Mechanics of UL Pads

some put a layer(s) of a reflective material inside.  This will inhibit convective air flow inside and also reduce radiative heat loss.

Assuming you mean inflatable and well-insulated lightweight air pads like the Thermarest XTherm & Women’s Neoair XLite, Exped Ultra 5R, and Nemo Tensor Ultralight Insulated. These pads generally have top and bottom fabric layers, with interior reflective fabric layers to maintain shape, warmth, and comfort.

Note: These are generalizations! YMMV!

Sleeping pad features that can affect weight and warmth (R-value), with other tradeoffs:

– Pad dimensions: Shorter and narrower pads with less fabric are lighter than longer and wider pads.

– Overall thickness: Thicker pads need more interior fabric, so weigh more.

– Pad shape: Rectangular pads weigh more than “mummy” pads.

– Fabric weight (denier, coatings, brushed top surface, etc.): Lighter-weight fabrics are usually more fragile. Treat all inflatable UL pads gently, and carry a patch kit or patch tape.

– Interior fabric layout: Simple lengthwise or crosswise tubes weigh less. More complex zig-zag, multi-level, or cell-like designs weigh more, but usually are warmer by reducing convection as Jerry pointed out. And they can be more comfortable due to less bouncing as you move.

– Reflective interior fabric layers: Adding a reflective coating slightly increases weight. More layers reflect more heat.

Having said all that, here’s some unsolicited advice.

– Test labs determine sleeping bag “EN” temperature ratings using R-4.8 sleeping pads. Anything less than R-4.8 means you are more likely to sleep cold. But EN ratings and tests have many problems, sigh.

– Some people can put up with low R-value, thinner-but-bouncy pads for a few days, until they get so tired they can sleep on ice blocks. Not me. YMMV.

– Spend the ounces (grams) to be warm enough and comfortable enough to get a good night’s sleep. But that doesn’t mean you should carry a 2-pound (1 kg) pad. An ultralight torso-length, narrow, mummy-shaped, high R-value pad, paired with an empty backpack under the legs, works for many people.

– Creative combinations of an inflatable pad and a closed-cell foam pad also work. R-values add, so layering an R-4 full-length inflatable with an R-0.5 CCF works. Or an R-5 inflatable torso with CCF under legs.

Whoops, my word counter is flashing red. Hope this helps.

— Rex

This is very relevant, pads cut open

https://imgur.io/a/ll6uObb

And the reddit discussion it came from

A bunch of cut open backpacking sleeping bags from Ultralight

Some inflatable pad designs claim lower weight because they don’t have interior fabric layers, substituting “dimples” connecting the top and bottom.

Problems:
– Needs extra fabric for the dimples, so not entirely a free lunch.
– Infrared cameras show lots of heat leaking through the dimples, reducing warmth. Don’t believe the claims that your sleeping bag will fill those in.
– Doesn’t stop internal air convection, or have inner reflective layers, also reducing warmth.
– Claims of “three-season” comfort anyway. Uh, no.
https://backpackinglight.com/sleeping-pad-r-value-not-that-useful/

Again: generalizations, YMMV!

— Rex

Thanks much, friends. I’m much better informed now!

Rex I completely agree with you on the dimpled pads. I have a first gen S2S UL Insulated and have always found it colder than the thermarest foam filled “self inflating” pads like the prolite and prolite plus, even though they have lower R values.  These pads predate the current testing standard, but I believe the similar current versions of both types still report a higher R value for the S2S UL insulated vs the foam filled self inflators.

David,

The misleading R-values could show an interesting limitation of the test setup. A large section of each pad is covered by heated plates, then various measurements turned into R-values.

So a dimpled pad, or similar pads with large air pockets above and below, should test somewhat higher, because those voids are more-or-less sealed by the plates.

In real life, people move around as they sleep. The warm air trapped in those pockets gets replaced by cold air each time. Doesn’t contribute warmth, and possibly makes matters worse because your body warms up that air over and over, compared to completely flat pads.

“Sneaky little hobbitses.”

— Rex

Dimpled pads and similar designs are very much like sewn-thru sleeping bags or quilts. Kind of good enough for warm weather – or layering with better insulation to extend temperature range.

— Rex

Per Sea to Summit Ether series, they have two layers though dimpled. Its the single layer dimpled pads intended to be cheap and thus poor quality (Rei Flash example). The Klymet design with long Vs also pump out warmed air. BA is coming out w new pads said to be much more competitive w Nemo, T.rest, S2S, Exped.

I’ve been experimenting with making my own vacuum insulated panels of late. I’ve thought about making a partial sleep “pad” partially out of these.  The idea is to make some VIP’s from 1/4th to 1/2″ inch thick (so roughly somewhere from R 6/7 to R 12/14 value just in the panel) for my core (more specifically from my head to butt area, the rest can be insulated and cushioned with EVA foam since my legs need less cushioning and insulation than the rest of me).  There will be 3 VIP’s that connect via velcro/hook & loop.

On top of these will be some strategically placed mini kapok filled pillows for cushioning/comfort (also velcroed/hook & looped on), which will also add some more insulation–plus the spaces in between the panels and the kapok pillows will allow for lofting of sleeping bag insulation.

Then, all this will be put in a sleeve of fabric with a highly waterproof/puncture and abrasion resistant fabric on the bottom and a breathable and somewhat puncture but more abrasion resistant fabric on top* (this will very slightly increase thermal insulation, but is more for protection of the VIP’s and kapok pillows).

If the VIP’s end up being a full 1/2″ thick I guessimate a total R value in the most insulated sections to be somewhere around R 16 to 17 or so.  But that is probably overkill and I will probably go closer to the 1/4″ thick because of bulk reasons.

The concept is for extreme cold conditions and not having to worry about a pad losing air etc. and providing significantly more R value per volume than traditional mats/pads.  Getting that kind of insulation in foam can get bulky.  Also, I’ve got to an age where I just don’t find foam comfortable enough anymore.

I’m thinking of making a thread about it in the MYOG section, once I’ve put more of it together.

* I’ll be using 1.1 oz Mountain Silnylon for the bottom of the fabric sleeve and 1.7 oz Mountain Hybrid Rip Stop Nylon for the top fabric of the sleeve. Course the kapok pillows will provide some protection to the VIP’s on the top at least as well.

I should modify the above–I don’t think the R values will be quite so high because unlike for home insulation use, I’m not using fumed silica at all in the above. Fumed silica has ridiculous thermal insulation properties even without being put within a vacuum condition, and industry uses a lot of fumed silica as the primary core in VIP’s.  I’m not using fumed silica in these particular VIP’s because of fragility of the material and I worry about it pressing on the plastic layers while my weight is on top. (That, and it is somewhat expensive).

(As to what I am using, the bulk of the core material is a natural hollow fiber with a bit of give/compression).

I have no real input on the technicalities. But – I do have a Neo air and an Xtherm. And the difference is significant. The Xtherm is warm, and I mean warm, directly on snow. The neo is great in the summer on dirt down into the 20’s, but sketchy at best, even on top of 1/4″ of closed cell, on the snow. so, whatever Thermarest is doing, it works!