Because of the versatility of outerwear, many people use all sorts of jackets, vests, sweaters and turtlenecks as mid layers. Many work just fine but with a little planning you can accentuate the engineering of your base and shell layers, expand the temperature range of your outfit and minimize the bulk around your body.
For the purpose of this article, mid layers are those between a base layer (one or two garments worn next the skin) and the shell (which directly faces the elements).
This mid layer could be made up of a single or multiple items of clothing, such as a thin fleece combined with a down vest, or just a single insulator like a heavy fleece jacket worn under a shell.
Insulation – The primary purpose of a mid layer is to hold warmth close to the body while moving moisture away. This creates dry, warm micro-climate near the skin.
The concept of micro-climate means it’s comfortable near your body regardless of whatever nastiness may be going on outside your clothing.
By regulating the flow of moisture and air near your skin while blocking wind, rain, snow and other crud far away, you create a happy place for your body to exist regardless of the elements.
Material – The most sought-after mid layers are constructed from materials that are light, insulating, and breathable. The most common fabrics for this layer are wool, fleece, and down.
Wool – A time-tested material with excellent insulating properties, wool is water resistant, very breathable and insulates when wet. Wool garments of many thicknesses provide great versatility. The drawbacks of wool are bulk and weight, especially once wet. Wool does not compress very well, so a heavy sweater will take up quite a bit of space in a pack.
Fleece – the term fleece originally meant wool, but in modern times it is used to identify synthetic materials with properties similar to wool. Fleece is usually made of polyester fiber.
Fleece, especially thin garments worn directly over base layers, makes excellent insulation. Fleece maintains warmth when wet and is breathable. Some fleece is designed with layers of GORE Windstopper or other wind-blocking material.
Like wool, fleece tends to be a little bulkier and heavier than down insulation.
Synthetic Down – Several types of insulation, such as Polartec Alpha, PrimaLoft and Climashield, have similar insulating properties as down. They are very breathable and work well even when wet.
Synthetic down does not compress as well as real goose or duck down so it will take up more room in a pack although they are closing the gap on natural down.
Modern synthetics are versatile and accepted by high-end manufacturers for their moisture management and durability.
Down: Still considered by many to be the best insulating material available, high-end down is warm, light and extremely compressible. Some well-designed down mid layers will compress to the size of a coffee cup or smaller for stowing in a jacket or pack.
For many years, the only negative associated with down was that it lost most of its insulating properties when wet. This is still the case with standard down, which must be kept dry to work properly.
With the advent of DriDown, this material became much more versatile and resistant to moisture. Hydrophobic treatments have created a type of down that dries quickly and maintains loft even when damp. These treatments are not a cure-all and down should still be kept dry if possible.
Down is rated by fill power. An entire article can be devoted to the types and qualities of down, but the short version is this: the higher the number, the lighter, more efficient, more compressible, warmer per ounce, and more expensive a down jacket will be.
Frank Kvietok, the Director of Advanced Development at American Rec (makers of Sierra Designs DriDown) explained the advances in hydrophobic down technology.
“Basically think of DriDown as a down with a Durable Water Repellant on it. You have not made the fabric waterproof, but the fibers are waterproof. It doesn’t turn a down jacket into a raincoat. It makes the down itself highly water repellent.”
Types – Mid layers are designed in many different jacket configurations. Hooded coats, vests and pullover type sweaters are all commonly used as insulation under a shell layer.
When choosing a mid layer style, consider things like interior pockets to keep phones or cameras warm and functional, hoods to insulate around your head under a shell and if the layer has a zipper or button down the front to open if you need to cool your body after exertion.
Weight – some jackets – especially wool and fleece – weigh more than others. Keep this in mind when choosing a multiple layer system. The combined weight of four or five layers can become cumbersome.
For layers that will be stowed in packs, natural down is usually much more compressible for the amount of warmth. Thin down sweaters also make for popular belay parkas, pulled on for extra heat during body chilling breaks during climbing, because of their light weight and compressibility.
Use these tips to help guide your choice of mid layer garments. Be sure to read reviews of items that catch your interest and find the most fitting clothing for your next adventure.