 
Bivy sack is common English now for
"bivouac sack." This is the most basic shelter you can use for
backpacking. Essentially a waterproof bag you sleep in, they
were originally used by soldiers and climbers who had to spend
the night in unexpected places without a tent.
Take a look in a store, a catalog or online, and you will see
that a good bivy sack can be both claustrophobic and expensive.
The solution for the first problem is simply to get used to it,
or use some other type of shelter. The second problem I have a
better solution for.
A Cheap Lightweight Bivy Sack
If your not wanting to spend $200 for a nice bivy, This was basically a "high tech"
plastic bag. On a rainy night in when camping, try out the bivy
sack and see if it works.
This got me thinking. When I later lost the bivy, I decided not
to pay $20 (or $200) to replace it. Instead, duct-taped two
extra large garbage bags together. Cut open one end, and I had
a three-foot by seven-foot bivy sack.
It weighed four ounces - lighter than anything commercially
available. Even my "emergency bivy" had weighed almost twice as
much. Of course the bivy sacks you buy are tougher, but so what!
I use my four-ounce sack as disposables. They cost less than a
dollar each, so it doesn't hurt to throw them away at the end of
a trip.
In fact, they are good for a week-long backpacking trip if you
are
careful. The small holes that may wear through in the bottom
won't allow moisture in unless you are laying down in standing
water. There's your lesson on making an ultra light bivy sack.
Four ounces, and it will fit in your pocket. (This isn't my only
disposable lightweight backpacking gear by the way.)
A bivy sack is best for in dry environments.
Whether you make one or buy one, the problem with bivy sacks isn't
getting wet from the rain. What will get you wet will be the
condensation on the inside of the sack. Even those that claim to
be waterproof and breathable can't handle much moisture.
One key to using bivy sacks, is to bring as little moisture into
them as you can. If you are sweaty, wait until you cool off
before entering the sack. If you have damp clothes, leave them
outside the sack. Put them in a plastic bag to keep them from
getting wetter if rain is coming.
Don't breath inside a bivy sack. This is the fastest way to make
the inside wet and uncomfortable. Some bivy sacks have a
screened pop-up canopy that will also keep the rain off your
head. Otherwise, you have to carry a small umbrella, use a piece
of plastic over your head, put your head under a ledge or tree,
or just let it get wet.
Check inside the sack every morning. If there is moisture, turn
it inside-out to dry during a break at some point in the day.
Some bivy sacks can be used with either side down. If yours is this way,
be sure to always use it with the same side down. Small holes
will likely wear through the fabric in time. These will allow
rain in if they are on top, but usually won't let much moisture
seep in from the ground - so keep the damage to one surface of
the bivy sack.
You will sometimes get your clothes and sleeping bag damp from
sleeping in a bivy sack. If it isn't raining or snowing, you'll
usually dry out in a quickly once you start hiking. You should
also get in the habit of taking a break to lay your sleeping bag
in the sun to dry out any dampness. I recommend against using a
bivy sack with a down sleeping bag (they lose more insulating
ability when damp).
Why Use A Bivy Sack?
The biggest advantage of a bivy sack may not be how light they
are. In fact, most of the ones you'll see in outdoor catalogs
weigh more than my tarp/groundsheet combination. They are a
light shelter, but a good bivy sack has one other big advantage:
It is simple. Unroll it and climb inside - you can't have a
faster set-up than that.
Key Points
1. Bivy sacks are expensive, but there are cheaper alternatives,
like making one.
2. A bivy is the simplest, and one of the lightest backpacking
shelters.
3. Moisture build-up is a real problem if you don't properly use
a bivy sack.
Pocket lint makes a good tinder for fire-starting. By the way, this is
why so many fires start in peoples clothes dryers. A spark will
usually take hold in lint and can be blown into a flame .
It's nice to have a cold drink when hiking in hot weather. One
way to do this, at least for the first day of the hike, is to
freeze your water bottle or sports drink before you go. It
should take most of the day to thaw if you keep it wrapped in
clothing in your pack. It is best to have another unfrozen water
bottles too. I have had to sit there thirsty more than once
while staring at my frozen water, waiting for it to thaw.
Pack your rain gear where you can easily get at it. You don't
want to be digging through the backpack in the rain.
Try and keep wood matches in old film containers to keep them
dry. You can but water proof matches and keep them in your
pocket incase of a emergency. Use large rocks around your fire
they will hold in the heat longer.
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