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Several years ago I purchased an Exped down filled inflatable sleeping pad. It is a more comfortable pad than Thermarest pads I've used in the past, so I would still recommend it. That being said, when I purchased the pad I remember Exped making a big deal about how their pads are filled with down as if this was a major improvement in terms of warmth (and possibly comfort) over other sleeping pads. At the time I bought this marketing pitch hook, line, and sinker.

However I was laying in my tent during a recent camping trip and I started to think about what possible benefit down really offers inside of an inflatable sleeping pad. As far as I know down is warm because it does a good job of trapping air and it is light weight. How would trapping air be helpful in an inflatable sleeping pad? The air is already trapped or the pad would deflate. Down's other claim to fame is being light weight. I don't know how that applies to a sleeping pad since not filling it with down would be lighter than filling it with down. Also getting down wet is a known problem and so Exped recommends that I fill my sleeping pad using the pad's stuff sack as an air pump so the moisture from my breath doesn't foul the down inside the pad. That leads to a bulky, specialized stuff sack that I didn't need for other sleeping pads. Another mild annoyance is there are two valves on the\h pad so that down doesn't collect on one side of the pad. This makes deflating quicker but it doubles the chance a valve will fail thus ruining my pad. The more I thought about it the more convinced I became that the increased warmth and comfort of my Exped pad was due to its substantially increased thickness vs Thermarest pads.

Does filling a sleeping pad offer any real benefit or is it just a marketing gimmick that exploits foolish people like myself who blindly accept down makes everything better?

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In terms of sleeping, a sleeping pad performs two purposes. The first is to provide warmth and down is a good insulator. The Exped XP 9 LW has an R value of 8.00, a weight of 41.3 oz, a length of 77.6 in, width of 25.6 in, and thickness of 3.5 in. The ThermaRest Prolite Plus, a fairly high end self inflating pad has an R-Value of 3.4, a weight of 31 oz, a length of 77 in, a width of 25 in, and thickness of 1.5 in. R values essentially add, so two Prolite pads would weigh 62 oz and only provide an r value of 7. The down seems to provide a substantial improvement to both weight and warmth.

It is worth noting that the thickness of a pad is only slightly related to the R value. Both the XP 9 and the Prolite provide about the same amount of warmth per thickness, this is because they are filled with insulating materials that trap the air and prevent/limit circulation. A pad like the Big Agnes Air Core is basically a pool float 3.5 inches thick that does little to limit air circulation. While the manufacturer states it is comfortable to 35 degrees, they do not provide the industry standard R value, which is likely essentially 0 (the two pieces of nylon will provide some insulation). For comparisons, Exped rates their pad down to -36 degrees F.

While down is a good insulator, for its weight, the ThermaRest Neoair provides an R value of 5.7 with a weight of 23 oz. It does this by creating a lot of small air compartments and lining these with a heat reflective coating.

This brings us to the second purpose, which is to provide a comfortable sleeping surface. While the Neoair provides better (marketed as the best) warmth for weight, many people complain these mats are loud. I am not sure how much quieter or more comfortable a down filled pad is.

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    Much quieter. :) – ppl Sep 12 '17 at 22:57
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Down reduces the convective heat loss substantially.

The movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat.

Rolling around is forced convection.

Down also reduces conductive and radiant heat loss.

They also put insulation between the studs in a house.

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    I didn't down vote either. More concerning to me is the deletion vote. This is clearly a serious attempt to answer the question so in my opinion there is absolutely no call for it to collect deletion votes. – Erik Sep 12 '17 at 17:56
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    @Erik seriously, the delete vote is totally uncalled for. In fact that is one of the reasons I gave it an up vote. – StrongBad Sep 12 '17 at 18:03
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    @StrongBad I totally agree that it was uncalled for. I try not to use my votes to compensate for other voters but I understand your impulse. As you can see in my comments either due to flaws in the answer or my own ignorance I question the validity of this answer. That is why I haven't voted either way. Hopefully I find out that I'm just ignorant and have a learning moment. If that occurs I'll gladly up-vote the answer and possibly accept it. – Erik Sep 12 '17 at 18:06
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    @Paparazzi Don't apologize. I fully recognize that my ignorance might be to blame. Where I'm having a disconnect is a jacket is an open system so the down's ability to retain loft with lots of space for air is what gives it its ability to keep us warm. An inflatable sleeping pad is a closed system that uses air pressure to maintain loft. This means the air is trapped and loft is preserved without the help of the down so what function does the down provide? Your answer seems to treat the down like a baffle blocking convection. I don't understand how it helps with conductive and radiant loss – Erik Sep 12 '17 at 18:09
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    I have a rep of 4000 and I cannot vote to delete a question not closed. This was voted to delete in the first 2 hours. How can that happen? – paparazzo Sep 12 '17 at 18:57
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You blow up your pad every night with hot humid air from your lungs. You deflate it every morning. That water vapour condenses in the pad. Getting the water vapour out requires letting the pad self inflate from a source of warm dry air, and then forcing the air out. Over and over and over. (The easiest way to do this is to fold it in half and put it under the cushions on a chair. Every time you sit down, you deflate it.)

How significant is this? My Thermarest is 60 cm x 150 cm and is about 3 cm thick. Volume of 60 * 150 * 3 = 27000 cm2 = 27 liters. Air at body temp holds about 40g water /cubic meter. If I'm in a hurry and add 20 liters of air to it, then I'm adding 40g/m3 * 20 l * 1 m3/1000 l = 0.8 g water per inflation. With foam I don't worry about it much. Foam isn't eaten by fungi.

Down is moderately good at absorbing water. A damp downbag isn't nearly as warm as a bone dry one. Down I think would be much weaker than foam for self inflating the pad.

I would worry about down mildewing in the pad.

So addressing your original question about a difference:

  • I expect a down filled pad to have a shorter life.
  • I expect it to be harder to dry out internally after prolonged winter use.
  • I don't see any reason for it to be warmer than the same thickness foam pad, and possibly worse depending on how much water the down has adsorbed.
  • Depending on construction details, I do expect it to be slightly lighter than a foam pad.
  • Your raise an interesting point, however this answer directly contradicts another (that has references), can you please cite references to backup your assertions and the presumption that you need to use you mouth to blow up a down sleeping mat – user5330 Sep 14 '17 at 2:04
  • @mattnz I don't think it contradicts my answer which also says foam and down given about the same warmth per thickness and that down is lighter. This adds some unsupported claims about lifespan, drying out, and mildew which I don't buy at face value. – StrongBad Sep 14 '17 at 2:43

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