[pct-l] Bivi Bag

[CHUCK CHELIN]

Good morning, Dave,



When you say, “…bivi bag instead of a Siltarp…” I presume you mean the bivi
would be your only shelter.  I’ve found that a breathable bivi is pretty
good against the possibility of splash, spray, and dew but as a primary
shelter for direct rain it has the same problem I find in all GoreTex-type
fabrics:  It breathes until it gets wet.



These fabrics rely upon the transpiration of water vapor across a barrier
that prohibits liquid water entry.  When the outer textile fabric of a
garment becomes totally covered with water – such as on the top of a bivi or
the shoulders and upper arms of a jacket --  the solid layer of water
becomes itself a barrier to, or a great resistance to, the transpiration of
water vapor.



This works OK in a garment such as a jacket because typically the jacket
isn’t totally covered by a sheet of rain water; maybe only 20%.  Other areas
may be only damp – or even dry – and will be available to breathe.  With a
bivi the top will be totally wet, and the bottom will be substantially
restricted from breathing because of the sleeping pad and ground cloth.  Some
transpiration may occur along the very limited sides of the bivi along the
ground.  The bottom of the bivi must be really water-tight because rain will
likely run down the sides and cover the top of the sleeping pad.



As always, your own experience is your best guide.  I recommend you try a
bivi by sleeping out in a substantial and sustained rain.  I live in the
lush, green Pacific NW so I can do that quite easily -- in fact more easily
than I really want -- but if you don’t have predictable rain, try sleeping
in the back yard with the lawn sprinkler running.  That will either tell you
what you need to know or the cops will drag you off to the funny-farm after
the neighbors call 911.



I use a 10 oz. SilTarp/poncho combination so I don’t have to carry separate
raingear which weighs even more than that.  If I relied upon a bivi I would
then have to carry separate raingear and the sum would be greater than just
the tarp/poncho alone.  Some hikers use a bivi under a tarp -- primarily to
protect the bag against wind-borne spray -- but I usually pitch the tarp low
to the ground where spray is very limited and is no worse than normal, light
dew.



Enjoy your planning,



Steel-Eye

Hiking the Pct since before it was the PCT – 1965

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

http://www.trailjournals.com/SteelEye09


On Wed, Mar 17, 2010 at 4:53 AM, Dave Potter <davepotter at webmail1.co.uk>wrote:

> Hi
>
> Advice please on suitability of use of bug-proofed bivi bag instead of
> Siltarp from KM to Truckee in July to shave some ounces?
>
> Many thanks
> Dave
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