winter closed cell pad advice

[Andrew Dolman]

So, after a little research.

Rate of energy loss through the mat in watts per metre squared of body contact = temperature difference between body and ground in Kelvin or Centigrade / R-value of mat

or wm2=K/R

so to keep w the same as K (the temp difference) changes

R2 = R1*K2/K1

e.g.

Assume a body temp of 37C, ground temp of 15C, mat of R=2.3 (Prolite 3)

wm2 = (37-15)/2.3 = 22/2.3 = 9.57

Say I'm comfortable with that and now I want to know how much mat I need to be as comfortable on my winter trip where the ground will be about 0C

New temp difference = K2 = 37-0 = 37

New mat = R2 = R1*K2/K1 = 2.3*37/22 = 3.86

Quick check:

wm2 = 37/3.86 = 9.57

So I need to upgrade to a TrailPro if I want the same comfort level (and I'm a Thermarest fanboy).

This all assumes that you keep your sleeping bag the same and wear the same clothes of course.

[Andrew Dolman]

The solid line is an example for someone who is comfortable at 15C, but no lower. If you are still comfortable on your Prolite 3 at 10C, then your line will be lower.

[David Olsen]

Richard-

Thanks.

So for example, a pad I have has a rating of W /m.K as 0.0392
and is 3/16" thick.

Whats the R Value?

[Richard Nisley]

David,

The W/m K value indicates that the foam has a composition similar to Zotefoam's VA35. The R value is .689.

[Steven Nelson]

I've had great success with blue closed-cell foam from Target (I've seen similar stuff at WalMart). It's about 1/3" thick and provides excellent insulation and good durability.

[Michael Martin]

Cool Graph! Thanks, Andrew.

One thing to bear in mind, though, is what happens if you're sleeping on snow.

The R-Value of snow varies depending on its density, but is in the range of R-1 per inch, IIRC.

So, after reaching thermal equilibrium, the sleeper/pad/snow system will rarely be below 0 Celsius at the pad/snow boundary. Proof of this is if you've ever had a depression melted below your pad while sleeping on snow. If the snow is melting, it's at 0C. (Well, alright, *around* zero, accounting for altitude, temperature gradient through the snowpack, etc., etc….) :)

The real issue on snow is thus "reaching thermal equilibrium" in the first place. The water content of snow gives it a high heat capacity or "thermal mass" and will suck heat out of your body to raise its temperature to the melting point.

FWIW, in the Winter, I value the simplicity and reliability of a single foam pad. My personal winter pad is a single cut-down and tapered Thermarest Ridgerest Deluxe with a claimed R-value of R-3.1. I've used it successfully down to -5 F.

Cheers,

-Mike

[Richard Nisley]

Soil temperature often depends upon the habitat, soil moisture and amount of snow or plant cover. Also, the temperature of the soil is determined by whether the soil is clay, sand (wet and dry), rock, etc.

Mike's assertion that the snow/pad boundary is near 0C is correct for deep snow. Typical birch forest winter ground temps with thin snow cover are near -10.5C. Hence, Andrew's x-axis graph range is appropriate.

[Dennis Park]

Just from feeling the pads, I have a suspicion that I (185#) would flatten the cells in the pad. Does this negate the insulating properties? I anticipate that I would lose the comfort of a nice cushion but can deal with that. I don't want to freeze from bottom up.

[John S.]

But it builds character!

[Stephen Klassen]

Richard's comments on stacking pads got me thinking.

Two ridgerests stacked would creat a lot of air pockets between the pads, or a ridgrest on a z-rest (or some other pads) – if a ridgerest is R 2.6, would two pads be significantly greater than R 5.2?

Alternatively, if using two pads, I wonder would throwing some spare clothing between the pads significantly affect the R value?

[Chad Miller]

I do not think that by stacking pads you will increase the R-Value beyond what the combined total of each pad is.

Furthermore I pose that a single warm pad will function better then combining multiple pad of the same R-Value. This is due to the shifting and inconsistent thickness of pad materials that you encounter when stacking pads. IE, you layer three pads and have a thin area around you legs. Over time more heat will be lost though the thin area of the pad creating a thermal bridge and causing you to become colder.

I have experienced this first had. One two separate nights of identical temperatures I used each of the following systems:
System 1
Full length Ridgerest: 2.6 R-Value
Torso length Ridgerest: 2.6 R-Value
Full length blue foam : 0.7 R-Value
Total R-Value: 5.9

System 2
Exped Downmat: 5.9 R-Value

I was considerably warmer using the second system.

[John S.]

I thought someone said once that R values between different manufacturers were not reliable, but R values within the same manufacturer did have more meaning as to relative warmth.

[Richard Nisley]

Chad,

There were two erroneous assumptions in your analysis. First the body surface area (BSA) of a torso length pad results in ~33% less BSA insulated than a full length pad. Second, Ridgerest pads have the indentations engineered to minimize convection currents only when the indentations are covered by a FLAT surface such as a tent floor or groundsheet on the bottom and a sleeping bag on top. Stacking two Ridgerests will not result in double the R-Value.

