Down vs Synthetic Sleeping Bags — Which Is Best?

Down is the insulating layer of a waterfowl — goose or duck — and it works because of its structure. Each bird grows thousands of tiny filaments that branch into clusters, and those clusters hold still air against your body. The warmth of a down cluster depends entirely on how big and lofty it is, which is what the fill power number measures. A 650 fill-power cluster takes up more volume per gram than a 400 fill-power cluster, so a bag can be both lighter and warmer. That is the entire down argument, and it is a strong one.

Synthetic insulation — most commonly hollow fibre — starts as a spun plastic filament formed into a coiled, spring-like structure. The coils trap air in their loops. Because the structure is a continuous filament rather than a cluster of separate bits, water runs off it rather than soaking into it, and the coils spring back after being compressed. That resilience is why synthetic recovers from a stuff sack and why it shrugs off a damp night.

Both are sold in the same three shapes — mummy, rectangular, and double — so shape is not a differentiator. Nor is the shell, which is usually nylon or polyester either way. The real dividing lines are warmth-per-gram, wet performance, longevity and cost. Here are the real listings we are comparing, side by side:

Read that table twice and one thing jumps out: the deepest cold ratings sit with the pricier bags, and the cheap bags cluster in the 20F to 30F band. That is not a conspiracy, it is a materials fact. Getting a -30F rating requires a lot of insulation, and insulation costs money whichever fill you use.

Put a down bag and a synthetic bag of the same weight side by side and the down bag is warmer, every time. This is the single strongest argument for down, and it is why every serious sub-zero bag you will see is down. The at $179.99 is the clearest example in the current listings: 4lb of 650 fill-power white duck down in a 400T nylon shell, with vertical baffles to slow heat movement, a zipper windshield to block convection, and a rating of -22F. Nobody puts four pounds of hollow fibre in a bag; the weight would be unmanageable for warmth that still fell short.

Fill power is where down bags separate from each other. 500 fill is the old standard and perfectly serviceable. 600 to 700 is modern performance territory. Above 800 you are paying a real premium for marginal gain. The 650 figure on the Qezer puts it squarely in the sensible middle: noticeably lighter than a 500-fill bag of the same rating, without the price tag of a boutique 850-fill shell.

Synthetic cannot match that ratio, but it can match or beat it in a different currency: total warmth per dollar. A synthetic bag does not need to be loft-efficient because it is cheap to add more of it. That is why a $54.99 30F synthetic bag can genuinely rival a used $64.99 down bag rated near 20F. You are not getting down's efficiency; you are getting more insulation for the same money, which lands you in a similar place for shoulder-season trips.

Verdict on warmth: down wins on weight, synthetic wins on dollars, and for most three-season camping the dollar comparison is the one that decides the trip.

This is the category where the argument stops being close. Down clusters are held together by static and structure, and water destroys both. Wet down clumps into dense masses, the trapped air escapes, and the bag that was warm at 30F becomes a cold, heavy sack of damp feathers. Recovery is the painful part: a down bag that has been properly soaked can take a day or more of gentle heat to return to full loft, and if you washed it in a front loader with detergent you have permanently sacrificed the oils that made the clusters work.

Synthetic hollow fibre has no such weakness. The filament is continuous, water beads off it, and the coiled structure returns to shape as soon as it dries. The care instructions show the difference starkly. The at $42.99 is listed as easy to care for by hanging to dry or machine washing cold. That is not a minor convenience — it is the difference between a bag you own for twenty years and a bag you are nervous around in the rain.

Counter-argument, and it is a fair one: you do not have to get a down bag wet. A 400T nylon shell, a proper draft collar and disciplined dry-bag practice mean down stays dry in ordinary three-season rain. The failures cluster in specific situations — river crossings, storm deluges, condensation inside a single-wall tent, or a bag that got damp and then got stuffed into a pack still damp. If your trips are alpine, multi-day and weather-driven, that is exactly your situation.

Condensation deserves its own mention. A sleeping bag is usually the coldest thing in your shelter, which means moisture from your breath collects on the inner surface. In a single-wall tent or a cold bivvy, that moisture can end up in the bag overnight even when nothing falls from the sky. Synthetic tolerates this without comment. Down needs the bag to be dry before you seal it into a stuff sack, and a stuff sack is an almost perfect container for rot.

Verdict on wet weather: synthetic wins outright. This is the deciding factor for anyone camping in the shoulder seasons, anyone without a dryer at home, and anyone with a tendency to lose track of which bag is dry.

Bag longevity is not simply about the age of the bag. It depends on how the fill has been treated. A down bag stored loose in a large breathable sack in a dry closet, and washed rarely and gently, can still be excellent at thirty years old. Synthetic loft, by contrast, degrades more steadily with heat exposure, repeated machine washing and time in a compressed state. After a decade a synthetic bag can feel noticeably flatter than it did when new, at the same printed rating.

