Generating Body Heat | Staying Warm to Survive

Outside, the wind blows and temperatures drop to near freezing. Suddenly the lights go out and the heater shuts off. Power has gone out in the neighborhood. How long will it take to come back on? The cold outside is already seeping through the windows, cooling the interior of your home…

Emergency Heating | Keeping Warm After the Power Goes Out

Until power is restored and the furnace can restart, you'll have to find a way to keep warm. Getting too cold can be dangerous and even deadly. You must keep your core temperature above 95°F (35°C), the point where hypothermia sets in and you will begin to physically suffer. Shivering and mental confusion occur first. If you're exposed to cold temperatures for too long, there’s a risk your heart will stop. You must act immediately to stay warm and survive.

When dealing with cold temperatures, you have to options—put on additional warm clothing or find a way to heat the room. Warming the body is easier and more efficient than warming the space. It can take a long time to heat the air in a room. It can take just seconds to heat the body, and space heating consumes a lot of energy. Here’s what you can do.

Generate Body Heat

Immediately put on a sweater. Layer up with more clothes. Put on a wool ski cap. Wrap a wool scarf around your neck. Put on a jacket and extra pair of slacks or jeans. Pull on a second pair of socks. Get out those gloves. Then look for your thermal underwear—you do have some—right?

When the temperature drops putting on more clothes helps—and a reduction of just 2°F (1°C) can reduce energy costs by 9 or 10%. And with no electrical power, and no consumption, the result is maximum energy savings. Just as long as you remain comfortable—comfort means 70°F (21°C)—even when the temperature of the skin is 92°F (33°C). Most loss of body heat occurs through the skin. The amount of insulation between the skin and the room atmosphere will determine the level of comfort. Clothing provides this insulation so lower room temperatures don’t sacrifice comfort.

Clo

Using clothing to better insulate the body can be evaluated using the measurement clo for “clothing.” It describes the thermal property of clothing measured at less than 50% humidity and in stagnant air. One clo defines the insulation required to keep a sitting person comfortable at 70°F (21°C). It corresponds to 0.155 square meters kelvin per watt. A clo of insulation is equal to wearing a three-piece business suit (light underclothes, shirt, trousers, and suit jacket).

In Europe clothing are often called “togs” so Europe adopted the “tog” measurement. A tog equals 0.645 clo.

In the U.S. the ability of a material to resist heat transfer through the material is called R-value. It’s a measure of thermal resistance. The higher the R-value selected, the better the insulating capability of the material. Stated in terms of clo, 1 clo is equal to 0.88 R, or an R-value of 1 is equivalent to 1.137 clo.

Clothing with a clo value of 2.7 will provide a neutral thermal feeling at 50°F (10°C). If the temperature drops to 32°F (0°C), 4 clo of insulation will be required to maintain the same sensation. Table 1 shows clo values for various articles of clothing. Adding the clo values for each item provides the insulation properties of a complete ensemble.

Using clo to measure heat (click to enlarge)

A person wearing underwear briefs (0.05), light socks (0.05), a short-sleeved T-shirt (0.10), long trousers (0.30), a heavy long sleeve shirt (0.25), and a medium sweater (0.30) has 1.00 clo of thermal protection. This person will feel comfortable sitting on a sofa watching a ball game at a temperature of 70°F (21°C). The complete ensemble serves as a heating system for human comfort.

Adding layers of clothing adds 1.6 clo for every 2 centimeters of thickness. Put another way, wearing 3 kg (6.6 pounds) of clothing provides 1.0 clo of thermal insulation—our reference for comfort.

Generating Body Heat With Thermals

Some clothing has been designed for specific levels of thermal insulation. Lightweight thermal underwear and outer wear considerably improve the warmth/weight ratio of clothing. A single layer of thermal long underwear allows the wearer to feel the same at 39.2°F (4°C) lower room temperature. Two layers of thermal underwear—top and bottom—more than doubles the clo value from 0.6 to 1.5 providing thermal comfort at 50°F (10°C).

Synthetic materials such as acrylic and polyester are 2.5 to 8 times better in the warmth/weight ratio than woven or knitted fabrics such as cotton or wool and are therefore better insulators. And quilt battings offer 13 to 17 times better warmth/weight ratios than cotton or wool. Perhaps this is why our ancestors created quilted blankets for use during cold winters in the Upper Midwest. A quilt is just warmer than several blankets.

Cotton long underwear adds 0.4 clo to thermal comfort. Use wool and the clo value is 1.0 easily allowing a reduction of indoor temperature of over 42°F (6°C).

Metabolic Effect

Human activity that increases body heat is the most significant factor influencing thermal comfort. A resting person may require 12 clo to feel warm at -40°C but only 4.0 clo if they’re walking. A 2.7 clo insulation factor at 50°F (10°C) can be lowered to 1.7 clo if the person is talking, typing, or actively painting. And if they’re jogging their body may only need 1.25 clo of clothing insulation.

Heat production can be measured in watts. Increasing metabolic heat production by 30 watts allows the temperature of comfort to reduce about 35°F (1.7°C). A sleeping person needs 2 clo of insulation at 68°F (20°C). Most sleeping bags insulate to more than 10 clo because of this fact.

This invites an interesting question, “What clo values could be used to survive comfortably and indefinitely without heating your home?”

In my next article we'll take a look at interior heating techniques.

In the meantime, take a look at these products that can help you retain body heat in an emergency:

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