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A Guide to Down Sleeping Bags | Buying Guide

By Rock + Run

Here at Rock + Run we have always had a general policy of recommending down sleeping bags for serious outdoor activities. The following article explains the ways in which manufacturers rate various weights and fill powers of the down used, the pro’s and cons of down bags over their synthetic counterparts (including how to get around the cons) and a few tips on care for your down product. Down sleeping bags are able to provide an incredible warmth to weight ratio and can be packed down to the smallest sizes – in comparison to synthetic materials, down can be condensed up to 25% more within the same area. The down used in sleeping bags - from conscientious brands - is a by-product from ducks and geese raised for food consumption. These feathers are fine and soft and grow close to the bird under its tough outer layer of feathers, keeping the bird well insulated from the elements. The natural insulating properties of these down feathers have been put to optimum use in down sleeping bags to keep you warm - whether that's for a weekend camping trip in the lakes or an expedition to the Himalaya. As a rule the best quality and regulated down comes from Eastern Europe. This is essentially due to the generally healthier livestock used, the expertise in handling and processing the down. That said, over the last few years, the improvement in manufacturing processes in the Far East has seen a vast improvement in Chinese produced down bags.

Fill Power

Whilst browsing through the ‘Sleeping Systems’ section of the Rock + Run website you’ll see every sleeping bag has a fill power rating ranging between 560 and 900. This number is the number of cubic inches one ounce of down will occupy. For example, in a sleeping bag with a 560 fill power rating; one ounce of down occupies a volume of 560 cubic inches. One thing to bear in mind when observing fill power stats is the difference between EU and US ratings. Where the European rating is 560+EU the American rating will be 610+US for the same product. This is because a different meter is used to provide the American measurements and can prove confusing for the customer – this is elaborated on later in the article. The higher the fill power rating, the larger the down clusters. The larger the down clusters the better quality of the down, as it can trap more air and have a greater ‘lofting’ ability and thus produce a higher fill power rating. Utilising larger down clusters means less down is needed to provide the same level of warmth as a lower power down, which is why fill power’s of 800-900 are used in expedition bags – maximum warmth and compressibility is achieved with the least weight per ounce. Fill power ratings are worked out using a Plexiglas cylinder with a weighted piston compressing the down. The standard method for measuring fill power is set down by the IDFL (International Down and Feather Laboratory). However, consumers should be aware that ratings produced by the American Fill Power Meter generally give a higher reading on top quality downs. This can be misleading if only one figure is provided on a down product as it can make down appear a better quality when it is in fact the same or slightly worse. All bags we sell are described with the US fill power rating, which is becoming the norm within the industry. 

Fill power is not the only figure that indicates the warmth of a sleeping bag; the weight also has to be taken into consideration as this indicates the quantity of fill. For example we can compare the Rab Expedition 1200 and 1000. Both bags have a fill power of 850+US. The 1000 (total down weighing 1000g) can be used in temperatures as low as -30 degrees Celsius where as the 1200 (total down weighing 1200) can be used comfortably in temperatures as low as -40 degrees Celsius because the 1100 has more down. In a similar vein, a down bag with a fill power of 650 compared to a down bag with a fill power of 750 can have the same warmth rating but the latter with the higher fill rating will be the lightest in weight.

Hydrophobic Down?

Some brands now exclusively use hydrophobic down in their bags and garments. So what is hydrophobic down?

Hydrophobic Down has a flexible Durable Water Repellent (DWR) finish on each individual down filament. This reduces absorption of moisture protecting down from perspiration, condensation and precipitation and maintaining loft and insulation in cold and damp conditions.

Keeping down free from moisture allows it to be effective for longer when in use. Additionally, it is protected from microbial action which can cause breakdown of damp down, therefore increasing the usable lifetime of your item.

