Base Layers, Wicking, and Backpacking (Member Q&A)

The only hydrophilic base layer tops that I have are some Nike DriFit based on a drop test. I have several hydrophobic tops that I regularly use without excessive wetting or inefficient evaporation while backpacking and winter walks. The other day on a 10F walk, I could feel myself sweating, but when I got home, my armpits in my base layer were dry.

I’m wondering how important hydrophilic characteristics are for my use. All of my base layers are skin tight, and when I did my drop tests I also pressed down on the drop. All of the hydrophobic tops absorbed the drop when I broke the surface tension like this and the water wicked well across the fabric.

I’ll listen in on the Jan 27 presentation, but is there something I’m missing about your analyses?

Thanks,

Mark

Just doing some math here.  Let’s say that the human body (at reast) puts out 100 W of energy and you are comfortable withthe cloths you are currently wearing.  Now let’s assume that you are doing an activity that generates an additional 100 W for a total of 200W  power output.  To remain comfortable, you need to dump the excess energy or you will heat up.  To do that, your body sweats with the idea that the evaporative cooling will get rid of the excess heat.

Tjhe latent heat of vaorization is 2254 J/g (Joules per gram).  So to get rid of 100W of energy, you would have to get rid of 0.044366 grams of water per second or 159.7 grmas per hour.

Now, if you have a perfectly (ideal) wicking material (with infinite fluid capacity) in contact with your skin, the water would wick away and your body would not cool down.  On the oposite end of the spectrum, in 100% humidity at near body temperature, the sweat will not evaporate and will not experience any evaporative cooling (been there, done that while living in Miami).

So evaporative cooling is required to keep the body cool.  Now the goal in cold temperatures is to control when and where the evaporative cooling takes place.  Additionally, since it is cold out, the water vapor will condense somewhere.  I have seen people on cross country skate skiing below freezing just wearing thin lycra gear. As long as they kept moving, they were fine as it is an efficient way to dump all that excess heat.  In that case, they really didn’t need to worry about where to water vapor -re-condensed.  The trick is other activities that do not require that amount of evaporative cooling.

Looking forward toi the next series of articles.

Jon, you wrote, “Now, if you have a perfectly (ideal) wicking material (with infinite fluid capacity) in contact with your skin, the water would wick away and your body would not cool down.”

Most wicking fabrics do not transport sweat away from the skin. They just spread the water out in a plane (of the fabric) perpendicular to the skin, but still in contact with it. A minority of wicking fabrics also transport liquid moisture away from the skin; this capability is called one-way transport (OWT) in the literature. There is a fancy piece of equipment that can measure OWT, using cameras and electrical currents, called a Moisture Management Tester (MMT).

But even if that is occurring –  sweat is being moved away from the skin and spread out there, rather than against the skin – won’t this still cool the body? After all, the evaporation is still occurring inside the clothing system microclimate.

“So, I have done a LOT of work on fluid transport using capillary action,  The ideal situation is to have good capillaries near the skin (say 2-3 inches of water (head)) with a thickness of say T1.  Bonded to that is a finer caplillary (say 5-6 inches of water).  The water would migrate from the course to the fine capillary.  Hopefully the finer capillary has surface area and ambient exposure to evaporate.  The thickeness of the good capillary (T1) would provide some insulation and capacitence to keep you dry and hopefully warm.  Complex stuff.”

What you’ve described is pretty much what Polartec claims for their Power Grid, although they specify three gradients: 1. A fiber gradient, that is opposite of the 2. thread gradient, as well as a 3. hydrophobicity gradient. I’ve played around a little with various Power Grids and the one that behaves most like the patent says it should is the one used for the ECWCS Level 2 tops and pants. That stuff wets very easily, then wicks very powerfully from the inner to (finely knit) outer surface, where water rapidly spreads out for evaporation.

The problem with this stuff is that wettability becomes impaired over time. The inner surface changes in texture with use, going from soft and smooth to rough and course. This might be a cause of impaired absorption. I think the hydrophilic treatment at the inner surface also probably gets worn off. The GIs are probably getting a new set of this stuff each year, so such degradation might not be a factor for them.

Jon, for winter use, is there any advantage, apart from “feeling dry” to having evaporation occur away from the skin (2-5mm away), rather than on it (sans base layer, or wearing fishnet, etc.)?

Thanks, Jon. I remember Richard Nisley writing that cooling down via sweat evaporation in sub-freezing temps was absurd and should be avoided. Why would we want to cool down when we’re traveling in air more than sixty degrees colder than we are?

