Re: [GBlist] vapor barrier

["John H. Alderman III" <mountice@bellsouth.net>]

According to Mike in Technical at ASHRAE in Atlanta this is not the total
picture at ASDHRAE. This is even though 1997 ASHRAE Fundamentals page 2310
talks about a study suggesting such. ASHRAE TC's are again researching the
ventilation affect. As a contractor in Georgia I can tell you two
absolutes: the air conditioner runs more and the roof puts sand in your
gutter faster on non ventilated roofs. I have generally found that a
hundred watts of exhaust can easily produce a ton load drop. There is a
need to prevent infiltration in and out across the ceiling by proper
construction. The use of vented ceiling lights on the top floor can produce
a whirl wind of hot air when the internal cold air column falls. I saw this
in Joe William's (Williams Brothers cousin) 19K house(4.5 stories) in
Suwannee , Georgia. A variable speed exhaust can solve this controlled by
the stalling of flow thru the ceiling.I can also assure you that there is
not a build up of moisture in the attics of ventilated attics as many of my
friends use their attic to dry produce for storage. One thing that is often
forgotten is the infrared transmission across ceilings. In my last home I
lined my entire attic space with discarded aluminized duct wrap on top of
the blown and lay-in insulation. With partial cathedral ceilings my home
stayed very cool on the hottest days. My power bill was about one third of
my neighbors. 
The one thing you will see immediately in my residential jobs is I do not
octopus my systems with flex duct in the attics. This is one of the most
dishonest things a builder can do to his home owner. The losses from the
duct wrap are tremendous. Not ventilating the attic only makes it worse.