So im confused as to whether it matters which mat i put on the ground and which one above. Now i realize there could be a million variants (length, thickness, width, type of mat)
The application i had in mind was:
in bivi - pad outside the bivi, no ground cloth.
full length CC pad (GG thinlight 1/8) and torso length TMrest (Prolite 4 s)
so my rational was to put the CC on the ground since it will protect the inflatable and since in theory pads are like resistors in series it shouldnt matter...
but then i saw this https://www.backpackinglight.com/cgi-bin/backpackinglight/airmat_sotmr_part2_2011.html where Roger and Will talk about foam overlay - they use it on top!!!

Whats the thinking behind this?
Mike

They don't work exactly Lile a series of resistors because of the larger surface area of the airmat. You want the foam on top so the temperature gradient between the air and airmat(with large surface area) is less.

If you are talking cold weather, snow etc. then foam on top

If you are talking about warm weather and hard surfaces, maybe desert conditions then I put the foam on the bottom for more protection of the inflatable.

Sorry, no absolute clear cut answer. Reasoning is given below if you really want to know.

Its Thermodynamics %'s. Whichever orientation gives the 2nd pad closest to the ground the smallest Temperature gradient. From one side being the "Hot" side to the other being the "cold" side. Of course this is too simplistic really.

Air mattresses without down in them create air currents transporting heat(convection). So, is said R-value of said air mattress given with said convection tabulated or did they "calculate" said R-value with conduction only?

Its calculated with conduction only and NOT Convection. This is a base Lie. It depends on temperature to determine how bad of a lie it is. Why most air mattresses come with foam inside.

Why? Convection depends entirely on HOW large the temperature differential is and therefore cannot be stated accurately for the common Jack/Jill to understand. The convection actually transports MORE heat than conduction. Therefore said air mattress R-rating are base lies. I used to carry one, I can vouch for that much! I now carry foam. Foam doesn't leak either!

So, back to initial statement. It comes down to %%%. If your air mattress transports more heat than your closed cell foam pad then it goes on the bottom and the foam pad on top. Therefore there is a SMALLER temperature gradient delta in said air mattress to create said convection air currents inside the air mattress. Of course this depends on how THICK your foam pad is. If it is very thin then placing said air mattress under you first would be preferable with the foam pad on the bottom.

If on the other hand you have a super thin down filled or equivalent air mattress and also a foam pad then it gets quite a bit more complex.

Sorry would really have to know EXACT setup and actual test environment. Consistent ground air temperatures in other words.

MOST people I know if they bring an air mattress, usually bring only a "half" air mattress and put it on top of their closed cell full length pad. Of course some cheat and use rope/slings etc at foot pad and don't bring full length pad at all, but rather bring half length closed cell foam pads. Besides half length closed cell foam pads are great for sitting on during a rest break. You wouldn't dare do so on an air mattress as you would quickly have a hole in said mattress.

Sorry, no absolute clear cut answer.

OK - i get what you guys are saying RE convection and it makes sense. Essentially the issue is both Temp gradient that leads to increased convection and most probably the fact that the think inflating mats have edge effects where there is heat loss that isnt through the whole thickness of the mat but rather through much smaller thickness in the edges.

So assuming I have a Torso length Prolite 4 and a full length Gossamer Gear thinlight1/8 and I guess i'll use my rucksack/drybag under the feet so the rational above would say put the Prolite underneath and the GG on top? (and i assume if im concerned RE punctures i could use a polycro GS)

???

I think that's it, Michael. You don't want a large temperature gradient all around a big airmat. It will transfer heat out the sides and top as well. Engineering principles,best as I remember, tell me that youwant you good thin insulator on top.

I've not done any calcs on this, but from an empirical (experience based) perspective, both myself and others I've hiked with have tried both ways and ended up with a rule of thumb that if there seems to be any chance of feeling cold on a given night, it's best to put the ccf pad on top of the inflatable.

I realize that this very un-rigorous conclusion was formed both subjectively and situationally, and based on a pretty small number of data points, but ... I nevertheless feel that it's a good & simple rule of thumb to go by.

Hokay - so i get the CCF+inflatable. now under my feet I usually would also put my rucksack - so i guess same logic applies here?

Largest resistor to heat flow on top and smallest resistor to heat flow on bottom in a truly simplistic manner. The better insulator will conduct less heat into the poorer insulating piece and less heat will be conducted. It goes from HOT to COLD. Not from COLD to HOT. =-) If you laughed at the last sentence well I have had folks talk like it was the last and not the former so... uh hem...

Only way to truly know would be to know the material of the foam pad and its W/m^2K value is compared to the air mattress.

