There are two sleeping bag classifications: camping and backpacking. The primary difference is the temperatures they are
intended for. Camping bags do not have temperature ratings
while backpacking bags use the En system. More on that later.
When I anticipate an evening low of less than 75° but above
55°, I pull out one of my favorite sleeping bags, an REI Travel
Sack (far left in the photo). The travel sack is not rated, but
60° seems to be its limit for keeping me warm (I’m grateful
for my low body fat percentage, but there are times when a
little extra would come in handy). The current model is not
as rectangular and has a larger footbox, theoretically making
it a bit warmer. You can still unzip the bottom and arm openings to loosen your appendages. Not only that, but you can
pull the bottom up and suspend it with a belt to walk around
in it, too—funky, yes, but practical. By strategically donning
my long-sleeve underwear I have a greater thermal range.
It’s quite roomy and this is my go-to bag for use in a tent on
modestly chilly nights.
I don’t stop camping when the summer ends and neither
should anyone else. When hiking with the right gear, I actually
prefer autumn and welcome near-freezing conditions (cold is
workable, ice can be a pain). The difference in body temperature
after sitting on a bike with cold 65 mph winds for four hours
versus climbing mountains with a 30-lb. pack on your back is
profound. I typically have my jacket off during the latter.
The human body loses heat via four mechanisms: Conduction;
energy transfer (hot to cold) from adjacent materials. Grab a hot
plate or piece of ice and you’ve got the idea. Convection; fluids
(air or water) will switch places when at different temperatures
(e.g. hot air rises and leaves cooler air in its place, for which an
adjacent warmer material will lose heat.) Radiation; the infrared
emission from our bodies, particularly on cold clear nights. A
hot fireplace from a few feet away is a great example of IR gain,
but we also lose heat this way, too. Finally, there’s evaporation;
the process of your sweat evaporating to a vapor, absorbing your
body’s heat. Interestingly, with passive, or insensible perspiration, you’re not even aware of it.
Well, there are five mechanisms, really. There’s one more
that’s sort of a combo of two of the above—respiration. Just
think of breathing as forced convection of the humidity in your
lungs. Anyone who has taken a deep breath at 20° below can
emphatically explain it to you.
Now let’s consider colder temperatures and backpacking
sleeping bags. There was once a time that pundits and salespeople
would advise you to pick a sleeping bag rated 10° below the lowest temperature you anticipate. Gone are the days when designers
and testers would spend the night in their local supermarket
freezer. Now bags are scientifically tested and rated using the
European Norm standard (EN13537). This testing is voluntary,
but I wouldn’t buy a bag from a manufacturer who wouldn’t test
its offerings. But why buy more bag than you’ll need and have
to climb out of when you’re too hot?
You’ll likely find at least two ratings and often three: a nominal comfort rating, a comfort rating limit for women and lower
extreme limit for men. There’s a reason why she’s taking the
covers at night. To maintain her body temperature during sleep,
she may need as much as 15° more insulation than you do. You
might also see an upper limit rating and an extreme rating. The
upper limit is for a standard man (healthy 25-year-old) that won’t
perspire excessively and the extreme is for similar women who
would survive, even with frostbite, for six hours.
Bags are filled with either down (bird feathers) or synthetic
material, or both. Taking it down (ahem) to the basic physics, the
smaller the air pockets in the insulation (that convection thing
again), the better. Air has extremely low thermal conductivity if
it doesn’t move by convection. If weight and packing size aren’t
an issue, synthetic is the way to go. It’s less expensive and still
insulates when wet.
Pound-for-pound, down—particularly the type that can create
a lot of loft (Goose is better than Duck)—is a better insulator.
And with recent water-repellant (not waterproof) coating technologies, the new down fill doesn’t lose as much insulative
utility when it gets wet. Another bonus is that it increases its
fill power rating.
Backpacking bags are rated by the En (European Norm) Standard.
Above is the tag on the Lamina 35. Camping bags don’t typically have
these temperature ratings.
Heat loss snapshot made with an infrared camera: The camper’s
head is losing the greatest amount of heat, from conduction, convection, radiation and also from breathing (evaporation and respiration).
The body, although insulated in a sleeping bag, is also losing heat
from conduction, convection and radiation.