Goose Down is the warmest, lightest, most efficient and durable sleeping bag insulation available, giving goose down bags the best warmth to weight ratio of any bag you can buy. Goose down is also highly compressible, allowing you to stuff a goose down bag into a much smaller stuff sack than a synthetic bag with a comparable temperature rating. However, goose down can temporarily lose most of its ability to keep you warm if it gets soaked.

Polarguard™ Delta and Polarguard™ 3D, the synthetic insulations used in Sierra Designs bags, retain most of their loft and warmth even when sopping wet. While bulkier than goose down, Polarguard™ Delta and Polarguard™ 3D are the most compressible and durable synthetic insulations available for sleeping bags. So if it's likely that you'll encounter damp conditions, Polarguard™ synthetic insulation may be your best bag insulation choice.