Foams vary ENORMOUSLY in their thermal conductivity. All it takes is a SLIGHT variance in the foaming method even on foams that have identical chemical properties to effect the conduction of said foam.

Just because some foam has a sticker that says say, polystyrene(common foam cups) does not mean you can look up said material properties in a book and "KNOW" its thermal properties. Said foam has a WIDE range of possibilities determined by how it was turned into foam in the first place.

How old is the foam and how "squished" has it become also adversely effects said thermal properties immensely. This is true of the air mats with foam inside. Thermarest. Why one must store your thermarest and equivalent air mattresses with internal foam in the inflated condition! Storing said thermarest all rolled up, you may as well not even have the foam inside of said air mattress after a couple of years. There went your $100 air mattress. This goes for your down air mattresses/sleeping bags/coats/pants as well. In fact that is true of ANY insulating material. Store it in its expanded state.

Well Brian - I do get that heat flows from hot to cold and not vise versa :)

RE storage - i store them both spread out and they are in "as new" condition.

regarding the resistor analogy: obviously the Prolite has a larger total thermal resistivity but the Evazote is better per volume (see below) so how does this fit in now?

______________________________________________________________________-
(I reference the work Roger and Will did where they tested a Prolite regular and not Prolite plus and it measured R=2 and the GGthinlight 1/8 was evaluated as R=0.6
http://www.backpackinglight.com/cgi-bin/backpackinglight/gossamer_gear_thinlight_foam_generic_foam.html
https://www.backpackinglight.com/cgi-bin/backpackinglight/airmat_sotmr_part2_2011.html

Alright, let's get all these engineering principles in order. The R-value of a mat is a measure of its resistance to conductive heat flow. The total R-value of multiple layers in series is the sum of those R-values. How the layers are ordered does not matter, at least as far as the conductance is concerned. That's the theory and it's correct as far as it goes.

Problem is, there's more than just conductance happening when two (or more) layers are stacked. Air can get in between the layers, especially with an active sleeper, and can act like a short circuit, making you colder than the summed R-value would suggest.

The engineer in me says that whichever order results in the least air movement between the layers will be warmer. That same engineer says that putting the higher R-value closer to me (i.e., moving the short circuit farther from my body) will be warmer. And those two factors may or may not work in the same direction. There may be other factors at play, too.

So the theory is of limited value because we can't know or control the details. Most reports I've seen from folks who've tried it both ways seem to favor putting the CCF on top of the air mat. It probably depends on the particular mats in question, their respective R-values and construction, the relative temperatures of the air and ground, and how much you move around at night.

As Todd states there are other factors as well.

I would point out that my old experience with air mattresses is that in full length inflated mode they tend to be a bit stiff. Since the ground we sleep on is not perfectly flat generally, this leaves air pockets under the air mattress. It therefore has less "area" to conduct to the ground(good). But it also lets air currents in and said volume of air generally becomes just as cold as the outside air temperature losing the insulating value of the earth/snow(VERY BAD). Main reason I never use an air mattress though with the 1/2 length NeoAir out.... hmmm.

Also been looking at Blizzard products recently as well[not air mattresses/bivies]. The Army has been handing them out recently and field reviews seem to be favorable.

In your case I would put the foam on top. Especially when its that thin as it won't have enough "body" to be rigid like the air mattress. On an uneven surface it would be likely that the Prolite wouldn't even be touching said foam pad in a large area and you would be losing its insulating value completely. Thus where reality overcomes theory regardless of R-value gradients. Why several folks I know only take 1/2 length(torso) air mattresses where its a guarantee that the majority of their weight will "flatten" said air mattress onto said closed cell foam pad.

PS. Ridgerest and z-rests are abysmal on cold snow as they INCREASE the surface area for conduction from your body and their apparent R value plummets drastically. That and the blasted things don't have a smooth top surface so any tracked in snow melts in said "buckets". Also any evaporation from your body or clothes overnight can condense into said buckets soaking your sleeping bag. This has happened to me twice(sweat from body not tracked snow). After which I got wise and never brought them with me when camping on cold snow. Here I am talking winter/spring. Summer snow is generally very close to 32F so said condensation never happens or has a Very infinitesimal probability of happening. In summer conditions, while a Z-rest won't be optimum it is workable. Besides they make wonderful sit pads/backrests. Besides you can punch holes in them for tie down points when its windy. Funniest thing you ever saw is a Giant gust of wind sending someones expensive air mattress flying over the cliff and their owner racing after it only to realize, ITS A CLIFF STUPID! STOP!