Ridgerest pads should be stacked with non-ridged pads to achieve additive R-Values. The insulation contribution of a torso-length pad should be reduced by 33% to determine the R-Value of stacked pads. Contact resistance between pad surfaces is fractional. It will be measurable in a lab but, it will be too small for an UL backpacker to notice.

Joe – R-Values, from reputable manufacturers, are the same and can be used to determine your pad stack value without any adjustments.

[te – wa]

i was wondering if anyone can enlighten me about the r-value of 2 pads. it is said that 1/4" ccf (ensolite) has an R value of 1 – thus a comfort rating of (?) on the ground. I have, in my experience used a ccf pad in 1/4" and found it to be nowhere close to the warmth of Reflectix, given the same dimensions when reflectix claims an R value of .6
these 2 pads were tested in my hammock over a period of 1 year. I can honestly say that below 45° the ccf fails miserably, while the .6 R value Reflectix is warm down to mid thirties. How can this be?
to avoid doing any math (even simple math) I beg you to enlighten me. THANKS!! mike

[Dennis Park]

For all of you experienced closed cell foam users, is the Thermarest Z Lite any good? Can it be used along for three seasons?

[A. B.]

Dennis, I've been using a z-lite recently and it works just fine. I like the accordian design because it is so easy to fold up.

I can't compare it to other foam pads though as this is my first foam pad.

[Roger Caffin]

Hi Michael

> it is said that 1/4" ccf (ensolite) has an R value of 1
Correction: some foam as a material has an R value of 1 PER INCH THICKNESS.
So a 1/4" layer of foam will have an R value of 0.25

Edited: it depends entirely on the foam of course. I was talking about some foam I had which had suffered similar misunderstandings. It was not a good insulator!

If Richard says that the R value of your foam is 4 per inch then a 1/4" layer will have an R value of 1. And so on. Michael – sorry I created more confusion than I solved!

Cheers

[te – wa]

thanks, Roger. I was quoting information on Speer hammocks product page. "–R-Value = 1 (57F rating)" regarding the 1/4" ccf pad they sell.
Are you describing the R value of ccf in a hammock as opposed to on the ground? because I really dont understand the "why and how" of the differences.

In your assessment, it would then take a 2.3 inch thick ccf pad to equal a Prolite 3 in terms of R value? i dunno

taken directly from Gossamer Gear:
1/8" has an R-value of ~.45
1/4" has an R-value of ~.90
3/8" has an R-value of ~1.42

[Richard Nisley]

Michael,

¼” Ensolite has an R-Value of ~.901. Reflectix at the standard product thickness of .28" has an R-Value of .670. If you were lying on the ground, the relative warmth would be determined by the R-Value.

Foil requires a still air layer (enclosed air space) of at least 25 mm (.98 inches) between your body and the foil for it to optimize radiation reflection. It is worthless if you use it on the ground but, if you drape it under a hammock it is very effective. Reflective insulation reduces heat transfer by radiation and forced convection (wind). There is slight conduction insulation from the air that adheres to the inner foil surface. Foil yields different apparent R-Values depending on your body’s heat angle relative to the foil. The reflective barrier, such as Reflectix, can have an equivalent R-value as high as 8 with a 1 inch gap if it placed below your hammock.

If you lay directly on the Reflectix in a hammock, the hammock arch typically provides gaps between your shoulder and but plus between your but and feet. This gives it an apparent R-Value higher than .670 but, it is no where close to its potential R-Value 8.

[te – wa]

i knew the reflected heat would be a bonus, my testing was in no way scientific and the pads are only used under my legs from mid-thigh down to feet. also to note is the pads have little compression, since a pad sleeve holds them close, but not tight. As tested in a Bridge style hammock, I can tell you that reflectix works better than ccf albeit there is a slight weigh disadvantage (~30% heavier)
thanks for the info, guys.

[Michael Davis]

Interesting points about the Reflextix and hammocking.

Correct me if I'm wrong (and I probably am) the efficiency of the Reflextix is the reflective property, not any insulation property.

If this is true, then a reflective mylar "space blanket" slung under a hammock would do the job and be fantastically lightweight. Am I on to something here, or is this too good to be true.

My Momma always said, "If it sounds too good to be true, then it probably is too good to be true."

[Richard Nisley]

Michael,

The bubbles in Refectix provide more conductive insulation than a "space blanket". A space blanket slung under a hammock is occasionally used. The challenges (what your momma said) that prevent more people from using this insulation approach are: the wind can easily billow out the warm air; and the frame structure to maintain the uniform 1" gap and enclosed environment is complicated to set up.

[Joshua Gilbert]

Man, once you guys get rolling on R values its like a science fair in here ;-) back to my earlier question: Foamy on top or under inflatable?

I am more interested in personal experience than science (well, science is good, but, you know, thread drift)

[Jim Sweeney]

Not sure if anyone has mentioned this before, but foam over inflatable is probably much more comfortable than the other way around. Also, since most inflatables consist of side by side tubes, the valleys between tubes can potentially rob heat, by convection but also by direct air flow, so foam over seems better.

[Dondo .]

"Foamy on top or under inflatable?

I am more interested in personal experience than science… "

I'm with James on this one.

In my experience, foamy on top is warmer as well as more comfortable.

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