To be clear, I'm certainly not trying to suggest that attic ventilation
does not make a difference, especially as regards moisture control.
Every home inspector sees numerous examples of veneration and moisture
control problems in attics, and there is no question that improved
ventilation can often help correct such problems.

Rather, my point was that you can't just assume that actual houses are
acting like the examples commonly depicted in of diagrams "attic
ventilation" under simple gable roofs, or that seemingly
"similar" systems act in "similar" ways.

For example, if you are depending on convection in rafter cavities to
move air from lower in the attic and up past knee walls to a ridge vent
in order to lower the temperature at the roof sheeting near the eaves
- a situation where you can't get the sort of cross-soffit
ventilation depicted the link in the post above - it's probably
especially important to pay attention to providing sufficient soffit
vent area, making sure every rafter cavity is vented, that you are
using one of the "engineered" ridge vents specifically designed to
assist in exhausting air when wind moves across it, and so on.

And, for example, to be aware that in some cases (ex: rafter cavities
obstructed by cripple jacks or below dormers):

http://www.tpub.com/content/engineer...4069_216_4.jpg
http://www.tpub.com/content/engineer...4069_216_5.jpg

you just can't ventilate the eaves via flow up to a ridge vent.

Homeowners not aware of such issuers are more likely to be frustrated
in their attempts to deal with problems such as ice dams.

Michael Thomas
Paragon Home Inspection, LLC
Chicago, IL
mdt@paragoninspectsDOTcom
847-475-5668