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Why are down-filled inflatable pads warmer?
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Home › Forums › Gear Forums › Gear (General) › Why are down-filled inflatable pads warmer?
- This topic has 20 replies, 14 voices, and was last updated 8 years, 2 months ago by Andy F.
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Feb 4, 2016 at 11:11 pm #3380291
Air is a good insulator. The down in bags and garments creates insulation by puffing out and creating trapped air volume. But an inflatable pad is already filled with trapped air. So why does down increase the R-value of inflatable pads, apparently quite significantly? Is it because the down keeps the air more static, reducing convection within the pad?
Feb 4, 2016 at 11:31 pm #3380295The down traps the air better. Inside an inflated bag, you get air currents that transfer the heat. Down stops the air from circulating (as well as adding to the amount of air). The NeoAir stops the air from circulating by using sharp angles, which was the innovation. They managed to trap the air without an additional substance (like down).
Feb 5, 2016 at 7:03 am #3380310Ross is right, it takes more than an air gap, it takes trapping air and keeping it from circulating.
Insulation in your house is no different. You already have an air gap created by the 2×4 (or 2×6 depending on where you live) wall but by itself that would hold little to no heat in. By putting fiberglass insulation in that cavity it keeps the air from circulating and holds heat inside your house.
Feb 5, 2016 at 7:48 am #3380320yeah
if the gap is 1/2 inch, or even 1 inch, there will be little air flow
above that, you’ll get circular currents of air. The down or synthetic or foam will prevent these currents
that’s how all outdoor insulation works like jackets and sleeping bags
Feb 5, 2016 at 9:12 am #3380344You have it right, Ralph. Heat is transferred primarily by conduction (heat transfer through static mass), convection (heat transfer involving transfer of mass), and radiation (this is radiating heat that requires no mass for heat transfer).
When opposite sides of the air pad are different temperatures, the mass of the air will swirl and allow a pretty fast heat transfer. The down slows down this convection. The down probably does little to affect conduction either way because there isn’t much mass in down and I would assume its not that conductive anyway. I have never heard this, but I would assume it helps slow down radiation as well since the down near the warm side of the pad is nearer in temperature to the warm surface of the pad than it is to the cold side of the pad.
Feb 5, 2016 at 10:02 am #3380356Not conduction , not radiation but convection. The pad has basically a “hot” side next to the body and a “cold” side on the ground. Over time, a convection current can form called a Bénard cell, this allows for efficient heat transfer. This effect can be seen when you get a hot bowl of miso soup. A pattern will form where fluids will rise from the bottom (along one edge) and then sink (Along another edge). Many times they create cells that look like hexagons. Some people have used this as an explanation of how basalt columns (Devils Postpile) are formed: slow cooling / convection currents. Down / insulation breaks up the size of these cells and slows the convection rate substantially. Best regards.
Feb 5, 2016 at 10:34 am #3380363Lest we not forget that “air” mattresses are around 2.5″ thick. This means that a “regular” sized pad might have over three square feet of EXPOSED edge, which can arguably be even colder than the ground itself. There better be something inside that pad to reduce the movement of air. Of course the second a person rotates from their side to their back, all that air will swish around once again.
Yet another reason why thinner ccf pads are soooo much better insulators than air mattresses.
:)
Feb 5, 2016 at 2:58 pm #3380427Matt brings up a point I hadn’t considered before – edges of air mattress would approach ambient temps (or even radiant heat away and be lower than ambient) and that cold edge could drive a convective loop. Also, if your bag/quilt doesn’t completely cover your pad, then portions of the top of the pad will cool to ambient temps. And down filling greatly reduces convective loops.
But mostly, the top of the sleeping pad is warm and the bottom is cold. That WON’T drive a convective loop. The cold air will stay low and the warm air will stay high. “Stratification” happens in those situations when there isn’t a driving force to convect the air.
Yes, down helps also reduce radiant heat transfer from the warm top layer of the pad to the cold bottom layer. Rather than radiating heat directly to the lower surface and getting less heat radiated in return, the upper layer radiates to the first down it “sees”, which warms a bit and reradiates to the next piece of down, etc. All those small steps reduce radiant heat transfer.
Feb 5, 2016 at 3:20 pm #3380431It makes sense to me that down would reduce radiant heat losses but I had really only considered the convection losses previously. I’m not sure if this amount is significant though.
I had thought about the heat losses along the edges of the pad. That’s why I use a 1/8 foam pad ON TOP of my air pad in cooler temperatures.
Feb 6, 2016 at 1:52 pm #3380595CONVECTION HEAT LOSS:
Down filled mattresses, like down clothing, work because the down traps warm air and prevents convection from carrying the heat away. Air mattresses, regardless of reflective linings like Thermarest designs, still lose heat due to convection (air currents) and down largely prevents that.
Heat Loss/Gain Mechanisms: Convection, Conduction, Radiation, Evaporation
Feb 6, 2016 at 2:25 pm #3380600the Neo air has internal baffles that limit the size of internal air spaces to less than 1 or 1/2 inch so there is minimal convection. Plus, they’re reflective so radiant heat transfer is reduced, more than down or synthetic insulation.
the down fibers actually allow some convection. the fibers are very small. the same thing that gives you better warmth per weight, also allows some convection. If you measure the clo/inch of down, it’s less than synthetic for example.
not that any of this is that important – neoair, foam, or down will make a sleeping pad warmer than just air
Feb 6, 2016 at 10:12 pm #3380668AnonymousInactiveJerry, maybe you, Roger, and/or Richard could answer this, but i’ve been wondering about CO2 gas as compared to air.
