The Insulating Layers
The objective of the insulating layer is to create dead air space to trap air near your body and help you retain heat. It also absorbs some of the base layer's moisture, which in turn keeps the moisture away from your skin, so you want it to pass moisture easily as well. This can be one or many layers, depending on the temperature.
- First Layer Shirts and pants are typically your first layer, and could be an extension of your wicking layer (i.e., wearing middle-weight to expedition-weight polpropylene that both wicks and provides insulation. Layers that permit you to unzip or zip, such as button-down or 3/4 zip shirts, allow for ventilation during intense heat-producing activities or extra warmth when needed.
- Second Layer If you are still cold and want more warmth, consider adding another top or bottom insulating layer such as synthetic fleece or wool.
- Outer Layer During very cold weather, you might consider adding an even thicker layer such as an insulated parka or pair of pants. These typically have an outer and inner layer of fabric that usually contains down or another synthetic insulation. These layers are typically worn at camp during the beginning and end of the day when activity level is low and/or temperatures around or below freezing.
Insulating Materials
- Fleece Fleece is a synthetic fabric typically made of plastic (polyester, polyolefin, and/or polypropylene). It has an insulation capacity similar to wool, retains its warmth when wet, does not absorb moisture, and drys quickly. Fleece is produced in a variety of weights, or thicknesses, therefore offering varying amounts of loft, insulation, and layering possibilities. While fleece provides an equivalent amount of warmth as wool at half the weight, it offers very poor wind resistance.
- Down Down, the soft underbelly plumage of geese or ducks, provides excellent insulation and dead air space for very little weight. Down is rated by its fill power, or how many cubic inches of volume an ounce of down will fill. Fill power ranges from 550 cubic inches up to 900 (although rarely seen - 800 is much more common). A down garment with a higher fill rate will loft better and be more thermally efficient than a lower-rated fill rate item (it will also be more expensive). It will also be highly compressible, which is an advantage when packing for a backpacking trip. Finally down garments typically have a long life span if cared for properly. However, you have to take care to ensure that your down garments do not get wet - when they do, they clump together, lose roughly half of their insulation value, and take a long time to dry.
- Synthetic Fibers Synthetic fibers are a variety of different synthetic fibers used for garments, most of which are based on some form of polyester. The fibers are fairly efficient at providing dead air space, although not as efficient as down. Unlike down, however, synthetic fiber-based garments do not absorb water, meaning they will retain their "loft" much more efficiently than a down item when wet, and also dry much quicker when wet. On the down-side, they are heavier than down items, harder to compress, and not as efficient in terms of insulation as down. Examples include: Polarguard 3D, Polarguard Delta, and Quallofil.
- "Superthin" Fibers "Superthin" fibers are based on the principle that the amount of dead air space around the fiber can be increased by making the fiber thinner. In fact, some superthin fiber garments are close to the weight of down for an equivalent fiber volume. These garments are lightweight and thermally efficient, compress down to a small size, and have a similar warmth-to-weight ratio as down without the wetness issue. Superthin fiber items also dry quickly. While very effective in smaller items such as gloves and boats, some superthin fibers are heavy and therefore are not good insulators for larger items like parkas (or sleeping bags). Examples include: Primaloft, Lite Loft, and Thinsulate.