Packbackers of the wood, Lend me your ears (eyes too).
Through many of my pursuits in life, I've always been one for trying to understand why and how things work or function. And when I became enthralled with hammock camping 4 years ago I have always tried to understand how to stay warm in the winter. I would like to share with you, knowledge I have gained through the many trips I have taken and interesting things I have learned in keeping myself warm. Let's first start off by saying that I'm sort of a math nerd and Love deriving equations that best represent a particular data set. So you may be challenged to use some math for these calculations. Let's jump right into it!
STAYING WARM!
As we know, our bodies produce heat as a form of exhaust to dissipate the heat we build from burning food (fuel) through exertion. Therefore different bodies put off different amounts of heat and also require different amounts of energy (food) to function properly. To find out how many calories your body expends while resting we use the Basal Metabolic Rate (BMR) equation to solve this,we will compare this quantity to a later calculated figure to check our work.
BMR Equation:
BMR= [(12.7*height in inches)+(6.23*weight in lbs)-(Age*6.23)]+66
E.g. I am 6'2"(74"), I weigh 190 lbs, and I'm a 23 year old male, my BMR is 2046 Kcal per day. My body in turn needs about 85 Kcal/hr while resting, take 2046 Kcal / 24 hours=85 Kcals/hr.
Something to note is that the calorie count that food is labeled with is actual a Kilo calorie which is 1000 calories, so a bar that says it has 200 calories is meant as 200 Kilo calories.
The next item to discuss is Body Surface Area (BSA):
Your body puts out some amount of heat per square unit of area of your body at some set activity level. The equation for BSA is the square root of your ((height in inches* your weight in pounds) divided by 3125), first take your height times your weight then divide by 3125, now take the square root of that number to get your BSA. This answer is in Meters squared which will be helpful in the next calculation.
Activity Level:
The level at which you perform activities is called a MET (Metabolic Rate Factor). For instance sitting still at a desk or a couch your are performing 1 MET of activity. If you are sleeping you are performing 0.8 MET, the average hiker exerts 6 MET while hiking (sometimes more, sometimes less due to pack weight and terrain).
1 MET can also be defined as 58 Watts/Meter^2, which represents the amount of "heat" your body generates per meter of body area.
Useful Conversions:
1 MET= 58 Watts/Meter^2
1 Watt= 3.41 btu/hr
1 btu/hr=252.16 calories/hr
1000 calories= 1 Kilo Calorie(Food Calories)
Bringing it all together:
Let's use my body stats for an example
I am 6'2" (74")
I weigh 190 lbs
I am sleeping and performing 0.8 MET (58 Watts/Meter^2 *0.8 MET= 46.4 Watts/Meter^2)
1st. Lets calculate my BSA... sq.root of((74"*190lbs)/3125)=2.12 Meters^2 of surface area
2nd. If I am performing 0.8 MET, then i am putting off 46.4 Watts/ Meter^2 of surface area. Well my body has 2.12 Meters^2 of surface area and therefore is putting out about 98 Watts of "heat". (46.4 W/M^2 * 2.12M^2= 98.4 Watts.)
3rd. Convert Watts to btu/hr... If 1 watt=3.41 btu/hr, then 98.4 Watts=335.5 btu/hr.
4th. Convert btu/hr to calories/hr. If 1 btu/hr=252.16 calories/hr, then 335.5 btu/hr=84,610.8 calories/hr.
5th. Since there are 1000 calories in a Kilo Calorie(Food Calories), then 84,610.8 calories=84.6 Kilo Calories(Food Calories).
What does this tell us?
While I sleep, I am burning about 85 Kilo Calories per hour (compare that to our BMR calculation from earlier, it's spot on), so it's reasonable to say that if we wanted to sleep for 7 hours and maintain a constant "heat" output, then I would need to take in about 595 Kilo Calories before going to bed. So that's 85 Kilo Calories/hr * 7 hours=595 Kilo Calories. Here's the catch though, imagine your stomach as a gas tank and if the needle is in the middle, then you are comfortable, if it is full then you have extra energy to burn, if it is empty then you are starving. If you get to camp, set up your gear and get ready for bed, and just before bed you are starving and say, " hey I'll eat 595 calories to get 7 hours of sleep and I'll stay warm" your making a mistake. Your gas tank is empty and so 7 hours from when you eat you will wake up with the same hunger that you had just before you ate and went to bed but you'll be colder because you aren't up moving around burning calories and creating "heat" like before. So eat a little extra so that when you get up and get moving around you're not starving while you make your breakfast. Well, that pretty much covers the warmth requirement for inside your body, how bout externally?
CLO's, R-Value's and bears OH MY!
No one wants to be left out in the cold wishing they had packed more gear to stay warm, so let's see if I can't help shed some light on a popular topic I've been seeing around, Insulation.