There are two live vintage listings that demonstrate this well. The at $64.99 is 3lb 12oz, retains what the seller describes as plenty of loft, and has a nylon shell zipped all the way around so it converts into a comforter — a 72 by 26 by 22 inch shape that is genuinely useful. Note the price relationship: a working vintage down bag costs about what a brand-new synthetic bag costs, which is the resale argument for down in a single line. The at $44.00 is from the 1970s or 80s, 71 inches long with a 56-inch circumference, with a draft tube, a double-ended zipper and a left-hand zip, described as a summer-season bag with wear near the top of the zipper. It is the same era as the REI bag and it is a summer bag, because the fill is synthetic and decades of compression have taken the edge off it.

That pairing is the honest headline on ageing: old down usually still works; old synthetic often does not. The corollary is the repair argument. A torn down bag can be patched in the field, and a partially flattened down bag can be fluffed in a dryer with tennis balls. A synthetic bag that has lost its structure is simply finished, because the coils have been permanently bent.

Washing and storage: the ten-year test

One more practical difference: washing. Down bags need a down-specific detergent, a delicate cycle or hand wash, and long low-heat drying with dryer balls. Many people skip this and the fill clumps permanently. Synthetic bags go in an ordinary machine on a normal cycle. If the number of things standing between you and owning a bag for a decade is what stops you buying one, that is an argument for synthetic and it is not a small one.

New, the two fills price differently in a predictable way. Synthetic occupies the cheap end, because its raw material is cheap and its rating requirements are modest. Down occupies the middle to upper range, because fill power, fill weight and shell quality all add up. In the current data the spread is stark: the at $54.99 and the at $53.31 sit at the floor, while the at $179.99 sits at the ceiling, with the at $145.00 between them.

Used, the gap closes and then inverts, because a down bag holds value where a synthetic bag does not. The two vintage down listings in this comparison are $64.99 and $52.50. The at $52.50 is 70 by 32 inches from Recreational Equipment Inc, described as being in as-is condition with some defects and slight stains that have not been washed, and not insect-repellent treated. That is honest seller language, and it is also a down bag for the price of a new synthetic one — a genuinely good deal for someone who does not mind a vintage shell and a wash.

The flip side of the used market is that the seller usually does not know the fill weight or the true rating. The REI semi-rectangular listing guesses at about 20F by judging the weight. Guesses like that are why a used down bag is often a bargain and sometimes a $64 disappointment. Synthetic used bags have the opposite problem: the price is already low, so the downside is capped.

The cheapest listings illustrate the floor. The is $30.35 for 100 percent polyester shell, lining and filling, used once and described as having a small pull on the edge. A sits at $37.99 new. Both are warm-season, low-effort, synthetic products, and both are perfectly sensible buys for a kid's first trip or a summer weekend.

Head to head: the differences that matter

Neither column is a winner in every row. That is the point. Down wins eight of the above on merit and loses on wet performance, washing and cost, and those three losses are the ones that decide most people's trips.

Buy down if you are carrying the bag on your back for multiple nights, you are heading into genuine cold below 20F, you are confident you can keep it dry, and you intend to own it for a decade. The at $179.99 is the flagship of that case: -22F rated, 400T nylon inside and out, vertical baffles, zipper windshield, and a fill weight stated plainly at 4lb. Read our sleeping bag buying guide before committing, because a down bag bought at the wrong rating is an expensive mistake.

Buy synthetic if you are car camping, your trips run shoulder-season in a wet climate, you share a bag with a partner or kids, you have no dryer, or you want a bag you will not think about. The at $42.99 is the case for synthetic: a 20-degree rating, room for two, machine washable, and no dry-bag ritual. The at $20.49 is the same logic at its simplest — 50F rated, synthetic, rectangular, one-person, machine washable, and priced so that a child can lose it in a wood without ruining your weekend.

Buy used down if the bag will live in a car or a garage and you like the idea of a $52 to $65 down bag. Check the loft in the photos, ask whether it has ever been washed, and accept that a seller estimating the rating by feel may be off by 10F. The at $64.99 is the best-value version of this, and its full-perimeter zip means it doubles as a comforter on a sofa, which no synthetic bag in this price range does.

Consider a liner first if you already own a bag that is 5 to 10F too cold. The at $47.22 is listed as adding up to 12C / 22F of warmth, which is the cheapest fix available and works with either fill.

Upgrade or travel: liners as the cheap option

If you travel rather than camp, the at $42.74 is a different product for a different job: 86.6 by 70.8 inches, 30.7 ounces, packing down to 7.8 by 7.8 by 3.2 inches in soft breathable cotton with a back pocket for your pillow.

One last note on a category that is neither: the at $199.99 is undyed, plastic-free and chemical-free with organic cotton inside and out. Cotton is neither warm nor light, so it is a nap and hygiene product for a child's bed or a summer camp, not a cold-weather bag — but it is the honest answer to the question for parents who want no synthetic and no down in the house at all.

Prices confirmed September 29, 2026. Every listing cited here was live on eBay that day, and the selection is not exhaustive. eBay prices change constantly, vary significantly by seller and by condition, and usually exclude shipping, so treat these as the prices on the day and re-check before you buy. Vintage items in particular may sell once and be gone. Whatever you choose, the fill is the last decision, not the first — get the temperature rating and the shape right, and then pick the insulation that matches your climate and your tolerance for maintenance.