Down vs Synthetic Fill in Sleeping Bags

There are pros and cons to both down and synthetic insualtion, so read on to make sure you're buying the bag that best suits your needs!

Down Sleeping Bags

"Down" is a pure, natural, thermal-insulation product grown by birds in cold climates enabling them to survive severe weather conditions. The insulation properties of down enjoyed by birds living in these conditions has clear benefits when applied to sleeping bag fill, and the performance benefit of down fill over synthetic is significant.

For a given temperature rating, a down bag will be both lighter in weight, and compress down to a smaller size when packed. The advantages are clear when you are carrying your own kit on a trek or expedition where weight needs to be kept to a minimum and pack volume is limited.

Perhaps the only drawback with down, in terms of performance, is the reduced insulation properties when the fill becomes wet. On long trips or in wet climates, keeping the bag dry can be difficult.  Manufacturers have solved this problem by use of advanced fabrics, and there are several down sleeping bags that use highly breathable lightweight waterproof materials to keep the fill dry and maintain the full potential of the insulation.

One way to describe the quality of down is by its fill power, which is the measurement in which an ounce of down will loft and expand to fill an empty space. Down quality is rated by cubic inches (cuin). The higher the fill power, the higher the quality of the down and the higher the thermal rating of the sleeping bag.

Fill ratios refer to the percentage of down plumes to support feathers in terms of overall fill
weight. Down is responsible for the insulating qualities of the fill, while support feathers create the
loft necessary to retain heat in the sleeping bag.This mixture is represented numerically as a ratio - for example 85:15, which means 85% pure down and 15% feathers. The greater the pure down % the better the insulation value, in theory, as long as you compare like-for-like qualities. In practice, above a 90:10 ratio it is quality differences which are of more significance in determining how good the insulation is.

Synthetic Fill Sleeping Bags

The most noticeable difference with synthetic fill sleeping bags is the price! However, it is well documented that synthetic bags provide less thermal insulation for a given weight and volume of fill, so if price, weight and pack size are not an issue then synthetic bags start to become more of a consideration.

There is another consideration, and that's when things start to get wet. Synthetic bags have a distinct performance advantage over down when wet, maintaining a higher thermal efficiency with a reduced tendency of the fill to 'clump' together. Synthetic fill also dries more quickly than down, with moisture transport away from the inside typically 20% faster than down.

Although synthetic fill does not compress as easily as down, this does have the effect of providing synthetic bags with higher insulation potential under compression, so the base of the sleeping is more efficient at retaining heat.

Marmot Down vs Synthetic Video

Those lovely people at Marmot have put together a series of videos about sleeping bags and insulation types. Keep an eye out for them on the other pages on this site.