Question for the science guys…

[jimmy b]

What would be the warmer scenario

1- setting up shelter and sleep system on frozen ground with layer of snow at temps to 20F or

2-  setting up on an AT raised wooden platform above the frozen ground in same shelter, sleep system  and temps.

passing some platforms on a Christmas day hike got me to thinking where I would set up. I must admit I was lacking for a scientific answer. The ground of course is a heck of a  heat sink but on the other hand that cold air exchanging constantly under the platform cant be all that warming either.

jimmyb

 

[David Thomas]

Bare, frozen ground versus wooden platform at ambient, sub-freezing temperatures?  Clearly the platform is better because the ground is (1) more thermally conductive so it takes away more heat even after steady state is reached and (2) has a higher heat capacity so more BTUs / calories are lost getting it to steady state.

But you asked about frozen ground with snow cover versus a wooden platform.  Snow is much more insulating than ground, until it melts.  So if the snow stays above 32F, you’re insulated from the potentially colder ground.  That is going to require a fairly good pad like R-4 or better in many conditions.  (And even if the snow melts, that means the bottom of your pad is at exactly 32F – that’s better than sub-freezing ambient temps).

Whereas the wooden platform, usually made with gaps, would, yes, let some air up against the bottom of your pad.  But that’s only in the 1/2″ gap versus 3-1/2 or 5-1/4″ width of a board – so mostly, your pad is resting on wood.  The top of each board would warm a bit from the heat lost through the pad, but not much.

Heat-loss wise, I’d estimate not much difference.  Whether on a frozen platform or frozen frozen ground or frozen snow, you have to have good insulation under you.  But whatever heat you lose to the wooden platform would dry it out, while heat lost to the snow will melt it.  Seeing as all your gear insulates better and weighs less when dry, I’d go for the wooden platform.

[David Thomas]

And I’d much rather reach out of my bag in the morning to operate a stove on a perfectly level wooden platform than deal with a hot flame and scalding water on a less stable base of packed stove.

[Roger Caffin]

David has covered some good points, and I agree that it could be a toss-up for warmth.

However, that is NOT the only thing you need to consider. Ever slept on a dead-flat concrete floor? Unless you are on a 6″ thick down air mat, it can be uncomfortable, even in mild weather. Very strange.

What we often find is that provided we have a warm air mat underneath us, the snow can be a lot more comfortable. The good air mat keps us warm overnight, but some heat does of course leak through to the snow. In the morning when we remove the tent, we find that the snow has been shaped by the small amount of lost heat from our bodies, making quite a nice shaped hollow for our hips. Very comfortable.

I don’t have the problem with my stove which David has mentioned, as I use a 3-ply stove base on the snow, and that stabilises the stove nicely.

Good question though.

Cheers

 

[James Marco]

This is about the break even point, 20F(~-7C.) You want an R5 below you, even on snow. So much will depend on other things at 20F: Any breeze or wind, surrounding cover, even the color of your pad, that this is not fixed but will change a lot. Air is a poor insulater compared to wood. It has an R value of about 1 at 3″ of thickness and wood has an R value of about 1.4 at 1″ of thickness. And snow has an R value of about 1 (it varies a lot) at 1″.

[jimmy b]

Very helpful. You guys did point out exactly the mental exercise I was struggling with including the comfort angle. I will normally avoid a platform if there is any semi flat pitchable ground around. Also we are using Xtherms and thought they would be fine for the described conditions but I wondered about snow melt. My first thought was to pitch on the ground but it seems there are opposing opinions. Thinking a bit more about the temps being at a breaking point, ease of securing the shelter may also be a valid reason for possibly choosing the platform.

[Jon Fong / Flat Cat Gear]

Here is a pretty straight forward test for you.  If you sleep on the snow and the next morning you found that the snow had melted, you would have been better off on the platform.  To melt snow, the water has to go through a phase change (latent heat) which requires a tremendous amount of energy.  My 2 cents.

[Roger Caffin]

Hi Jimmy

There is a pretty big difference between an XTherm and a good snow mat. Some caution is indicated.

Hi Jon

I have to disagree. Heat will travel through your mat, whatever it is. And over a 12 hour period it is very likely to melt a little bit of the snow under your mat. What i am reporting is our experiences over many years of ski touring with comfortable nights.

