transverse heat loss in insulated air pads

That could be why the Big Agnes pads have a R-Value of 4.1 and are rated to 15* but many people find them cold at much warmer temperatures. I find my Exped Downmat very warm but I assume the down fills all of the space within the pad when it is inflated.

Hi Brad,

I don't have an answer but I do have an opinion. :-) If I understand your question it's how much the vertical (edge) surface area contributes to air mattress heat loss. Having used all three brands I suspect some loss likely occurs but is very minor compared to how much heat is lost through the ground-to-pad interface. If someone wanted to essentially eliminate air-to-pad heat loss a bivy sack should do the trick.

On the general topic, I've always wanted to see thermographs of the diecut self-inflating pads showing how much heat is lost through the holes in the foam. So many experiments….

Cheers,

Rick

From Exped :
Welded baffles between the down filled chambers, unlike traditional airmats, eliminate cold spots and create a comfortably supportive and stable air cushion.

Franco

Franco, yeah, that's kinda where I was going… down-filled air mats, baffled, filled entirely w/insulation are the way to go for true insulation. Extrapolating from the Exped bit, "pool toy" style air mats are "welded through" instead of baffled, so even if you insulated them, you'd get cold spots at that narrowing. Baffled keeps even loft of down top/bottom, down insulates throughout depth of pad/all volume of pad.

People mention how their IACs feel warmer when mostly deflated; I could feel the cold passing through my NeoAir. I just wonder how much heat loss occurs proportionately in the uninsulated portions of some air mats, or if there's a way to set up the testing equipment to check it cross-wise. I mean, w/no insulation in the air space, it oughta have an R-value of 0+ or so for, say, 2/3 of the thickness of an IAC, it seems.

"I've always wanted to see thermographs of the diecut self-inflating pads showing how much heat is lost through the holes in the foam. So many experiments…."

Yes, indeed, to the thermographs. For an experiment: I wonder if lining self inflating pads with mylar or something similar would reduce the loss, especially if it were facing upward on the top wall of the pad?

I think that with almost all pads, most of the warmth will be lost through the bottom. There is a much bigger surface area there, as well as contact with a cold, solid surface (instead of air). I'm not sure, but I think both of those contribute to a lot more heat loss.

I get the feeling that with the NeoAir, the loss isn't huge, but it is significant. Because of that, people suggest laying a closed cell pad on top of the NeoAir, instead of underneath it (when you want to add a little more warmth). The big advantage of putting a closed cell pad underneath is that it protects the inflatable. Because of this desire to do both, some have suggested creating a closed cell envelope. Such an envelope wouldn't weigh much more (since there isn't that much along the edges). I've run through some ideas in my head, but haven't come up with anything that would work well as a sit pad and be easy to apply in camp.

"I get the feeling that with the NeoAir, the loss isn't huge, but it is significant. Because of that, people suggest laying a closed cell pad on top of the NeoAir, instead of underneath it (when you want to add a little more warmth)."

My thought is that if you're putting a CCF pad on top of the NeoAir to add more warmth, as compared to putting it underneath to add more warmth, then the strong implication is that the NeoAir would lose the added warmth of the foam pad before the benefit could be realized on the person-side of the pad. By putting the CCF on top, you're saying the Neo would lose the heat before the heat could do any good… which makes me wonder if the pad does much of any good for insulation. CCF on top, as described above, makes it seem as though the CCF pad is clearly the primary insulation.

I'm not a scientist, nor do I play one on TV… I did spend some time laying on snow recently though… These are just my opinions (guesses).

Three modes of heat flow:
-Conduction. Any sort of pad blocks conduction. The lower the mass, the better- so a plain air mat is probably very good because of its low mass. Lay down directly on dense snow and you can feel the heat flow out of your body rather quickly- a great example of conductive heat loss.
-Radiation. Any sort of mass will either reflect or absorb and re-radiate the heat in all directions. Quilt sleeping on an uninsulated air mat you only have your clothes and top layer of the mat, so probably a fair piece of radiant heat loss. Neoair is low mass but has reflective layers. Foam pads have much more mass so they block radiant heat loss pretty well- they have the effect of many layers, each absorbing and re-radiating the heat energy. Down filled inflatables might lose some heat to radiation but I'm just guessing.
-Convection. This is the biggie for air mats. The smaller the air spaces the better, so foam or down are both good at stopping convection, a few baffles like the Neoair are probably much less effective.

