north-north-west wrote:All this sciencey stuff has got me thinking (never a good idea) about the 'wet foot' phenomenon with the sleeping bag.
Now, the idea of it being from vapour emitted by the feet overnight, passing through the bag and collecting on shell doesn't make sense to me. Maybe thatt's because I doon't know enough about the science but . . . with poor peripheral circulation especially, wouldn't the majority of the vapour be from the torso and, therefore, the outer dampness (and I'm talking about those mornings with the shell wet when the inside is dry) should be more on the upper area than around the foot.
So . . . but with warmth being transmitted (for want of a better word) ing through the inner lining and the insulation into the shell material, if the feet are colder and the torso/head area warmer (and this could be exacerbated by the greater dead air space around the feet), could there be a sufficiently significant temperature differential between the material of the outer shell in those areas for vapour inside the tent to condense on the foot rather than elsewhere?
It seems logical to me, but I don't know enough about how this works. Does the science contradict that?
Yep, you're pretty much on it.
The footbox of the bag, having less meaty warmth inside it, is the most likely place of the bag to hit dew point first (assuming uniform insulation distribution, which often isn't the case, but the rule still holds (usually).
The point here is that it's not a case of vapour from one given source or another that's more likely to condense there. Its more a case of 'what is the general distribution of humidity owing to mixing of various sources and environmental conditions, around the bag shell, at a given temp?'.
If you were sleeping in the atacama, but went to bed with a humidifier in your footbox, enough of that vapour would seep through the bag, to increase the humidity in the air around the bag skin, despite the environment being very very dry. The temp may stay at the same, but the humidity added to this cold air saturates it , and bam, droplets on yo foots, everything else dry. Its would LOOK like they 'fell" onto the bag, because the drops precipitate on the colder outer surface (hence the frequent misunderstandings).
Conversely, hypothetically go to sleep in an alpine bog, with some icebags and a vapour liner round your feet, you could have a footbox that emits NO vapor, but becomes so cold compared to the environment, the already present cool, humid air would condense out on the bag skin too. Both inner and outer conditions contribute, its about where the equilibrium winds up on a given night
So, getting back to the real world, what we have is a combination. The feet aren't armpits, but they do sweat. For some more, others less. The footbox is part of the overall bag inner too, so unless you're in a VBL, its pretty much a given that the relative humidity inside the footbox is going to be higher than environmental after a few hours. Its also a given it will be warmer. Less so for some, but even if you *feel* like your feet are cold***, they're still warmer than outside. So - there will be SOME humidity elevation around the bag skin due to escaping moisture..
Does this form drops? Depends - on whether you reach dew point for that particular air sample, with whatever temp and RH it has due to the combined effects of foot temp and vapour, meeting and mixing with the environment
. If your foot vapour was adding to an already very humid air, then it may well condense with no temp drop. In this case, you may not see any on the bag skin of the body, because its warmer and thus keeps the air able to hold that higher moisture content.
Similarly, if the temp drops, the footbox (having less meaty heaty bits) will hit dew point sooner and you get drops. However even the chilly feet are pushing out more sweat than say your pack, so it remains dry - the air around it isn't as close to 100% RH and would thus need to be colder for droplets. Although its likely that yourdoarm breath rising to the underside of the tent skin will also condense out up there - the tent skin is even colder than the footbox, so it can chill air easily to dewpoint. If you tuck your bag close to your chin, you'll get dampness around there too - super moist breath needs muuch less cooling to reach saturation. And indeed breathing for long enough will increase the RH inside the whole tent, which of course also contributes to conditions at the bag skin, so you may even increase the RH enough to provoke condensation on the whole bag.
***having feet that *feel* cold is another matter. Unless you're a corpse, they're still comparatively warm. They just resupply that heat far slower than elsewhere, especially horizontal, so other tiisue and nerves stay cooler. Meaning extra insulation often won't do anything - its supply that's the issue, not loss. If cold feet an damp footbox are a chronic problem, carry a small plastic bottle (old peanut butter jar) in a stubby cooler. Fill with boiling water, wrap in dry socks. Slow release heat source increases effective heat supply. Needs to be slow, or although your feet feel warm at first, you dump most of that heat into the insulation, which of course just bleeds it outside in a few hours.
If you want to go REAL toastyfoot, combine this with a garbage bag, and stick the bottom 1/3 of a foil emergency blanket to the insid of it with double sided tape. Line the bottom of your bag/quilt with it, wear some thin wicking merino socks. All up inc PB jar, its only abou 200g and will make a maaasive differnce. Should all but eliminate footbox condensate, and if you get cold feet after doing that, see your doctor,- I suspect you are in fact the undead.
Make sense? Sorry these turn out so long, but I find if you gloss over stuff or don't flesh out examples it just confuses people more