Heat loss time curves?

if this is survival, one of those mylar “space blankets” is good – small volume and weight, good warmth for the weight.  Wrap it around you on the outside

I carry one with me but never use it, never been in a survival situation

I have and I can tell you from the painful experience that those mylar things are worthless and no better than any other plastic bag.  They just give you a false sense of security

They only provide a little warmth, preventing radiant heat loss

May be the difference between surviving and not

Protect against rain and wind.  But you can have condensation problems.

People do typically over-rate the emergency blankets, but they can be useful. I did a number of nights with just emergency blankets as an experiment.  I can’t find my notes anymore where I recorded temps. The expensive/double layer with air (can’t remember the brand / model) were  more effective, but I would rather have my down quilt :) Technique: You want some dead  between you and the blanket otherwise the radiation insulation doesn’t work well and you the you cool by conduction. I found squatting down with the blanket like this diagram was the most effective provided I had a warm enough hat. You need to be off the ground… e.g. have a foam pad / leaves etc under you. Running a candle was a  big help.  For me, the “candle” was a small backpacking oil lamp called an ultracandle which hasn’t been made for years :(

Yeah, if there’s an air space between you and the blanket it will work better

If you put the space blanket on the outside, then there’s an air space between the space blanket and ambient – less radiant heat loss to ambient

I can’t prove the following but have experimented enough to know that it is true for me.

(other things being equal) A full length bag will be lighter than equivalent warmth from  a half bag combined with anything else.

Yes, Mylar film helps a LOT. A person can radiate about 50W of IR energy. This is around half your bodily energy. The rest is used in chemical transformations/other processes. So, a Mylar or Space Blanket that totally encompasses you will allow collection of roughly 90% of this energy, or in round numbers, around 45W is about what they add to your system…or roughly what a 30W to 40W lightbulb puts out. Putting yourself totally into a reflective mylar skin or bubble (not recommended for breathing, condensation, etc) would help a lot.

Insulation around the bubble would help a lot, too, of course. But anything non-transparent to IR would work. However, even an air space can absorb IR and turn the “heat” energy into a mechanical energy…molecular movement, increased Brownian motion, excited energy states (making them more reactive,) re-radiation of IR energy, etc.

Hi James.

I wonder if you have ever sat out a Scottish snow storm wrapped in one of those RFL “survival” bags; shivering your **** off and wondering if you’ll live though the night?

Australia isn’t unlike Scotland in it’s winter conditions when you get above the snowline.

Given the expertise on this forum I was hoping somebody could direct me to some black body radiation graphs so I could make an extra layer of insulation that was “just enough” in keeping with the UL ethos, rather than making something I know would work but would be 500 grams heavier. I just made one of those and it weighs 900 grams and is too wide to fit inside my UL WPB Bivvy bag.

As stated in my first post my day pack always has in it the UL bivvy bag, my DAS trousers and an insulated parka, usually my trusted Polargard DAS

A FF Vireo would work if I could afford one, even though they are a bit on the skinny side to wear the DAS in it and I would need to select a less warm parka

Edward et al.,

With a DAS Parka and Pants, and a WPB bivy (my current favorite is the MSR Pro Bivy), I can be “OK” through a night of stormy weather down to about -5 or -10 C with a synthetic quilt that’s about 3 cm thick (single layer, Climashield) on a pad of 1/2″ evazote. This is my typical “long-day-possible-night-out” alpine climbing kit. The quilt weighs around 600g, uses 7d fabrics (homemade).

This doesn’t provide tons of comfort. I’ll get up 2 or 3 times during the night to do jumping jacks or make some hot tea or food. It does allow me to survive the night well enough that I don’t have to worry about getting sick or trashing my immune system. I mostly use this setup during spring and fall when dark nights are 10+ hours long.

Thanx Ryan, that does give me better perspective and maybe getting the warm layer lower in weight than 600 grams and still being effective isn’t feasible using synthetic insulations.

I’m aware of the effects of severe cold stress on the immune system and it’d why I need to carry a bit extra.

Would that have been the 160gsm APEX?

Next ski season is still a maybe here but these projects keep me busy

Nope, never said they were all that warm, they reflect IR at about 80-90%. Insulation does not normally reflect IR. There are three forms of “heat”. IR energy that can be absorbed, is the first form. Think a large wood shop with radiant heating. The wood can be frozen together but the perceived temp is close to 60F. Most energy will devolve to IR after being absorbed once or twice. Most of the heat a human produces is IR. Most of the heat from a flame is IR, initially.

