Choosing an insulation is not a matter of picking the warmest material on offer. It is a matter of deciding which compromise your programme can live with. Every family holds still air in a different way, and each behaves differently when it takes on moisture, when it is compressed and carried, and when it is worn under load-bearing equipment. Those three questions, not the material name, decide whether a garment still works after the wearer stops moving.
What insulation is actually doing
Insulation does not generate heat. It slows the rate at which the body loses it, mostly by holding still air close to the body. Loft structure decides how much air is held, quilt or baffle construction decides whether the air stays distributed, and moisture decides whether the air is still there at all: water displaces air, so insulation that has taken on moisture has partly replaced the thing it was chosen for with the thing that conducts heat away. Any serious comparison between insulations therefore has to fix the test conditions rather than compare numbers from different datasheets. Our insulation comparison test sets out those conditions and the limits of what such a test can tell a buyer.
The main families and how they differ
| Family | How it holds air | Where it performs well | Where it struggles |
|---|---|---|---|
| Down | Lofted clusters held in baffles | Warmth for weight, compressibility | Loses function when wet, slow to dry |
| Synthetic, continuous filament | A bonded structure that resists collapse | Damp conditions, repeated packing | More bulk for the same trapped air |
| Synthetic, short staple | A batt of short fibres held in a scrim | Cost control, even distribution | Can migrate and settle with wear |
| Fleece | Brushed knit that traps air between fibres | Active use, damp tolerance, drying | Bulk in the layer stack, wind passes through |
| Reflective or radiant liners | Returns some radiated heat inward | Adding a light boost inside a shell | Not a substitute for a trapped-air layer |
The table is deliberately about behaviour rather than performance figures. We do not publish warmth values for any insulation, because a figure measured on a material does not describe what a finished garment does with it.
Down: the best warmth for weight, and a moisture penalty
Down holds an exceptional amount of air for its weight, which is why it is used where weight and packability dominate. Its weakness is structural: wet down clusters collapse, the trapped air is lost, and drying is slow. In a dry cold programme with the ability to keep the insulation dry that trade can be correct; in a wet cold or high-output programme, down needs a shell and a moisture path that can be relied on.
Synthetic fills: continuous filament and short staple
Synthetics trade some warmth for weight in exchange for structure that does not collapse when wet. Continuous filament constructions resist migrating and hold their distribution better under repeated packing, which matters for garments that are compressed and carried. Short staple battings spread cost more evenly across a panel and are easier to specify in different amounts by body zone, but they can shift and settle with use, which is why quilt spacing matters as much as the batt itself.
The practical question for a buyer is not which is better, but which failure mode your programme can tolerate: more bulk for the same warmth, or uneven warmth after a season of compression.
Fleece: the layer that keeps working wet
Fleece is the least glamorous option and frequently the most useful. A brushed knit traps air between fibres, continues to insulate when damp and dries relatively quickly, which is the combination needed for the layer a wearer moves in and out of all day. Its limitations are bulk and wind: it is a poor outer layer because air passes through it. Read more on the fleece material page.
Reflective liners deserve a separate caution. They return a portion of the heat a body radiates back toward it, which is a useful increment inside a shell and also easy to oversell. A reflective liner is not a replacement for a trapped-air layer, and where a quotation rests on a reflective claim the buyer should ask what the layer beneath it is doing.
Quilting and baffle construction decide the outcome
Two garments with the same insulation can behave differently because of how the insulation is held. Baffle height and spacing, quilt line placement, seam type and how much insulation is mapped into each panel all change where warmth sits on the body and how well it survives compression. Mapping by panel, rather than applying one uniform thickness, keeps bulk away from the shoulders and inner arms where it costs movement. This is a product development decision and belongs in the specification, not in the sampling discussion.
Matching the insulation to the programme
Work from the failure mode, not from the material name. In dry cold with shelter, weight and packability may dominate. If the wearer alternates between exertion and waiting, recovery after compression and behaviour when damp matter more than maximum loft. If precipitation is likely, the shell and the moisture path become the priority. Insulation choice also interacts with the base layer, and a base layer that cannot move moisture will defeat any insulation above it; see our moisture wicking fabric page for how the innermost layer is specified.
What you cannot choose from a datasheet
A material datasheet describes a material in a condition your garment will never be in. It does not describe the insulation sewn into a quilted panel, compressed by equipment, worn against a wet base layer for a shift and then packed wet overnight. That is why we describe insulation behaviour in words rather than in figures, and why any comfort conclusion has to come from your own field trials in your own conditions. Where a tender requires measured performance, agree the test method first and have it run on the construction being offered.
Our cold weather tactical jackets are developed around these choices, and the full system view, including how the insulation layer works with the layers above and below it, is set out in the extreme cold weather programme.
FAQ
Is down or synthetic insulation better for cold weather clothing?
Neither is better in the abstract. Down holds more air for its weight and packs smaller, but collapses when wet and dries slowly. Synthetics give up some warmth for weight in exchange for keeping their structure when damp. The answer depends on whether your programme can keep the insulation dry and how much packability matters to the wearer.
Does higher loft always mean a warmer garment?
No. Loft is a proxy for trapped air, and trapped air only insulates while it stays in place. Quilt spacing, baffle construction, compression and moisture all change how much of that air remains where the wearer needs it. A high loft panel that collapses under a strap can deliver less than a lower loft construction that holds its structure.
Is fleece warm enough to be an insulation layer?
Fleece is best understood as active warmth rather than as a replacement for an insulation layer. It keeps working when damp and dries quickly, which suits the layer a wearer moves in and out of, but it is bulky and air passes through it, so it needs a shell over it.
Can we specify insulation amounts by body zone?
Yes, and in a cold weather garment you generally should. Mapping insulation by panel keeps bulk away from shoulders and inner arms, where it costs movement, and puts more warmth where the body loses heat fastest. It is a pattern decision, agreed before sampling and documented so repeat orders match.
How do we verify that the insulation performs as specified?
Agree the test method first, then have it run on the actual construction rather than on a material sample, and keep the report with the order. Where the tender requires independent verification, that test has to be commissioned from an accredited laboratory. For comfort in real conditions, the evidence that counts is your own field trial.