A topic never mentioned: Sleeping pad compression r-rating loss.

Interesting observations.  Sounds plausible.  Makes me want to finish making and test my weird hybrid vacuum insulated panels/kapok mini pillows sleep pad system.

There wouldn’t and couldn’t be any kind of issues with this system like you described with the pads in your OP.

. Best Sleeping Pad for Backpacking 2020 | Complete Guide .     ALAN DIXON . mentions having your pad fully inflated to get the full R rating out of it, this is about 7minutes and 45 seconds into this video but watching the whole video is good.

having your pad fully inflated to get the full R rating out of it,
This was covered extensively in our review of air mats:
https://backpackinglight.com/airmat_sotmr_part1_2011/
https://backpackinglight.com/airmat_sotmr_part2_2011/

Individual mats were covered thus:
http://www.backpackinglight.com/cgi-bin/backpackinglight/big_agnes_air_core_clearview
https://backpackinglight.com/exped_downmat_pump_synmat_ul/
http://www.backpackinglight.com/cgi-bin/backpackinglight/klymit_x_frame
http://www.backpackinglight.com/cgi-bin/backpackinglight/kooka_bay_goosepad_insulated_taperlite
http://www.backpackinglight.com/cgi-bin/backpackinglight/mammut_light_pump
http://www.backpackinglight.com/cgi-bin/backpackinglight/montbell_ul90_ul120
http://www.backpackinglight.com/cgi-bin/backpackinglight/nemo_zor
http://www.backpackinglight.com/cgi-bin/backpackinglight/poe_ether_elite_peak_oyl
http://www.backpackinglight.com/cgi-bin/backpackinglight/stephensons_warmlite_down_air_mat
http://www.backpackinglight.com/cgi-bin/backpackinglight/thermarest_neoair_prolite_deluxe_le_mats
http://www.backpackinglight.com/cgi-bin/backpackinglight/vaude_norrsken_insulated_mat
https://backpackinglight.com/gossamer_gear_thinlight_foam_generic_foam/

Um – I am not sure whether all of these URLs work as they were created before the current version of the web site.

BUT, and this is very important, a fully inflated air mat is hard and uncomfortable to sleep on. You won’t like it. You will find an air mat a lot more comfortable if you let just a little bit of air out, so it can adapt to your lumps and bumps (hips, shoulders, etc).

Cheers

This 2019 BPL R-values article and long thread go into a lot more detail, including:

Therm-a-Rest’s previous practice of inflating to only 80% thickness to reflect real world use (vs ASTM ~100%), plus the ambient air change, helps explain significant R-value increases for their pads under ASTM testing.

In short, inflate harder for more insulation. Might not be as comfortable, but depends on the pad and sleeper.

I’ve slept fine on maximally-inflated Nemo and Exped pads. YMMV.

— Rex

I’ve been using my uber lite for a while

I think it’s more comfortable when inflated more.  If it’s under inflated then it feels like I’m going to fall off.  But I’ve only used it a few times.

I was thinking of your mattress testing Roger.  What ambient temperature did you do it at?  If there are convective air currents inside driven by cold surface on the edge of the mattress, you’d need to do testing at cold temps to check that.  Probably a small effect.  Great testing and article.

If there was a baffle around the edge of the mattress that was a little bigger, that would keep you from rolling off of it.  I wonder if anyone does that.

If there was a baffle around the edge of the mattress that was a little bigger, that would keep you from rolling off of it. I wonder if anyone does that. 

I’ve owned a couple of Exped mats that had bigger baffles on the edges.  Now I have a Big Agnes Rapide SL that also has that feature.  It works. The baffles keep you cradled in the center of the pad.  The fact that you don’t have to worry about rolling off relaxes you and enables better sleep.

hmmm… I’ll have to check those out.

another problem is if you’re not level side to side  you can fall off.  The baffle around the perimeter would help with that also

I bet that a mat filled with down like some of the Exped mats retains its R value better with slight under inflation than any other system.

Fully inflated, the S2S Ether Light XT Extreme is the best cold weather pad for side sleepers I’ve tried, but it’s heavy. For approx the same weight you could take an Xtherm plus an EVA pad, which would help mitigate the reduced heating area when side sleeping and would provide redundancy in case of pad failure.

I was thinking of your mattress testing Roger. What ambient temperature did you do it at? 
Shoulder season temps. Certainly not sub-zero.
The R-value testing was done in the middle of the pad. I am not going to try to guess what effect really cold air would have on the results. I just don’t know.

To my mind, the really significant results were the huge differences between empty air mats and those with some sort of ‘filling’ inside to block air movements and the way the R-value fell as the thickness of the air mat was lowered.

Side baffles: many modern air mats have these now, as their value has been proven.

We still love our Exped UL7s, even at sub-zero. We do have self-inflating mats and DAMs, but they are heavier.

Cheers

Well, I guess the non-operative word in my title of this thread is NEVER. ;)

Looks like you guys have caught on to this effect in your own testing and experiences.

I would be a pretty wild experiment to slit apart an Xlite pad and fill the edges with a light layer of fuzzy insulation to block edge bleed and then measure compared to a stock pad. I guess one could also somehow blast some down into there through tiny holes that can patch up tight. If I had an old pad to test this with, I would probably try it.