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I am very curious. I have frequently used a very small summer-rated sleeping bag, but have now bought a somewhat warmer bag, which is less compressible and takes more volume. It will be tested soon. Some sleeping bags in that category used 80 g/m2 polyester fiber as fill, while mine uses 150 g/m2, which is almost 2 times more. Both are similar priced and rated, but mine takes more volume -- it's less compressible and weighs 13 percent more.

Is a more wide, sparse synthetic sleeping bag better, because there is more volume to isolate you from the cold? Or it is the same if you have more dense filling with less volume taken? Is it better to have more dense filling, or to have it more widely distributed with more volume taken for similar weight? What are your experiences?

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Welcome to outdoors.SE! This is a great question.

I would say that there are maybe five different criteria involved here: warmth, cost, weight, compressed volume, and resistance to moisture.

You don't mention in your question whether you're car camping or backpacking. If you're backpacking, then weight and compressed volume are important. If you're car camping, then you won't care about those criteria.

If you have enough money to buy what you want, and moisture resistance is not an issue (e.g., you plan to sleep inside a tent), then a down bag will be superior to a synthetic one for all purposes. Usually the reason people buy synthetic bags is because of cost.

For warmth, it's not the filling material that keeps you warm, it's the volume of air. The only function of the filling material is to maintain more air volume in there. So when the bag is in use, you want the maximum volume you can get. However, if you're carrying the bag in a backpack, you want the compressed bag to have the minimum possible volume (and weight).

To compare different bags in terms of comfort, there are EN/ISO ratings. These give three temperatures, defining a range in which men will generally be comfortable (but many women may not), a range in which you're safe but probably uncomfortable, and a range in which you're in danger.

In any sleeping bag, the part underneath you will compress a lot and basically not do you much good in terms of insulation. This is why it's important to have a pad.

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  • I am backpacking, so weigh&volume are an issue. I use synthetic for now, so I am interested in that.
    – Al_
    Aug 6 at 18:28
  • Ratings are +12, +8 and -5 for 930 grams and 8 liter volume. My old bag was really light summer one, so it was great, but if you hike a lot you can get cold and that is not good. I used survival blanket as outer part if it got cold. It helped for a few degrees and I managed. New bag has much more volume so I am interested how that will help. In my opinion your answer that the volume of the air keeps you warm nailed it. Of course I look forward to test it soon. 2 cm sleeping pad works the best for me, 0,7 cm is too little.
    – Al_
    Aug 6 at 18:39
  • One reason against down in some cases: allergies Aug 7 at 2:25

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