Eventually an IR photon will strike something and be absorbed. The energy has to go somewhere so the so called electron shells (probability zones) usually enlarges(de Broglie.) The molecule is in an “excited state” but it must be a function of ity’s electron shell distance. This is not heat as we know it, and is the second form of “heat.” When it drops out of an excited state it emits a photon of energy, usually close to what was absorbed but usually at a lower wave length…think IR. Example a dark rock will absorb light energy and emit IR energy. It sort of drops down the wavelength.

The third form is the actual shaking of the molecule, for example Brownian Motion. At some state energy is transformed to motion on an atomic level. Mass increases slightly (almost irrelavently.) And the whole molecule will “vibrate”… a lot like a bell but in discrete wavelengths/amplitudes. This is what most people feel as heat and the third form of “heat,” the shaking of molecules/atoms.

None of this is a good explanation of the numbers and actual happenings, but close enough to use. I suspect electrons are actually energy themselves hence the absorption and release of photons soo easily… Skip it…

Soo, a combination of insulation and IR reflector would likely be much warmer than insulation without an IR reflector. Both will help…

Look up “heat output of IR in a human”, “Images for heat output of IR in a human”, and Wikipedia (though always suspect.)

Many years ago , IR reflective layers were a thing for  while in synthetic sleeping bags. Moonstone for one made several bags with an IR reflective layer. I had a buddy who worked for them, he got me some of the material they used, called Texolite if I recall correctly. It was aluminized Mylar perforated for breathability. I made a bag using fairly thin Polargurad insulation, nylon lining and Texolite shell. Still have it. It weighs about 28 or 29 oz. ; probably the nylon is about 2 oz/yd – couldn’t get much lighter back in the late 70’s . It kept me warm down to about freezing, wearing some clothing. the saem polarguard insulation, with just a nylon shell, would I think have been about 10 degrees less warm. Clearly the IR reflective aspect was helping. BUT. Even with the perforations, I got condensation inside. So with a non-perforated layer, that could be massive unless you also do VBL layer close to the skin. Personally, if I were making a semi- emergency sleeping setup, I’d go with very light bivy , light synthetic quilt, and nice warm closed cell pad. The closed cell pad might not be the lightest option for the warmth, but it is utterly reliable. And a warm pad is step one, in my opinion, if you are going to be on snow. If I could only take one – bivy, quilt, or pad – I’d take the pad.

Consider the possibility, with synthetic insulation, of a quilt with lining but no shell. If used inside a bivy sack, you don’t really need the shell. Rather fragile, and maybe not really worth it to save the weight of one layer of .67 oz nylon, but if you after the lowest weight, maybe.

I’ve thought about using 2 layers of my Thinsulate 40DS [ 40 GSM + double scrim] with no shell and I’ll sew one up but it has a disadvantage of being hard to compress into a small package. Heeding what Ryan said and the comment from Daryl&Daryl perhaps an UL quilt with an enclosed footbox and different insulation weights might be a better solution and could double as my warm weather sleeping bag, I do have 3.5 metres of 2.5oz APEX  that was going to be a large size picnic blanket.

Double layer up to the waist and a single layer above that?

If I use EPIC as the outer shell perhaps I can leave out the bivvy sack and use a cheap plastic drum liner instead

Sounds like what you need Edward is some of that new fangled polymer based (polyimide) aerogel flexible films.  Both joking and not joking.  Joking cause the stuff is so dang expensive.

This is more of an aside:  One of the things I’m going to be experimenting with in the nearish future are lightweight vacuum panels made out of composites.  Not very practical for backpacking (these are more for coolers and mini fridges), but I could see use for maybe in combo with a foam or air ground pad if one is also using a pulk (since volume would be less of a factor/issue).

Along more practical lines.  While I don’t particularly like mylar space blanket material because it’s so flimsy and fragile and can build up moisture vapor so much, the SOL Escape Bivy has been nice in combo with down quilt.  I don’t know how weight to warmth efficient it is–it likely would be more efficient to just put more goose down in the quilt or bag.

I actually bought a SOL Pro bivvy, the breathable one, the problem was that I needed to be almost naked to get inside it. They are not built for blokes with big chests and wide shoulders while wearing winter layers.

It did make a very good liner but not what I needed so I sold it on.

I’m experimenting at the moment with sewing a half bag using a double layer of 40GSM LiteLoft. I am sandwiching each layer with 10GSM non-woven scrim and basting it together. I’ll see what the result is in a few days. Foot box size is predicated on my not taking my ski boots off, just loosening the buckles.

I seem to recall a Chouinard half sack similar from the late 1970s that used a very thin layer of something, perhaps Thermolite as used in the S2S liners, a big wall half sack for summer climbing