I turned my garage into a studio.  The cathedral roof has an 18" void
between the plywood sheeting on top (which the roof shingles are tied
into) and the tonque and groove planks which serve as the interior
cathedral ceiling. Batt insulation (9" of it) has been stapled to the
base of the rafters., which are spaced 24" apart. My question has to
do with providing ventilation to the void. Was it necessary?  My
concern about pushing air through the void has to do with the
introduction of moisture and airborne molds into the void;
specifically onto the batt insulation as the air is pushed along the
unpapered surface of it. Living in Houston, where there is high
humidity and high mold counts, would I be inviting a mold problem by
pushing airflow through the void.  The batt insulation seems like it
would be the perfect nesting area for molds to settle into.  Mix some
nice moist air into it and ....??? Did I create a mold cocktail? Or
are my concerns unfounded? The air velocity is created by a ridge vent
at the top of the roof and one 2" circular soffit vent on the sidewall
of  each cavity.

2mark-ga:

Thank you for your question regarding roof ventilation in your studio.

The general consensus in the construction industry is that the
ventilation you provided (specifically the ridge vent and soffit vent
approach) is exactly what the (house) doctor ordered.

As a rule, the air underneath your roof will be warmer than the air
outside the house, and the air inside the house. This is due to the
sun beating down on the roof and heat being conducted through the roof
to the air gap, and the insulation under the air gap preventing any
heat conduction to the rest of the studio. Even without the vents,
this air gap is not air-tight, so the heating of the gap, followed by
cooling at night, will cause outside air to be sucked into the gap.
This means that the air in the gap will have moisture in it, as well
as mold spores from the general environment. At night, this moisture
would condense against the cooler surface of the roof, leading to
swelling and/or rot in the plywood sheath as well as the batting.

By having the vents in place, the warming of the air in the gap
promotes an air exchange with fresh air from the outside. As the gap
heats up and cools down throughout the day, the temperature in the air
gap will maintain a reasonably close relationship to the temperature
of the environment, meaning that there is less opportunity for
condensation to occur, and lessening the opportunity for a mold
outbreak.

A ventilated air gap also helps to keep the inside of the studio
cooler, a good thing in the hot Houston summers.

Finally, if ever your roof develops a leak, having a ventilated air
gap will help to prevent moisture build-up in the air gap and the mold
explosion that would be sure to occur.

All in all, having the air gap is a good thing, which is why the
majority of building codes in North America now requires it.

Here are some articles that you may find of interest on this topic:

Attic Ventilation: Why It's Necessary
http://www.rci-mercury.com/files/ab071229.pdf
- see page two in particular


FAQ's About Ventilation
http://www.rci-mercury.com/files/faqs.pdf
- a general set of common questions about why roof ventilation is a good thing.


Roofing Experts - Roof Ventilation
http://www.renovationexperts.com/roofing/thor_roofvent.asp
- an expert's view on the problems associated with insufficient
ventilation and the damage it can cause


Other Ventilation Articles:
http://www.rci-mercury.com/index.cfm/fuseaction/browse_docs/category_ID/10/ctitle/Ventilation


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Regards,

aht-ga
Google Answers Researcher