If manufacturers came out with a narrower air mattress that would fit inside a sleeping bag, I would buy it in a jiffy minute. That and quit wasting down on the bottom of their sleeping bags! Everytime I pull my BBag out I am constantly trying to shift the down from the bottom to the edges to get more warmth out of it.

Well Brian - I do get that heat flows from hot to cold and not vise versa :)

RE storage - i store them both spread out and they are in "as new" condition.

regarding the resistor analogy: obviously the Prolite has a larger total thermal resistivity but the Evazote is better per volume (see below) so how does this fit in now?

______________________________________________________________________-
(I reference the work Roger and Will did where they tested a Prolite regular and not Prolite plus and it measured R=2 and the GGthinlight 1/8 was evaluated as R=0.6
http://www.backpackinglight.com/cgi-bin/backpackinglight/gossamer_gear_thinlight_foam_generic_foam.html
https://www.backpackinglight.com/cgi-bin/backpackinglight/airmat_sotmr_part2_2011.html

I love tunnel vision. While everyone make good points about the r-value benefits to having the foam on top of the air mattress, one major benefit is often overlooked.

Perceived cold usually comes from point sources, not an overall lack of warmth from your pad system. Hips and shoulders compress your pad which effectively lowers the r-value and makes them colder than the theoretical max of your system. The benefit of having a CCF on top of an air mattress is that the CCF tends to be stiffer material, so it distributes your weight over a larger surface area. This mitigates the cold spot point sources and allows more of the air mat to loft fully, thus increasing the pad's effective r-value as far as perceived cold is concerned.

Basically imagine you take a thermal picture of you air mat on top of a CCF. You would have small blue (cold) points at contact areas where your shoulders and hips compress the air mat most. By putting the CCF on top, those blue spots would turn green but cover a slightly larger area as your weight is distributed area. This allows the air mat (and low pressure achieved by inflation) to work more effectively and create an overall higher average loft...ie warmer.

Dustin,
Yeah, I agree mostly, but I think you may be overlooking the compression of your sleeping bag as part of the sleep system. The stiffer distribution of the CCF pad will also cause increased compression of any insulation, synthetic or down on a specific area, like your hips, and shoulder or buttocs and shoulders. It might very well negate any real R value increase due to distribution of loads. It might not, too. Roger & Will admit to having some strange results testing the Klymit pad, as an example, that uses part of the sleeping bag's loft/insulation to warm a sleeper. (Thus achiving higher R values than a simple pad test will indicate.)

One of the problems I have the testing of the individual gear items, pads and R value in this case and as Roger & Will have done, is that it does not test other related items a hiker/camper uses to perform a task. Focusing on R-value of a pad, and adding them up, does NOT necessarily give a true picture of the entire sleep system. Distributing compression across an air pad will help. Focusing on order, as you say, makes a slight difference with two different pads as the OP was taking about.

James,

Good point. I was working under the model of using a quilt which has no insulation on the bottom, but you're concern is absolutely valid for a full sleeping pad.

Which means a ridge rest or GG Nightlight type CCF may have extra benefits. The peaks and valleys could mitigate the compression of the bag insulation while still providing a better distribution of weight for the sleeping pad.

What I would really enjoy is seeing a FLIR type thermographic image of sleeping pads during a test. While it's more qualitative, I think these images would be very informative on the behavior of various systems.

Foam on top, because the foam will insulate your body heat (you can actually make warm winter clothing out of foam). The inflatable will then keep you farther away from the ground, but it won't insulate your heat.
Sleeping on just air can leave you with a cold back in colder temps due to loft compression.

For the combo you bring up is should make little difference. I would do whichever is most comfortable.

The resistor analogy is pretty reliable and does tell you you can stack the pads either way with equal results (regardless of each's rvalue).

The analogy begins to break down with:
1) increasing heat lost to outside air -- this is the case with large and thick pads (more area in contact with the air).
2) increasing importance of radiation heat loss. This mode of loss becomes more promenent with non/partially-insulated/ airmats (b/c radiation heat loss is occuring btw the top and bottom fabric) and with colder temps (b/c radiation losses to air become more and more important).

Generaly, my criteria for picking which pad to put on top (order of importance):
1)put the mat that has highest resistance per unit thickness on top.
2)put the mat that matches my frame best on top
3)foam on top (more comfortalbe for me)

Regardless of the above I would put the foam on bottom, if I was really worried about the the air pad popping (never slept on ground that really worried me but I am sure its out there).

Edited by jnklein21 on 12/30/2011 20:37:31 MST.

"Ridgerest and z-rests are abysmal on cold snow as they INCREASE the surface area for conduction from your body and their apparent R value plummets drastically."

This is an excellent point for these types of mats -- hadn't considered that nuance.