Our Guide To Down Jackets And Synthetic Insulation

By Hati Whiteley

Down jackets keep you toasty, warm and dry outside. The first down jacket was made by George Finch for the 1922 Everest Expedition. Reportedly it was mocked at first but won great respect for its performance in mountain conditions.

Since 1922 they have become ubiquitous in the outdoors and a puffer down jacket has become a fashion statement.

In recent years breakthroughs in man-made fibres mean some synthetic insulation now rival down in terms of performance.

In this article we look at types of down and synthetic insulation and also the features to look out for in an insulated jacket. Have a read of our buyer’s guide to help you decide what insulation you need.

What is a down jacket?

Down jackets are jackets that are filled with the light fluffy feathers found under the breast and underbelly feathers of ducks and geese.

They are so warm because they trap warm body air and hold it close to our skin. And because the feathers are so small and airy jackets made from down are lightweight and can be packed incredibly small. This makes them incredibly effective mountain clothing as you can pack it small in your backpack.

What do fill power and fill weight mean?

Fill power indicates the quality of the down and how warm a jacket can be. It is a measure of how fluffy down is. It measures the volume of air that 20g of down occupies under strict scientific conditions. (In case you were wondering, the number represents how many cm/3 a gram of down lofts to. So, 750 fill power = 1 gram of down lofts to 750 cubic centimetres).

The higher the fill power the warmer it will be. 600-650 fill power is good. 700-750 is very good and over 800 is excellent.

Fill weight as the actual weight of the feathers used in the garment.

Jacket warmth is a combination of the quality of the down (fill power) and how down is used (fill weight).

Down is generally goose down or duck down. As goose down are larger feathers they tend to have a higher fill power. Goose down tends to be more expensive and is seen as a premium fill.

The colour of down feathers range from off-white to black. Generally down feathers used in jackets are white or grey. Some jackets with ultralight fabrics show the colour of down within a jacket.

Types of synthetic insulation?

Synthetic insulation is made with man-made fibres. These are designed to retain body heat and so are incredibly warm and breathable.

There are two main types of synthetic insulation. The first type is a sheet type insulation (sometimes called “mat” insulation) such as Primaloft Gold Active or Primaloft Silver Active. The second type of synthetic inslation mimics the loft and fill qualities of down. Examples include Primaloft Thermoplume which is used in our Talini jacket.

Which is better down or synthetic insulation?

There’s still no man-made fibre that comes close to the comfort and warmth-to-weight ratio and breathability of down. However down can clump when wet and lose all warmth retention. It becomes a cold soggy jacket.

No natural fibre comes close to the performance of synthetic jackets in wet conditions.

However recent breakthrough both in hydrophobic treatment of down and also the lofting of synthetic jackets have closed much of the gap.

Hydrophobic down has been treated to make it retain some of its thermal qualities when wet and recovers better from exposure to damp conditions. This makes it more resilient than regular, untreated down, and better for more humid or damp conditions.

There are different kinds of hydrophobic treatments available, we use Down Tek Water Repellent down. Unlike other treatments, DownTek down is plucked, treated and stuffed in the same country (China), reducing the distance that your down has to travel to get to you.

As with pillows and duvets, all insulation has to be sewn-in. This means that the shell fabric must be fibre-proof to stop the insulation fill escaping. This has the benefit that it is completely windproof. Some synthetic insulations, such as Primaloft Gold active, have been designed to be ‘migration resistant’. This means they can be used with non-fibreproof fabrics… fabrics that are more breathable. Jackets with this technology are ideal for active use over a wider range of temperatures, preventing that annoying ‘on again, off again’ thing… This is where jackets like our Katabatic fit in.

Down insulation performs better when you’re going to be in cold, dry conditions or you know you can keep your jacket dry despite the rain. Choose down when comfort, low bulk and low weight are important.

Synthetic insulation performs better in damp or wet conditions for extended periods and your jacket is likely to get moist. So go synthetic when performance and recovery when wet is more important than weight and packability.

How warm should my jacket be?

‘How warm’ should a jacket be is a funny question: your jacket doesn’t impart warmth of itself, it just helps retain the warmth your body generates. And the warmth your body generates depends on your level of activity.

So the better question is, how much insulation do I need for a specific activity?

If you’re buying a down jacket for activities where you’ll generate more heat (say hillwalking) you’ll probably want something less insulating. A mid-weight jacket like Filoment or Tailini is ideal.

Similarly, if you’re an generating a lot of heat on a mountain marathon or riding a bike, you might go for a little less insulation too. An ultralight layer like Kanyo is an example. Ultralight insulation is ideal of layering too.

