What Is Hydrophobic Down & Why We Don’t Use It

A recent thread on UKClimbing.com’s forums wondered why we at Mountain Equipment don’t use hydrophobic down in our sleeping bags. It’s a worthy question and one we addressed. Here is our outlook on the use of hydrophobic down in both clothing and sleeping bags.

What is hydrophobic down?

Hydrophobic down is the term usually given to down which has been coated with a hydrophobic (water hating) chemical to make it absorb less water and dry out faster. In many conditions down is the best insulator available, but its one drawback is that in sustained wet conditions it loses some of its insulating properties. Thus, making it hydrophobic seems a good idea.

Hydrophobic down has been around for years, having been developed by the US military and then made in various forms ever since, though the popularity of hydrophobic down coatings used in the outdoor industry have increased greatly in the last ten years. This is partly as a result of the trend for narrow baffling and lighter mid-weight down garments, meaning down is being used in a greater range of environments than it traditionally might be. These modern hydrophobic coatings are either PFC-based, silicone-based, or wax-based and have been fairly widely adapted by the outdoor industry.

Why we don't use hydrophobic down in our sleeping bags

Firstly, we do occasionally use hydrophobic down, but not very widely. That’s because it isn’t a magic bullet: hydrophobic down increases how long you can use a product for in damp conditions, but it doesn’t make down the perfect thing for, say, open bivvies in the rain (shudder).

In most conditions you’re not likely to notice much difference between hydrophobic and non-treated down bags, and we’ve plenty of evidence of non-hydrophobic down bags being used very successfully on prolonged and serious trips.

For example, when Tom Livingstone, Aleš Česen and Luka Stražar climbed Latok I over 7 days last year they used a Fireflash Sleeping Bag – ie. not hydrophobic down – and a two-person bag we made in the office, again filled with non-hydrophobic down.

Similarly, Ben Saunders’ and Tarka l’Herpiniere’s frankly outrageous 105 day Antarctica crossing, packing away frozen sleeping bags every day, was done with non-hydrophobic-down bags.

Conversely, we also make fully customised sleeping bags for multiple Piolet d’Or winner Paul Ramsden; he requests hydrophobic down in his sleeping bags as it gives an extra degree of insurance for unplanned bivvies and if packing away bags soaking wet day after day. Paul is a perfectionist, very knowledgeable about kit and really knows what works for him, but he’s an anomaly amongst our athletes in requesting for hydrophobic down in his sleeping bags.

It comes down to construction

If you want to keep something completely dry, you start on the outside: A fair few of our bags have water repellent shell fabrics, and sometimes these line the hood and footbox too, in case of spindrift/snow, or moisture coming off damp socks or inner boots kept inside your bag. A water repellent shell is a far better way of keeping something dry than coating what’s inside.

If you’re sleeping somewhere really wet, even if the hydrophobic down inside your sleeping bag stays dry, all the fabrics, stripping, and construction of the bag will get soaked, which is just grim: if you’re driving a convertible car you don’t put a waterproof jacket on when it starts to rain; you put the roof up.

It’s also worth noting that, just like with down, hydrophobic down also has a ‘breaking point’ where it too gets overly wet and saturated.

A longer lasting alternative

Down sleeping bags last a long time and hydrophobic coatings don’t tend to. We have plenty of customers get in touch about bags that are 20 or 30, even 40 years old. We all know that DWRs don’t usually last very long, and even though hydrophobic down lasts longer because it’s inside a sleeping bag, it won’t last anywhere near the lifetime of the bag. Coated fabrics like Drilite Loft will last much longer, and membranes such as GORE-TEX INFINIUM longer still.

Like a duck to water...

Down is hydrophobic to start with: Down is very water repellent in its ‘natural state’: both geese and ducks have extremely hydrophobic feathers. Go down to your local park and look at the feathers beading on a duck when they clean themselves: it puts man-made technology to shame.

The problem is, down smells and can rot easily if it isn’t cleaned of a lot of these natural oils, so we thoroughly clean it, but this also reduces its hydrophobicity. It’s a pretty inelegant solution to then dunk a load of synthetic chemistry on these cleaned feathers to make them more hydrophobic again, especially when, as an industry we are trying to reduce the number of coatings, treatments, etc. being used to try to reduce environmental impact.

We’ve tested a huge number of different options and in general, the more environmentally-friendly the technology at application, the less long it lasts; the more environmentally-harmful treatments last longer. By the way, contrary to popular belief, down doesn’t saturate easily: it floats on water for starters, despite being denser than it.

There are other ways to stop down getting wet inside a sleeping bag: this is the clever stuff which not a lot of people understand (and we try not to tell everyone details of!), but you can do a lot with where and how you put down in a bag to make it more water resistant. This is where the most interesting development stuff happens, as down density affects moisture moving through down and you can, to a certain extent, mitigate moisture transport.

Having said the above, on occasion we do use hydrophobic down in our bags – for example in the one-off prototypes mentioned above – and we have made commercially-available bags using hydrophobic down in the past. We keep a keen eye on the development of hydrophobic down technologies and are in constant contact with down suppliers and testing labs regarding hydrophobic down testing and performance.

We also use hydrophobic down in some of our lightweight down garments (the Arete series) which are fairly likely to be used for short periods in damp conditions (think of jogging back to the car as the rain starts to fall), and it’s good for that sort of use. Similarly, those sorts of garments are fairly likely to be used above freezing in dank weather, and having a coating that makes them dry out faster is beneficial.

So in summary, hydrophobic down certainly has its place, we just don’t think it’s nessesary in sleeping bags.

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