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How to Choose a Sleeping Bag: Fill

Fill type is an important consideration when choosing a sleeping bag. We have it narrowed down to three types; synthetic, down and treated down. Knowing the differences in fill will help you determine the right sleeping bag for your needs.

How to Choose a Sleeping Bag: Fill

One of the first things to determine when choosing a sleeping bag is what type of fill you prefer. There are many different types of fill, but just three main types: synthetic, down and treated down. The following how-to considers the key points of each fill type to help you determine what type is best for your needs.


Synthetic fill is made to mimic the structure of down, but it is a man-made material that uses thicker and more durable strands to provide a framework to trap your body heat. There is a variety of different types of synthetic fill on the market, and with rapidly changing technologies, there are new brand and trademark names out there all the time. To name a few names, you might find Cloudloft, Thermal Q, Thermalite and Syprofill synthetic insulations, among others.


One of the biggest benefits to synthetic fill insulation is that is has a water resistant property that continues to provide insulation even when damp or wet. When synthetic fill does get wet, it dries quickly because the moisture gets trapped in between the fibers rather than getting absorbed into the fibers. Another benefit of synthetics is that they are man-made, so they are completely hypoallergenic (unlike down, which is an animal product). Synthetic sleeping bags are relatively easy to care for and are usually machine washable in a front loading washer. Since they dry quickly, you can hang them outdoors in direct sunlight for fast drying.


There are also some downsides to synthetic fill bags. They do tend to be bulkier, heavier and not as compact as their down counterparts. They are heavier because there is not a high warmth-to-weight ratio that you get in a down bag. With the advancing technologies of synthetics, they are getting closer, but they still cannot compete on warmth-to-weight ratio with down fill.


Down fill keeps you warm by trapping in an abundance of body heat in its tiny clusters of fibers. There is no other material that is as thermo-efficient for its weight than down. Compared to a synthetic-insulated bag, down is a lot more efficient ounce per ounce. Down comes in a variety of fill powers, typically in a range anywhere from 500 to 900. It takes more of 500 fill down to have the same thermal efficiency as 900 fill. For example, say you have a 20-degree rated sleeping bag that is filled with 550 fill down and you also have a 20-degree rated sleeping bag that has 900 fill down in it. Both of these bags are equally warm, but the 900 fill bag is going to be much lighter in weight.


There are obvious pros and cons with down insulation; already mentioned was the unbeatable warmth-to-weight ratio, and down also has a very, very compressible nature. Your down-insulated bag will easily compress to save space in your pack on the trail. Down definitely loses its insulating properties when it gets wet or damp and it is slow to dry, so it requires maintenance when it actually does get wet. This is a major negative. Down may harbor allergens, or even cause allergic reactions itself. If you are prone to allergies, it might be wise to invest in better quality down products, as lower quality down seems to cause more reactions. The last downside to down-insulated sleeping bags is that they tend to be more expensive than comparable synthetic sleeping bags. So if you are on a budget, you may want to look towards a synthetic bag.


The last type of fill we we’ll review here is treated down. Treated down is essentially a microscopic, hydrophobic finish that is applied to each plume of down. This is a permanent treatment, so it is not going to wear off and it is not going to wash off. What the treatment does is makes the down plumes function more like a synthetic material, because it sheds water; your treated down bag is going to perform more like a synthetic bag than like a traditional down bag. You can take treated down into damp or wet conditions without worry. If you are on a long trip where the bag might absorb water day after day, the treated down-insulated bag is going to stay drier longer, and it is going to dry out faster than traditional down. Treated down is a great option for anyone who wants something very packable and very lightweight, but also wants to keep water away. The only downside to a treated down bag is that it is going to be more expensive than traditional down.


With these pros and cons of each type of fill—synthetic, down and treated down—hopefully you will be able to have a good starting place to decide what type of fill is best for your needs.