Re: [GBlist] vapor barrier

["John Salmen" <terrain3@home.com>]

an increase in heating load and sand in the gutter I think refers more to
the type of roofing than the ventilation approach. A duroid or fiberglass
roof, especially darker colour will deteriorate faster in an unvented
situation - the deterioration is a result of a slight temperature increase
at the sheathing level. I think the figure was a 2% temperature increase
translating into a 3-4 yr loss on the life of a 25yr duroid roof. I don't
encourage the use of duroid for a lot of reasons - especially how they deal
with heat so I've not really considered this to be important. The air
condition running more can work both ways. IF the ducts are in the attic
(very very common) the loss comparison is in favour of unvented. Even if
they are not - there tends to be a lot of leakage into the attic which can
also result in more losses, especially if a fan is venting - its a lot
easier to create a pressure barrier at the roofline. It also depends on
effective insulation levels. Use of foam, sips or dense pack cellulose in
joist cavities or on top of joists can provide 'more effective' insulation
levels than loose blown material in an attic. Also we have not discussed the
kind of insulative losses within a vented attic as a result of air movement.

I agree that basically it all depends on how things are constructed but I
think in terms of conventional building there would be more potential for
energy savings with unvented attics - specifically there would be less
potential for loss.

It would be interesting to go into this more as there has been only spotty
conversation on the list over the years.

John Salmen
TERRAIN EDS



----- Original Message -----
From: "John H. Alderman III" <mountice@bellsouth.net>
To: "John Salmen" <terrain3@home.com>
Sent: Tuesday, July 31, 2001 12:18 PM
Subject: Re: [GBlist] vapor barrier


> 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 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.
> From Certified Hot and Humid Georgia,
>
> John
> Ga Conditioned Air Contractor Two CN3634
> USN NEC 3361/4245 SUBLANT
>
>



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