Whether it be a top quilt, under quilt, sleeping bag, or bivy, they all adhere to the same principles of thermodynamics. They keep you warm by trapping and slowing down the dissipation of heat that exits the body by utilizing their loft. Generally, the thicker a sleeping bag is, the warmer it will be, and yes I know new materials allow less loft with the same warmth, but the principle still applies. Up until the early 1900's air conditioning didn't exist, many clothes were tailored with varying thicknesses, and the Industrial Revolution was just around the corner. People who wore suits in office buildings, used to sweat their butts off while working in the summer, as you can imagine. When air conditioning was implemented a new standard of living came along with it, inside temperatures in the summer were in the 70's and so clothing design came to the 20th century with a big leap. They said a man sitting at his desk writing is performing 1 MET, and so therefore we must design a suit that will keep him thermally equal and we will call that amount of clothing worn 1 CLO (clothing coefficient). Enough with the history lesson, how does this help us? Well, we can break down some of these equations they wrote and determine what amount of clothing should be worn for a specific temperature. I have converted and transposed one of these equations into units we can understand just for you guy's and I'll explain it below.
Comfort Temperature(°F)= 87.8-0.255*( Watts/Meter^2 * CLO)
let's do an example, ( I love examples):
I am planning a trip on the Ozark Trail and the low for trip was going to be 25°F, how much clothing insulation do I need to pack to sleep Warm?
Well, substitute 25° in for Comfort Temperature, Watts/Meter^2= 46 because, remember while sleeping you are putting off 46 Watts/Meter^2, and solve for CLO. You will get 5.4 CLO. Well, I can pack my KAQ New River and Owyhee quilts which I have calculated with 3 separate tests to be an conservative estimate of 4.7 CLO. So I need 5.4 CLO and my quilts account for 4.7 CLO, therefore I only need to wear 0.7 CLO to bed to be warm. Well, what makes up 0.7 CLO you may ask? Well, a pair of long johns top and bottom have about 0.2 CLO per piece, I wear a thin pair of polyester socks and a thick pair of wool socks that accounts for 0.08 CLO together, boxers account for 0.04 CLO, I wear a fleece balaclava and a pair of wool mittens that account for 0.2 CLO together. Covering your head is an important factor, because your brain consumes a lot of your required energy, it puts off a lot of heat so keep it covered to stay warm. I have stayed many nights at 30°F or colder and wore the right amount of clothing and stayed toasty warm all night long. The issue becomes with estimating CLO values for your clothes and Bags, right? Well, that's because the majority of CLO values for clothes are from the 40's and 50's and advances in materials allows some pretty incredible CLO to be had, but not many company's are willing to fork over money for testing...which is only like $600 for basic CLO value with the copper manikin test which gives us ratings like the EN system. So, if you're curious about your CLO values you can fork over the money for the tests and shipping, or you can use these general guidelines to help you with getting started.
For articles of clothing, a rule of thumb is that 1 pound of clothing=0.155 CLO, this only applies to single layer fabrics like non insulated jeans, socks, underwear, thermals. It has been used in the past and been declared a rough estimate at best, but hey it's something!
Another rule of thumb for estimating CLO is that 1 inch of fabric thickness= 4 CLO. This is measured from your skin to the outside of your coat or sleeping bag, DON'T measure the total thickness from the back of the coat to the front of your coat, that would be wrong and if you do, use the rule of thumb 1 inch= 2 CLO.
Another good rule of thumb is that 1 CLO = 0.88 R-Value, this has been used quite a bit and seems to be acknowledged as "good enough" for rough estimates.
Now a few things to consider, this is all fine and dandy as long as there is no wind and humidity is around 50%. It is very important that you have some way to block the wind as with a tarp, soft shell, wind jacket, just something to stop the wind from carrying away your heat. Humidity plays a big role in temperature as well. As Loaf has stated, in the winter the higher the humidity the cooler it feels, the lower the humidity the warmer it feels. Another concept to take in is that compounding insulating layers also compounds CLO. An example would be wearing a puffy jacket in your sleeping bag. Not only are the jacket and Sleeping bag working independently to slow the heat transfer but now we are integrating air layers between the two to insulate even more. Similar to a wet suit, water gets into the suit between the neoprene and the skin allowing the body to heat a few millimeters of water to help hold in the heat instead of losing all the heat to open water.
Well, I hope that you take from this some form of information that will help you be better prepared in your travels. As with any anything, take this with a grain of salt, it's not the Gospel, it's merely a guideline to help you guy's get out there and do some real world testing of your own, just like I did. As always, be safe,have fun, and keep it hangin'!
Dill Pickle
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hang easy!
Originally Posted by Dill Pickle
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