The Rating System Is Confusing on Purpose
A sleeping bag rated to 20degF sounds straightforward — it keeps you warm at 20 degrees, right? Not exactly. Temperature ratings are one of the most misunderstood specs in outdoor gear, and the confusion has cost a lot of people a lot of sleepless, shivering nights.
The problem starts with the fact that there's no single number that captures how warm a sleeping bag is. Your body, your metabolism, your sleeping pad, the humidity, wind, what you ate for dinner — all of these affect whether you sleep warm or cold. But we need some standardized benchmark, and that's where EN and ISO testing come in.
EN 13537 and ISO 23537: The Standards
The European Norm (EN) 13537 standard — now superseded by ISO 23537, which uses the same methodology — tests sleeping bags on a heated mannequin in a controlled environment. The mannequin wears a base layer and lies on a standard sleeping pad. Sensors measure heat loss at various temperatures.
The test produces three ratings:
- Comfort rating: The temperature at which a standard "cold sleeper" (based on female metabolic norms) will sleep comfortably in a relaxed position.
- Lower limit (or "Limit") rating: The temperature at which a standard "warm sleeper" (based on male metabolic norms) can sleep for 8 hours in a curled position without waking from cold.
- Extreme rating: The temperature at which a standard female can survive 6 hours without risk of death from hypothermia. This is a survival rating, not a comfort rating. You will be cold, possibly dangerously so.
Which Rating Matters?
Here's the critical part: most manufacturers advertise the lower limit rating, not the comfort rating. That "20degF" bag? Its comfort rating is probably 30–32degF. The 20degF number means a warm-sleeping male can survive the night curled in a ball — not that anyone will be comfortable.
My rule of thumb: use the comfort rating as your planning temperature, or add 10–15degF to the advertised lower limit rating. If you run cold (and many people do), add 15–20degF.
The extreme rating is meaningless for purchase decisions. If you're using a sleeping bag at its extreme rating, you're in a survival situation, not camping.
Bags Without EN/ISO Ratings
Not all sleeping bags carry EN/ISO ratings. The testing is optional and costs manufacturers money. Bags without standardized ratings use the manufacturer's own testing, which may be optimistic, conservative, or completely made up.
Big brands (Western Mountaineering, Feathered Friends, Marmot, REI Co-op, Kelty) generally publish honest ratings even without EN/ISO certification, because their reputations depend on it. Unknown brands on Amazon? Treat their ratings with extreme skepticism. I've tested $60 "0degF" bags that left me shivering at 40degF.
Down vs Synthetic Fill
Down
Down (goose or duck) is the gold standard for warmth-to-weight ratio. A quality 800+ fill power down bag weighing 2 lbs can match the warmth of a 3.5 lb synthetic bag. Down also compresses smaller for packing and lasts decades if cared for properly.
The catch: Down loses almost all insulating ability when wet. In humid conditions (coastal camping, lots of condensation in your tent or roof tent), down can absorb moisture and leave you cold. Hydrophobic down treatments (Nikwax, DownTek) help but don't fully solve the problem.
Best for: Dry climates, weight-conscious overlanders, 3-season use, anyone willing to manage moisture.
Synthetic
Synthetic fills (Climashield, PrimaLoft, Polarguard) insulate even when damp, dry faster than down, and cost less. Modern synthetics have closed the warmth-to-weight gap significantly, though down still wins.
The catch: Synthetic bags are bulkier, heavier, and have shorter lifespans. After 500–700 nights of use (or 3–5 years of regular compression in a stuff sack), synthetic fill degrades and loses loft. Down bags last 1500+ nights with proper care.
Best for: Humid climates, wet-weather camping, budget-conscious buyers, hard use where the bag might get damp.
The Overlanding Context
For overlanding specifically, weight matters less than backpacking — you're not carrying the bag on your back. But moisture management matters more, especially in roof tents where condensation is a persistent issue. A synthetic bag or a hydrophobic down bag is often the smarter choice for roof tent sleeping. If you run a hardshell like the iKamper Skycamp 3.0 with good ventilation, condensation is less of an issue and down works well.
Layering Strategy: The Smart Alternative
Instead of buying one sleeping bag for every possible temperature, consider a layering system:
- Primary bag: A quality 30–35degF (comfort-rated) down or synthetic bag. This covers 80% of your camping nights.
- Liner: A fleece or silk liner adds 8–15degF of warmth. A Sea to Summit Reactor liner adds 14degF to any bag.
- Overbag or quilt: For true winter camping, a lightweight quilt layered over your primary bag extends your range to sub-zero temperatures.
This system is more versatile and often cheaper than buying multiple bags for different seasons. You can dial warmth up or down depending on conditions, and on warm nights you sleep with just the liner.
What You Wear Matters
A sleeping bag doesn't generate heat — it traps the heat your body produces. What you wear inside the bag affects how warm you sleep:
- Base layer: A merino wool or synthetic base layer (top and bottom) adds meaningful warmth without restricting movement.
- Socks: Warm wool socks in a mummy bag keep your feet warm — extremities cool first.
- Head coverage: You lose significant heat through your head. A fleece beanie or balaclava is transformative on cold nights.
- Don't overdo it: Too many layers inside a sleeping bag compress the insulation beneath you, reducing its effectiveness. One base layer is usually optimal.
Your Sleeping Pad Matters More Than You Think
The insulation beneath you is more critical than the insulation above you. Cold ground (or a cold roof tent floor) conducts heat away from your body far faster than cold air. A sleeping bag compressed under your body weight has near-zero insulating value.
Sleeping pad R-value determines how well it insulates from the ground:
- R-value 1–2: Summer only
- R-value 3–4: 3-season use (down to ~30degF)
- R-value 5–6: Cold weather and winter camping
- R-value 7+: Extreme cold / snow camping
In a roof tent with a built-in foam mattress, the mattress provides some insulation (typically R-value 3–4), but on genuinely cold nights, adding an insulated sleeping pad on top of the mattress makes a dramatic difference.
Buying Guidelines
- Use the comfort rating as your baseline. If the bag only lists one temperature, add 10–15degF for realistic comfort.
- Buy warmer than you think you need. You can always open a bag to vent heat; you can't add insulation you don't have.
- Try before you buy if possible. Mummy bags, rectangular bags, and quilts all feel different. Some people can't stand the restriction of a mummy bag.
- Invest in your sleep system holistically: bag + pad + pillow + layers. A $300 bag on a $20 pad will leave you cold. A $150 bag on a quality insulated pad will keep you warm.