I've read related articles on this site that were really helpful. In particular, the review methodology for testing a sleeping pad's stated R-value and its value after compression. I'm having some difficulty however reconciling various streams of data.

What is the minimum required R value for a pad to stay thermally neutral camping on snow?

On Mt. Rainier last summer, I slept on the Cowlitz glacier in a Firstlight with just a RidgeRest SOLite (R value of 2.8) and a 30 degree bag. Temps were slightly below freezing, with a decent windchill. I was slightly cold on my hips and shoulders, but not unbearably so. Was able to sleep for quite a few hours (between people shuffling about getting ready to climb).

I'm looking at getting a new pad for alpine climbing at established bivies (currently think the new Thermarest ProLite Plus Women's is the perfect mix), but I'm confused on the required numbers I need. Some people say I need a crazy R value of 6+ to be neutral on snow at freezing temps, but that wouldn't explain my experience where perhaps just 3-4 R-value would have resulted in warm, comfortable camping on Rainier.

Thoughts, anecdotes, and evidence?

Edited by whitenoise on 03/14/2012 13:02:29 MDT.

What does "thermally neutral" mean?

There's the plot of clo vs temperature that Richard posted http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/9378/index.html

Given the amount of heat produced by a human body, you need a particular insulation value to keep your core temperature at 98.6 degrees F

Thanks for the response Jerry. Basically, thermally neutral means very little heat from my body is going to the snow beneath me, and very little cold is felt by my body from the snow.

Essentially, what does it take for me not to feel the cold beneath me when camping on snow at freezing temps?

The estimates and data I've seen seem all over the map, let alone personal experience. I know lots of climbers who only use a foam pad during the late spring/early fall, even on snow/glaciers. Steve House only brought a Thermarest ProLite 3 on his ascent of the Rupal Face on Nanga Parbat, though admittedly he slept cold. Not using that as a real example, just to say that high up on an 8,000m peak, some people's sleep systems have no more than 3-4 R value.

Edited by whitenoise on 03/14/2012 13:50:15 MDT.

Hi Sean

Your question does not make a lot of sense, which is why you may have been getting different answers.

Snow can have any temperature below freezing it wants. It can be at 0 C (32 F), or it can be at -40 C (-40 F). You will need different amounts of insulation for these different conditions.

Cheers

Thank you Roger, I didn't know that! I'm not surprised by that fact, but I am surprised I didn't know that. :)

So then, for snow/ice that is approximately freezing (0C or 32F), what is the required R value for a pad?

And similarly, for snow/ice that is approximately -17C or 0F, what is the required R value for a pad?

Thanks,
Sean

An awful lot of people seem to have been happy with a total r-value around 5. The classic combination is a ridgerest (2.6) and an old thermarest ultralite (around 2.2).

there was another BPL post I cannot find, that discussed combining two pads does not necessarily equal the combined R value.

example: foam pad 2.6 + air pad 2.2 does not have to equal R value = 4.8

unless...

if the foam pad is on top, air pad on bottom, better R-value.
if the air pad is on top, then you will get colder lower combo R-value, because air is a poor insulator.

your experience may be different, but it's something to consider.

I prefer the 2 foam (no air) pads method, because of the versatility, you will have to rig something to keep the two pads from slipping. but that's another thread (hint: velcro)

Air expands when hot and shrinks when cold. You can fully inflate an air pad, but at 3AM, the air padding will change.

Edited by RogerDodger on 03/14/2012 17:36:13 MDT.

Anecdotal evidence: with a pad that has an r-value >5, I've not been cold on the snow in temps down to the low teens with appropriate clothing and bag. Though, I have other issues in winter (too much moisture). I think if you keep the r-value upwards of 5, you should be ok provided you're not getting into extreme temps.