The main benefits are:

  • Dries faster
  • Repels water for longer
  • Retains Loft even when damp, to keep you warm

Benefits of Down over Synthetic

When purchasing a sleeping bag you will be choosing between a down and a
Rock+Run Yangra 800 Sleeping Bag - Blue. Goose down
synthetic bag. Although down bags are more expensive, they hold many advantages over synthetic bags and can last a lifetime if well looked after. Below is a condensed list highlighting the pros and cons of a down sleeping bag when comparing it to a synthetic.


* Down is warmer weight for weight when compared to any synthetic material and has a wider comfort temperature range, meaning you won’t feel too hot when using your -5 degree Celsius bag in warmer climates. * Down bags are more durable. Down bags hold their loft and insulating ability over time whereas synthetic bags can lose a considerable amount of insulating ability in just the first 30-60 days of use. * As synthetic bags can lose their insulating ability, they need replacing frequently if you are counting on them in cold conditions. Down bags have a much longer life if looked after properly. * Down sleeping bags are highly compressible and lightweight. A down bag can weigh up to 25% less and compress 25%, or smaller, than a synthetic bag of the same temperature rating. * Down is generally more breathable, wicks body moisture and allows it to escape.


* Down loses its insulating properties when wet and is slow to dry. Synthetic fills are water resistant and manages to retain the majority of its insulating properties when wet. However the storing of your down bag in a dry bag (when compressed), and purchasing a bag which has been DWR treated should prevent any major problems. * Down requires special cleaning. Normal household detergents can break down the down feathers making the bag lose its natural lustre and efficiency. This can be easily overcome by purchasing a cotton or silk sleeping bag liner. * Down is not entirely hypoallergenic. Lower quality down can harbour dust particles or other non-down materials, causing an allergic reaction. However, higher quality down is cleaned to strict industry standards so choosing higher quality down can almost eliminate these problems – see the earlier comments regarding European and Far Eastern downs. * Down is more expensive than synthetic materials.


Looking after your sleeping bag and keeping it in good condition increases its life considerably. You can wash your bag if it eventually gets filthy but prevention is always better. Just take care when you’re out and about and keep it away from excessively dirty conditions, if necessary with a bivi or survival bag. You can keep the inside of your bag clean by investing in a thin sleeping bag liner to reduce the amount of natural oils your body transfers onto the bag - cotton is more economic but heavier, whilst silk is lighter and has a higher warmth to weight ratio. When storing your sleeping bag you should always keep it in the large cotton storage sack that comes with your bag. This will ensure the down in your bag maintains maximum loft. When your bag is compressed tightly into a small stuff sack, it pinches and squashes the filling which can cause gaps which in turn result in cold spots. If you were to store your bag like this, it could reduce the loft of your bag and ultimately it’s effectiveness at maintaining its temperature rating.

Wherever possible do not keep your down bag stored compressed for more than 24 hours. If you are traveling with your bag make sure you keep it in the stuff sack only when you need to. For example; if you are backpacking you will obviously need to put your bag in the stuff sack but if you are traveling in the car to go camping, your bag can be stored in the storage sack in the boot. If your bag does get wet dry it in a tumble drier as soon as possible. You can wash your sleeping bag as long as you follow these rules: you must use an oversized front loading washing machine like the ones found in a launderette. You must also use a non detergent soap, like Nikwax Down Wash which is specifically made for items such as your sleeping bag. This is more gentle and less likely to damage down filaments or strip the shell of its DWR treatment. You can hand wash your bag in warm water in the bath tub. This is time consuming but works well as long as you make sure you have rinsed out all the soap. When it comes to drying your bag, air drying (only in warm summer conditions) can be used, although you must keep turning and 'plumping' the bag to prevent the down clumping together. More practically electrical tumble dryers are recommended (on a mid to low heat setting), but again they must be the larger front loaders found at the launderette, as the heat build up in household dryers may melt the outer.

View our range of Rock + Run Sleeping Bags

Check out our full range of Down Sleeping Bags