The Three Numbers on Every Bag
EN 13537 / ISO 23537 testing uses a heated mannequin in a climate chamber:
- Comfort: Temp for a standard woman (25th percentile cold sleeper). Use this number.
- Lower Limit: Temp for a standard man in curled position. Marketing uses this.
- Extreme: Survival-only. Never plan around this.
Marketing Trick
A "20°F rated" bag usually means Lower Limit — the Comfort rating may be 30°F+. Always check both. If no EN/ISO rating, be skeptical.
Down vs Synthetic
Down: smaller pack, 10+ year lifespan, loses insulation when wet. Synthetic: insulates when damp, costs less, bulkier, 3-5 year loft life.
REI Co-op Magma 15
850-fill goose down, Comfort 24°F/Lower 15°F, 2 lbs 2 oz. Best warmth-to-weight under $350.
Kelty Cosmic 20
550-fill duck down, Comfort 28°F/Lower 20°F, 2 lbs 11 oz. Best budget down bag.
Marmot Trestles Elite Eco 20
Synthetic, Comfort 29°F/Lower 20°F, 2 lbs 15 oz. Best synthetic — performs when wet.
Sea to Summit Spark SP2
850+ down, Comfort 35°F, 14.5 oz ultralight. Gram-counting warm-season only.
NEMO Disco 15
650-fill down, Comfort 25°F/Lower 14°F, spoon shape. Best for side sleepers.
Sources & Further Reading
- ISO 23537 sleeping bag requirements — international sleeping bag test standard for temperature-rating claims.
- Therm-a-Rest sleeping bag temperature ratings — manufacturer explanation of comfort, limit, and extreme ratings.
- Sea to Summit sleeping bag testing FAQ — manufacturer explanation of ISO testing, comfort ratings, and real-world warmth variables.