Last updated: June 24, 2026 — BestCampGear Editorial Team | See our sleeping bag guide • sleeping pads guide
Down = warmer per ounce, compresses smaller, lasts 2× longer, expensive, useless when wet
Synthetic = insulates when wet, cheaper, heavier and bulkier for the same warmth, wears out after 5-8 years
Verdict for most campers: Buy down if you backpack and your total pack weight matters. Buy synthetic if you camp in wet climates (Pacific Northwest, UK, Southeast Asia) or you are on a budget under $120.
Fill power measures how much space one ounce of down occupies in cubic inches. 800-fill down is loftier than 600-fill down—meaning 12 ounces of 800-fill down fills the same space as 16 ounces of 600-fill down. Higher fill power = lighter bag for the same warmth, because less down is needed to trap the same amount of air.
| Fill Power | Quality Level | Typical Price Range | Real-World Example |
|---|---|---|---|
| 550-600 | Budget down | $60-120 | Kelty Cosmic 20 (550 fill, 2 lbs 12 oz) |
| 650-700 | Mid-range | $150-250 | REI Co-op Trailmade 20 (650 fill, 2 lbs 1 oz) |
| 750-800 | Premium | $300-450 | Feathered Friends Flicker 20 (800 fill, 1 lb 7 oz) |
| 850-900+ | Ultra-premium | $500-700 | Western Mountaineering Alpinlite (850 fill, 1 lb 15 oz) |
Key insight based on spec analysis: The Kelty Cosmic 20 ($120) at 550-fill down weighs 2 lbs 12 oz for a 20°F rating. The Feathered Friends Flicker 20 ($399) at 800-fill weighs 1 lb 7 oz—less than half the weight for the same temperature rating. You are not paying for warmth; you are paying for weight reduction and packability.
Modern synthetic fills—PrimaLoft Silver, ThermoBall, and CoreLoft—are fundamentally different from the hollow-filament polyester in a $50 Coleman bag. They use short-staple fibers (staple = cut to a specific short length) that mimic down clusters. The best synthetics today achieve fill-power equivalents of roughly 600-625, meaning they are within striking distance of budget down on warmth-to-weight. The gap has closed significantly over the last decade.
| Insulation Type | Continuous Filament | Short-Staple |
|---|---|---|
| Used in | Budget bags ($40-80) | Premium synthetic bags ($130-250) |
| How it feels | Stiff, clumpy | Soft, mimics down |
| Durability | Loses loft after 2-3 seasons | Holds loft 5-8 years |
| Wet performance | Insulates OK when damp | Insulates well when damp |
| Attribute | Down (Kelty Cosmic 20) | Synthetic (REI Trailbreak 20) |
|---|---|---|
| Temperature rating | 20°F (EN comfort: 30°F) | 20°F (EN comfort: 31°F) |
| Weight | 2 lbs 12 oz | 4 lbs 5 oz |
| Packed size | 8×14 in | 10×18 in |
| Price | $120 | $100 |
| Wet performance | Zero insulation when soaked | Retains ~60% warmth when wet |
| Expected lifespan | 10-15 years (with proper care) | 5-8 years (compression cycles degrade loft) |
Down's fatal weakness is water. When down gets saturated, the individual plumules collapse into wet clumps and the bag loses virtually all its insulating ability. This is not a minor inconvenience—it is a safety hazard. A down bag that gets soaked in a 40°F rainstorm becomes an emergency. A synthetic bag in the same conditions still provides roughly 60% of its rated warmth, based on studies by the U.S. Army Natick Soldier Center and manufacturer testing data (PrimaLoft, 3M Thinsulate).
Practical implications: If you camp in the Cascade Range, Olympic Peninsula, Great Smoky Mountains, or anywhere with persistent humidity and rainfall, the weight penalty of synthetic is the price of safety. If you camp in the Colorado Rockies or Utah desert, where humidity averages 20-30% and rain is rare, down is the clear winner.
Every time you stuff a sleeping bag into its stuff sack, you compress the insulation. Down springs back thousands of times before losing meaningful loft. Synthetic fibers lose loft progressively with each compression cycle. After roughly 200-300 compression cycles (equivalent to 4-6 years of one-weekend-per-month use), a synthetic bag loses 20-30% of its loft, based on manufacturer care data and long-term user reports on Backpacking Light forums. A down bag of equivalent use still has 90%+ of its original loft. This is why the 1970s-era down bag in your grandparents' attic still works, but a 2016 synthetic bag from the same attic is flat and worthless.
A $120 Kelty Cosmic 20 (down) lasting 12 years costs $10/year. A $100 REI Trailbreak 20 (synthetic) lasting 6 years costs $17/year. Down costs more upfront but less per year of useful life. This math only holds if you care for the down bag properly—store it uncompressed in the included large mesh sack, never long-term in the stuff sack—and if you do not get it soaking wet.
Several manufacturers now treat down with a durable water repellent (DWR) coating on the individual plumules. The most common is Nikwax Hydrophobic Down, used by brands including Rab, Mountain Hardwear, and Sea to Summit. Independent testing by Backpacking Light (2023) found that DWR-treated down absorbs water 50% slower than untreated down and dries 30% faster. It does NOT make down waterproof. It is a buffer, not a solution. A DWR-treated down bag still needs a waterproof stuff sack and careful moisture management.
If you camp in Colorado, Utah, Arizona, or California east of the Sierra: buy down. The weight savings, packability, and longevity are impossible to beat in dry conditions. If you camp in Oregon, Washington, North Carolina, Tennessee, or the UK: buy synthetic. The wet-bag safety margin is real, and the weight penalty doesn't matter for car camping. For your first bag, a $100 synthetic REI Trailbreak is the safe choice—you will not freeze if it rains, and you can always buy down later if you get serious. Read our full sleeping bag comparison for specific models.
Disclosure: BestCampGear is a participant in the Amazon Services LLC Associates Program. As an Amazon Associate we earn from qualifying purchases. Insulation data comes from manufacturer spec sheets and independent testing by Backpacking Light and the U.S. Army Natick Soldier Center.