Insulation -- How Jackets Keep You Warm
A jacket does not make heat -- it traps the heat you already produce. Insulation works by holding still air in tiny pockets. This guide explains the two families (natural down and synthetic), the numbers that describe them (fill power and fill weight), and the qualities that decide how warm, light and packable a jacket feels.
Natural Down
Down Insulation
The soft, fluffy clusters from under a bird's feathers -- the warmest, lightest, most packable insulation there is.
Why it is special
Down lofts into a three-dimensional cluster that traps huge amounts of still air for almost no weight. Nothing synthetic quite matches its warmth-to-weight ratio.
The catch
Untreated down clumps and loses loft when wet. Modern jackets use water-resistant down or a protective shell. See the canonical down insulation glossary.
Duck Down
Down sourced from ducks -- excellent, widely used and slightly more affordable than goose down.
How it performs
Duck down delivers most of the warmth of goose down at a lower price, which is why it fills the majority of down jackets.
Cluster size
Duck clusters are generally a little smaller than goose, so very high fill powers are rarer -- but for most jackets the difference is small.
Goose Down
Down from geese -- larger clusters, higher achievable fill power and a premium feel.
Why pay more
Geese are bigger birds with bigger down clusters, so goose down can reach the highest fill powers (800+), giving more warmth for less weight.
Best for
Premium winter coats where lightness and packability matter most.
Fill Power
The number that measures down quality -- how many cubic inches one ounce of down lofts to fill. Higher means warmer for its weight.
Reading the number
600 fill power is good, 700 is very good, 800+ is premium. It measures quality (loft), not quantity. A small amount of 800-fill can out-warm a lot of 550-fill.
Common confusion
High fill power does not automatically mean a warmer jacket -- you also need enough of it (fill weight). See the canonical fill power glossary.
Fill Weight
The actual amount (grams) of down stuffed into the jacket. This is the 'how much', where fill power is the 'how good'.
Why it matters
Two jackets can share the same fill power but differ wildly in warmth because one holds twice the down. Fill weight is the quantity dial.
Warmth = power x weight
Real-world warmth comes from fill power and fill weight together, plus how well the baffles stop cold spots.
Synthetic Insulation
Synthetic Insulation
Man-made fibres that mimic down's lofting clusters -- warm when wet, faster-drying and lower cost.
How it works
Fine polyester fibres are crimped and layered to trap air like down does. They keep insulating even when damp, which down struggles to do.
Trade-off
Synthetic is bulkier and heavier than down for the same warmth, and packs down less small. See down vs synthetic.
PrimaLoft
A premium synthetic insulation engineered to behave like down -- soft, packable and warm even when wet.
Why it is known
PrimaLoft's ultra-fine fibres get closer to down's warmth-to-weight than most synthetics, while keeping synthetic's wet-weather advantage.
Best for
Active and wet-climate use where down's wet-weakness is a risk.
ThermoBall
The North Face's synthetic insulation built from small clusters that mimic down's loft and trap warmth even when wet.
How it works
Instead of continuous sheets, ThermoBall uses round synthetic clusters that loft like down balls, holding warmth and resisting damp.
Who makes it
Found across The North Face jackets -- see the brand technologies hub.
Fibre Ball Insulation
A general term for synthetic insulation shaped into small round clusters rather than flat sheets, for a down-like loft.
Why the shape
Round clusters move and loft more like natural down, so the jacket feels softer and traps warmth more evenly than sheet insulation.
Where you see it
An increasingly common construction in mid-range puffers chasing the down look and feel without natural down.
Hybrid Insulation
A jacket that mixes down and synthetic -- down where it stays dry, synthetic where sweat or rain hits.
Best of both
Down keeps the core light and warm; synthetic guards the cuffs, shoulders and hood where moisture collects. The result resists damp better than pure down.
Best for
Wet British winters and active use where pure down would struggle.
Measuring Warmth
Loft
How much an insulation puffs up and the thickness of the air layer it creates. More loft, more trapped air, more warmth.
The core idea
Warmth is really about trapped still air. Loft is the visible sign of that air layer -- a flat, compressed jacket has lost its loft and its warmth.
Keep it lofted
Storing a jacket compressed kills loft over time -- see the care hub on restoring loft.
Warmth-to-Weight Ratio
How much warmth you get per gram of jacket. The headline reason down is prized and synthetics keep improving.
Why it matters
A high warmth-to-weight jacket keeps you warm without weighing you down or bulking you out -- vital for travel, walking and layering.
Down still wins
Gram for gram, high fill-power down leads; premium synthetics close the gap but rarely beat it.
Compression
How small a jacket squashes down for packing. Down compresses best; synthetic less so.
Why you care
A jacket that compresses small lives in a bag or rucksack until needed. Down's compressibility is a big practical advantage for travel.
Care note
Long-term compression damages loft -- compress to carry, store loose at home.
Packability
Whether a jacket folds into its own pocket or a small stuff sack. Driven by insulation type and fabric weight.
The convenience factor
Packable jackets stash away when the sun comes out and travel without taking a bag slot. Most lightweight down and windbreakers pack; heavy parkas do not.
What enables it
Thin face fabric plus compressible insulation. See materials.
Thermal Efficiency
How well a jacket converts its insulation into felt warmth, accounting for baffles, cold spots and wind.
More than fill
Even great insulation underperforms if baffles leak heat, seams let wind in, or the outer is not windproof. Construction matters as much as fill.
See also
Baffle design in the construction hub and the canonical thermal insulation glossary.
Frequently asked questions
Is down or synthetic warmer?
Gram for gram, high fill-power down is warmer and lighter. Synthetic wins when it gets wet, dries faster and costs less. For dry cold choose down; for wet or active use consider synthetic or a hybrid.
What fill power should I look for?
600 is good, 700 very good, 800+ premium. But check fill weight too -- a high fill power with little down is not a warm jacket.
Does down work in the rain?
Untreated down clumps and loses warmth when soaked. Modern water-resistant down and a weatherproof shell solve most of this, but a hardshell over down is the safest in heavy rain.
Why has my puffer gone flat?
It has lost loft, usually from compression or trapped moisture. Washing and tumble-drying with dryer balls re-lofts most down -- see the care hub.