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Passive vs. active attic cooling

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Posted by John Ladasky on November 8, 2004, 7:18 pm
Hi, folks,

I just found this newsgroup.  Great stuff!

I wish that I was building a house from scratch, so that I could
incorporate the more complete solutions I've been reading about here.
I'm doing something a bit less exciting -- remodeling a California
suburban tract house.  Still, I hope that you folks can help.

Our remodeling projects are mainly about improving energy efficiency
and comfort.  We'll be replacing all of our windows with double-pane,
low-E glass.  We're replacing our 30 year-old furnace.  If it is
needed (I'm having trouble finding a home energy efficiency rater who
will answer my inquiries -- yes, I know about www.cheers.org), we'll
add some more insulation.

All of that should help with heating issues, but we're also concerned
about cooling.  The house is a split-level.  The upper story gets
unbearably hot on certain summer days, retaining heat well into the
night, which makes sleeping difficult.

We'll also be generating our own electricity.  I'm about to sign a
contract for a solar PV installation.  I've had three contractors
on-site, and I've narrowed it down to two.  Both of these contractors
have offered to make improvements which should assist with summer
cooling, while they're up on the roof.  I am trying to find out which
of the options I have been offered is best.

On the basis of what I told these folks about summer heating, and the
fact that the gratings over the intake vents under the eaves of our
roof are rusted (a sign of moisture retention, and thus inadequate air
flow), both companies have recommended that I try to increase attic
ventilation.  One company advises that we replace the essentially
passive turbine vent that we have on the roof right now with other
passive devices -- namely, eyebrow vents or a ridge vent.  The other
company suggests that we install self-powered solar attic fans, which
would not require a strong temperature differential to move air.

I like passive solutions, as long as they actually work!  But if the
passive approach won't be as effective here, I have also read warnings
on-line that the motors in the solar attic fans can have fairly short
lifespans, only a few years.  I suspect that I might find the
maintenance of these fans to be a bit more trouble than replacing the
fans in my bathrooms...

Adding active cooling is not out of the question, though I'd of course
prefer to avoid it.  I'm in semi-arid San Jose, California, so I might
be able to use a swamp cooler as opposed to central AC.

Thanks for your advice!

Rainforest laid low.
"Wake up and smell the ozone,"
Says man with chainsaw.
John J. Ladasky Jr., Ph.D.

Posted by Robert Morien on November 8, 2004, 11:23 pm
 ladasky@my-deja.com (John Ladasky) wrote:

I like ridge vents along the complete length of the roof. and the proper
amount of soffit vents. You don't say what type of roof and attic
insulation. If the insulation is attached to a sloped roof you need to
make sure that there is a ventilation space between roof and insulation.

Also a whole house fan to use at night: cools the house and should cool
down the attic, or at least ventilate it.

Posted by sno on November 8, 2004, 11:56 pm
Passive works ok...as long as you have vent along the whole
length of roof plus soffit vents....but unless this is built
into the house originally it is usually pretty expensive to

I have found that the combination of a powered roof vent that
is thermostat controlled....(they usually come with them) plus
a powered whole house fan that you turn on at night, when it is
cool enough to do so is the most best way to go....so basically
agree with robert....the small amount of electricity they use
in comparison to what you will save in air conditioning costs
makes them very inexpensive....

My experience with the powered fans is that they will last about
4/5 yrs.....however replacing them is not something that you
need to hire someone to do....they can be bought at home builders
places and are easy to install....you do not need to replace
the flashing or anything around the hole that is in the roof....
just the fan is replaced....

hope helps......steve

Robert Morien wrote:

Posted by Robert Morien on November 9, 2004, 12:18 am
It occurs to me that in the proper circumstances one could build a
thermal chimney to replace the powered attic fans...and they have the
perfect thermostat (based on size of course).

Posted by ladasky on December 9, 2004, 4:08 pm
 Hi there,

I'm picking up this thread again after a month of further research.
I've made my decision.  I've signed my solar PV contract.  I'm bringing
in a specific roof contractor to install a ridge vent.

sno wrote:

Steve, your comments were indeed helpful.  Here's my followup.  The
powered attic fans themselves may not be very difficult to replace when
they fail.  However, walking on certain kinds of roofs can be a

My PV contractor drew me a picture of how my Cal-Pac composite-shingle
roof was constructed.  He wanted to show me how they would be
installing the mounting brackets for the PV modules.  Underneath each
row of shingles is a fairly narrow stringer.  Between each stringer is
about 10 inches of empty space!  I had one of my other PV contractors
tell me, "when I walk up on your roof, I'm going to break a few of your
roof tiles."  Now I see why.  I asked the contractor to whom I gave the
PV job how they were going to work up there.  His answer: "You just
have to know that you're working on a Cal-Pac roof.  You have to know
where to step."

In another post I mentioned that the ridge vent would add about four
square feet of ventilation in the attic, which would be important if I
chose to add a whole-house fan.  (With the ridge vent plus the soffit
vents, I can probably move about 4000 CFM of air through the house.)  I
don't trust myself to climb up on this particular roof.  So this gave
me a second reason to choose a ridge vent over powered attic fans.

Rainforest laid low.
"Wake up and smell the ozone,"
Says man with chainsaw.
John J. Ladasky Jr., Ph.D.

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