Down lofts further for its mass than synthetic fibre does, so a bag reaches the same stated rating with fewer grams of fill and less packed volume. That single property is what the whole down-against-synthetic argument runs on, and everything else in the comparison is a consequence of it.
Loft is what does the work
Insulation works by holding still air. The material itself is almost incidental; what matters is the volume of trapped air a given mass can create and hold. Down clusters open into a three-dimensional structure that traps a great deal of air for very little weight, and that is why a down bag is lighter and smaller for a given stated rating.
Synthetic insulation is made of fibres arranged to do the same job. It takes more mass and more bulk to reach the same loft, which is why a synthetic bag at the same stated figure is heavier and packs larger.
The two down numbers, and why one is useless alone
Down is rated by fill power — the volume a fixed mass lofts to. A higher fill power means better-quality down that opens further per unit of mass.
On its own it tells you nothing about the bag. The figure to read beside it is the fill weight: the grams of down actually in the thing. A high fill power with a small fill weight describes a light bag, not a warm one, and comparing two bags on fill power alone is the commonest error in this family. Both numbers are published by makers who take the specification seriously.
Around the fill, read the shell denier and the baffle construction, which is what stops down migrating to one side of the bag and leaving a cold seam where it left.
What synthetic buys in exchange
Three things. It keeps more of its loft when damp, which is the argument for it in wet climates and in a bivy bag where your own moisture has nowhere convenient to go. It costs less for the same stated figure. And it is far easier to care for: washing a down bag is a careful job, while synthetic tolerates ordinary handling.
The cost is weight and packed size, and both are published. Compare them directly rather than by reputation.
What the temperature figures are
Bags tested to EN 13537 or its successor ISO 23537 carry a comfort figure and a limit figure. Those are laboratory values, measured on a heated mannequin rather than on people, and they are an indication rather than a promise. A third figure sometimes appears at the bottom of the label; it is not a temperature to plan a night around.
Read which standard the maker names beside the numbers. A bag quoting a bare figure with no standard cannot be lined up against one quoting EN 13537, and the difference is not in your favour. Whatever the label says, the manufacturer’s own guidance governs how the bag is meant to be used.
What sits underneath is a separate number
A bag’s rating assumes the sleeper is not losing heat straight into the ground, and that job belongs to the mat. Mats publish an R-value, and since ASTM F3340 gave the industry a common test those figures can be compared directly between makers. R-values add, which is why a thin closed-cell mat under an inflatable one still exists as a product.
So the honest comparison is a set rather than a single number: fill power with fill weight, the temperature figures with the standard that produced them, and the mat’s R-value with the standard behind that.
What people ask before buying
Does a high fill power alone make a bag warm?
No. Fill power describes the volume a fixed mass of down lofts to, and it says nothing about how much down is in the bag. The figure to read beside it is the fill weight in grams, because a high fill power with a small fill weight is a light bag rather than a warm one.
What does hydrophobic down change?
It is a treatment the maker states, and the claim belongs to the maker. It is quoted here as published, alongside the fill power, the fill weight and the temperature figures, rather than restated as a property of the bag.
Last reviewed 6 September 2026