[pct-l] Re: Gear Tradeoff -- Tents versus Sleeping Bags

[reynolds@iLAN.com (Reynolds, WT)]

Brett-

I think we agree on what to take on a hike. You chose Ron's tent for one and
I chose Brawney's tent for two for probably the same reason. Weight. The
single-wall sil-nylon tents are the lightest shelter available that provides
some weather and bug protection. The downside is that they must be open,
like a tarp or horrendus condensation will result.

On to Physics.
Condensation when warm moist air encounters a cold surface. The air falls
below its dew point and condensation occures. A tarp avoids condensation due
to air exchange. The logic is that more air passes over the sleeping
bag/person removing the moisture without significent increase in air water
content or air temperature. In a perfect world a tarp does not affect air
flow at all. The heat/moisture exchange between bag/sleeper is identical to
a person sleeping out.

The Stephenson works by allowing the inner wall of the tent to attain the
same temperature as the inside of the tent. In a perfect world this means
ZERO condensation because there is no cold wall to condense the water out of
the air. The warm, moist air is vented out of the tent where it escapes to
the atmosphere.

In a typical double wall tent the warm-moist air passes through the inner
wall and condenses on the outer wall, the rain fly. That is why a standard
tent APPEARS to have no condensation. The inner wall raises to the
temperature of the inside air. The rain fly is cold and condensation
occures. However, standard tents rely on air movement between the inner wall
and the fly to remove the condensation. Obviously there is a tradeoff
because this air movement also cools the inner wall resulting in
condensation.

With the Stephenson, however, there is no air movement through the inner
wall and, in theory, no air movement between the inner and outer wall. In a
perfect world the air between the inner and outer walls acts as an insulator
the same as a storm window. Of course, the world is not perfect. The ends of
the Stephenson tent are single wall and their is much condensation on them.

Still the physics of the Stephenson tent is that the warm, moist air is
vented out of the tent without condensing for the most part. This air is
10-15 degrees warmer than the air in a standard tent. The physics of a
standard tent is that air passes through the inner wall that is somewhat
colder than the air so some condensation results and condenses on the inside
of the rain fly. The physics of a single wall tent is that air will condense
on the tent if it is closed off. To the extent it acts like a tarp the moist
air will be swept away via air movement without condensing.

On to RAIN
The secret of both tent and tarp is to keep the ground cloth/tent floor dry.
In both cases one does this by proper placing of the tent and trenching
[using a trowel to build a trench around the tent/ground cloth to direct
water away from it.] if necessary. While it is true that the bathtub design
of some tent floors will resist ground water to some extent, nobody should
count on this in a serious storm.

The problem with the tarp, obviously, is when one is required to close it
down to stop rain from the outside thus creating, in effect, a single wall
tent.

The bottom line
In a TARP in a COLD three day storm: You sit under the tarp in 25-35 degree
weather. You will be subject to the wind blowing causing heat loss. You
certainly won't freeze to death but it won't be fun. Generally, during the
day, you will have to choose between stuffing the sleeping bag to protect it
from the elements or using it to keep warm. Most people won't sit. They'll
walk.

In a Stephenson tent in a COLD three day storm: You will sit in 40-50 degree
still air. During the day you will be able to stuff the sleeping bag to make
a pillow. At most you will need a jacket to stay warm.

Fortunately COLD three-day storms are rare

-----Original Message-----
From: Brett Tucker [mailto:blisterfree@earthlink.net]
Sent: Thursday, September 26, 2002 12:02 PM
To: Pacific Crest Trail
Cc: reynolds@iLAN.com
Subject: [pct-l] Re: Gear Tradeoff -- Tents versus Sleeping Bags


Well, Tom. Looks like Ron Moak is back, and I like what he's been saying
lately. So I'll just have to defer your offer onto him. :)

Actually, I don't know that I agree with the physics you're employing with
regard to moisture and temperature within the shelter and bag
micro-environment. If the shelter micro is high humidity, then the bag micro
is gonna be likewise. What we need is a temperature gradient, in order to
drive moisture away from us before it condenses into heat-sapping liquid.
That gradient is likely to be higher under a tarp than inside a tent,
because the tarp user is not able to heat the shelter's micro-environment as
efficiently. So, bag warm + shelter cool = moisture transfer away from
person and out through bag. At least, this is the physics as I understand
it, mostly from what I've learned about waterproof-breathable fabrics. These
work best in cold humid weather, as opposed to warm humid weather, again
because of the temperature gradient idea. That said, if the temps will be
below freezing for extended periods, then both tarp and tent users might be
inclined to use a VB to avoid the frozen-insulation syndrome.

As for the idea of a quilt ventilating better than a bag, not necessarily.
The quilt's (or a bag used like a quilt) main advantage is its weight
savings, due to its elimination of a bottom layer. To perform correctly, a
quilt still must cover the person, and this only permits about the same
degree of ventilation that we would find in a fully-enclosed bag. A more
influential factor might be the amount of insulation in the quilt or bag,
the shell material's porosity, and whether or not it employs a DWR or
membrane of some sort. All of these things, as you know, affect
breathability, and sometimes they can cause more harm than good. Like, I
wouldn't use a down bag that had a W/B covering, unless I were also using a
VB, since otherwise the bag might became a condensation sandwich. Ideally, I
want the most breathability I can get from a quilt or bag, while at the same
time being able to keep its exterior free of weather and external
condensation. Again, a properly-pitched tarp often handles the job
wonderfully - IT is the "DWR."

I am willing to bet the Stephenson is a very good tent in terms of
ventilation. Right now, I can't confirm what you're saying, although someday
I hope to. But in 3-season conditions, a tarp vs. Stephenson's best would be
an interesting challenge indeed. In the dead of winter, forget it. Give me a
tent. Tarps do have their limitations in very cold weather, or when it's
snowing with spindrift, or the wind is blowing at whole gales. Not that they
suffer structurally under these conditions. Again, it comes down to the
performance of the micro-environment.

Sometime I'll come up with a list of various weather scenarios, and explain
which shelter I would choose for each. Often you would see the tarp listed.
But a tent would be on the list, too. And not only in winter. Actually, I'll
be leaving for the Appalachian Trail in a few days, and what kind of shelter
am I taking? Why, one of Fallingwater's specials - a silnylon tent. I'll
tell you why when I get back. :)

- blisterfree


From: "Reynolds, WT" <reynolds@iLAN.com>
To: "'Brett Tucker'" <blisterfree@earthlink.net>,
   Pacific Crest Trail
<pct-l@mailman.backcountry.net>
Cc: Bighummel@aol.com
Subject: RE: [pct-l] Re: Gear Tradeoff -- Tents versus Sleeping Bags
Date: Wed, 25 Sep 2002 14:17:15 -0700

Well, Brett I challenge.

I say I can ride out cold, moisture and any combination thereof, in my
Stephenson than you can in any tarp, no matter how well pitched. In fact I
say that there is NO comparison

You see anyone doing high peak assaults using tarps? No ! But Stephensons?
yep!

For typical PCT cold and wet storm conditions I say that I will be able to
use my bag like a quilt when you are trussed up like a mummy because the
inside of the tent is 10-15 degrees warmer [and, yes, higher humidity] so my
body moisture will vent directly to the air not thru the bag. Finally, I say
that the venting system deployed by Jack Stephenson is superior to a tarp.
Jack, in fact, advises people to NOT OPEN the windows because it will
disturb the carefully designed air flow characteristics.

That being said, I experienced one of those cold pockets while sleeping out.
The temp hit 24. My Feathered Friends Rock Wren II was warm enough, but just
so. It was a wet cold-very uncomfortable.

Tom