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Down vs synthetic insulation: which is best for keeping warm?

Down vs synthetic insulation
Insulted jackets are one of the best ways to keep warm on the trail. (Image credit: Rab)

If you want to stay warm in camp or on the trail, insulated jackets are usually the best option, offering superior warmth for weight compared to polyester fleece, natural wool or other fabrics. For similar reasons, a sleeping bag or backpacking quilt has become pretty much the universal sleep system of choice for nights in the great outdoors, supplanting old-school blankets, bedrolls and swags.

But if you’re in the market for a new sleeping bag or an insulated jacket – what Brits often call a ‘puffer’ jacket and what Americans usually refer to as a ‘puffy’ – one of the most important buying decisions you’ll need to make is negotiating the down vs synthetic insulation dilemma.

Each has its pros and cons, related to factors such as warmth for weight, packability, versatility, durability, sustainability and price. It’s about weighing up all of these different aspects in regard to your intended use, to help you choose which option is right for you. To make that decision easy, we’ve looked at each of these specific areas and highlighted the benefits and drawbacks of down and synthetic fills, pitting them against each other in a head-to-head.

Down vs synthetic insulation: warmth for weight

In this regard, there is really only one winner – at least at the top end of the scale. High-quality down remains the best insulator for clothing and sleeping bags. Despite huge technological advances in synthetic fibres, no man-made fill can compare to premium down, which is why down jackets and sleeping bags are still typically the preferred choice for those visiting the coldest places on Earth, from climbers in high-altitude environments to researchers working in polar regions.

It is important to talk about warmth for weight rather than just outright warmth alone, because technically you could make a synthetic jacket that would be just as warm as a premium down jacket. The problem is that it would require so much fill that it would be bulky, awkward and downright uncomfortable to wear – basically, you’d be waddling around like a penguin.

Down, on the other hand, is very light, yet delivers exceptional warmth in relation to its weight. How? The key lies in its composition. Contrary to common belief, down is not made from feathers. Down is actually the soft, fluffy plumage found beneath the feathers of waterfowl like ducks and geese. These super fine, wispy clusters trap air, and it is this trapped warm air that provides down’s exceptional insulating value.

Of course, not all down is equal. The warmth of down depends on its ability to loft, or trap this warm air. This is indicated by fill power (FP), which is a measure of how many cubic inches one ounce of down fills inside a standardised testing device. The fill power of down for outdoor gear typically ranges from about 550 FP (meaning that one ounce of that down fills 550 cubic inches of space) to 900 or occasionally even 1000 FP.

Down vs synthetic insulation

It's important to factor in the fill weight as well as the fill power. (Image credit: Rab)

Why does fill power matter? A higher fill power means that a sleeping bag or jacket requires less down fill to deliver a stated warmth. And less fill equals a lighter product. So, a two-season sleeping bag rated to 0°C (32°F) that uses 700-fill-power down will be lighter than a 0°C bag using 600-fill-power down – assuming the fabrics and other features are the same. But this also means, of course, that when you’re comparing down jackets or sleeping bags, it is important to look at the total fill weight as well as the fill power.

There’s also one other thing to consider: down composition. As noted previously, down is not quite the same as feathers – and feathers are not nearly as good insulators as down clusters. But they all come from the same source, ie geese or ducks. And collecting down without also getting a few feathers in the mix is very tricky. As such, most insulated jackets and sleeping bags will list a down-to-feather ratio in addition to a fill power (FP) figure and a fill weight. The down-to-feather ratio can vary from 70:30 at lower grades (i.e. the fill composition is 70% down and 30% feather) to 90:10 or higher for the best grades of premium down.

So, when it comes to down, there’s lots to consider. Surely synthetic insulation is a lot simpler? Well, perhaps a little, though there are still a huge number of different synthetic fills around. These include proprietary materials like Primaloft Gold, Silver and ThermoPlume as well as products from other brands like Thinsulate and also ‘in-house’ fills from the biggest outdoor manufacturers: Patagonia PlumaFill, The North Face’s ThermoBall and Arc’teryx Coreloft, plus a host of others.

Broadly, however, these are all made from hollowfibre polyester. They can also be split into two main categories:

Firstly, there is block insulation, which is essentially a sheet of wadded synthetic fibres (as used in many jackets and almost all synthetic sleeping bags). It is sometimes called continuous filament insulation. 

Secondly, there are loose fills, which are silky tufts of synthetic fibres that are closer in terms of their characteristics to natural down. This is often referred to as short staple insulation. The latter can be blown into baffles just like down, resulting in synthetic jackets that look and feel a lot like a conventional down jacket. Good examples include the Montane Icarus and Phoenix ranges of jackets, The North Face’s ThermoBall jackets and Patagonia’s Micro Puff jackets. All are loose fills that offer similar warmth-to-weight as 550 FP down. This isn’t quite as warm as the top synthetic sheet insulations that you find in sleeping bags and heavy belay jackets, but they are far more compressible.

