer overall comfort and protection. The classic layered clothing system is well adapted to outdoor activities, from hiking and backpacking to cycling.
Nowadays, waterproofness is not considered a feature necessary at all times anymore. On average, a water resistant garment will be necessary 10% of the time whereas high breathability is fundamental in 90% of sports activities. Rain protection has been overestimated in the past, especially in summer collections where total waterproofness is rarely necessary and often reduces clothes breathability.
Breathability becomes the main performance feature and the focus shifts to the second layer which is becoming the pivotal garment in outdoor collections. This new mid layer is designed to offer: elasticity, wind protection and a degree of thermal insulation or water resistance. This is just one of many possible configurations of softshell jackets.
Softshells situate between mid and outer layers. They are comprised of densely knitted base fabrics laminated with a membrane and wicking, or thermal fleece material on the reverse. Such fabrics are usually more breathable than water-resistant fabrics and provide warmth, flexibility, and wind resistance.
The renewed focus on breathability over waterproofness implies reorganizing the layered clothing system in such a way that the main item is no longer the outer layer, but the second layer, the softshell.
Few people outside the outdoor industry understand the terms hardshell or softshell when applied to clothes. Softshells feature many functions and have been made possible through advances in textile technologies. Fleeces can be very warm, but they are not capable of protecting the wearer from cold wind. Their thermal properties need to be supplemented by a windproof shell either as a separate item of clothing or by bonding the fleece to a membrane or a textile (thus getting a softshell).
Softshells, which are advanced composite textiles are often sold at prices similar to, or higher than, hardshells. Designed for intense activities, they combine ease of movement and protection. In many cases, softshells are also water resistant. They generally resist water penetration for about half an hour, time enough to find shelter, or to put on a waterproof jacket. A softshell jacket designed for lightweight and compact protection is easy to store in a backpack and pull on when conditions get rough. The waterproof and breathable jacket (once the most important item of a layered clothing system) has shifted to the light, fast and efficient softshell. The arrival of this new category means the outdoor market is moving to a four-layer system in very cold and extreme conditions. Then, the wearer will combine four garments: base layer, fleece, softshell and an insulation jacket for maximum warmth and protection.
Nowadays, it’s recognized that only complete layering concept can ensure the optimal performance of hiking clothing.
In a layered clothing system, base layers are worn as a constant. A variable number of mid layers are carried, depending on the weather. Outer layers are only worn in cold, windy, and wet conditions. To maintain comfort, items must be adjusted according to weather conditions and perceived exertion. Any adjustments should be effortless and efficient; garments must support comfort and performance.
It is crucial that clothing fits well while accommodating layers. However, it must also allow freedom of movement while performing activity specific movements.
When adjusting layers, the whole construction (comprised of all three layers) has to be considered as the different layers interact with one another. The thermal resistance of any two layers is usually higher than the sum of the two single layers.
To sum up, in order to offer an optimal protection, hiking clothing basically has to provide good insulation, be waterproof (water resistant), windproof and breathable.