Inflatable mat over Foam? or vice versa?..

[Secondmouse]

I want to increase the warmth of my inflatable mattress without buying new. advice is to add a foam pad and, to me, it seems logical to put the foam under the inflatable.

however, I have seen several people say to do it the opposite - put the foam pad on top of the inflatable mattress.

why?..

[nsherry61]

. . . I have seen several people say . . . put the foam pad on top of the inflatable mattress. why?..

Only one reason. Because it works better! And, noticeably so.

Actually, from what I gather, there are some inflatable mattresses (e.g. x-therm) that are supposed to work as well or better with the foam underneath.
With my experience with non-winter type inflatables, I have experimented in very cold conditions (-10 and -15 degrees) and in each case, I was notably warmer when I moved my foam pad from the bottom to the top. It was not a small improvement, it was a very notable one.

[Tipi Walter]

Foam on top, as mentioned. Noticeably warmer.

[Secondmouse]

Only one reason. Because it works better! And, noticeably so.

Actually, from what I gather, there are some inflatable mattresses (e.g. x-therm) that are supposed to work as well or better with the foam underneath.
With my experience with non-winter type inflatables, I have experimented in very cold conditions (-10 and -15 degrees) and in each case, I was notably warmer when I moved my foam pad from the bottom to the top. It was not a small improvement, it was a very notable one.

hmm, I accept that it is so, too many people have said the same thing. but I'm having a hard time understanding why. do you have an explanation?

to my reptile brain, it just seems that putting an inflatable on top of a foam would be warmer...

[Ercoupe]

The foam adds more protection from heat loss to air movement (convection). Any inflatable will have you or a flattened bag next to the moving air in your air mat, with only a thin poly sheet seperation. The foam acts as a extra layer of fat with no nerves.

If you own one of the old neo-airs with the silver on one side, place it silver side up. Big difference in warmth.

[DuneElliot]

For the same reason as an air mattress without some kind of insulation on top (blanket or pad) is colder. You wouldn't want to put the pad or blanket underneath as it would do nothing but protect the bottom of the pad. Air circulation within the pad is what causes you to cool off. It's obviously less of a problem in inflatable pads vs an air mattress but the principal is the same.

[pyroman53]

I've participated in this question several times here on WB and I'm still not sure of the answer. For the neoair thermorest, the web site states "Patent-pending, reflective ThermaCapture™ technology traps radiant heat while Triangular Core Matrix™ construction minimizes convective heat loss–all without the bulk, weight or durability issues of down and synthetic fills"

Thus, in order to use the advantage of this technology I put the foam pad under the inflatable. Any other pad I would put the foam pad on top. Works for me.

[Secondmouse]

For the same reason as an air mattress without some kind of insulation on top (blanket or pad) is colder. You wouldn't want to put the pad or blanket underneath as it would do nothing but protect the bottom of the pad. Air circulation within the pad is what causes you to cool off. It's obviously less of a problem in inflatable pads vs an air mattress but the principal is the same.

inflatable pads vs an air mattress? sorry, I don't get the distinction. do you mean a self inflating pad?..

I have an Exped 6R air mattress. it's dreamy but it weighs 2.5lbs. I was thinking of replacing it with something much lighter in the 3R range for cool weather and supplementing that with a CCF in really cold.

I've never slept on just a foam pad. does it cause you to sweat against it?..

[Another Kevin]

I've found that whether the foam goes under or over my ProLite doesn't make that much of a difference. But the ProLite is a self-inflating pad with the open cell foam inside, so it's basically foam on foam.

[BobTheBuilder]

For better insulation, air mattress on the bottom, foam on top. To protect your air mattress better from punctures, foam on bottom, air mattress on top. The first part is thermodynamics, the second part is just experience.

[DuneElliot]

inflatable pads vs an air mattress? sorry, I don't get the distinction. do you mean a self inflating pad?..

Not all inflatable pads (Neo Air, Klymit Static, BA Air Core) are self-inflating. I have to blow mine up. An inflatable pad is basically a really (really) small and light version of an air mattress. Air next to your body conducts heat away as it moves.

[Franco]

"An inflatable pad is basically a really (really) small and light version of an air mattress. Air next to your body conducts heat away as it moves."
Not exactly.
There are non insulated inflatable mats but most do have some insulation that being foam stuck under the top or, as with the Neo Air series , reflecting layers . Some of the warmest mats are inflatable, the Exped 9 is about as warm as you get (down insulation) even when you move over it.

