According to A. C. Burton’s Man in a Cold Environment, published in 1955, a sleeping person requires approximately the thickness of insulation shown in the following table for a good night’s sleep. This thickness is measured from the skin to the outer surface of the insulation.

Even though this chart was published over 60 years ago, we feel that based on our experience with down insulated gear that this chart is fairly accurate. However, you will note that the table does not say that the insulation has to be down, polyester, wool or some other insulation material. Why is that so? The guy at the outdoor store told you that the 800 fill power down jacket that was a quarter of an inch thick was the warmest thing around and that this was due to the fact that the 800 fill power down was the best insulator around.

Understanding heat loss will allow you to understand what to look for in an insulated article, be it a coat or a sleeping bag.

Heat loss per unit time by conduction through an insulated wall is described by the equation

 

Q/t = -kA(T_H - T_L)/(X₂ - X₁)

 

Where:

• Q/t is the heat flow per unit time
• k is the coefficient of thermal conductivity of the insulating material
• A is the area through which the heat flow passes
• T_H - T_L is the temperature difference between the inner and outer surfaces of the insulated wall
• X₂ – X₁ is the thickness of the insulating wall

So minimum heat loss is achieved by making Q/t as small as possible. To do this you want to make all the quantities in the numerator to the right of the equals sign as small as possible and the quantity in the denominator as large as possible. In the numerator the quantity (T_H - T_L) is simply the difference between the temperature inside the bag (using a sleeping bag for demonstration purposes) and the temperature outside the bag. This temperature difference will be fixed for our discussion as will the A which is the area through which the heat is flowing which would be the area of the surface of the bag. Since these quantities are fixed the only quantity left in the numerator is k, which is the coefficient of thermal conductivity of the insulating material. The table below presents some values for k for several common insulating materials.

Consider these values to be approximate since they vary slightly depending on the reference. Obviously goose down is the best, since k for goose down has the lowest value of those listed. Also, though not shown here, k for 550 goose down is approximately the same as k for 800 fill goose down. Therefore, for the same thickness of insulation, that is X₂ - X₁, the heat loss is the same. So from a insulation standpoint 3 inches of 550 goose down will keep you as warm as 3 inches of 800 fill power goose down. What you are getting with the 800 fill power down is that it takes less down by weight to give you the same thickness, since fill power is defined as the volume that one ounce of down will fill. This is measured according to the specification for down filling power. So given that your bag is filled with goose down, warmth just becomes a function of the thickness of the bag, thickness being warmth. This obviously also means the puffy, wide tube down jacket is much, much warmer than the thin, narrow tube down jacket regardless of what fill power goose down is used.

From the table above, since k for polyester is, on the average, approximately 1.68 times k for down, to have the same insulating value (heat loss) the polyester bag will have to be 1.68 times as thick as the down bag to have the equivalent insulating capability. Synthetic insulations are constantly improving.

We hope that this discussion clears up much of the misinformation that is floating about today regarding insulating materials and their capabilities.