Down, PrimaLoft or Fleece? How Warm a Kid's Jacket Really Needs to Be

Stand in front of a rack of kids' winter jackets and the labels start talking at you. Down. Goose down. 650 fill. 750 fill. PrimaLoft. Synthetic insulation. Fleece-lined. Brushed. Most of it is true, some of it matters, and almost none of it is explained. So parents do what anyone would do — they pick the puffiest one, or the most expensive one, and hope.

The puffiest one is often wrong. Warmth for a child isn't about how thick the jacket looks; it's about how the insulation behaves when a kid does what kids do — run until they're sweating, then stand in the cold, then fall in something wet. Down, synthetic insulation and fleece each answer that differently. This is a plain-language guide to which one belongs on which kid, on which day, and what the numbers on the label are actually telling you.

First, how warmth works on a moving kid

Insulation doesn't make heat. It traps the heat a body is already producing, in tiny pockets of still air. The more air a material can hold for its weight, the warmer it is per ounce. That's why a light down jacket can be warmer than a heavy wool coat: the down is holding more air.

The complication is that kids don't produce heat at a steady rate. A child on a sideline is producing very little; the same child ten minutes into a game is producing a lot and sweating. A jacket that's perfect for the first state is a sauna in the second — and the sweat that builds up becomes cold and damp the moment they stop. The best insulation for a child isn't the warmest one. It's the one that copes with both states, and lets moisture out while it's holding heat in.

Down: the warmest per ounce, with one weakness

Down is the fine under-plumage of ducks and geese, and nothing made by people has matched it for warmth-to-weight. It compresses to almost nothing, which is why a down jacket can live in a backpack, and it lasts for years if it's cared for.

Its one weakness is water. When down gets soaked it collapses — the clusters clump together, the air pockets disappear, and the jacket stops insulating until it's fully dry. That's manageable with a water-repellent shell fabric and a kid who mostly stays upright, and it's a real problem for a kid who rolls in wet snow. Our seam-sealed waterproof down jacket solves it from the outside, with a fully sealed shell so the down inside stays dry; the lighter down jackets in the range rely on a water-repellent finish and are best for cold, dry days.

What "fill power" means — 650 vs 750

Fill power measures how much space one ounce of down takes up when it lofts fully. A 650-fill down expands to about 650 cubic inches per ounce; 750-fill expands to 750. Higher fill power means more air trapped per ounce, so the same warmth from less down, and a lighter, more compressible jacket.

Two things follow from that. First, fill power is a quality measure, not a warmth measure — a 650-fill jacket with a lot of down in it can be warmer than a 750-fill jacket with a little. The amount of down matters as much as its quality, and that's not always on the label. Second, for a child, the practical benefit of a higher fill power is weight and pack size, not survival: the 750-fill goose down jacket in our range is lighter and squashes smaller than the same warmth would in a lower fill, which for a kid carrying it to school matters more than any number.

Synthetic insulation: the one that keeps working wet

Synthetic insulation — PrimaLoft is the best-known name — is a web of fine polyester fibers built to mimic down's air-trapping structure. It isn't quite as warm per ounce, and it doesn't compress quite as small. What it does that down can't is keep most of its warmth when it's damp, and dry out fast.

For a lot of kids, that trade is a win. Wet mittens wiped on the chest, a puddle at recess, snow down the back of the collar — a synthetic jacket shrugs it off. Ours is the Primaloft puffer jacket, and it's the one we'd point a family toward if the climate is wet rather than bitterly cold, or if the child is the type who ends every outdoor hour soaked through regardless of the weather. The same insulation shows up in a vest for kids who run hot; the case for that is in our guide to kids' vests.

Fleece: the layer for the running kid

Fleece isn't insulation in the same sense — it's a knitted fabric with a brushed surface that traps air in its pile — but for an active kid it's often the most useful of the three. It breathes better than either down or synthetic fill, so a child who runs hot doesn't stew in it. It dries quickly. It goes through the wash without fuss. And it's warm enough on its own for most autumn days, and as a middle layer under a shell for real winter.

The limitation is wind and wet. Fleece lets air through, which is why it breathes, and that means cold wind goes straight through it too. On its own, fleece is a cool-weather jacket; in winter it's what goes under something windproof. We covered how to choose one in our fleece jacket guide.

The short version, by kid and by weather

• Cold and dry, kid mostly walking and standing: down. Lightest, warmest, packs smallest.
• Cold and wet, or a kid who gets everything wet: synthetic insulation. Warm when damp, dries fast, forgiving.
• Cool weather, kid constantly moving: fleece. Breathes, doesn't overheat, washes endlessly.
• Real winter, active kid: a system — quick-drying base layer, fleece or light insulation in the middle, a windproof or waterproof shell over the top. Each layer can come off as the day warms up, which is exactly what one heavy coat can't do.

Caring for insulation so it keeps its warmth

Insulation stops working when it's dirty or compressed. Body oils and grime coat the fibers and stop them lofting, so a jacket that's never washed gets colder over a season. Down and synthetic jackets should be washed cold on a gentle cycle with a mild detergent — no fabric softener, which coats the fibers — and tumbled dry on low with a couple of clean tennis balls to break up clumps and restore the loft. Dry completely before storing, and store loose on a hanger rather than compressed in a bag. Fleece is simpler: wash cold, skip the softener, and it comes out as good as new. For the everyday routine across the rest of the closet, see how to wash kids' activewear.

Frequently asked questions

Whichever insulation you land on, the test is the same: does the kid keep it on? A jacket that's warm on the hanger and abandoned on the bench isn't doing its job. Browse the down and insulated jackets, or start with the whole winter collection, and build from the base layer up. The jackets that pass that test most often are in best sellers.