[pct-l] Tent vs Bivy Sack

[CHUCK CHELIN]

Good morning, Tim,

Shelter and warmth should not be confused.  Whether hiking on a cold day or
sleeping through a cold night it is necessary to retain body heat to some
extent:  That’s the job of garments and sleeping bags.  The outer layer of
each must be sufficiently tight to discourage warm air from moving away,
yet somehow allow body-generated water vapor to exit rather than condense
inside the insulation making it damp and ineffective.

The purpose of shelter – a shell coat, rain gear, a tarp, or a tent – isn’t
to create insulation per se, it is to reduce the heat-robbing air movement
across the insulating surfaces, and to keep off the rain.  I would say if
your tent got frosty in the inside, and kept the outside of the sleeping
bag substantially free of frost, it did its job.  Then if you slept cold
the blame should be hung on the insulating ability of the bag.

I find it unwise and inefficient to add several pounds of shelter weight –
tent or bivy – trying to increase warmth when much less weight of down in
the bag will provide warmer sleeping.

>From my experience, bevies are possibly useful to resist occasional
moisture, such as when sleeping under the stars to keep off frost and dew,
or when a tarp isn’t fully trusted to keep out blowing rain.  I don’t use a
bivy when sleeping out – which is most of the time.  I just let frost or
dew form on the bag cover and brush it off in the morning.  If I pitch my
tarp, I keep it sufficiently low to the ground that blown-in rain is
minimal.

A simple waterPROOF bevy will collect body moisture condensate constantly
so it is off my list unless I crawled in during a brief rain shower and
crawled out after it had passed.  That’s a pretty limited usefulness for
the weight.  Waterproof, but breathable, fabrics are also available for
shelter, but they are relatively heavy.  Those are the ones with the magic
membrane that passes vapor but keeps water out.

Living in the damp Pacific Northwest I use coats of that fabric with mixed
results:  They do a good job – when they don’t have to.  When the top,
shoulder and sleeve area of a jacket -- or the entire top of a bivy sack --
is exposed to rain, the outer shell beyond the membrane soon becomes
totally covered with a solid layer of water.  That water layer won’t get
inside, but it also forms a barrier to any vapors that are trying to escape.
Result: Condensation and reduced effectiveness of the underlying insulation.


With a coat that mostly happens across the shoulders and down the outside
of the arms, leaving the remainder of the coat’s surface to reject vapor;
but with a bivy the top can’t breathe because it’s wet and the bottom can’t
breathe because it’s on the ground cloth.

Enjoy your planning and your trip,

Steel-Eye

-Hiking the Pct since before it was the PCT – 1965

http://www.trailjournals.com/steel-eye

http://www.trailjournals.com/SteelEye09/


On Mon, Jan 16, 2012 at 8:14 PM, Tim Gustafson <tjg at tgustafson.com> wrote:

> Hi,
>
> So I went for a "training" trip this weekend at Henry Coe park with my
> Eureka Solitaire tent.  The tent was woefully inadequate for the cold
> temperatures overnight - I'm not sure exactly, but I would say it got
> down to perhaps 25 degrees or so.  There was a significant
> accumulation of ice crystals inside and outside the tent.  Since it is
> a 3-season tent, it doesn't ever "seal shut" - the tent is no-see-um
> mesh covered with a second layer that is essentially a rain fly, but
> since the rain fly doesn't zip shut (it stakes to the ground
> separately from the rest of the tent), there was no good seal around
> the perimeter of the tent and I got a constant draft through the tent
> all night.  I wound up "cocooning" myself into my sleeping bag and
> holding closed the top as I slept.
>
> So I'm thinking about perhaps purchasing something that would be more
> suitable for 4-season use, and I was wondering if I should get a
> 4-season backpacker tent, or maybe go for the bivy sack option.  The
> bivy sacks are way lighter than the 4-season tents I'm finding, they
> pack smaller, and they're a lot cheaper too.
>
> I'm not thrilled with the prospect of my pack and boots getting rained
> on, but I suppose I could fix that with a waterproof pack sack, or
> perhaps a tarp that I could erect a simple shelter for them in the
> event of rain.
>
> Does anyone have any wisdom related to using a bivy sack rather than a
> tent?
>
> --
>
> Tim Gustafson
> tjg at tgustafson.com
> http://tgustafson.com/
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