How to Layer for Cold-Weather Camping: Down vs Synthetic Sleeping Bags in 2026

Short version: Down beats synthetic on warmth-per-gram and packed size by roughly 2:1, but it loses most of its insulation when wet and costs 3x more. Synthetic wins in humid climates, on multi-day rainy trips, and for anyone whose gear budget is tight. Below is the honest breakdown of when each is the right choice — and what the “800 fill power” number actually means.

What fill power really measures

Fill power is a lab measurement of how much space one ounce of down occupies when compressed. 800 fill means one ounce of that down fills 800 cubic inches. Higher fill power = more insulation per gram, which is why 850-fill down gets you the same warmth in a 900 g bag as 650-fill down does in a 1200 g bag.

What fill power does not measure: total warmth. A 20°F bag stuffed with 800-fill down uses less down than the same 20°F bag with 650-fill, but both are equally warm. Fill power is about weight and packed size — not warmth rating.

The wet problem — and how down brands are solving it

Traditional down loses 60-90% of its loft when saturated. This is the historic argument for synthetic in wet climates like the Pacific Northwest, Scotland, and Patagonia. The last decade of “hydrophobic down” (DWR-treated) has narrowed the gap: modern treated down retains 60-70% of loft when wet, and dries in half the time. It is still not synthetic-level, but the gap is no longer catastrophic.

EN 13537 comfort limit — the only rating you should trust

A sleeping bag rated “20°F” tells you nothing without knowing which standard. Look for EN 13537 (European standard) or ISO 23537 (its successor). Both give three temperatures: comfort (warm sleeper), lower limit (cold sleeper), and extreme (survival). Marketing usually quotes the lower limit — the “20°F” often means the average person will feel cold at 20°F, and comfortable only above 30°F.

Our Cirrus 20°F Down Mummy Bag has an EN 13537 comfort limit of -6°C (20°F). That is the third-party tested comfort number, not the survival number. Buy your bag by comfort limit, not lower limit — you will sleep warm.

Down vs synthetic — decision matrix

• Dry climates (Rockies, Sierra, alpine): Down. Weight and packed size matter more than wet performance.
• Wet climates (PNW, Scotland, Patagonia): Synthetic, OR hydrophobic-treated 800-fill+ down with a good bivy.
• Winter mountaineering: Down. Cold + dry conditions favor down.
• Sea kayaking / packrafting: Synthetic. Water immersion risk is too high.
• Budget under $200: Synthetic. Down at this price point uses low fill power and heavy fabric — no advantage.
• Budget $250+: Down. This is where 800-fill starts being available.

What about quilts?

Quilts remove the down under your back (which gets compressed and provides zero insulation) and rely on your sleeping pad for the R-value under you. Result: 30% weight and pack-size savings for equivalent warmth. The trade-off is a small learning curve for side sleepers and a slightly draftier setup in high wind.

See our Meadow 30°F Down Quilt for a starter quilt at 620 g — 40% lighter than an equivalent mummy bag.