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Location of foam pad in bivy

9:00 p.m. on February 17, 2004 (EST)

a.k.a. Karl

When sleeping on the snow or in a snow cave, I normally have been putting my foam pad inside my bivy sack. This has resulted in condensation from my body between the bottom of the foam pad and the inside of the bivy sack. It hasn't been a big problem, but it could be if I had to sleep in in many days in a row. The only solution I can think of would be to move the dewpoint location outside by putting my foam pad outside underneath the bivy. I haven't tried it yet. Any experience out there?

11:01 a.m. on February 18, 2004 (EST)

Hi Karl,
I've tried both ways and I think it is just a personal choice.
1. You may want to (i.e. should) air out your sleeping gear whenever possible, so your pad-in-the-sack approach is fixable.

2. If you put your pad outside the sack, just make sure you use a trash bag etc as a ground sheet, or you'll have a good chance in freezing your pad onto an icy ground. Also conside the possible of snow collecting on top of your pad, especially if you're using a ridge/Z rest.

Just a sidebar: I ditched my thermarest years ago and use closed-cell pads instead. If you do a subjective "hand conductivity test" to compare the warmth between a thermarest and a closed-cell pad. The closed-cell pad wins every time. Maybe that's why they use it as toilet seat up in Denali

Have fun and play safe :-)

12:32 p.m. on February 18, 2004 (EST)
2,234 reviewer rep
5,182 forum posts

Hmmmm. I have never had a problem with condensation between the pad and bottom of the bivy, either inside or outside the sack. I have sometimes gotten condensation on the top, thanks to gtx not really being all that breathable. That gets cured by using a VBL (I use the 3 oz ID VBL that can also be used as an emergency bivy, which means I can cut the weight way back for those alpine pushes). The VBL vents through the face hole of the VBL, sleeping bag, and the bivy, so there is little or no moisture to condense inside the bivy, or more important, in the insulation of the bag.

The questions that occur to me are whether you are venting the bivy sack as you should (gtx and other waterproof/"breathable" fabrics really aren't all that breathable, so be sure to keep the face area open to exchange your breath freely with the outside world), whether your pad is really thick enough, and are you absolutely sure it is a closed cell pad (well, that's probably not the case - you are using what, a Ridgerest, Z-rest, "blue foam", ??? - an open-cell pad or one of the ones in a nylon shell will allow your body moisture to go through it and that would allow condensation).

A couple things about winter camping, whether in a snow shelter or tent - first thing in the morning when you get up, roll your sleeping bag tightly to squeeze all that humid air you sweated off during the night out of the insulation (On Will Steiger's first trans-Arctic expedition, they gained an average of a pound a day in frozen perspiration in the bags, so that by the time they reached the pole, the bags weighed about 75 pounds and could not be stuffed or even folded for packing). Another is to turn the bivy inside out (in the tent, snow shelter, or nice sunny day, not outside in a blizzard). This will allow drying, shaking off any condensation, etc. If it is a sunny day, open your sleeping bag and lay it out in the sun (on a dry surface, of course, perhaps draped over the tent). Winter sleeping bags should have black (or at least very dark) inner liners so they absorb the sun better and dry faster.

I generally use 2 layers of pad, a full-length blue foam and a 3/4 Thermarest. But then I tend to be camping in really cold temperatures. I do not use the bivy in the tent, but do use it in snow shelters (which I prefer to tents), and I use a VBL if I am going to be out for more than a couple nights. Some people find VBLs clammy, but I find that using them with a light to medium set of wicking long johns and being sure to keep my nose and mouth pointed out the face hole of the VBL and sleeping bag prevents that. I generally prefer to use the pad or pads inside the bivy in a snow shelter, and always put the pad inside when doing a real bivy (well, for a real bivy, the "pad" is either my pack or a small square of blue foam).

8:09 p.m. on February 19, 2004 (EST)

a.k.a. Karl


Thanks for the comments.

I am curious about the heat transfer test you mentioned. I'll look more into it. The hand test may be flawed, however. In a short term test, the skin of your hand on the nylon thermarest cover is sensing the heat capacity of the nylon, not the foam pad. The nylon heat capacity, granted, is more than a bare foam pad, so it sucks the heat out of your hand faster than the raw foam and thus causes the sensation of being colder. However, that doesn't mean that overall the thermarest "system" is a worse insulator than the bare foam pad. I'll do some actual heat loss tests with some temperature instruments at work and find out.

