The primary function of a sleeping bag is to keep you warm. As such, it is an excellent insulated envelope/cocoon-like heat-retention chamber in which to sleep. Factors to consider when choosing the proper sleeping bag are directly related to: temperature, weight vs. warmth and insulation.
Casual home or backyard and light car campers require minimal sleeping bags. For general family camping, car/tent use, the traditional rectangular bag is common. Cabin or hunting/fishing camp sleepers might prefer the heavier, warmer rectangular bags while backpackers and other lightweight campers usually prefer the benefits and warmth of lightweight down. Extreme polar conditions require expedition quality winter bags.
At the foundation of the mechanical factors regarding levels of comfort/warmth in a sleeping bag are the human elements of a body’s metabolism (producing heat), blood circulation (distributing heat), gender (women tend to be colder than men), and hydration (we tend to stay warmer when we are adequately hydrated). These human factors will determine at which temperatures we feel the most comfortably “warm.” That personal sense of warmth can be used to determine which sleeping bag might best provide that comfort zone while sleeping.
Generally then, the following construction features all combine to provide a range of temperatures at which each bag should provide an accepted range of comfort. Summer/Indoor bags are best suited for temperatures above 40 degrees F; general 3-season bags range between 10 degrees F and 40 degrees F; winter campers should consider bags within a –10 degrees F to +10 degrees F. In the most extremely low temperature ranges, bags should be rated even lower!
A new standardized rating system for sleeping bags has been introduced from Europe called the European Norm (EN) 13537 Protocol that sets comfort limits for both Men’s and Women’s sleeping bags (remember, women are typically colder than men):
* Upper EN Limit: the highest temperature at which the average man can sleep comfortably;
* Comfort EN: the lowest air temperature at which the average woman can sleep comfortably;
* Lower EN Limit: the lowest air temperature at which the average man can sleep comfortably.
* Extreme EN: the “worst case scenario” rating — a bag designed to keep a woman alive!
Your safest bet in determining the proper sleeping bag temperature range is to decide on the coldest temperature you expect to encounter and pick a bag with a low-end rating of about 10 degrees colder.
Here are a few basic terms that are used to describe function, materials and uses of most sleeping bags:
Insulation doesn’t provide warmth by itself but is the material used to create the barrier of trapped, non-circulating “dead” air that retains our body heat. Bags are filled with either natural (down, duck), synthetic insulation and in some cases simply cotton.
Weight is a primary factor when choosing a bag for backpacking or personal carry beyond a vehicle. Most often the main factor in a sleeping bag’s weight is the amount and type of fill. The shape of the bag can also affect the overall weight.
This represents the amount (height/thickness) of fill in a sleeping bag. The higher the loft (which helps form air pockets), usually the warmer the sleeping bag. And when comparing the same type of fill, more loft typically means more weight and more warmth. Therefore the amount of trapped heat (warmth) relative to the amount of fill (weight) equals the warmth/weight ratio of a sleeping bag.
The outer, exposed cover, the “skin” of the sleeping bag. Shells are commonly made of synthetic fabric (taffeta or ripstop nylon, polyester) and are often treated with a durable water-resistant (DWR) finish. Some inexpensive/summer bags shells are made of heavier cotton.
A lining is the inner surface that makes contact with the sleeper’s body. Linings can be silk, cotton flannel, fleece, insulated fiber or other or other synthetic materials. These general features, and others, are incorporated into a sleeping bag’s design to offer a range of function in the different types of sleep systems. “Liner” is also the term used for a separate, interior bag added to a sleeping bag (see below).
Shapes Of Sleeping Bags
When we are in a sleeping bag the heat from our body warms the pockets of air that envelops us inside the bag. That warming process is more efficient and effective when that “dead” air between our body as well as the air trapped in the sleeping bag insulation doesn’t move around a lot or fill too large of a space. A good-fitting sleeping bag will envelop the body in a warm but not excessive pocket of dead air. This is why mummy style bags are warmer than the larger, more spacious rectangular bag designs.