The Ultimate Guide To Down Jackets and Synthetic Insulation

The Ultimate Guide To Down Jackets and Synthetic Insulation

By Mark Roberts

We evolved in to live at 28°C.  Without clothes, we’d shiver below 25°C. That's why insulated jackets are an essential part of our outdoor wardrobe. Here’s everything you need to know to choose an insulated jacket.

How to choose an insulated jacket

  1. What are insulated jackets?
  2. Down jackets guide
  3. Synthetic jackets guide
  4. Which is better, down or synthetic?
  5. How to choose an insulated jacket


Four people wearing down jackets and Primaloft synthetic insulated jackets in an ice cave in Chamonix

What are insulated jackets and what are they for?

Insulated jackets are jackets filled with either down or man-made synthetic insulation to trap the heat generated by your body. The insulation works by creating ‘loft’, a deep fluffy structure that creates lots of tiny pockets of air between the feathers or fibres. Still air is a fantastic insulator, which is why insulated jackets are so much warmer than other outdoor layers.

The main purpose of insulated jackets is to keep you warm.

A woman wearing the Filo down jacket with the hood up over a climbing helmet in the Slovenian Alps

Down jackets

Down jackets are filled with the light fluffy feathers found under the breast and underbelly of ducks and geese. Because they’re so fluffy, these down ‘filaments’ create loads of insulating loft for very little weight. Despite innovations in synthetic insulation, down is still the best insulator we have with incredible warmth for its weight. Down is also highly compressible, squashing down small and rebounding back into shape again without getting damaged. These properties are really useful when you’re trying to pack light or you’re short of pack space.

A woman wearing the Fantom down jacket with the hood up in the snow
A woman wearing a Fantom down jacket with the hood over a climbing helmet in the French Alps
A woman putting a Kangri 4-season mountain tent up in her Fantom down jacket

What do fill power and fill weight mean?

Fill power (FP) measures the volume of air that 20g of down occupies under strict scientific conditions*. It tells you about the down quality and is a measurement of how much loft the down creates (how fluffy it is). 

(*In case you were wondering, the number represents how many cubic centimetres a gram of down lofts to. So, 750FP means that 1g of down lofts to 750cm³). Higher fill power down has a better warmth to weight ratio so 100g of 800FP down will be warmer than 100g of 650FP down.

Fill weight is a measure of how many grams of down is inside your jacket. This is where it gets a bit complicated. Jacket warmth depends on both the fill power and the fill weight. This can make it quite hard to compare the warmth of down jackets with different fill powers. That said, when you’re comparing between high quality down (e.g. between 650FP and 750FP), fill weight will give you the best indication of how warm each jacket will be.

Which is warmer, duck or goose down?

It depends on the fill power of the down. As geese have larger feathers, they generally produce higher fill power down which is more expensive. The best quality duck down is around 650FP whereas the highest quality goose down can sometimes reach 900FP (but only in very rare circumstances). 

All the down we use is ethically sourced and certified by the Responsible Down Standard as meeting the highest standards of animal welfare. 

A woman walking in the Filoment micro baffle down jacket with stitch through contstruction

What do box wall and stitch-through construction mean?

Box wall and stitch-through are different types of baffle construction. Down jackets are made with lots of baffles, separate compartments that hold the down in place and stop it migrating.

Stitch-through means exactly what it sounds like: each baffle has been stitched all the way through the jacket fabric (like in the picture above). Stitch-through construction is the lightest and the most compressible, but it can create cold spots along the lines of the stitching.

Box wall construction uses a divider of fabric between each baffle which means each baffle is shaped like a… well, box. Box wall construction allows the down to loft better, but the extra fabric does add weight and makes your jacket slightly less compressible.

What is hydrophobic down?

Hydrophobic down is down that has been treated with a durable water repellent (DWR). The downside (ha!) of down is what happens to it when it gets damp. When down gets wet, the filaments absorb water, collapse and clump together, completely losing their warmth-trapping structure. Down also takes a long time to dry out and usually needs to be tumble dried to effectively de-clump it. Hydrophobic down repels water and body moisture for longer and dries quicker.

The DownTek™ PFC-Free Water Repellent Down we use stays dry 13.5x longer than untreated down.

A man pulling on the Heiko Primaloft lightweight insulated jacket with quilted baffles

Synthetic insulation

Synthetic jackets are filled with man-made polyester fibres that are designed to replicate the properties of down. Like down, synthetic insulation is warm, lightweight and compressible.

Unlike down, the polyester fibres don’t absorb water so synthetic insulation keeps its warmth-trapping properties even when wet. This makes synthetic jackets ideal for wet climates, or for active use when you’re likely to get extremely sweaty!

There are lots of different types of synthetic insulation – from carpets of fibres to individual ‘filaments’ designed to mimic down – each with their own advantages.

How is synthetic insulation measured?