8:35 p.m. on February 19, 2004 (EST)

a.k.a. Karl

Thanks for the insights! I was using a 3/4 deluxe LE thermarest foam pad with a real gortex topped bivy. I had my head under the bivy "vestibule" flap that was open and just laying out past my face, so air was free to go in and out, but I wasn't directly breathing out. The temps were not cold--upper 20's outside. There was not condensation on the underside of the top side of the bivy--I don't attribute that to Gortex, which I too don't find that breathable. I find the same problem inside tents--condensation on the tent floor under the foam pad. However, when it was very cold (<0F) it was just frost.

10:41 a.m. on February 20, 2004 (EST)
2,234 reviewer rep
5,182 forum posts
Thermarest vs foam

The insulating "power" of Thermarests (and other inflatables) depends on which model you are talking about. If you look on Cascade Designs' website, you will see that they rate their pads according to both comfort and insulation. Some of the thicker pads (the LE, for example) are rated for less cold conditions than the medium thickness (Standard, for example). The reason is the internal construction. All of their pads use open cell foam internally. This means that air can convect inside the pad, albeit slowly, whereas closed cell pads like their Ridgerest do not allow any internal convection (how much the ridges allow is another question). The Standard is "solid" foam, while the LE and other light-weight pads have various styles of open air pockets in them. The LE has long cylindrical openings, for example. One other family has the foam sliced into a kind of open gridwork with the pockets running vertically. These open air pockets allow a fair amount of internal convective flow, hence lower the insulation value. Which is why the LE (2 inch thick) is rated for warmer temperatures than the Standard (1.5 inch thick). While I have not conducted any kind of controlled test, my impression is that a 5/8 "blue foam" ($7 at WalMart, and less than 8 ounces) is about the same insulating value as a Standard Thermarest (mucho bucks more and something like 24 ounces). The blue foam is not as comfortable, of course, and the LE is more cushy than the Standard. So if the temperature is reasonable, you might want to choose comfort. By adding the blue foam to the Standard for winter and the Alaska Range, I end up with both thermal insulation and comfort, even though the combination is bulky and heavier than I would like.

2:09 p.m. on February 20, 2004 (EST)


Hi Karl,
I don't know how heat capacity, conduction on the nylon surface, or micro-convection play out in a complex system like an inflatable pad. It is great if you can get some measurement. You may deposite your test result under "community" at: http://www.backpacking.net/index.html

Both MEC and REI list R-values in their websites, but they don't always square with each other.
At MEC, it is listed under "Gear & Activity Info" or got to

Without surfing with my Javascript on, I don't know if this is listed on the "Comparison Chart" at REI.

Have fun with your testing :-)

7:59 p.m. on February 20, 2004 (EST)
2,234 reviewer rep
5,182 forum posts
Re: R-values

Les -

The MEC chart is interesting, but the question is how they obtained the R-values. They show a range for the inflatables, but what does that mean? Is one end of the range for partially inflated and the other for fully inflated? (no, I don't expect you to have the answer, but it would be nice if MEC said something about their methodology. I suspect that having a -20F surface to lay the pad on might give a different number than a 32F surface with the pads that have the cavities in them, since that would affect the efficiency of the convective transport).

I was a bit surprised at size of the difference in the Z-Rest, Ridgerest, and blue foam. But the range matching so closely for the Standard and the LE was also interesting given the large difference in weight.

By the way, for those who don't know, R-values add, so to get the equivalent R-value of a blue foam plus a Thermarest (or two pads stacked), just add the R-values together. R-value is also called "heat resistance", meaning something analogous to electrical resistance. There is a formula similar to Ohm's law for heat flow rates using R-value, temperature differential, and thermal energy flow rate.

4:58 p.m. on February 22, 2004 (EST)

a.k.a. Karl
Re: R-values Ratings

I'm with you on the need for a methodology. How can you compare a compressible foam pad like a thermarest with a barely compressible more rigid foam? Sure the 1 1/2" thick foam will insulate better when full thick, but when my hip is compressing it down to a 1/4" to the snow, what then? But my torso and thighs might still be seeing an inch or more. So do we do an "average" R-value??? This is where simple ratings are interesting, but not that important. Using some common sense and talking to others is likely the best information.

I love my thermarest and sleep a lot better with it compared to my stiffer thinner foam. I can add clothes to get warmer, but clothes do little for aches and pains. Its inflatable foam for me!

April 18, 2014
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