Down is one of nature’s finest insulators. Nothing matches its warmth for weight, compressibility, and resilience. Its ability to absorb perspiration and cozily drape along the contours of your body is unparalleled by synthetic materials.
Down is more expensive than other insulating materials. However, when you divide its cost by the number of cozy nights it provides, down is a bargain. A down bag costs approximately 50 percent more than a synthetic bag of the same temperature rating, but down insulation lasts about three to five times longer.
When it gets wet, down insulation is compromised. In damp conditions, a waterproof-breathable shell fabric can protect sleeping bags or jackets from external wetness.
Down is harvested from geese that are being slaughtered for food.
Most reviewers recommend the Western Mountaineering UltraLite ($300) and the Marmot Fusion 15 ($180) as the best down-filled bags. Good sleeping bag reviews can be found at: Backpacker Magazine and Backpacking.Net.