Hi ame

Selecting a sleeping bag can be difficult despite the huge range of excellent down and synthetic sleeping bags on the market. A top quality down sleeping bag rated at -10 C will weigh around 1.0 Kg. A top quality synthetic sleeping bag rated at -10 C will weigh around 2.0 Kg. If your expecting conditions to be around -10 C then a -15 C specification bag will probably be required. This means a 1.1Kg down or 2.3 Kg synthetic bag. The difference is around 1.2 Kg or to put it another way is around 3500-4000 Kcal heating fuel contained within 1.2 Kg of additional food, which could be carried in addition to a down sleeping bag instead of carrying a synthetic bag.

A 2.3 Kg synthetic bag will be considerable larger when packed down compared to a 1.1 Kg down bag.

One thing to remember is there is no cold just lack of heat. Sleeping bags don't provide any heat. They just keep the heat that is generated from the body confined around the body within the sleeping bag. If you are consuming an extra 1000 Kcal worth of food a few hours before sleeping, the difference in perceivable warmth will be very noticeable especially if it's been a tough tiring day trekking. There is a real difference between being cold, tired and hungry and just being tired.
A down bag is more expensive than a synthetic bag, typically twice as expensive, but this has to taken in context because a down bag will generally last twice as long or longer.

Down bags when they become wet will loose most of its insulation properties, much more so than synthetic bags. You just have to be careful to ensure the down within the bag does not get wet. Many manufacturers of high specification down sleeping bags will use down proof, breathable and water resistant outer ripstop fabrics to help provide protection against this problem. An additional dry storage bag with a roll closure for the down bag is a good idea.
There is also a huge difference in quality of the down available. Down is rated by the loft rating. The best down has a loft rating of 800 (rare European Eider down). The loft ratings vary from 500 through 600, 650, 700 and 750. Anything above 700 is generally considered to be highest quality. US loft figures are generally 100 above European. i.e. an 850 loft bag is equivalent to a 750 European loft rating.

Sleeping bag liners are very useful. They keep the bag cleaner and add additional warm. A silk liner may add 2-3C to the bags rating. The silk liner is also the lightest and most compact. Liners made from Pertex are useful as well being similar to the silk liners. Liners made from Fleece are the warmest adding up to 5-7C but are much heavier (800 grams) and bulky being almost as bulky as another separate single season sleeping bag.

There are some excellent down sleeping bags from companies like Rab, PHDesigns, Marmot, Western Mountaineering and Mountain Equipment etc. They are all excellent manufacturers.

There is an excellent article available at http://www.planetfear.com/article_detail.asp?a_id=417