Pad on top or bottom!

"don't you end up with a very much colder inflatable this way?"

I am sure you do, and that is the point. The inflatable provides a large area to loose heat from. The less heat you pump unto it from your body the less you will loose.

Except that, with my old Exped downmat, using a thinsulite underneath, the pad would actually warm ME up–I would often unzip my bag and use it like a quilt, with the downmat staying warm underneath me.

Granted, I'm talking about three season use, but I often used this downmat to below freezing (with my bag zippered up) and felt that it stayed warm.

I thought that the whole idea of a downmat was to keep you warm; or at least, retain heat. If not, why pay for the down?

I may be missing something here…

Insulation NEVER warms you up. There is no exothermic reaction within insulation. It's purpose is to "trap" heat, to keep it from transferring from one side of the insulation to the other. When we use it to keep us warm we are attempting to keep too much of our body heat from away from us. We are "trapping" our own body heat next to us.

Insulation NEVER warms you up

Yes technically correct HOWEVER on the practical side I understand and agree with what Jeffrey is saying mostly because I have a DM and know how it works for me when I use it as opposed to when I think about it in front of my computer…

All right fine it doesn't warm me up. But my downmat is a far sight warmer than other inflatables that I've used. But I doubt that it would be if I had a blue mat on top of it. It seems that a blue mat on top would make all inflatables equal,in terms of warmth. So again, why pay for the down?

You want whichever is more insulating next to you. If the down mat is more insulating than the foam pad then that is what you want on top to keep you the warmest. Many of the air mats (non-down ones at least) are not very insulating and therefore you want them on the bottom.

Based on a quick google search the OP's pad appears to have an R value of 3.5 while the Exped Downmat has an R value of 8. Therefore, the pad belongs on the bottom for that combination. That gives the best thermal performance and protection for the Downmat. :)

WWWWW (the OP) has a DM XP9, R8

OK, here's a simplified explanation as far as I understand it. I make no claims of being as eloquent or educated as Drs. Jordan and Caffin. as I understand it:

An insulated, inflatable pad's primary contribution to insulation is through the prevention of heat loss through radiation. The best means for this is thick insulation to trap air and limit it's movement. Down is quite efficient for it's weight for this purpose. Notice I said limit air movement rather than prevent. There is still air moving within the down, due to the temperature gradient between the top and bottom of the pad, and this is carrying heat away from your body through convection. This convection is greatly mitigated by the down in the pad, but it is not completely eliminated.

By placing a layer of ccf between your body and the pad, these air currents can no longer come in contact with your body surface. This does not eliminate the insulative capabilities of the down in the inflatable. The thin layer of ccf will fail to retain enough body heat in cold conditions, thus the down provides for further insulative loft; the purpose for inflating the pad is to preserve this loft beneath the pressure of your body weight. Without any insulation value in the inflatable, the ccf probably won't be enough. It seems a couple of members tried using BA Clearview pads with ccfs a few years ago with poor results.

This may differ regarding the XTherm, as it uses very different principles to provide insulation.

….now I don't know whether I'm afoot or horseback.

Both sides have provided convincing arguments, both scientific and practical, why the above method vs. the below method is preferred.

We've come full circle.

What about the neoair xtherm? It uses reflectors instead of insulation like the microfiber of a exped synmat.

After trying many different pads and combination of pads and what combination of pads go on top or bottom; even without a PHd I came to the conclusion the the CCF is better on top:
1. Technically it is, as stated above.
2. More important- for me, it is far more comfortable on top. A pad on top fills in the "rolls" or voids in the inflatable and makes for a smoother, more like my bed at home feel. And thus, I'm more comfortble and if I'm more comfortable, I am warmer (inside and out). It may be all in my head, but that is all my body needs to be warmer; or the tought of such.
I think it, thus I am.
Tad