At the other end of the spectrum, activities where you’ll be still for extended periods and not generating much heat call heavy weight garments such as Fantom or 0Hiro. Think belay stances, winter camping, manning checkpoints on the High Peak Marathon.

Insulation construction: box wall, stitch-through, wide or micro baffles?

Down and down-like insulated jackets and sleeping bags are constructed with pockets that hold the insulation in the place its needed and stop the insulation migrating around.

The pockets are called baffles. And baffles come in various shapes and sizes depending on the purpose and spec of the jacket.

Wider baffles are used for products with a higher fill weight, giving the extra down plenty of space to loft (giving it more insulating capacity) and need less seams. Seams are often stitched through that can create cold spots. A box wall construction overcomes this by creating 3 dimensional chambers within a jacket. Box wall are warmer, heavier and bulkier than stitch through.

Narrower baffles are used when the fill weight is lower, and the down needs less space to loft.

These ‘micro’ baffles are easier to layer, more compressible, and have a low-bulk, slimmer profile that makes them much more suited to active use than wide baffles. However, your insulation has less space to loft and there are more seams, so more cold spots, on your jacket.

Opt for box-wall if… it is exceptionally cold and warmth is a critical factor.

Opt for wide baffles if… warmth is a priority over manoeuvrability, you won’t be moving about much so you’re generating less heat and you’re less worries about freedom of movement. Or if it’s really cold, you’re moving and generating heat but it’s so cold that you need the warmest jacket possible.

Opt for micro-baffles if… You’re out in UK spring and autumn conditions, or a slim fit for active use is your priority.

What to look out for in an insulated jacket

The anatomy of a modern insulated jacket incorporates many technologies and components. Here we discuss the key elements.

The Fit

Fit comes down to what you’ll be using it for. If your down jacket is for active use and you’re going to be climbing, hiking or biking in it, you want an active fit. This fit can be achieved in the following ways:

  • Zoned insulation: less down in the arms for more freedom of movement.
  • Articulated arms and shoulders: This prevents the hem from rising and letting in the draughts when you lift your arms.
  • Slim baffles: To keep a low-profile that’s easier to move in
  • Or you could just get a jacket with no sleeves (AKA a vest or gilet) that keeps your core warm with the most freedom of movement possible.

For more stationary cold weather days, on belay ledges or when winter camping, comfort comes in the form of warmth rather than freedom of movement. These kinds of jackets help you to stay warmer and cosier (a longer cut for more coverage, wider baffles) with less of an emphasis on freedom of movement.

Fabric – tough, breathable and water resistant.

All down and most synthetic insulation must be sewn in.

As with pillows and duvets, the insulating fill can easily escape from jackets and sleeping bags, so your shell fabric needs to be tough and abrasion resistant and ‘fibreproof’.

Fibre-proof fabrics stop the down or fibres from poking out through the fabric itself. As a result, these fabrics are completely windproof: great for cutting out the chill, but not so breathable. In theory, if you are cold enough to be wearing a warm down jacket, breathability is much less of a concern than warmth. In practice, however, your output levels are likely to change over the course of the day, in which case you may be looking for something a little more breathable.

Some water resistance (look for hydrostatic head) or a DWR coating is ideal for protecting your down from the elements if you do get caught out in a drizzle.

Hood or no hood?

The main benefit of having a hood is comfort and warmth – they keep the wind off your neck and your head and ears warm. However, they also add bulk and weight to your jacket and can get in the way when you’re layering other garments over the top of your jacket.

A removable hood gives you the option but are draughtier than a permanent hood.

If you decide that you do want a hood, you also need to work out whether or not helmet compatibility is important to you. A helmet compatible hood is ace when you’re wearing a helmet but should be adjustable so that you can wear it without a helmet too.

Hoods usually feature either crown adjustment (around the back of your head), front adjustment (around your face), or both.

Think about how you’ll use your hood, how important is it that you can get it snug around your face?

Some jackets can be adjusted from within their pockets, so you don’t have to expose your hands to the cold to get your hood nice and snug around your face.


They may not seem like a big deal, but irritating pockets are, well, irritating. Do you want to be able to access your pockets when you’re wearing a harness? Do you want them lined so that you can warm your hands in them? Do you want an internal valuables pocket to keep your batteries or head torch from freezing or an external valuables pocket that you can get to without undoing your jacket? How about internal mesh pockets for your gloves or rock boots?

2-Way Zips

A 2-way zip is ideal for throwing your jacket on over your harness at a belay because you can zip it up and still access your loop to belay.

Hem drawcords and cuffs

Most down jackets are pretty adjustable so that you can seal out any draughts.

Cuffs may be elasticated or adjusted using hook and loop tabs. Consider how you prefer to wear your gloves – under or over – and whether that will work with your cuffs.

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