Down vs synthetic jacket verdict At the top end of the market, premium down still beats the leading synthetic rivals in terms of warmth to weight. But there are now advanced synthetic fills that can match the insulating value of lower grade (550 to 600 FP) down.

Down vs synthetic insulation: packability

Down vs synthetic insulation

Down offers superior pack size hands down. (Image credit: Rab)

Along with its impressive warmth for weight, down is highly compressible. That makes down jackets and sleeping bags well suited to lightweight backpacking and other adventures where weight and pack space are at a premium.

Generally, down clusters also survive repeated compression well, so your down jacket or sleeping bag ought to retain its loft even after multiple uses – although for optimum results it’s best not to store it compressed when not in regular use. That’s why most premium down sleeping bags are supplied with a larger, breathable mesh or cotton storage sack in addition to a small stuff sack.

Synthetic fills are not as compressible as down. As a result, synthetic insulated jackets and sleeping bags tend to be bulkier than comparable down jackets and bags, though newer loose fills are slightly more packable than synthetic sheet insulation.

Nor do they tend to survive repeated compression as well. As such, you might notice that over time your synthetic jacket or bag does not seem to have quite the same degree of loft – or deliver the same warmth – as it did when new. As with down, the best way to mitigate this is to store your synthetic jacket in a closet or wardrobe rather than rolled up in its hood or tightly bundled in its stuff sack. Similarly, if you have the space, a synthetic sleeping bag will last longer if stored loosely folded rather than kept in its compression sack.

Down vs synthetic jacket verdict Although synthetic loose fills offer improved compressibility over other synthetic sheet insulations, they still cannot match the packability of down. If packed size is a primary consideration for your adventures, a down jacket or sleeping bag will be less bulky than a synthetic alternative.

Down vs synthetic insulation: versatility

Here, arguably, is where synthetic insulation has the upper hand over down. Synthetic jackets and sleeping bags are more effective in a wider range of conditions than down products, largely because of their superior wet weather performance.

The traditional Achilles heel of down is that although it is a superb insulator when it is dry, if it becomes saturated, its performance drops off drastically. That’s because sodden down clusters tend to collapse, meaning they can no longer trap warm air. Moreover, saturated down clumps together and takes a long time to dry out, so if it gets wet, it stays wet. That’s not so great if you’re wearing a down jacket in pouring rain or find yourself huddled inside a damp sleeping bag in a wet tent.

In such situations, you’d undoubtedly be better off with a synthetic fill. Hollowfibre polyester is more water-resistant than down, insulates when wet, and is also much faster drying, since the fibres tend to transport or ‘wick’ away moisture better.

To help offset down’s poor wet weather performance, in recent years outdoor brands have started to use hydrophobic down. This refers to down that has been coated to make it absorb less water and/or dry out faster. This is usually a DWR (durable water repellent) treatment similar to that applied to the face fabrics of outdoor clothing. This increases the time that a down product will continue to insulate in damp conditions, but it still doesn’t make down the ideal option for prolonged rainy weather.

Down vs synthetic jacket verdict If it’s very cold but also dry, as in an Alpine environment, then down is a great choice. In cold but wet weather – like a Scottish winter – a synthetic fill is a better option. This means that arguably, synthetic jackets and sleeping bags are slightly more versatile than down alternatives, since even if you get caught in a downpour, you’ll stay fairly warm. That’s still the case despite the emergence of hydrophobic down, although this does mean that a bit of drizzle on your down jacket or moisture in your down sleeping bag is now no longer the hazard that it used to be.

Down vs synthetic insulation: durability

Down vs synthetic insulation

Synthetic fibres can take a bit more wear and tear (Image credit: Outdoor Research)

Individual down clusters are very fragile compared to synthetic fibres and are easily clogged with dirt, grease, sweat and body oils, all of which impair performance. As such, knowing how to look after a down jacket or sleeping bag can be a complicated affair. Synthetic insulated jackets or sleeping bags are certainly easier to look after and less susceptible to rough treatment than down. So, if you’re hard on your kit, a down jacket or sleeping bag might not be the best choice.

The very fine nature of down means that it also tends to escape from the seams of jackets and sleeping bags (and can sometimes even work its way through face fabrics). Over time, this can lead to down loss, which will impair loft, since there’s less fill to occupy the internal baffles. Another problem is down migration, which is when the down fill moves around inside the baffles of your jacket or sleeping bag, clumping together in some areas and leaving ‘cold spots’ in other areas. Both issues are less of a problem with synthetic gear, especially with sheet insulations.