Today I did brief hikes in -30F to -5F temps. About one mile each between bouts of work. For the temps between -15F and -5F I wore a thin Mammut Klammath Power Wool t-shirt (~5oz), Alpha 90 hoody and either Arcteryx Incendo or BD Alpine Start Hoodies. ECWCS L2 Power Grid hose under stretch-woven pants with warm boots. I broke trail on level ground with ~15″ crusty snow, so somewhat vigorous. Never noticed appreciable skin coolness or moisture buildup. After each hike (20-25 minutes) I stripped down. Each time there was visible/palpable moisture on the inner surface of the wind layer. In warmer temps I think this would have transported through, but today perhaps that’s where the perspiration froze. Both of these wind layers are in the top 5% of MVTR, as measured by S. Seeber. The Alpha layer was damp to the touch, a bit like Alpha comes out of the washer, ever so damp at the tips:). The base layer was pretty much dry, with little or no evidence that it had wicked or absorbed moisture. A bit of moisture palpable on the skin, mostly along the trough of the spine – base layers never reach this valley.

Overall, it seemed to me that the bulk of the moisture was on the inner surface of the wind layer, the next most in the Alpha, and the least in the base layer/on the skin. So it seemed that the majority vaporized into the outer layers. I’m not sure what to make of this, other than that I was comfortable throughout in very cold temps with insulation that I feared would be insufficient for very cold temps. I was also surprised by how much perspiration was condensed on the inner surface of the wind layer after only brief walks. Given how little insulation I had on – underdressed as they say – would this qualify as “insensible perspiration”? The volume seemed quite significant for being so insensible.

I’m away from home and don’t have a gram scale. Otherwise, I would weigh each layer after each hike.

Caffin, there was no perception of sweat, nor overheating. It was between -5F and -15F. That’s -20C to -26C. These are the same layers I wear at 30F (0C). A t-shirt, a fleece and a wind shirt. I was underdressed. If anything, I was moving too fast, but I was trying to push a bit to test this layering scheme, having just read this thread. I was testing gear. And as I said, I was not aware of overheating, nor of sweating.

But the point is that my insensible perspiration got caught in the outer and mid layers, not my base layer. This happened quite quickly, even on a brief walk in very cold conditions. That surprised me.

You might try reading a bit before veering off topic with the predictable sanctimonious reply. I ‘hike cold’ every day in winter, and need a “message” on this topic about as much as I’d like another screed on tunnel tents.

I think that the reason it is hard to manage layers in cold environments is “what is your first clue that you are sweating?”.  The obvious answer is when you can “feel” wet, but that is pretty far down the path.  The second observation is when taking a layer off, you feel the cold sweat due to evaporation, again to far down that path.  I can image that the local humidity near your skin is pretty high and in the cold, it would be hard to tell the dofference between 50%, 75% and 100% RH near your skin.  You are more likey to know that you are warm and toasty, so that may be the first sign.  However; removing layers when your in your happy place is not the most intuative process.  As I stated earlier, it may help if we were able to sense temp/humidity near your skin to provide guidancce.  Food for thought.

That is the first rule for Arctic/Antarctic travellers: DO NOT SWEAT.
That means stripping down so you are travelling cool, not hot.

That’s exactly the reason why even in chilly temps (say 40°) I always start with a minimum set of clothes (a T-shirt and perhaps also a wind shirt). Yes, the first 5 minutes of the trip I feel cold but then my body produces enough (not too much) heat that I’m comfortable.

[BRIEFLY putting on his silly-con valley hat]

We need clothing that senses moisture on the skin for us, then sends an alarm through our smartphone if it’s too high. Must search for prior art …

MIT is working on something like that:
Sensors woven into a shirt can monitor vital signs
https://news.mit.edu/2020/sensors-monitor-vital-signs-0423

A bandage with built-in moisture sensor looks promising:
Wireless Textile Moisture Sensor for Wound Care
https://www.frontiersin.org/articles/10.3389/fphy.2021.722173/full

But moisture sensors might need more power than NFC can provide. Like this one:
A Camel Nose-Inspired Highly Durable Neuromorphic Humidity Sensor with Water Source Locating Capability
https://pubs.acs.org/doi/10.1021/acsnano.1c10004

“Honey, why are you up so late?”
“Sorry, can’t go to bed until I recharge my clothes.”
“THAT’S why it smells like a camel in here!”

