You're likely already in the habit of layering backcountry clothing, mixing and matching items as conditions change. Similarly, you can augment the warmth of your sleeping bag by adding a liner, an overbag, extra sleeping pad, or by wearing a thin layer of clothing to bed.
A sleeping pad is the foundation of any sleeping bag system – it is not an option. Sleeping bag temperature ratings assume the use of a pad. Without one, heat is drawn away by the cold ground and the bag's performance is undermined. Even a thin, inexpensive closed-cell foam pad will boost a bag's performance. A lightweight inflatable pad, adequate in summer – might be augmented with a foam pad in winter.
See Sleeping Pads for information about the insulation they provide.
Made of either polyurethane-coated or silicone-impregnated fabric, a Vapour Barrier Liner (VBL) is placed inside a sleeping bag. By raising the ambient humidity around your body, a VBL reduces or stops vaporous perspiration and its accompanying heat loss. This can add from five to ten degrees Celsius to a sleeping bag's warmth.
To keep garments from becoming clammy, VBL users normally wear little clothing. Liners are best at below-freezing temperatures, when you sweat very little.
An overbag increases the warmth of a sleeping bag by five and ten degrees Celsius and prevents dampness in the inner bag.
A sleeping bag's insulation is often dampened by precipitation, humidity, damp clothing, or vaporous body perspiration. While you sleep, your body heat pushes moisture vapour outward. At a certain distance from you, water vapour condenses into liquid. The point where condensation occurs is called the dew point. Where it occurs depends on the outside temperature. An overbag moves the dew point further away from the body, and hopefully outside the inner bag. Its added insulation helps keep the inner bag drier and you warmer.
An overbag works well on longer trips in cold conditions, where completely drying out a sleeping bag is impossible. After many nights, moisture can accumulate to the point where a sleeping bag's performance is severely compromised.
Clothing such as fleece is not crushed by snug-fitting bags and absorbs little moisture. Wearing full fleece will add about five degrees Celsius of warmth to your bag. Socks and long underwear deliver heat dividends, too. Clean, dry long underwear and socks will add about two degrees Celsius. Wear a hat or balaclava to prevent significant heat loss, particularly if you are using a barrel bag or claustrophobia prevents you from cinching up the hood.
Your shelter can also be considered part of your bag system. Depending on the wind, it's five to ten degrees warmer inside a tent than outside. Though not as effective as a tent, a natural windbreak or a tarp helps keep you warm. In still air, a tarp rigged as a roof reduces radiant heat loss.
Read more tips about Staying Warm