r value

According to the Exped website 4.1 is -12c and 4.9 is -17c.

Are you talking about mattresses? typically they use R value

Are you talking about the minimum temperature you can sleep comfortably?

I wonder how they test these things, and if a MYOG testing rig would be out of the question…?

I assume you're talking about a ground pad. The equation for heat loss (Q, Btu/hr-ft2) is:

Q = (Temp_above_pad – Temp_below_pad) / R

If we assume your body is about 70F (not 98.6 because there's compressed sleeping bag under you) and the R-4.1 pad keeps you comfy down to a ground temperature of 32F, the heat loss is:

Q = (70 – 32) / 4.1 = 9.27 Btu/hr-ft2

To find the ground temperature that gives the same heat loss with the R-4.9 pad:

9.27 = (70 – Tg) / 4.9

Solving for Tg gives 24.6 degrees. That's rough, of course, but it suggests the R-4.9 pad is worth *about* 7 or 8 degrees of ground temperature. Who knows either (a) how close to reality these simplified numbers are or (b) how close to the manufacturer's ratings they are.

I love physics….

Thanks all for the quick knowledgeable responses! Yes, I was talking about ground pads. Trying to decide if it's worth the money to give up my insulated BA (which I got for a good price here) for an all season neoair. Doesn't sound like enough of a difference (as I suspected) to warrant the expense at this time. I guess I'll just add a closed cell to my shopping list. Thanks again

If I need to know the meaning of whatever greek mumbo-jumbo you just posted, I'm assuming a MYOG test rig is out of the question after all.

MYOG is never out of the question Ozzy- if you're so inclined. Let us know what you come up with and we can see how it compares to Todd's assesment. I'm bettin' they're close.

Roger wrote an article about testing pads. I wrote an article about testing sleeping bags or jackets or whatever. It's a bit complex. There's uncertainty about the results unless you get expensive equipment.

Idea is to have a heat source, reference insulation, test insulation, and a cold surface. Measure temperature difference across the two insulations. Ratio of temps is the same as the ratio of the insulation values.

Hi Ozzy

> I wonder how they test these things, and if a MYOG testing rig would be out of the question…?

We have two comprehensive articles about this. The first comes in many parts.

http://www.backpackinglight.com/cgi-bin/backpackinglight/airmat_sotmr_part1_2011
http://www.backpackinglight.com/cgi-bin/backpackinglight/airmat_sotmr_part2_2011
http://www.backpackinglight.com/cgi-bin/backpackinglight/big_agnes_air_core_clearview
http://www.backpackinglight.com/cgi-bin/backpackinglight/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
http://www.backpackinglight.com/cgi-bin/backpackinglight/gossamer_gear_thinlight_foam_generic_foam

It's all there if you want it.

Cheers

But have you tested the R value for sleeping on your rope?

What is the difference in R value for an 8mm vs. 10mm?

ugh.. ever tried it? used mine as a pillow once with clothes on top when a day trip turned into an impromptu 2 day trip. maybe for a 3/4pad extension but hips.. shoulders.. owww.

now back to your regularly scheduled maths.

"ugh.. ever tried it?"

Yes, sort of. I spent a night in a tent with two other guys. The tent had a bathtub floor, but the tent owner had neglected to bring the rainfly for the tent. After it had been raining on us for a while, the rainwater was pooling in the floor of the tent right about where my nice down sleeping bag was. Once the water got about three inches deep, we got smart enough to place our climbing rope in the water. Then I could balance the foot of my sleeping bag on the rope, and that kept it out of the deepest of the water. That helped, although the damage was already done. Not recommended.

The rope was half-inch diameter and 150 feet long.

–B.G.–

It works much better than nothing, especially on a cold granite slab or on snow. It also
works well under a pad for extra insulation.

Flaked out it works much like one of the open space klymit pads, only firmer, much firmer.

Good wiki article on r values, including the important point that there are two types, euro and usa, and the standard way of expressing them fails to note which we are talking about.

I read most of the article to once and for all satisfy my curiosity about whether the new Lawson eva pads can actually be 0.75 R value at 1/8″. Answer, yes they can, judging by the r value charts there.

