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re: metal storage building ideas wanted
19 dec 2000
phxbrd@home.com  wrote:

> attic hotter than outdoor ambient is simply inadequately vented. 

i replied:

>you might as well say "every unshaded attic is inadequately vented."
 
>>you must have both low and high vents in order to allow convection to
>>allow cool air in to replace hot air leaving at the top.
 
>what's the indoor-outdoor temperature difference when 250 btu/h of sun
>shines on a square foot of uninsulated dark attic roof on an unwindy day,
>with 1.5 btu/h-f of thermal conductance to outdoor air, if the attic has
>2 vents, each a ft^2, with h feet of height difference?

a 1000 ft^2 dark south roof might be modeled like this on a 0 f day:
    
         ---                 ---
 0 f ---|-->|----*----*-----|-->|--- 0 f
         ---     |    |      ---
     250k btu/h  |    t    i --->
                 |
 0 f ---www------ 
      2/3000

which is equivalent to this:

                             ---
 167 f ------www------*-----|-->|--- 0 f
           2/3000     |      ---
                      t    i --->
                  
with h = 8', i = cfmt^1.5 = 47at^1.5 = (167-t)/(2/3000), so

     250k = 47at^1.5 + 1500t. 

with 8'x8' vents, 250k = 3008t^1.5 + 1500t, so t = 17.7 f.

what size do we need for "adequate venting"? :-)

nick




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