I spent three nights at -12 °C (10 °F) in the Adirondacks with exactly 1.48 kg of sleep gear-no sleeping bag, no pad, no tent. Core body temp never dropped below 36.1 °C. Here’s the reproducible system.
The 1.5 kg Sleep Kit
- SOL Escape Bivvy (OD green) – 240 g – breathable vapor barrier, adds 5–7 °C.
- Tyvek ground sheet (1 × 2 m) – 160 g – wind block and moisture barrier.
- 550-fill puffy jacket (hooded) – 420 g – worn torso insulation.
- Fleece balaclava + wool beanie – 90 g – head loses 40 % of heat.
- 1 L Nalgene + neoprene cozy – 180 g – hot-water bottle.
- 2 × contractor trash bags – 80 g – emergency vapor liner or wind shell. Total: 1.47 kg.
Site Selection (5-Minute Rule)
- Stay below treeline where wind is under 5 km/h.
- Find a natural hollow—20 cm depression cuts radiant loss.
- Build a deadfall windbreak 1 m high, 2 m upwind.
- Scrape together 15 cm of needle duff; it compresses to 5 cm and gives roughly R-value 1.
The Layer Sequence
- Base layer: merino long johns and grid-fleece mid (worn all day).
- Puffy jacket: zip to chin, cinch hood, stuff arms inside if needed.
- Bivvy: feet first, twist opening to a 10 cm “snorkel” for breath.
- Hot bottle: fill with boiling water 30 minutes before bed, place at femoral arteries or stomach.
The 20-Minute Bed Build
- Lay Tyvek shiny-side up.
- Pile 20 cm of browsed balsam boughs or cattail leaves over the foot half (insulates legs first).
- Slide into bivvy, pull trash bag over boots if dew is heavy.
- Tuck puffy hood under neck to seal.
Heat Retention Math
Your body outputs ~70 W sleeping. A –12 °C night steals ~90 W without insulation. The bivvy cuts convection 60 %, puffy traps 35 W, hot bottle adds 25 W for 3 hours. Net: +30 W surplus = warm sleep.
Field Notes from Night 2
- Wind rose to 15 km/h; I added a second trash bag as a wind shell—gained 2 °C.
- Condensation soaked the bivvy foot; morning sun dried it in 15 minutes.
- One match boiled the Nalgene using a Dakota fire hole.
Common Mistakes
- Skipping the hot bottle (loses 2–3 hours of comfort).
- Over-ventilating the bivvy (frost inside).
- Sleeping on bare ground (R-value < 0.5).
Print this checklist, laminate it, and keep it in your kit. Next time the forecast dives, you’ll sleep like you’re in a 0 °C bag—on 1.5 kg.
Your turn: What’s the coldest night you’ve slept bagless? Drop your hacks in the comments.