Cheers

 

[Jon Fong / Flat Cat Gear]

Roger,

If you sleep on the snow, the maximum temperature differential across the sleeping pad will be between you’re body and 32 F (the melting point).  If you sleep on a wooden platform and the surface of the wood directly underneath your pad is above 32 F then the wood is a better surface.

I sleep in a tent on the snow in a regular Neo Air and have never experienced a cold spot.  I agree with you that the snow compresses and matches your body shape providing a nice cradle.  Best regards.

[Roger Caffin]

Hi Jon and Jimmy

As far as temperature differentials go, the surface of the wood might be slightly warmer. But that is not the full story. So, an explanation.

What matters to you, the sleeper, is how much heat your body loses downwards during the night. That is governed by the product of two things: the temperature differential across your mat, and the insulation quality of the mat. So we need to look at the mat as well as the surface underneath.

When you sleep on a hard surface you get a thin spot under your hips, and sometimes under your shoulders as well.It is not hard for a nice 3″ thick mat to be reduced to <1/2″ under your hips. That’s where you lose heat and get cold.

But when you sleep on snow, the snow melts a bit, or maybe compresses a bit, under your hips. So the thickness of the mat under your hips increases, back towards the nominal thickness of the whole mat. It is very easy for the thickness under your hips to go from 1/2″ to 2″ when the snow softens and shapes to fit your body. And that increase in thickness can give you a FOUR times increase in insulation; a four times reduction in heat loss. Small changes in the temperature of a wood platform don’t come anywhere near that improvement.

The same applies to sleeping in a hammock. You don’t put your down between you and the hammock material: it will just get compressed and leak heat. You put it under the hammock, so it stays fluffed up and a good insulator.

HTH

Cheers

 

[Jon Fong / Flat Cat Gear]

Roger,

 

I agree with you 100%.  Beyond the science aspect there is the practical, real world usage.  Snow is great as you can conform it to your body shape and build wind breaks around your shelter.  In the last 15 years, I don’t think that I have every slept on a wooden platform.

[jimmy b]

Roger/Jon,

First off I am glad the real world or practical app to this question is leaning more to the ground. As stated I avoid platforms as well. As far as a true winter mat Roger, are you suggesting something different all together or supplementing with a  closed cell foam as many here seem to have good luck with. And if supplementing is fine what thickness would you first experiment with for snow camping. As you can see I’m new to the winter BPing but eager to learn and get started. That is if we actually get any amount of  snow this year :)

Thanks again for helping with replies, jimmyb

[Roger Caffin]

Hi Jimmy

OK, first rule: there are no magical answers. For someone who has a couple of good Ridgerests, that might be the best FOR THEM answer. For someone who has a down air mat (DAM), you will probably have to wait until you can take it out of their cold dead hands. And so on.

We (my wife and I) have used Exped Synmat UL7s with 1/8″ foam underneath down to -7 C, and we were OK. We have used Thermrest Deluxe very happily, although they are heavier and I think they are out of production. We have used DAMs as well, but they are not light.

What matters more is the total R-value of your mat(s). For snow use you need something up around 6 I think. A good starting point would be to read our survey of air mats:

http://www.backpackinglight.com/cgi-bin/backpackinglight/airmat_sotmr_part1_2011

http://www.backpackinglight.com/cgi-bin/backpackinglight/airmat_sotmr_part2_2011

Yeah, written in 2011, but the underlying theory is still 100% correct and relevant.

One technical point to remember. Your groundsheet WILL be at 0 C. So there is a very good chance you may get some water condensing on it, under your mat. That is not a leak of any sort. So, some like to put a foam layer down first. We often do that ourselves.

Cheers

 

 

[Matt Dirksen]

Perhaps it’s been said in different ways, but I’d thought I’d chime in:

In buildings with exposed floors, our codes require a much larger amount of insulation than would be necessary when the same floor is sitting directly on the ground. This is due to the fact that air temps have much greater extremes than ground temps, not to mention the soil under a built slab ultimately “stabilizes” the soil temp underneath of it. So on a cold winter’s night, the underside of an exposed floor could be almost 10-15 degrees colder than a slab on grade. Obviously the this isn’t quite the same as a tent platform, but I find it interesting to think about, none the less. The best way to answer this is to better understand the relationship between soil temp vs. air temp. One can typically look up these differences on-line, depending on where you are.

