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[Bill Jones <billj AT harborside.com>]

Just yesterday I was out looking at the new solarium for animals (guinea 
pigs, iguanas?) that's part of my new shed, and I pretty much concluded 
the same thing while standing there, so the timing is amusing.  Every 
morning the guinea pig cage tarps are coated with condensation (at this 
point it functions as part of the desperation heating system, like 
condensation in an igloo).  But we're stuck with the wood frame, with 
roof but no walls yet (I really shouldn't hug treated wood too much, 
probably).  Some folks are solving the problem by using a steel frame 
around here.

So as you say, the interior space must be sealed from the space between 
the walls, and between the roof and ceiling.  But isn't that what makes 
for a comfy house anyway?  Fortunately I only have to do it for one 
room, since the storage room will be well-ventilated.

I decided to add a few other features.  Since a solarium is likely to be 
very warm in sunny weather, I'm *inclined* to have the interior ceiling 
slope upward to adjustable screened vents near the tallest point of the 
roof.  Each of the two vents would lead into a short conduit that would 
allow humid air to leave the solarium without mixing with the air in the 
wall space.  It goes without saying that the space between the roof and 
ceiling must have outward vents of its own (if it can't be perfectly 
sealed).  It appears that if our sloping ceiling is to be flush against 
the rafters, then each rectangular _section_ of that air space must have 
at least one vent hole, I would think (unless each section were somehow 
completely sealed on six sides, which is unlikely).  Then the paradox 
is, the more ventilation there is to the roof space, the less insulation 
that the intervening air space will provide, because the air temperature 
in that space will approach that of the outside air.  Maybe in humid 
Eastern climes it would be better to completely seal the air spaces, but 
we can't easily do that, because our roof was already laid without any 
sealing strips.  Fortunately our winter's never too bitter, and when it 
is, it's bone dry; and the summer is entirely dry.  And during the 
rainiest times, the temperature is warm enough to leave the vents wide 
open all the time.

I can thing of some more fancy but cheap solutions.  What about a long 
fiberglass wick at the low ends of the roof that runs along and dangles 
down into an eave-protected space that's open to the breeze?  If I'd 
thought to do so, it would have been easy to at least give my purlins a 
coating of used motor oil or old deep-fryer grease to seal them, or try 
some of the less toxic wood preservation methods we've discussed.

But I still don't see what's to be so discouraged about, not having to 
hire a bozo to put on a new roof every few years.

Bill
S. Oregon coast

> When warm, moist 
> interior air hits cold metal, the moisture condenses out of the air and 
> clings to and runs down on the metal. Any untreated wood--like 
> purlins--that is continually wetted will rot, and informed people do not 
> want treated wood inside their homes. Any insulation such as fiberglass 
> becomes soaked and looses its insulation quality. One solution is to leave 
> a dead-air space next to the metal and to have adequate insulation and 
> interior air-tight barrier to keep the warm, moist air inside the structure 
> and away from the metal. 
>
>