[pct-l] Sleeping Bag Ratings

[Eric Lee (GAMES)]

Patti wrote:
>
actually the more layers u wear the colder u are from what I was told b/c the bag uses ur body heat to warm up. So if ur wearing more layers then it's that much more work for the heat to get to ur bag...just what I was told by several bag reps that's all
>

I'm pretty sure that's nonsense, no offense.  The idea here is to keep the heat from leaving your body.  It doesn't matter what kind of insulation you use to accomplish that, or what temperature the bag itself is.  The more clothes you wear, the more total insulation you have and the less heat escapes.

Now, there are a few important caveats that I suspect people get mixed up with the main idea, resulting in the advice you got from the bag reps.

The first is that you don't want to wear really tight clothes, or so many layers of clothes that they become tight.  If you do, that will restrict blood flow to parts of your body and those parts will end up feeling cold.

Second, you don't want to wear damp clothes to bed, because as the moisture evaporates off of them it takes your heat with it.  If all you have are wet clothes, you may be better off wearing no clothes at all.  But dry clothes are always better than no clothes.

The third is that you don't want you plus your clothes to be too big for the bag, which compresses the bag's insulation.  Sometimes people think that it's the actual feathers or synthetic fibers themselves that trap heat, but it's actually the still air trapped inside the insulation that traps the heat.  If you compress the insulation, there's less trapped air and so it works less effectively.

Fourth, you don't want a bag that's too big for your body, not because it takes more work to "heat up" a big bag, but because a big bag has more surface area.  Assuming your bag is relatively uniform, each square inch of your bag will leak a certain amount of heat based on the relative difference in temperature between the inside and the outside.  If you add more square inches of surface area, that's more total heat that's leaking out.  If you reduce the surface area, that's less heat that's leaking out.  At least, until you hit the third caveat above.

Eric