Regardless, before I would plop my shelter down, I would consider:

1. Is the air temp at night going to become colder at night? or Warmer? Usually, if the day temp is mid 30’s and the night temp is mid teen’s, then I would wager that sleeping on the ground would feel much more comfortable. However, if a weather front is on it’s way, where the air temp might actually rise at night, perhaps the platform might be better.
2. Is the platform/tent space shaded by trees? Or exposed to the night sky? (In general, a shaded spot might provide more temp. stability no matter what, due to nighttime radiation issues.)
3. How much moisture do I want my shelter to take on? While tenting on snow might generally provide more temperature stability, do I want to take on the excess water issues if I don’t quite have enough R-value under me?

[Roger Caffin]

While tenting on snow might generally provide more temperature stability, do I want to take on the excess water issues

I remember waking up one sunny morning – early, in the snow, to find the tent was bone dry. A light breeze and sub-zero overnight temperatures were responsible.

Cheers

 

[Adam Kilpatrick]

Reading through this, I was going to point out two things.

1. Thermal mass of water (frozen or otherwise) plus the ground underneath is far, far greater than wood, especially relatively thin wood suspended in air (both material and thickness are critical factors here). Matt is spot on about this. The wood has far greater ability to cool down overnight to a lower temperature. If you go down a small way into the snow (probably just a few inches) you are hitting temperatures approaching the average temperature over the past few days. Go down 3-5m into the soil and you are hitting ~ the average annual temp (that’s the average of every moment of the year, night and day).
2. As per the frozen snow vs frozen ice problem, increasing R value a small amount decreases the energy flux across the pad significantly. The amount of energy transfer is easy to calculate, and you could also calculate the temperature rise of an area of frozen snow based on rough snow density, area, and the calculated energy transfer (over say a 12 hour period). I would not assume at all that body temperature is the temperature on your side of the pad. A more realistic temperature would be much lower. Higher in a double skinned tent. Lower if you are in a sleeping bag vs quilt. Lower the more clothing you are wearing. Lower the more you personally toss and turn! Likely, over your torso area, where your body runs hotter and you put the most pressure and area on the pad, will be where the most energy transfer happens. Hence, add a little more insulation there.
3. Wind speed and subsequent chill. Wind speed decreases significantly the closer you are to natural ground. Small, minor undulations in terrain, vegetation, random pebbles, even tiny blades of grass…all decrease wind speed close to the surface. There is a huge difference between say 10cm off the ground and 20cm. The first metre or so off the ground is a big curve. Getting closer to the ground for protection is a big deal! If you are sitting a metre or so up on a platform that might be bad news. Of course, you don’t want to sit your tent in a hollow either (cold air accumulates), but find protection. Increased airflow will amplify the affects mentioned above with a low thermal mass thin wood platform. Its bad news. If you are sans tent (and thus exposed more directly yourself to the wind), R value for R value, a thinner mat is always going to be warmer. You’ll always be warmer on the foam than on a big inflated job. That few inches of height is huge. (I know all about this from studies in soil science, agriculture and desert ecology…a tiny bit of grass/groundcover stops continental size dust storms…). As mentioned by others above, on the snow you can dig in snow breaks around your tent. Even a couple of minutes of effort to do so would make a big difference, worth it every time.

 

[Anonymous]

I’ve seen a better rundown in the BPL archives but wiki has the quick answer:
Snow R-1 per inch
Hardwood (most) R-0.71 per inch
https://en.wikipedia.org/wiki/R-value_%28insulation%29

Discussion (the R rating above does not include the airgaps so its probably lower for your platform. I think someone in BPL researched a formula for multiple inches of snow. It’s not an infinitely linear curve.

Also I agree full heartedly with Roger. Flat wood is tortuous to sleep on unless you have a nice inflatable. Snow is deluxe.

Since you’re not supposed to cook in your tent (let’s agree to disagree BPL :) I’d say sleep on the snow and cook on the platform if you’re in a rush. if you’re not in a rush, and have plenty of snow I recommend constructing a nice recliner out of snow.

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