On the other hand, if well looked after, down kit can be very durable indeed. With a bit of regular care and maintenance, there’s no reason your down jacket or sleeping bag won’t provide many years of good service.

Down vs synthetic jacket verdict If you’re after fuss-free outdoor kit that can be used and abused, synthetic puffer jackets and sleeping bags are a top choice. If you’re prepared to look after your gear, then a down garment or bag should last you over many years of adventures. It’s worth noting that there are also a number of specialist companies who can rejuvenate old and tired down gear by cleaning and/or ‘topping up’ the down fill inside your jacket or sleeping bag.

Down vs synthetic insulation: sustainability and ethical concern

As a natural product derived from ducks and geese, down has its drawbacks. Campaigners have highlighted significant animal welfare issues within the down industry, after it emerged that in some cases wildfowl had been mistreated and even live-plucked for their down. These unethical practices still go on, although increased supply chain transparency as a result of various outdoor industry initiatives have drawn attention to these problems and started to address them.

‘Ethical’ down is occasionally gathered from nesting sites, but this is an expensive and time-consuming method. As such most commercial down is recovered as a by-product of the meat industry, when birds are killed for food. Most reputable brands now use fully traceable down, which is one way to ensure that producers adhere to a set of production welfare standards. One of the pioneers of traceability was Mountain Equipment, with their Down Codex scheme. Other brands use certified down suppliers who are signed up to initiatives such as the Responsible Down Standard, a scheme created by global non-profit organisation The Textile Exchange. The Responsible Down Standard aims to ensure that down and feathers come from animals that have not been subjected to unnecessary harm, incentivising practices that respect the humane treatment of ducks and geese. The standard also provides companies and consumers with a tool to know what is in their products, and to make accurate claims about their origins.

Proponents of down argue that responsibly sourced down can be an environmentally friendly choice for outdoor gear, as it is a natural and biodegradable product that is a by-product of the food industry. Similarly, they argue that it is also renewable and has a lower carbon footprint than oil and petroleum-derived synthetic fabrics, though a comprehensive comparative analysis has not been published.

Basically, if you’re buying a down jacket or sleeping bag, look for evidence that the down has been responsibly sourced. In some cases, down products will have a coded swing tag that allows you to check where, when and how the down in the product was produced.

If this seems a bit of a minefield, you might be tempted to go for a synthetic puffer jacket or sleeping bag instead. But the vast majority of these fills are made from polyester, which is itself derived from petrochemicals, meaning it is not a very environmentally conscious choice either. Fortunately, most brands now use fills made from recycled polyester (in varying degrees – check the label). But this is of course only a partial solution.

A further major criticism of synthetic fibres is that they do not degrade. Unless recycled, they persist in the environment at the end of their lifecycle. In addition, there is evidence that synthetic fabrics can leach microscopic fibres into their surroundings throughout their lifetime, especially when they are washed. Ultimately, these microplastics can end up in our seas and oceans – an issue that is now at the top of the environmental agenda.

Down vs synthetic jacket verdict There’s no clear winner here, as there are pros and cons to both natural down and synthetics. As ever, the consumer has the power – if you’re considering buying a down jacket or sleeping bag, ensure the down is responsibly sourced. If you’re looking at synthetics, check for recycled content in the insulation.

Down vs synthetic insulation: price

When it comes to outdoor kit, down fill is almost always more expensive than synthetic fill – and that’s true whether you’re shopping for a puffer jacket or a sleeping bag. For example, a four-season down sleeping bag can easily cost upwards of $500/£500, while a box-wall baffled down jacket is likely to set you back at least $250/£250. You can find cheap down goods, but these have their own drawbacks – not just in terms of quality but also in terms of dubious ethical and animal welfare standards. High-quality synthetic alternatives are a better choice for those on a budget.

Down vs synthetic jacket verdict When it comes to the price tag, synthetic fills are almost always cheaper than down fills. Lifetime value is a little harder to quantify, since the high initial outlay of a down jacket or sleeping bag can often be offset against the years of good service it ought to deliver.

Down vs synthetic insulation: results

It’s close run thing – whereas down has the edge when it comes to warmth for weight and packability, synthetic fills are cheaper, easier to look after and arguably more versatile, given their superior wet weather performance. It’s hard to pick a clear winner when it comes to considerations such as sustainability, durability or value, as much depends on the footprint of an individual product and how well it is looked after during its effective lifespan.

Ultimately, your choice should reflect the activities and conditions in which you’re likely to be using your puffer jacket or sleeping bag. If warmth, weight and packed size are vital, then down is likely to win out. On the other hand, if you’re adventuring in wet and wild conditions or are on a budget, then a synthetic piece should serve you well.