The Problem With Temperature Ratings
Walk into any outdoor shop and you'll see sleeping bags labelled with temperature ratings like "Comfort: 2°C / Limit: -3°C / Extreme: -18°C." These numbers come from a standardised test (EN 13537 / ISO 23537), which is a significant improvement over the wild west of unverified manufacturer claims that existed before it. But even standardised ratings require interpretation to be useful.
What the EN/ISO Standard Actually Measures
The standard uses a heated mannequin to measure how a bag retains heat at different ambient temperatures. It produces four ratings:
- Upper Limit: The temperature at which a "standard man" can sleep without excessive sweating. Relevant for warm sleepers.
- Comfort Rating: The temperature at which a "standard woman" can sleep comfortably in a relaxed position. This is the most useful practical reference.
- Lower Limit: The temperature at which a "standard man" can sleep for 8 hours in a curled position without waking from cold.
- Extreme Rating: Survival threshold only. Extended exposure at this temperature risks hypothermia.
Key takeaway: Most manufacturers advertise the Lower Limit, not the Comfort rating. This is technically accurate but misleading for average users — especially those who sleep cold.
How to Adjust Ratings for Your Reality
The mannequin test assumes a standard set of conditions that may not match yours. Adjust your selection based on these factors:
Sleeping Warm vs. Cold
Some people naturally sleep warm; others sleep cold. If you consistently need extra blankets at home, select a bag rated significantly warmer than your expected conditions. A practical buffer of 5°C beyond your coldest expected temperature is a reasonable starting point for cold sleepers.
Tent Type and Ground Insulation
Bags are rated in combination with a sleeping pad. Your sleeping pad's R-value matters as much as the bag — cold ground conducts heat away from your body rapidly and no bag can compensate for inadequate insulation beneath you. For 3-season camping, an R-value of 3–4 is recommended; winter camping demands R-5 or higher.
What You're Wearing
Sleeping in thermal base layers, a hat, and dry socks can effectively add several degrees of warmth to any bag's performance. This is a useful tool for extending a bag's range into colder conditions.
Moisture and Humidity
A damp bag performs significantly worse than a dry one. This is especially critical for down bags, where moisture collapses the loft that provides insulation. Keep your bag dry, store it uncompressed, and air it out after each use.
Down vs. Synthetic Fill
| Property | Down Fill | Synthetic Fill |
|---|---|---|
| Warmth-to-weight | Superior | Heavier for same warmth |
| Pack size | Smaller | Bulkier |
| Performance when wet | Poor (improved with hydrophobic treatment) | Retains warmth when damp |
| Durability | Long-lasting if cared for | Degrades faster |
| Price | Higher (especially high fill power) | More affordable |
Fill Power Explained
For down bags, fill power (e.g., 650FP, 800FP, 900FP) indicates how much one ounce of down expands in cubic inches. Higher fill power = lighter weight for the same warmth. An 800FP bag will be lighter and pack smaller than a 650FP bag with the same temperature rating — but will cost more.
Bag Shape and Its Effect on Warmth
- Mummy: Tapers from shoulders to feet, minimising dead air space. Most thermally efficient. Best for cold conditions.
- Semi-Rec/Tapered: Slightly roomier than mummy, slightly less efficient. Good three-season compromise.
- Rectangular: Maximum space, minimum warmth efficiency. Fine for car camping in mild weather.
The Practical Rule of Thumb
When choosing a sleeping bag, always buy for the coldest night you expect to face, not the average. Being too warm is easy to manage (unzip, vent). Being too cold in a sleeping bag is a much harder problem to solve at 2am on a hillside.