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What is the Fill Power of Synthetic Insulation?

What is the fill power of synthetic insulation?

How does synthetic insulation compare to down insulation in terms of warmth? You might be surprised how lackluster it is in comparison, even with recent advances in the types of synthetic insulation available.

Fill power is used to measure the warmth of the goose-down and duck-down insulation in down jackets, down sleeping bags, and down quilts. Expressed as a number, fill power measures the ability of down to trap the warm air heated by your body in a down jacket or down-insulated sleeping bag. For example, a 900 fill-power down sleeping bag will be lighter weight, more compressible, and much more expensive than if the identical sleeping bag were filled 650 fill-power down and rated for the same temperatures. This is because it takes less 900 fill-power down (by weight) to retain the same amount of warmth as 650 fill-power down.

Unfortunately, the warmth of synthetic insulation is not measured using fill power, which makes apples-to-apples comparisons difficult. But as a rule of thumb, the synthetic insulation found in insulated jackets and synthetic sleeping bags is equivalent to 550 to 600 fill-power-down insulation. meaning that it requires more synthetic insulation by weight to give you the same warmth as a higher fill power down.

This is a useful comparison when it comes to buying insulated jackets and sleeping bags, especially if cost is a consideration. It means you can buy a much less expensive 650 or 750 fill-power down jacket or down sleeping bag that is warmer, lighter weight, and more compressible than an expensive top-of-the-line synthetic jacket or synthetic sleeping bag. The manufacturers of synthetic insulation such as Primaloft (Polartec), Coreloft (Arc’teryx), VerticalX (Outdoor Research), and others don’t go out of their way to explain this, but I thought you should know, especially if you have budgeting constraints.

But the superior weight-to-warmth ratio and cost aren’t the only reasons to choose 650 or 750 fill power down over synthetic insulation, but they are important factors to consider depending on your location and activity level, as shown below.

Best ForCold, dry weatherCold and damp weather
ActivityStationary activitiesStart-stop activities
MaterialGoose or duck downPolyester fibers
ProsHighly compressibleDries quickly
ConsLoses insulation ability when wetHeavier, Not as compressible
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  1. In the third sentence, “lighter weight” should be removed from the advantages of the 900 fill-power sleeping bag over a 650 fill-power sleeping bag “ if both are insulated with the same amount of down by *weight*”. It’s like the old riddle: which weighs more, a pound of lead or a pound of feathers? Which weighs more, a pound of 650 fill-power down or a pound of 900 fill-power down?

    Alternatively, change “the same amount of down by weight” to “the same amount of down by volume” for a better comparison.

  2. I’m not sure how FP measures anything other than packability. If you’re only looking at 1 number to judge warmth it should be baffle size. A 900fp quilt isn’t going to be warmer than an 800fp or synthetic by virtue of FP, it’ll only be warmer if that 900fp is in a bigger baffle.

  3. Josh the point of this article is to point out that synthetic insulation is only as insulating as 550-600 FP insulation irrespective of the garment or quilt it’s put into. FP is completely independent of the garment or quilt the down insulation is in.

  4. Would like to hear your thoughts on long term loft retention. People fuss over the storage of down items. But it is my synthetic items that “pancake” over time with no ability to regain loft.

    • Manufacturers recommend washing and then tumble drying on LOW heat with tennis balls to help fluff things up. The results will differ depending on the type if insulation you have. If its a matted type that comes in a roll, you’ll probably have less lock than of its looser and mimics down. But it will never be the same as when new.

    • Try popping them (synthetic insulated items) in the drier. That’s all I can think of.

  5. How does Climashield Apex compare with the ones you mentioned in the article – it always seems to be the ‘chosen one’ amongst ultralight and MYOG users. I wondered if this is to do with its insultation vs weight properties or simply that it is easier to work with than some other materials?

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