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[Bill Jones <billj AT harborside.com>]

I looked at the shed roof some more, and there is barely room to fit a 
false ceiling beneath it, with a layer of fiberglass insulation and 
Tyvek housewrap outside that.  I say that because every further inch of 
space would take away light from the shuttered, 16" tall, 7' wide 
Solacryl UV-transmitting window at the top of the south wall.  The 
interior walls and false floor are less constrained.

I examined the material they put between the metal roof and the 
purlins.  It feels like plastic canvas.  But that certainly doesn't 
exempt us from dealing with internal condensation, since moisture would 
now just collect on the plastic!  However it does protect the purlins 
from moisture that might seep through the unsealed roof.  It will also 
keep water vapor from cooling quite as rapidly as it would if it struck 
the metal directly.

Thanks to that plastic layer, the purlins do indeed create separate 
"boxes" in which water vapor could be trapped.  But the plastic is 
somewhat loose, so we could maybe insert some small plastic tubes here 
and there to allow water vapor to flow uphill against the plastic and 
into the next box (the idea being to get away with just a single vent 
hole).  Then maybe we'll just cover the small rectangular opening, where 
the exhaust air flows out along the center spine, with a screen on one 
end.  Thanks to the warmth inside, the air flow should consistently be 
upward, so if there's no draft in the roof space from cross-ventilation, 
the air space should retain some warmth, and yet still be open at one 
end (especially since the opening is well under a good eave).

Then there's always dehumidifiers...

Bill
S. Oregon coast

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