Drying Shed, White Roof

Name: Felix
Status: other
Grade: other
Location: ME

Question: Years ago I read a popular science article on construction
of an efficient wood drying shed. It specified a white roof, with no
details as to why, and I have since wondered why, if there is any reason.
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Good question, Felix.
A dark roof would make the interior warmer, and you would think that would speed drying.
Or a clear roof, like a greenhouse...

Two half-angles:

Pro:
The shed definitely works OK with a white roof.
Drying (evaporation of water) does happen even with no temperature differences applied,
even with the circulated air at ambient (<100%) humidity.
Water leaps into the air to become vapor,
leaving the place behind it cooler and less able to evaporate further.
Then a breeze carries in ambient air to erase that temperature deficit,
and this frees more water to make its escape.
No distinct heat need be applied.
The whole world could be 68.3645 degrees F, as long as the humidity is only 50%.
And drying would still happen.

Con:
The shed with dark roof might get quite humid
if sun-heating somehow forces ground moisture up into the interior air.
This could happen quite easily, then your wood might dry considerably less and maybe get moldy too.
A clear roof, in particular, matches the classic "solar still" used by survivalists,
in which sunlight passes through a clear membrane, heats the earth, and evaporates water.
The water vapor then condenses on the sheet because the sheet is in contact with the cooler exterior.
Your wood would be drier than the dripping clear sheet, but not by much.

I imagine this ground-moisture problem could be cut off by having a good floor
such as elevated wood planks plus plastic sheets for vapor barrier.
Who knows, maybe then a clear or dark roof would dry wood fastest.
But many practical people will not want to sweat that hard on a backyard drying shed.
For them it should be a white roof.

Jim Swenson
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A white color keeps a roof the coolest when the sun shines on it.  I do not
know why a cool roof would be especially important for a drying shed.

Robert Erck
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Drying sheds are usually open structures to allow free air movement.  Light roofs
such as white granuated or silver coated modified roofing is preferred in most 
cases simply because by reflecting light and heat, the roof last longer.  Absorption of heat
will spped the demise of the roof.  I think this is the only reason they mentioned 
white roof.  Drying sheds have roofs to protect against sun damage.

Steve Sample
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