I also now understand why cheap blue foam pads will provide less r value, that’s a good article, well worth the time to read it if you’ve heard these terms and now actually wonder what they are referring to.

Interesting take away, proving that myth will always find a place in the internet:

“In summary: claims for radiant barrier insulation are justifiable at high temperatures, typically when minimizing summer heat transfer; but these claims are not justifiable in traditional winter (keeping-warm) conditions.”

In other words, those aluminized materials are useless and a waste of weight. Good to know. I believe if I understood that article right, which is not certain, that what happens in the cold is that the convection makes up for the radiance, ie, they equal, or something like that, so it doesn’t matter if there’s a radiant barrier, it will just convect out.

Nothing like actual information to brighten one’s day.

To the point of this topic though, my experience with the claimed r values of neorest type air mattresses is that you can largely ignore them. Why? Because to be able to sleep on the pad you have to deflate it enough to make it comfortable, at which point who really knows what that r value is. Plus of course, how both companies determined this r value, how much air was in pad, etc. I’m sure if you make it so hard you can’t sleep a minute on it, ie, default in testing probably, it may have the claimed r value. the prolite’s however, since they actually work better at greater inflation, might actually end up being a touch low in their real world r values I’d guess. Sandwich two lawson 1/8″ evarests around an airpad of whatever type and you’ve added r 1.5, which should cover all but the coldest circumstances.

Thanks Roger. I really need to buy a membership, not sure why I haven't yet.

Herald, check out http://en.wikipedia.org/wiki/Blackbody_radiation for example

An aluminized mylar "space blanket" will keep you a significant amount warmer

Or the aluminized layers in a neo-air mattress – there is radiative heat loss going on inside insulation

Richard alluded to this in his recent comments

I've been fooling around trying to measure this, and if I feel masochistic : ) and have anything I'm happy with, I'll write something up. An aluminized layer on the outside produces maybe 15 degree F of warmth, and an aluminized layer inside produces maybe 5 degree F more warmth than just a layer of fabric (which has a small amount of air space)

Roger's articles did some measuring of non-fully inflated air matts.

Bob

That's why they call it a "bathtub floor"

Floorless tent lets the water flow away better

Regarding Jerry's post, I would note that radiant bariers are ONLY usefull as a single layer. Trying to avoid confusion.

"I would note that radiant bariers are ONLY usefull as a single layer"

That's what I thought, James, but

I measured it and determined that an interior layer of reflective material does something more than an interior layer of non-reflective. For example, measure fleece+eVent+aluminum, and measure fleece+aluminum+aluminum and that's 5 degrees warmer.

And, the neo-air has several radiant layers in their mattresses which they say is better. They wouldn't say that if it wasn't true would they? Well, it could just be marketing spin.

And Richard's comments:

The second hint is that as down is compressed there is interplay between the increase in conduction heat transfer and a decrease in the radiation heat transfer. One of the many flaws in your tester design is that it negates the ability to measure the radiation component.

The third hint is that a blackbody is a perfect emitter, meaning that it has an emissivity of 1.0. Everything else emits less than perfect and we say it has an emissivity factor. Human skin has a .99 emissivity factor and your diffuser must possess similar emissivity.

That is a little difficult to interpret, but I think it says radiative heat loss is happening inside the insulation.

And if you think about it, there are planes of equal temperature inside the insulation, adjacent planes at different temperature, so radiative heat transfer between them…

"That's why they call it a "bathtub floor" "

I was all ready to take my knife to it to make some drain holes, but the tent owner refused to have his tent damaged that way. The bathtub floor would have been fine if he had simply brought the rainfly.

Eight of my ten toes were numb, and I didn't get the feeling back for about two weeks.

–B.G.–

I would have been ready to take my knife and make some drain holes in the guy that didn't bring the fly :-P

I have a couple of tents with bathtub floors

Yeah, 90+% of the time they work

Then I screw something up, water gets in, no way to get out

"Then I screw something up, water gets in, no way to get out"

Sump pump

–B.G.–