[Secondmouse]

"An inflatable pad is basically a really (really) small and light version of an air mattress. Air next to your body conducts heat away as it moves."
Not exactly.
There are non insulated inflatable mats but most do have some insulation that being foam stuck under the top or, as with the Neo Air series , reflecting layers . Some of the warmest mats are inflatable, the Exped 9 is about as warm as you get (down insulation) even when you move over it.

I see. I was confused above because he made a distinction between "inflatable pads vs an air mattress". I consider CCF foam a pad, and anything that contains air (whether self-inflating or manual) a mattress. but anyway, it's just a matter of semantics...

I have an Exped 9. It's warm but it's also heavy at 40oz so I'm looking for a lighter alternative. I understand R-values are additive so stacking lightweight pads (or mats) should get me there.

I was just looking for an explanation of why a foam pad on top of an air mattress is warmer than the opposite...

[Franco]

"I was just looking for an explanation of why a foam pad on top of an air mattress is warmer than the opposite..."
My impression is that apart from the Neo Air mats, it does not really matter because R1 + R4 is the same as R4+R1.
I suspect that many perceive it differently simply because they feel more comfortable one way than the other.
A bit like having a couple of shots of brandy before going to sleep should make you feel colder (alcohol cause the body to lose heat) but most feel in fact warmer, in other ways.
Of course, I could be completely wrong...

[zelph]

[note to daughter] Air mattress on top. Foam pad conducts heat much more than air space in air mattress.

[Tipi Walter]

I don't believe adding up R values with sandwiched pads equals, say, the warmth of one pad with a high Rvalue. Example: Sleep on an Exped downmat at 8R at 0F and it's tremendously warm. Now do the same with two separate pads equaling 8R and it's just not as warm.

Another test: On a recent January trip I got to field-test my pad setup with my Ridgerest Solar pad either under my Thermarest Trail Pro or on top. It was 5F for several nights and using my system I found the Solar on top of the Trail Pro to be noticeably warmer. It may be because the Solar pad has a silver reflective surface on top next to my body.

A Cautionary Tale: Be careful using a ccf pad in conjunction with an inflatable pad. Why? Because most people strap their ccf pads exposed on the outside of their packs. They are bulky. These foam pads pick up thorns and sharp briar tips easily which get embedded into the foam. At night in the tent you place your inflatable pad on top of your foam and your body weight causes these thorns to poke up and out of the foam pad and into your inflatable, causing a leak.

[DuneElliot]

I see. I was confused above because he made a distinction between "inflatable pads vs an air mattress". I consider CCF foam a pad, and anything that contains air (whether self-inflating or manual) a mattress. but anyway, it's just a matter of semantics...

And it's she...not he!

[la.lindsey]

I don't believe adding up R values with sandwiched pads equals, say, the warmth of one pad with a high Rvalue.

Tipi is exactly right. When you layer pads, you're adding R values in parallel, which means they add in inverse. I.e.,
1/Rtotal= 1/R1 + 1/R2 +...+1/Rn

So you can't say that you have an R value of 2 and an R value of 4 and you'll get 6. It's 1/2 + 1/4= 1/ Rtotal.

Now, as for whether it's warmer to have the foam on top or bottom, I'd postulate that foam on top is warmer. The inflatable pad will have conductive and convective heat loss over a larger area (more area means greater heat transfer). The air inside the pad will have convective heat transfer on all four "sides".

The foam pad will have conductive heat loss (with a lower R value, so some increased heat transfer) but no convective heat loss.

The reflective cover on the foam pad will prevent some heat transfer.

So, if you think of yourself as a heat generating body insulated by two materials, you'd want the foam pad with only one type of heat loss next to you.

Consider heat transfer as a line with decreasing slope as more heat is lost (heat transfer v time). With an inflatable pad on top, heat is lost more quickly, then slows (less steep slope) when it hits the foam pad. The foam pad on top would have the better arrangement with less heat loss (less steep slope), then increasing when it hits the inflatable pad. This keeps more heat next to you.

The energy balance is the same- heat in is heat out, but you'll feel warmer and the process will be slower.

(Disclaimer: heat transfer is not my strongest subject, but the general concepts should be correct.)

[Tipi Walter]

Good post where science reflects field use.

[Secondmouse]

And it's she...not he!

lol, sorry...