[pct-l] condensation on inside of homemade tarptent

[elee at microsoft.com (Eric Lee (GAMES))]

Scott wrote:
>
Put another way, the air temp inside your tarp is warmer due to body
heat and the air outside tends to be cooler. Coupled with a high
enough humidity inside(from your body or other sources), you'll get 
water condensating on the cooler tarp surface, just like a cold can of
soda.
>

I'm not an expert in this subject but I think you have it kind of
backwards.  A tarp can collect an enormous amount of condensation even
if no one's sleeping in it.  I've pitched tarps in my back yard, left
them overnight while I slept indoors, and come out the next morning to
find them absolutely soaked.  They couldn't be any wetter if I sprayed
them with a hose.

The bottom line is that if there's heavy dew on the grass around the
tarp, I can pretty much guarantee there will be heavy dew on the tarp as
well, both on the top side and the underside.  Doesn't matter who's
sleeping underneath or what they're sleeping in.  In this case, it has
everything to do with the tarp (and the grass and everything else)
radiating away its heat to the sky and becoming cooler than surrounding
air.  There's nothing you can do about this short of heating the tarp or
putting something between the tarp and the sky (like trees).  This is
probably one reason why double-wall closed tents often have less
condensation problems in the conditions we're talking about (clear, cool
nights in the open) - they trap heat and the inner wall doesn't get as
cold.

I struggle with this same phenomenon in another hobby, too.  In
astronomy, it's a constant struggle to keep dew off my telescope lens.
The only reliable way to do it is to heat the scope, either with an
electric blanket type wrap, or an infrared light source, or some other
way.

Eric