A sleeping bag does not produce warmth. It slows the rate at which the body loses heat, and the temperature printed on the stuff sack describes that slowing under conditions nobody sleeps in: a standard mat, standard clothing, still air and a mannequin that lies perfectly still. The obvious reading of the figure is that it states the temperature at which the bag keeps a person warm. The useful reading is narrower. The figure is a laboratory comparison between bags measured the same way, and it says nothing about the mat underneath or the wind on the tent wall.
Most disappointing nights have the same origin: a rating treated as a promise. The bag is one part of a system, since the mat beneath it and the clothing worn inside carry a share of the insulation, and a good bag on a poor mat will still be cold. Loft, dryness and fit matter more than the printed number, and all three can be judged before any money changes hands.
What the standard actually measures
The European rating standard began as EN 13537 and now sits in the ISO 23537 family. It produces its figures from one laboratory procedure. A heated mannequin wearing a standard base layer lies on a standard mat inside a climate chamber, the chamber is cooled in steps, and the power needed to hold the mannequin at a steady skin temperature is measured.
Four temperatures come out of it.
- Upper limit is the warmest temperature at which the bag can be used without overheating.
- Comfort is the lowest temperature at which a standard woman is comfortable in a relaxed posture.
- Lower limit is the lowest temperature at which a standard man is comfortable, curled up.
- Extreme is a survival figure, the temperature at which a standard woman is judged to be at risk of hypothermia within six hours.
The distinction that decides how the numbers are used is the one between comfort and extreme. Extreme is not a sleeping temperature and does not belong in trip planning. It marks a point at which a cold and tired person starts to be in trouble, and it assumes the accepted posture and the standard clothing. A bag bought so that its extreme figure matches the forecast will be a bad night for anyone who has to rely on it.
Two qualifications follow from the test. It uses one standard mat, so a thinner mat changes the outcome more than the choice of bag does, and it uses a still mannequin, so it cannot account for a restless sleeper or a day that included too little food.
Fill power and fill weight
Down bags carry two numbers that are often confused. Fill power measures the volume a fixed mass of down occupies, quoted in cubic inches under American practice or as a European fill power figure under European practice. The two methods use different cylinders and loads, so the same down can read higher on one scale than the other. Fill weight is the mass of down in the bag.
Neither number is a warmth rating by itself. High fill power means more loft for each gram, so a bag can be lighter for the same warmth or warmer for the same weight. Price rises quickly with fill power, and the gain flattens once the shell, the baffles or the sleeping mat set the limit. Walking bags commonly use down between roughly 550 and 900 fill power, and figures quoted without a test method compare only within one maker’s range.
Synthetic fills divide into two families. Short-staple fills imitate the structure of down and give better warmth for weight, but they migrate and clump over years. Continuous filament fills are knitted into a sheet, stay in place and last longer, with slightly less warmth for the same weight. Both are heavier and bulkier than down, and both keep a useful share of their insulation when damp.
Loft is the insulation
A sleeping bag holds still air. The fill traps air in the gaps between fibres, and the thickness of that trapped layer is what resists heat loss. This is why loft, not the label, is the honest measure of a bag.
Loft is easy to check. Lay the bag flat, give it a few minutes to recover from the stuff sack, and measure the height of the fabric above the bed of the bag at chest level. A bag that has lost loft has lost warmth, whether through damp, through years of compressed storage or through the fill breaking down.
Baffles hold the loft in place. Sewn-through construction stitches the inner and outer shells together, which saves weight and creates a line of zero thickness where the stitching runs. Box wall construction inserts a strip of fabric between the shells so the fill can expand fully, adding weight and raising the warmth of a bag with the same fill weight.
Damp decides how much loft survives the night. Down loses most of its loft when wet and is slow to dry. Synthetic fill loses less and keeps working while the shell is clammy with condensation.
Shells and linings
The shell is the bag’s defence against the damp that destroys loft. Shells of roughly 10 to 20 denier save weight and resist abrasion less well, while heavier fabrics take more punishment at a weight cost. A water-repellent finish sheds splashes and condensation, and needs renewing as it wears. Linings are usually nylon or polyester taffeta, chosen for how they feel against the skin. Down treated to resist moisture holds more loft in damp conditions and costs more for the same fill power.
Shape is a weight decision
A bag is a closed tube, and its shape trades space against weight.
- Mummy bags taper from the shoulders to the feet and enclose the head in a hood. The taper removes fill the legs do not need, and the hood covers the surface that loses the most heat when it is bare.
- Rectangular bags are roomier and usually cheaper, and heavier. They suit car camping and anyone who cannot sleep in a narrow cut.
- Quilts remove the underside fill, on the basis that it is crushed beneath the sleeper, and rely on a mat and straps to seal the edges. They are the lightest option and the least forgiving of draughts.
Two fittings decide how a bag feels at three in the morning. A draft collar is a ring of insulation that closes around the neck, and without it warm air escapes whenever the shoulders move. A draft tube behind the zip does the same job along the opening, which is why a bag with a thin zip baffle feels colder than its rating suggests.
Length and girth decide whether the fill can do its job. A bag that is too narrow compresses its own insulation against the shoulders and hips, and compressed fill insulates poorly. Where makers publish internal dimensions, those are more useful than a size name.
A buying rule
Read the comfort figure first and treat the extreme figure as irrelevant. Compare fill weight and loft rather than a single temperature, and check the girth against the sleeper and the mat it will lie on. Choose down for dry cold and synthetic for wet, mild conditions, and count the mat and the bag as one purchase. A bag that is warm enough and slightly heavy will always beat a lighter bag that is not.
