At night when our bodies are inactive, we lose body heat a great deal faster than we produce it. A sleeping bag insulates that heat, by trapping the warm air inside, which reduces cooling.

A sleeping bag slows down the loss of body heat.

We lose body heat in 3 ways:

Convection: This occurs when air moves away from the body, via openings in the bag.
Radiation: This is the natural process of heat leaving the body through the materials.
Conduction: The transerence of body heat through still air, via the insulating materials.

A sleeping bag will help to combat these factors by reducing the loss of body heat. This is acheived by a layer of still air being trapped by the insulating fibres between the inner and outer lining materials.

What is Lofting?

Lofting is the fillings ability to trap air by expanding. The greater the volume of air trapped, the better the insulation (warmth).

Types of Sleeping Bag.

Rectangular:
The most common type of bag. These are ideal for light camping and caravanning. A full-length zip allows for two bags to be joined together forming a double, or it can be opened fully and used as a duvet or blanket. Rectangular, or standard, bags are generally larger and more bulky than a "Mummy" style bag, and consequently have more space to heat.

Mummy style:
These are excellent for all camping and backpacking needs, when a lower weight and smaller pack size is important. They are of a tapered design, with a 'hood', which increases thermal efficiency under colder and more demanding conditions. They tend to be less roomy than a standard bag, but this means less space to insulate.

Types of Construction.

Stitched Through - Inner and outer of bag are sewn together at intervals, giving a quilting effect. At these intervals, cold spots can occur.

Single Layer - A single layer of synthetic filling, which is stiched to the inner lining to avoid cold spots. Very small pack size.

Double Layer - A double layer of synthetic filling , eliminates cold spots by ensuring no point of contact between the inner and outer linings.

Box Wall - Used in high performance down and feather bags. It allows the filling a deeper loft, thereby increasing thermal efficiency.