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One of the selling points of a synthetic sleeping bag is that it retains its thermal properties even when wet. However, one of the most miserable nights I have ever had was in a wet synthetic sleeping bag in a rainstorm.

Would there be a percentage reduction of the temperature rating or another way of predicting how much insulation it will provide?

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    There is more than one synthetic insulation. None retain they just lose to a lesser extent. Get a bivy that does not leak. – paparazzo Aug 31 '16 at 2:41
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    A more accurate statement would be "A synthetic bag retains more of its thermal properties when wet." key here - "more of". If your sleeping bag gets wet, you will be less miserable in a synthetic than down, but you will still be miserable. – user5330 Aug 31 '16 at 4:45
  • Synthetic sleeping bags provide some insulation when wet. My guess, around 10-20% of the warmth are retained. The big selling points of synthetic bags are that they insulate when wet, but I would not bet my life on it. Rather get a proper down sleeping bag, keep it dry and take a wool sweater or two. Wool retains much more warmth when wet. Merino for the win! :) – Marcel-Is-Hier Sep 7 '16 at 7:42
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Really its all about the loft. So if you have a specific bag you're wondering about, next time you launder it look at it when its wet and see how much loft remains. I know my winter bag loses about 40% of its loft when wet, which isn't bad, especially because it dries quickly even if it relatively humid. There are many different types of synthetic insulation, different types of face fabrics that can affect the loft. There are too many variables to give a flat number. I've heard people say that synthetic base layers are 40% effective when totally wet, but I don't know how that carries over.

Really the best thing to do is take your dry sleeping bag, and fluff it out until its fully lofted. Take a ruler and measure 3 points on the sleeping bag to see how much loft you have and average them. Launder the bag, and measure the same 3 points while the bag is wet and average them. Take the averages and you can calculate the percentage difference.

i = initial measurement average
w = wet measurement average

(i-w)/i

This will give a pretty good idea of how much insulation your bag losses when its wet.

Synthetic will most certainly be more insulating when its wet than a down bag. Down is literally useless when wet. A wet synthetic bag, I'd still sleep in if I had to, a wet down bag I wouldn't even bother(luckily never had this happen). If weight isn't a huge concern, you could try adding a wool blanket. Wool is better than synthetic at keeping its insulation when wet. Wet wool barely looses any insulating properties even when completely saturated. It dries slower than synthetic, but synthetic is generally said to dry faster. But if you're in a rain storm and can't keep dry then wool sounds like it would have been a good option.

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I think this is one of the biggest myths in outdoor gear. No insulation is "warm even when wet", the real difference is whether it's life threatening or just really unpleasant when wet.

The useful difference is when the garment/bag is just slightly damp. (i.e. if you get somewhere dry, wring the water out of the garment and does it then provide any benefit?)

The answer for synthetics is mostly yes, but IMHO that's a long way from 'warm even when wet'. Your case is a something we all learn the hard way, being dry is warm, being wet is not. Doesn't matter what gear you have; you have to get in a dry environment to allow whatever insulation value remains in a damp garment to be useful.

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    No insulation is "warm even when wet" -- This is patently false. Wool retains much of it's insulation value when wet. This is because even when "soaking" wet, it still has air pockets, many of them. The same could, but may not be, true of any of a number of synthetic materials. Though to be super technical, insulation isn't "warm" anyways. It insulates. That's completely different from something that is warm (ie - radiates it's own heat). – Russell Steen Aug 30 '16 at 21:53
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    The point I'm trying to make is that if you continue to come in contact with a constant source of cold water ( i.e. rain/leakage...etc), it doesn't matter how well the insulation functions, you'll be cold and uncomfortable. The difference between wool and cotton is what happens after the rain stops. "Warm when wet" is no substitute for staying dry in the first place. – Fred the Magic Wonder Dog Aug 30 '16 at 22:54
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    Yes, if you are sitting in the rain or a creek, that is a problem. Re-reading the question I do see that what should be a relatively straightforward question is muddled by the addition of a story to go with it. – Russell Steen Aug 30 '16 at 23:09
  • I think the idea with natural and artificial fibers is that artificial fibers dry quicker. Wool will retain warmth even wet but in the outdoors, it would take days to dry while fleece is likely to mostly dry overnight. – Desorder Aug 31 '16 at 22:04

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