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Down Insulation 101

Finding the right insulated jacket for a particular trip can be overwhelming – there are many aspects to consider, including different construction styles, weights, and fabrics. Here, we break down the fundamental properties of down insulation so that you can better navigate the options and come prepared for your next mountain adventure.

Why Down Keeps You Warm

When you put on your down jacket, the down plumes fluff up (plumes are soft fuzzy clumps of down- not actually feathers nor quills!), trapping millions of tiny air pockets throughout the garment. When you crawl into your sleeping bag, or put on your jacket, these air pockets heat up from your body warmth, trap your heat, and keep you warm.

Types of Construction

Down jackets are designed with “chambers” which keep all of the down from settling to the bottom of your jacket. These chambers are created in two different way- with baffles or sewn-through closures. Each construction method offers pros and cons:

Sewn-Through

A sewn-through jacket is just as it sounds; the outer fabric is sewn completely through to the inner fabric lining, forming down chambers between stitches. While there is no extra fabric added and thus weight is saved overall, this can form “cold spots” in the jacket, as there is no down in the seams which border the chambers of down. In addition, the shape of sewn-through baffles tend to compress the down on each “edge” of the down chamber, preventing the down from lofting fully. Most lightweight jackets and down sweaters use sewn-through construction. In profile, sewn-through down chambers look like a series of ovals joined at the narrow end. Example: Rab Neutrino Endurance.

Box-Baffled

A box-baffled jacket has different chambers of down which are fully separated by a wall of fabric. This allows the down to loft throughout the entire garment, eliminating cold spots. Some baffled constructions have small holes in the fabric walls to allow down to move between chambers, and even loft “in” the wall. These thin walls of fabric do add extra weight to the garment, but also create a warmer overall structure than sewn-through construction. Most warmer expedition-grade jackets are made using box-baffle construction. In profile, box-baffles look like a series of squares which sit next to one another. Example: Rab Expedition 8000 Jacket.

How do I know how warm my down garment is?

There are two factors which tell us the warmth of a down jacket- the fill weight and the fill power. Think of fill weight and fill power as two parts to a story: you can’t understand the full story if you only look at one aspect!

Fill Power

Fill power refers to the quality of down used in a jacket; specifically it measures the volume (size) of one ounce of down. So, when a jacket label shows that it has “850 fill power” (or “850FP”), it means that the quality of down used to make it fills 850 cubic inches of volume per every ounce of down used. “650 fill power” would mean it has 650 cubic inches of volume per every ounce of down. Generally, the higher the fill power of down used, the less overall down needed to keep you warm because it offers a greater volume per ounce- more down plumes to trap air to keep you warm! Additionally, higher quality down generally lasts longer, and packs smaller than its lower fill-power counterparts.

Fill Weight

Fill weight refers to the total weight (quantity) of down used to fill a jacket. This is usually measured in grams or ounces. The greater weight of down your jacket has, the warmer it will be because it will trap more air between the down plumes. Fill weight plays a huge factor in indicating how warm your jacket is will be. This is the first thing you want to find out when shopping for a down jacket.

Alpine Ascents Mount Vinson Climb
The summit of Mount Vinson, Antarctica – an appropriate locale for a box-baffled jacket

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