[GSBN] windscreen

[Derek Roff]

I don't think you can get the protection Darrel is needing via a screen 
with smallish openings alone.  I've done some informal experiments, and 
I think the results will be familiar to anyone who has stood on a 
screened porch during a rain storm.  Window screen (fly screen) has 
quite small holes.  On the order of 50 per square inch, if I remember 
correctly.  It has a pretty high ratio of open area.  This screening 
can block most of the force of driven rain, but it is much less 
effective in decreasing the amount of rain that reaches a surface 
behind it.

When stinging rain hits the screen, it is transformed into something 
resembling the output from a heavy-duty plant mister.  But that mist 
will still be lifted and driven by wind gusts.  A wall four feet behind 
the screen will soon be covered with moisture, at least on its lower 
quadrants.  A vertical surface one foot behind the screen will be very 
wet.  In some cases, turning an erosive blast of water into a heavy 
mist would be adequate protection.  In many cases, it would be totally 
insufficient.

Placing a second layer of window screen in a plane 2" behind the first 
cuts most of the mist.  In my experiments, almost no water reached a 
wall 1 foot behind the second layer of screen.  However, some water 
would moisten a surface 2" behind the second screen.  The amount of 
water was much less, but not zero.

I imagine that an attractive trellis grill, backed by two layers of 
window screen, could be very effective against wind-driven rain.  It 
would offer great drying potential between rain events, and might open 
up some aesthetic possibilities, in some cases.  But it doesn't seem 
like an approach that would chosen all that often.

Derelict


--On Friday, January 21, 2011 10:12 AM -0800 Bob Theis 
<bob at bobtheis.net> wrote:

> John Swearingen's comment about bending rainscreens brings up an
> interesting point I'd like clarified:  If the primary function of
> rainscreens is to break the force of the wind, then the "screen"
> needn't be shingled planks as we generally see, but any manner of
> screen with smallish openings set away from the wall behind, yes?
>
> If that's the case, is there any guidance regarding  how small the
> openings should be?
>
>   Yes, yes, it will depend on wind speed,  the thickness of the
> screen,  and variables I'd never imagine, but I'd be happy with an
> oversimplified  generality to guide evaluating screen material
> possibilities. If, for example, I want to make the screen a grill on
> which I grow vines, how dense would it have to be to work, absent the
> growies?

Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm.edu