I think i remember that CO2 gas is about 40% less conductive than air. But CO2 gas is also denser than air. Being denser, wouldn’t that limit convection some as well?
Reason why i was thinking about this, was i was thinking about Dave Thomas’s idea of a combo pulk/shelter for extreme cold weather trips. If you lined it with Neoair pads filled with CO2 gas, you could get some crazy heat retention at fairly low weight, though foam would be safer/more reliable.
Feb 7, 2016 at 4:44 am #3380684A couple of issues with CO2…
- For a CO2 supply, you’d have to carry some of those bicycle tire refill cartridges and rig up an adapter.
- CO2 molecules are smaller and pass through plastic/rubber much faster than air, which is about 78% N, so the air mat would go flat quite quickly. If you’ve ever done a roadside or trailside repair of a flat bicycle tire using CO2 to reinflate, you’ll recall that the tire has to be refilled the next day with an air pump.
There might be some advantages I’m not aware of, but these are a couple of practical/technical challenges that must be dealt with.
Feb 7, 2016 at 6:57 am #3380693…For a CO2 supply…
What exactly do you think you are filling your inflatable with, when you are blowing it up by mouth? Pure Helium?…
As a side note, I consider the NeoAir XLite overrated and ineffective in reducing convection, no matter what the manufacturer claimed. Used one on a single trip, where night air temps went down to 40°F, gained unpleasant experiences and happily disposed of it as soon as possible.
Feb 7, 2016 at 7:14 am #3380694What exactly do you think you are filling your inflatable with, when you are blowing it up by mouth? Pure Helium?…
I don’t blow it up by mouth, especially not in the winter, and down air mats should never be inflated by mouth.
I realize we breathe out CO2, but the air we breathe in is still 78% nitrogen and therefore exhaled air will contain only a small amount of CO2. And, even then, unless you hold your breath for a long time in the lungs, only a small percentage of the O2 is converted to CO2. This why mouth-to-mouth resuscitation works; if no O2 was being delivered to the victim it would be pointless.
Feb 7, 2016 at 7:43 am #3380698If you were carrying your pad inflated, that would totally change things. Being able to deflate pad to carry it is a huge design constraint
Maybe styrofoam would be better? with enough of a foam pad on top for comfort? With gasses, it’s the weight of the container to consider.
If you’re doing an arctic or antarctic expedition, maybe that’s not the time to experiment. Better to study what other explorers used and do that? Although carrying a pulk and using foam might be safe. Maybe test new ideas in Alaska in the winter near a cabin.
http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
notrogen and oxygen – 0.024 W/Km
argon – 0.016
CO2 – 0.015
R12 refrigerant – 0.007
radon – 0.0033 – except the lead container would be a problem
vacuum – 0 – again, heavy container
helium – 0.14 – but it would help that the gas is bouyant
hydrogen – 0.16
Feb 7, 2016 at 8:25 am #3380702AnonymousInactiveThat’s good to know Bob–i wasn’t aware that CO2 permeates out that quickly (i’ve never used it for bikes or anything else). You could probably epoxy on a special film or two to deal with that, but it’s probably better to use Argon in that case. Maybe that’s that’s the primary reason why Klymit uses Argon instead of CO2?
Feb 7, 2016 at 8:39 am #3380704AnonymousInactiveIvo, i’ve only ever used the Neoair All Season pad, but going from that, it worked and works extremely well and has kept me toasty warm (in conjunction with a thin foam pad) down to negative temps.
Feb 7, 2016 at 9:10 am #3380709AnonymousInactiveIs Styrofoam the most weight to warmth efficient foam? Hmmm, if bulk is less an issue, then why not get two birds with one stone. Heat resistant, aerogel bonded blankets (Pyrogel) or ceramic fiber blankets for the inner part of the insulation (and foam for the outer)? That way, if you wanted to cook in the pulk shelter, you’d have some extra protection.
Jerry, anything i came up with, i would try several times in a more safe environment before bringing it to a potential survival situation.
Anyways, i’m over derailing this thread so i’ll stop talking about this here.
Feb 7, 2016 at 9:49 am #3380720No point in over-thinking this. Take your Ridgerest ccf pad and throw it on frozen ground with air temps at 0F. Go to sleep. Squirm, possibly. The next night take your Exped Downmat 9 and do the same thing. Tubes of goose down make a huge difference. Prove this by real-world testing.
Find what works when push comes to shove and settle for that.
Sadly, the major problem with Exped is their tendency for tube baffle seams to rip resulting in malfunctioning air bladders. See—(one of those pics is mine)—
Feb 8, 2016 at 11:57 am #3380902“Take your Ridgerest ccf pad and throw it on frozen ground with air temps at 0F. Go to sleep. Squirm, possibly. The next night take your Exped Downmat 9 and do the same thing. Tubes of goose down make a huge difference. Prove this by real-world testing.”
I did this on a trip once, except I skipped the Ridgerest. The Downmat 7 was a bit chilly even. A 1/4″ 17×17″ ccf sit pad under my torso and on top of the Downmat allowed me to warm up enough to sleep eventually.
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