I believe Just Bill has pretty much dominated this conversation with excellent analysis - and more words than BillyBob58 has ever mustered. That's hard to believe!
I believe Just Bill has pretty much dominated this conversation with excellent analysis - and more words than BillyBob58 has ever mustered. That's hard to believe!
"A foolish consistency is the hobgoblin of little minds." Ralph Waldo Emerson
When I was in the Nat'l Guard in the late '80s I volunteered for a training exchange with the Canadian Reserve Army. It was a Winter Warfare Training class in northern Ontario during the first two weeks of February (10 days "camping" with no fires allowed). It never got above 10F until the last day in the field. We were instructed to wear only our long johns in our down sleeping bags otherwise we would be cold all night. That's what I did and I was warm and toasty every night. Since then I've always done the same thing so I don't know what impact wearing multiple layers would have but I can absolutely vouch for one layer working well in sub-zero weather.
Of course these days you couldn't pay me to sleep out in the cold. Well you could but it would have to be a large sum.It's the whole frozen boot thing in the morning. I just can't deal.
"Behold, as a wild a** of the desert, go I forth to my work." -- Guerney Halleck
I'm not going to argue against the stove analogy but would offer my simple (overly simplistic, probably) explanation.
When we get cold, the problem isn't the cold getting in, it's the heat getting out. This is why we are cold in damp layers no matter how much insulation is piled on top of us, as some have mentioned up-thread. Moist clothing is sucking out big BTUs to convert liquid water to vapor. (Which then re-condenses in the insulation — or on tent/hammock cover walls, depending on ambient temp and relative humidity, airflow, etc — when it hits dewpoint, but that's another barrel of VBL-clad worms.)
My feeling is that thicker insulation is warmer because it widens heat loss gradient thereby slowing heat loss. With a too-thin quilt there is less buffer in the gradient and heat is lost more quickly.
Five Basic Principles of Going Lighter (not me... the great Cam Honan of OZ)
“If everybody is thinking alike, then somebody isn't thinking.” ~ Gen. George S Patton
Not a competition, but for sake of conversation; ' We're both right'.Originally Posted by cmoulder
In the case of all insulation- you're correct on the thickness increasing the temperature gradient.
Someplace in there I mentioned roughly 10* per inch in 6" stack vs 1" of clothing for example.
The further (thicker) the spread between each extreme... the slower the energy transferred along the whole thing.
Loose fill attic insulation is directly measured in thickness and converted to R-value.
Down fill insulation is also directly measured in thickness (Loft), though converting to R or CLO is trickier.
The exception though is synthetic. 5/8" or so of 3oz Primaloft Gold is roughly equal to 1.25" or so of down.
This is why synthetics are measured in CLO... again roughly 2.76 CLO.
Apex is not as efficient as PLG and would have 2.46 CLO.
Doesn't sound like much but keep in mind that a basic base layer is .5 CLO.
And of course closed cell foam blows them both out of the water with an R value of about 6 per inch. 6/.88=6.82 CLO per inch or 3.4 for a half inch.
Basically... it's not how thick exactly... but how much 'stuff' do you need to trap a given volume of air.
A loose fill like blown in cellulose isn't very efficient. A dense fill like Closed Cell foam literally creates perfectly trapped bubbles of air.
The problem with water is that it can damage the insulation we choose and reduce it's air trapping efficiency.
A bit of moisture in celluose... and it sags and clumps together.
A bit of water in open cell foam... well that's basically what a wetsuit is. The water next to skin rises to body temp and then can't escape.
Or think of a foam cover for a hot tub... or the cozy for Freezer bag cooking. So long as the moisture doesn't displace the air, it doesn't affect the insulator... it's not a problem.
It does take much more heat output to dry damp down. Or perhaps more accurately, water is horrible at heat transfer so it slows down your ability to heat trapped air... or if very damp acts as a heat sink because as you mention... takes way more BTU's to heat up water than air. And even more to heat it to the point that it can escape.
But since a synthetic like PLG actually traps air pockets smaller than water molecules... it retains 92% of it's insulating value when wet.
Closed cell foam... nearly 100%.
The VBL thing is a mess. It really doesn't mean much when discussed here unfortunately.
A little humidity actually increases warmth...and a VBL can help with that sorta.
I am not a VBL expert. But very few are in truth. It's tossed around a lot, but folks with practical knowledge of them are slim. And I rarely hear folks talk of using them properly.
The right way to use a VBL is not to sweat at all. Your goal is to balance your heat so you do not overheat.
The point is so that your natural body vapor does not enter your insulation in sub-freezing temps.
This applies to multi-day trips of 4-5 days typically in zero or lower temps. Skurka's North country trip a fine example.
That's why I'm not an expert. I don't do 1-2 week deep winter trips. Few do.
Because the thermal gradient is so extreme, portions of your insulation will be below freezing at all times.
This means that every night a few ounces of moisture from body vapor will build up in your sleeping gear.
A true VBL is used to prevent this issue. And not for anything else. It is to protect your insulation from yourself as you have no way to dry it during those extreme temps.
When it's 20* or above you can likely sun your bag or air it every few days if you're out for longer.
I bump my insulation about 10 * and typically suffer a 5* loss over a long weekend from this. If it is colder or I'm out longer... then the issue would be come deadly.
At this point multi week zero degree trips aren't really in my future... so I don't see anytime I would find a practical use of a true VBL.
On the flipside of the temp scale....
A synthetic sleeping bag with a Semi VBL (bivy sack) with WPB fabrics is actually a good fast and light way to sneak 5-10* by creating a moist humid environment for sleeping.
This is done in safer temps (45*-65*) to 'push' a summer weight bag further than one normally could.
As there is no danger of the water vapor freezing, or the synthetic insulation going poof... once can simply air the bivy out and sun the bag to prevent any mold/mildew from forming.
I think this is the more accurate description of how many use VBL's and there are some SUL folks pushing the idea of sleeping in your WPB shell (next to skin) as a way to both push down bags and to push a system.
But it has nothing to do with traditional VBL use. In Nordic countries fishnet long undies baselayers with a VBL are used in deep cold clothing systems similar to sleep systems.
But again... it's the long term moisture accumulation issue in constant sub-freezing temps... not the 'sweatsuit' that is of value.
Great stuff and many thanks to the thread contributors!
By the way, there will be a quiz on this material at the next group hang. So study up!
The game is the best teacher.
If there is a quiz, I'm pretty sure Just Bill is going to ruin the curve for the rest of us!
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As they say at Whiteblaze...
I greatly abuse what is implied when I push ‘post quick reply’
What I learned from all this is that I am cold because I'm old and not exactly the brightest light on the porch either.
Good info here, really like the light bulb analogy as a way to conceptualize it
Interesting concept. Whenever I had to rough it in the Army I would strip my top layers and keep my pants/ drawers around my ankles so I could pull them up quickly if need be. My only reasoning for doing this was so I wouldn't sweat in my clothes and wake up wet out in the cold. I guess I was ahead of my time!
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