A few thoughts-
It seems filling a NeoAir with down would make a pretty warm, still very light pad.
Neoair plus foam: Use the foam underneath on harsh ground to protect against puncture and to add a bit of warmth, on top when your camping on snow so you don't have to warm all that air up!

I read a comment somewhere from a guy who was involved in product development of the Neoair that the CCF should definitely go under the pad. Reason has to do with the Neoair taking advantage of the conductive heat loss from your body to warm the pad, which the nifty proprietary reflective baffling traps. This actually provides most of its R-rating. Without the radiant heat from your body, which would be substantially blocked by the top CCF, the Neoair is pretty much just an uninsulated air mattress.

I think anyone connected with developing/selling the Neoair will always advise that the CCF pad goes under the mat. They would be more worried about punctures than heat loss. :)

I would think that heat loss out the sides of a mat must be a factor in any air mat.

"They would be more worried about punctures than heat loss. :)"

Not exactly. It's pretty obvious that the Neo feels warmer than a totally uninsulated mat, so those reflective inside layers must work.
(they do for me…)
Franco

Just FWIW, the radiant heat barrier in the NeoAir is a single horizontal layer running through the middle of the pad… it isn't used as the "corrugated" baffling or anything. The baffles are a porous poly fabric that waffle from the outer shell to the middle radiant heat barrier. It does definitely work to a degree, but…

One of the early threads about the NeoAir discussed whether it made sense to put the closed cell pad on the top or the bottom. Several people who have used similar inflatables said that you should put it on top (for warmth). This didn't make sense to me, and it disappointed me. It didn't make sense to me, because I intuitively thought that it wouldn't make a difference from a heat perspective. This is true if you two pads made out of identical material, of course. I believe it is also true if you have loss only due to the ground (regardless of how it is lost). This makes sense, when you compare the two situations suggested by the last two sentences. In the first case, for example, let's say you have a two inch foam pad and a one inch foam pad (with identical material). Does it matter what side is on top? Of course not. Now, lets replace the one inch pad with something that has the identical R Value. Ignore heat loss to the side. Again, does it matter which side is on top? No. If it does, why? Where could the heat be going (again, ignoring heat loss to the sides)?

Now, let's consider the NeoAir and heat loss to the sides. The NeoAir is fairly efficient, from a heat retention/weight perspective. However, it is not efficient from a heat retention/volume perspective. One of the ways that it gets its good R Value is by being so thick. So now, take a closed cell pad with the exact same R value. It is much thinner. Since it's much thinner, it has less surface area on the side. Thus, it stands to reason that it would have less heat loss to the side. As a result, the NeoAir loses a lot more heat to the side. Now, the key to this thing (and I had to think about this a while) is the difference between the temperatures. Assuming that a NeoAir loses more to the side than a closed foam, which one do you put on top? If you put the NeoAir on top, then heat loss to the sides of the NeoAir is no different than without the pad. However, if you put the NeoAir on the bottom, then there is less of a heat difference between the top of the NeoAir and the sides. In other words, the top of the NeoAir is no longer at body temperature, but at some value between the air (and ground) temperature and body temperature.

OK, all of this assumes that a NeoAir loses more to the sides than a closed cell pad of the same R value. It also assumes that my physics is correct. It has been a long time since I've taken physics, so it's quite possible that my arguments are wrong. If someone out there would correct (or confirm) my work, I would appreciate it very much.

What we need is a set of 100 identical twins and of the same sex. They will need to exert in the same way for the day and eat the same food,wear the same clothing and so on. One will sleep with the solid foam mat on top, the other on the bottom. The next night they swap. A 10 day test should suffice. I think that Ben has a list of twins from the G Spot experiment.
Franco

"What we need is a set of 100 identical twins and of the same sex."

Octomom is working on that.