Gear · Sleep systems
For trips where every bit of pack weight and volume matters, down usually offers the stronger warmth-to-weight ratio and compresses smaller. In persistently damp conditions, rougher use or on a tighter budget, synthetic insulation is often the more practical choice. The key is to compare bags with similar temperature ratings rather than choosing from the words “down” and “synthetic” alone.
The shortest answer
PriorityLook at firstWhyLow pack weight and volumeDownDown generally delivers more warmth per unit of weight and compresses very well.Persistent damp, boating or wet climateSyntheticSynthetic insulation performs better when damp and tends to dry faster.Tight purchase budgetSyntheticIt is generally less expensive at a comparable temperature class.Frequent multi-day backpackingDownLow weight, small packed size and good longevity can justify the higher price.
Start with the temperature rating
Insulation type is not a temperature rating. Two bags can use different fills and still be designed for the same conditions. First choose the temperature class, fit and shape that make sense for the trip, then compare total weight, packed volume and insulation.
Fill power is also not a direct measure of how warm an entire sleeping bag is. For down, it describes the loft efficiency of the fill. Overall warmth still depends on fill weight, baffle construction, fit, shell materials and the complete design.
Where down has a real advantage
REI highlights down’s strong warmth-to-weight performance, compressibility and long service life with proper care. Those benefits become especially valuable when backpacking, where a sleeping bag competes for space with food, shelter and the rest of your kit.
The trade-off is moisture management. When down becomes seriously wet, clusters clump, loft falls and insulation performance drops. Hydrophobic treatments can improve resistance to moisture and drying behavior, but they do not make a down bag equivalent to synthetic insulation in persistently wet conditions.
Where synthetic is more practical
Synthetic fills are typically made from polyester fibers. REI and Therm-a-Rest emphasize two useful properties: synthetic insulation retains useful insulating performance better when wet and dries faster than down. That matters on river trips, in very damp climates, with repeated condensation or whenever drying opportunities are limited.
The usual penalty is weight and bulk. At similar warmth, a synthetic bag is often heavier and less compressible. The exact gap varies considerably by design, so avoid universal percentages and compare the published specifications of the two actual models you are considering.
Practical rule: if you can reliably keep your sleep system dry and carry it on your back, down is a compelling option. If sustained moisture is part of the trip itself, synthetic insulation buys useful resilience.
Storage matters too
Neither type should spend months tightly compressed in its transport sack. For long-term storage, keep the bag dry and loosely stored or in a large breathable sack according to the manufacturer’s instructions. Down requires particularly careful washing and complete drying so that its loft can recover.
Choose in one minute
- Define the real temperature range and compare similar ratings.
- Check total weight and packed size for the actual models.
- Ask not simply whether rain is possible, but how hard it will be to keep the bag dry for several days.
- For dry multi-day backpacking, give down serious priority.
- For persistent moisture, a tighter budget or less delicate use, start with synthetic.
- If you choose down, check sourcing and animal-welfare certifications when those are important to you.
Bottom line: there is no universal winner. Down optimizes weight and packed volume; synthetic optimizes wet-condition resilience and upfront cost. Choose around the route and how you will actually manage your gear.
