Best Ultralight Emergency Shelter
Bivy Sack (e.g. SOL Emergency or OR Alpine)
$50–$250
Weight10-18oz — dramatically lighter
SetupNo setup — slide sleeping bag inside
WeatherWaterproof outer but breathability varies
LivabilityMinimal — sleeping only, no sitting up
CondensationHigher — limited airflow
Best useEmergency kit, summit bivouacs, ultralight multi-night
No independent score
Best for General Backpacking
Backpacking Tent (e.g. Big Agnes Copper Spur UL2)
$350–$700
Weight1.5-3.5 lbs — significantly heavier
Setup5-10 minutes
WeatherFull rain fly, vestibule storage, ventilation
LivabilityFull shelter — sit up, wait out storms
CondensationManaged with ventilation and rain fly gap
Best useAll backpacking use cases
Bottom line: Bivies save 1-3 lbs but eliminate livable shelter — you cannot cook under cover, wait out a thunderstorm, or sit up comfortably. The AT regularly features unexpected weather. For anything beyond fastpacking and summit bivouacs: use a tent. The weight savings do not justify the experience reduction for multi-day trips.
Head-to-head differences
Weight
Bivy10-18oz — 1-3 lbs lighter
Backpacking1.5-3.5 lbs — significantly heavier
Bivy wins on weight by a real margin.
Livability
BivySleeping only — cannot sit up, no vestibule space
BackpackingFull shelter — sit up, organize gear, cook in vestibule
Tent wins decisively on livability.
Condensation management
BivyHigher condensation inside — breathability is limited
BackpackingRain fly gap + ventilation manages condensation effectively
Tent wins on sleep quality in typical conditions.
Setup speed
BivyNo setup — lay bag inside bivy
Backpacking5-10 minutes full setup
Bivy wins on speed — useful for single-night bivouacs.
Choose Bivy if:
- →Fastpacking and summit bivouacs where weight is critical
- →Emergency backup shelter in a daypack
- →One-night technical routes where tent is unnecessary weight
Choose Backpacking if:
- →Any multi-day backpacking trip
- →AT section hiking where weather is variable
- →You want a livable shelter to wait out rain
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View Full Category →Common Questions
Is a bivy sack waterproof?
Quality bivy sacks (Outdoor Research Alpine Bivy, SOL Escape) are waterproof on the outside with breathable fabric. Budget emergency bivies are not breathable and collect significant condensation. For overnight use: invest in a breathable waterproof-breathable bivy, not an emergency Mylar sheet.
Can you use a bivy sack instead of a tent on the AT?
Some ultralight AT thru-hikers use bivies, primarily relying on three-sided shelters along the trail for storm protection. For section hiking where you may not always reach a shelter: a tent provides significantly better weather protection and flexibility. Most AT hikers use tents or hammocks.
How much does a bivy sack reduce pack weight?
Replacing a 2-lb tent with a 14oz bivy saves approximately 18oz — a meaningful reduction. The question is whether the livability trade-off is acceptable for your specific trip. For multi-day trips on variable weather terrain: the tent weight is worth carrying.