Most brands measure synthetic insulation by grams per square metre – you might see this listed as either gsm or g/m². However, because there are so many different types of synthetic insulation, each with a different warmth-to-weight ratio, gsm only really helps you to compare between jackets using the exact same insulation.

A woman tying in to her climbing harness wearing a Talini Primaloft jacket

What is PrimaLoft®?

PrimaLoft® is a brand of synthetic insulation, first developed in the 1980s to provide a water-resistant alternative to down for military jackets and sleeping bags. Their fibres are specially treated to make them water repellent for even better wet weather performance. PrimaLoft® Gold provides the same amount of warmth-to-weight as 550FP down.

PrimaLoft® became the market leader for synthetic insulation in the 1990s – to such an extent that people often refer to all synthetic insulation as ‘primaloft’ (in much the same way we call vacuum cleaners 'hoovers'). We exclusively use PrimaLoft® fill for our jackets as they provide the highest quality of synthetic insulation available.

A Guide To PrimaLoft® Insulation

PrimaLoft® group their insulation into three broad categories based on how insulating it is:

•  PrimaLoft® Black – A standard of fill for everyday wear, only really used in fashion clothing
•  PrimaLoft® Silver – Excellent quality but good value insulation used in outdoor clothing
•  PrimaLoft® Gold – The highest quality of synthetic insulation available, used in performance outdoor clothing

We use the following types of PrimaLoft® insulation in our jackets:

•  Silver Eco – Insulation using 100% recycled content (Heiko)
•  Silver ThermoPlume® – Synthetic plumes designed to mimic the loft and compressibility of down feathers. Treated with space-age Cross Core Technology™, an ‘aerogel’ that increses its insulation. (Talini)
•  Silver Hi-Loft Ultra – Incredibly lofty and hardwearing insulation that retains its loft well, even after repeated compressions. 70% recycled. (0Hiro)
•  Gold P.U.R.E.™ – 100% recycled insulation using a brand new manufacturing method that reduces carbon emissions by 70%. (Sierra)
•  Gold Active – A mat insulation that can be combined with more open, air permeable fabrics for better breathability. 45% recycled. (Katabatic)

A man walking in the snow on Kinder Scout in the Peak District wearing a Primaloft Gold Active breathable insulated jacket

What is ‘active insulation’?

Active insulation is a special type of synthetic insulation that can be combined with more breathable or stretchy fabrics for high intensity activities. Most forms of insulation (including down) need to be combined with dense-weave fabrics to stop the insulation from escaping. These fabrics tend to be windproof but not very breathable. Active insulation stays together in one sheet which means we can make more breathable insulated jackets like the Katabatic that allow sweat and excess heat to escape.

Two people standing on a platform in the French Alps wearing the Fantom Down Jacket and the 0Hiro Primaloft Jacket

Which is better, down or synthetic insulation?

Generally speaking, down jackets are better for dry, cold environments and synthetic jackets are better for damp, cold environments. But with synthetic insulation technology closing the gap on down’s warmth, weight and compressibility, and hydrophobic down improving down’s wet performance, it’s not quite as simple as it used to be.

Down Jackets
Pros Cons
The best warmth-to-weight ratio Loses all its warmth when wet
More compressible than synthetics Harder to look after in the damp
Lasts for years with good care Takes a long time to dry out
Natural, renewable and biodegradable More expensive

Synthetic Jackets
Pros Cons
Keeps its insulation when wet Not as warm as down at the top end
Water repellent and fast-drying Heavier for the same warmth as down
Very easy to wash and care for Less compressible than down
Cheaper than down Can lose loft after repeated heavy use

A woman hunkering down in her Filoment down jacket in the Slovenian Alps with the hood up

How to choose an insulated jacket


Ask yourself what your priority is: weight or durability? Hardwearing fabrics like the 0Hiro’s nylon 6,6 are useful if your jacket is going to take a beating. Otherwise, you can probably get away with a lightweight nylon ripstop. Water resistance is always useful which is why all our insulated jackets are treated with a PFC-free durable water repellent.


Are you going to wear your insulated jacket over other layers, or do you need it to fit under your shell as a midlayer? Jackets designed for moving in tend to have zoned insulation (more insulation in the body and less in the arms) and narrower baffles.

A two-way zip on the Filo down jacket


Narrower, smaller baffles like those on the Filoment and Talini are easier to move in but they don’t provide as much loft and warmth as larger baffles (like those found on the Fantom). Micro baffles usually use a stitch-through construction which creates more cold spots.


An insulated hood provides vital extra warmth in the very coldest conditions. However, they do add extra weight and faff to your jacket which may be unnecessary if you’re intending to use it as a midlayer under your shell. Helmet-compatible hoods are useful if you’re going to be using your insulated jacket for climbing or mountaineering.


Two-way zips are useful for climbing and mountaineering as they make it easier to get to your belay device without your jacket getting in the way.

Walking into the distance in the snowy French Alps with crampons on and a 0Hiro Primaloft insulated jacket

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