Insulation is the middle layer of a 3-layer cold-weather clothing system. Such a layering system involves:
* Base layer such as a T-shirt or long underwear.
* Insulation layer such as a fleece jacket or down vest.
* Outer layer such as a rain jacket. Outer layers may also be insulated.
Typical insulation choices:
* Down: nature's best insulator.
* Synthetic fibers: engineered to mimic down's natural loftiness.
* Fleece: usually made from synthetic fibers, though some wool jackets/sweaters exist.
Down
Advantages: Impressive warmth for minimal weight. Goose down plumules (a mix of feathers and puffy clusters) exhibit a natural loftiness that is exceptionally efficient at trapping "dead" (noncirculating) air and retaining warmth. Can be compressed into a very small shape. Luxurious feel. Long lasting.
Disadvantages: Loses its warmth-retaining abilities if it gets wet. Very slow to dry. Expensive.
Overview: Down garments make an excellent choice for dry, very cold conditions and are well-suited for moderate activity in dry weather such as skiing or snowboarding in powder. Also good for dry, chilly mornings when camping, belaying or backpacking. Relying on down in wet or damp conditions is risky, though; down garments must be carefully shielded from moisture.
Not all down is created equal: Down is graded according to fill power, which indicates how many cubic inches 1 ounce of down occupies when placed inside a container. Down ranges from 450 to 900 fill power. Higher numbers indicate a higher quality of down, with more air-trapping ability. Down with higher fill-power numbers includes fewer feathers and uses bigger, more mature down plumules. Larger down clusters are more durable and can better withstand repeated compressing.
Synthetic Fibers
Advantages: Water-resistant, will dry much more quickly than down and even retains some thermal resistance when damp. Less expensive, too. The most advanced synthetic fibers (e.g., PrimaLoft) have drawn close to down in breathability, weight, texture and compressibility.
Disadvantages: Down still trumps synthetics in minimizing bulk and weight, though an innovator such as PrimaLoft continues to narrow the gap. Less durable than down, especially if repeatedly compressed.
Overview: A very good insulation choice if wet conditions are expected. It performs quite nicely in dry conditions, too, of course. Personally, I wore a lightweight jacket lined with PrimaLoft during a midsummer climb of 14,411-foot Mt. Rainier and found it to be quite comfortable in what I estimate were 40F (or less) temperatures on the summit. Coworkers tell me they also like synthetic insulation during cool mornings when hiking, camping or sea kayaking. Despite advances, synthetic insulation still can't match high-end down for warmth in extreme cold. Nearly all synthetic insulation is made of polyester.
Like down, not all polyester is identical. The science of synthetic insulation fabrics continues to evolve. At the moment, the PrimaLoft family of insulations (explained in more detail later in this article) is widely considered the most highly evolved "species" of the synthetic world, often besting other synthetics in weight and low bulk, though the differences are not always hugely apparent. Clothing manufacturers routinely create their own proprietary variation of polyester: Coreloft and ThermaTek from Arc'Teryx, for example, Heatseeker (The North Face); Thermal R (Marmot); Thermogreen (Patagonia); and so on. Most of these examples also have a green/eco version, meaning they were manufactured from recycled materials.
Fleece
Advantages: Very good breathability, making it a good choice when insulation is needed during vigorous, highly aerobic activity. (Down and synthetic jackets/vests are best worn for moderate to sedentary activities.) Dries quickly when wet, usually faster than a puffy-style synthetic garment.
Disadvantages: Not for serious or prolonged cold. While most synthetic fleeces dry quickly, a few are prone to retaining water (and it's not always easy to predict which fleece items are the exception to the dries-quickly rule). Fleece is also bulky and heavy when compared to down and synthetics. Wind can also permeate fleece pretty easily (which leads to chills) unless it contains a wind-blocking membrane (which inhibits stretch) or is worn under a jacket.
Overview: Fleece comes in various weights (light, mid and heavy). Heavier garments, logically, are better suited to colder conditions. Polartec is one of the best-known brand names in fleece. Its Classic fleece categories—100 (lightweight), 200 (mid) and 300 (heavy)—remain popular and are in widespread use. Its Thermal Pro and Thermal Pro High Loft products offer next-generation benefits in terms of lower weight and reduced bulk. Some fleece-like pullovers are specially engineered to provide extra stretch, wind-resistance, water-resistance or some combination of all of these. Ultimately, though, even the heaviest fleece is not as warm as a jacket insulated with down or a synthetic such as PrimaLoft.
A recent trend: Fleece middle layers made out of actual fleece—natural, 100% wool, that is. Already a huge hit with active outdoor types in socks and base layers for its adaptability to cool or warm conditions and its odor-free nature, mid layers made from soft, finely textured merino wool are worth a look. Just be aware that heavier cuts of wool tend to dry slowly. One suggested use is as a downhill skiing layer in dry conditions.