How long until a portable battery charger is the heaviest single piece of gear in our packs?

— Rex

Way more than you wanted to know about sweat, and measuring sweat:
https://onlinelibrary.wiley.com/doi/10.1002/advs.202103257

[QUICKLY removes silly-con valley hat] Whew, that was close!

Powered-by-movement is Version 2.0, wireless brain interface is 3.0. Minor surgery required. Incompatible with other popular brain implants. :-)

— Rex

Lifted from the article linked by Rex. All you ever wanted to know about sweat. Rex where do you find this stuff?

“Thermoregulation is the most important function of sweat loss as it maintains the normal physiological activities of the human body. Under different thermal conditions, the human body with normal thermoregulation always maintains the core temperature within a narrow range of 36–38°C at rest and up to 41°C during heavy activities.[89]The heat loss caused by the evaporation of the two sweat types plays an indispensable role in thermoregulation.Generally, the control of sweating for thermoregulation is referred to as the integration of core and skin temperatures (mean body temperature). Under very mild conditions (e.g., resting,sleeping, and low-intensity exercise), the dry human body mainly relies on radiation and only needs to evaporate nearly constant insensible sweat to maintain the body temperature. As the thermal load increases and exceeds the onset threshold, the body begins secreting sensible sweat from the sweat gland to the skin surface, which evaporates for heat dissipation, because the latent heat of evaporation of insensible sweat is insufficient to regulate temperature. During this process, the relationship between change in mean body temperature and sweat rate can be briefly described in Figure 5. Thermal challenges that can induce the thermoregulation response of sensible sweat include high-intensity exercise and sport, passive heat exposure, and intake of hot and spicy food. Additional details on the physiological mechanism of thermal sweating can be found in other specialized reviews. The secretion rate of sensible sweat increases linearly with an increase in body temperature for heat balance. In this case, effective cooling of the body is mostly achieved through the latent heat of water vaporization from sensible sweat. In terms of the health role of sweat loss in thermoregulation, heat balance at a low core and skin temperature is partly attributed to the evaporation of insensible sweat, whereas heat balance at a high core and skin temperature is mostly attributed to the secretion and evaporation of sensible sweat. Although the secretion and evaporation of sensible sweat play a vital role in proper thermoregulation, excessive sensible sweat loss can in-crease the risk of dehydration and electrolyte disorder.This condition is common in athletes, military personnel, and industrial workers whose sweat loss for thermoregulation exceeds fluid intake during prolonged heat stress.In athletes, a low level of dehydration begins to compromise physiological conditions and reduce sports performance.”

So seems like the body needs to thermo-regulate to prevent over-heating from high levels of exertion. Problem is that in cold, windy conditions the phase change/cooling effect of evaporative cooling might cool the body off too much too fast so sorta need to regulate the evaporative process. OTOH being wet from sweat is uncomfortable too plus that phase change /evaporation is gonna happen sometime and with it the big chill so….?  Sounds like Stumphges has a working system.

^^ “sensible versus insensible sweat also depends a lot on local humidity.”

I’m with you there but kind of in reverse. Growing up and spending most of my life in the Miami like conditions Jon refers to I’m used to sweat and basically for @ 5 months of the year it just means you are outside and not in a hammock in the shade. OTOH On my trips and in the time periods I lived out west it seems that baring running or a really sustained steep climb I almost never get dripping wet with sweat but I’m losing water by the pint or quart so I have to constantly remind myself to water up. Also think you’re on to something about losing moisture just from breathing.

It seems to me that Woubeir and Roger and Stumphges and Jon are all saying sort of the same thing. To avoid a dangerous core cooling situation leading to hypothermia it’s important to try to keep your body below the edge of the temperature that results in copious sensible sweat and also layering is a series of tactics to disperse sensible sweat when it does occur and manage the pace by which it is evaporated or phase changes into vapor so there isn’t a drastic and rapid evaporative cooling event that might really lower your core into hypothermia range. Gotta maybe be a little uncomfortable to stay comfortable?

But the other big problem is getting wet and staying wet in a wet/cool, maybe windy environment where getting back to dry is difficult. Danger Will Robinson, danger!

Karen’s mention of -25 caught my attention. It seems to me to be a major transition point like freezing or zero. I guess that’s covered by Roger’s Antarctic reference. Slow down, no sweating allowed, and keep skin exposure to an absolute minimum.