[VACList] Re: white roof

[John Sellers <sellersj@xxxxxxxxxx>]

Hex,

OK.  For the benefit of anyone who just tuned in, the original question
was, will an Airstream sitting in the sun be cooler with a white roof or
a polished aluminum roof?

The answer is the white roof will be cooler, because both polished
aluminum and white paint do a good job of reflecting the heat from the
sun, but white paint radiates heat back to the sky better than polished
aluminum.

Now--without wanting to get _too_ much into the physics--about the
reference to near-room temperature.  All objects at temperatures above
absolute zero emit some radiant energy, but the amount of energy
radiated is proportional to the 4th power of the absolute temperature. 
So hot objects radiate a lot, and not so-hot-objects radiate a little. 
Radiation is a relatively small part (say, 10-20%) of the heat loss of
objects at relatively low temperatures--say, an Airstream roof at 120
degrees.  Most of the heat loss to the surroundings is by
convection--heat transferred to air flowing over the surface.  (Or maybe
I should say "air streaming" over the surface. . .)
 
> And what exactly does the breeze do? Are you saying it helps us 
> or hinders us from having a "cooler" trailer(Assuming it is closed up)? 

The breeze helps cool the trailer.  In a 10 or 20 MPH wind the
convection becomes large enough that heat radiation from the roof would
make little difference. 

> If it lowers the temp, then the trailer in motion on the road would be cooler 
> inside on a 103 degree day than the trailer sitting still given the same sun 
> exposure..Right?

Right.  We notice this when we make a quick lunch stop at a rest area in
the sun.  If you get into the trailer as soon as you stop, you will find
that the trailer is at the outside air temperature (cooled by a 50-60
MPH air flow).  But it very rapidly begins to heat up in the sun. I make
sure to open the trailer windows and roof vent as soon as we stop.  

> If significantly so would it not make sense to place a blower upon the roof to 
> create breezewhen parked in the open. My layman's guess is the breeze would not 
> lower with much significance. Is that what your post says?

Well, yeah, you could set up a big fan to blow air over the roof of the
trailer, and that would help. But for a given amount of power spent on
the fan it would probably help more to blow air through the trailer--e.
g. a Fantastic Fan.
 
> What would setting an oscillating sprinkler do for the temp. Assuming that the 
> water was somewhat to slightly cooler than the ambient? I have given thought to 
> doing this to my home in Houston on occasion, but am sure the homeowners assoc. 
> would go nuts. Just always wondered if it would lower attic temp and subsequent 
> house temps. 

It depends on the humidity.  If the humidity is low it would help
enormously since a great deal of heat is absorbed in evaporating the
water.  (But you would end up with water stains on your Airstream!)   A
similar strategy would be an evaporative cooler, (sometimes known as a
"swamp box") which used to be popular in states like Arizona.  It's
basically a big humidifier, which lowers the temperature but raises the
humidity.  In a really dry climate they do a pretty good job of cooling,
at a much lower energy consumption than air conditioning.  If the
humidity is fairly high to begin with, they just make it more miserable. 

I guess the simplest solution is to buy an air conditioner. . .

Best,
John Sellers
WBCCI/VAC #1587
1960 Pacer
(Dayton, OH)