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Passive vs. active attic cooling - Page 2

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Posted by John Ladasky on November 9, 2004, 4:16 pm
Hi, Robert,

Thanks for the reply.

According to one of the solar contractors who visited our house, we
have an adequate number of soffit vents.  What we need is better
exhaust at the ridge line.

The roof is a bit of a hodgepodge, though I don't think that any fatal
errors have been committed.  The original roof is shake.  About 15
years ago, before I owned the house, a composite roof was installed
directly on top of the shake.  The solar contractors have identified
the roof as one manufactured and/or installed by Cal-Pac.  I found a
few of the shingles in the garage.  The shingles are corrugated metal,
and have been sprayed with some sort of rubbery asphalt material.

Installing a ridge vent or attic fans will cost me a bit more because
they have to cut through two layers of roof instead of one, but I
didn't find the cost prohibitive.

Our attic insulation is installed on the level surfaces, on the
ceilings of the rooms.  It's not installed directly under the roof.
So the attic space is above the insulation, which according to you is
the correct way to do it if you want to use passive roof cooling.

I'll look into these.  It only helps at night, though?  Pity.  I'd
like to start cooling a few hours before sunset on a summer day,
though I might manage without it.

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 9, 2004, 11:00 pm
 ladasky@my-deja.com (John Ladasky) wrote:

Examine each soffit vent. They easily get clogged with spider webs and
other stuff, which tends to diminish the effect. Also examine behind
them in your attic to see if anything is making a nest that can

If you can manage it, you can install thermal mass somewhere and that
will help moderate the temperature. Especially if you place it so when
the whole house fan is going most of the air goes past/thru the mass
(think hundreds of one gallon water bottles but it could be any
sufficiet mass) I suppose if it got too hot you could always have a fan
blow thru the thermal mass until you could use the whole house fan.

Posted by ladasky on December 9, 2004, 4:29 pm
 Hello again, Robert,

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 choosing
to install a ridge vent, in anticipation of installing a whole-house

Robert Morien wrote:

should cool

I may have just the spot for cooling thermal mass... but it might be a
little tricky to use.

Our house design is a bit strange.  It's a split-level of sorts, even
though we're on a level piece of ground.  The garage, family room,
laundry room, and downstairs bathroom are built on a ground-level slab.
The living room, dining room, and kitchen are four steps up from the
ground level.  So I have a big, empty space underneath this part of the
house.  It's shaded, and in contact with the ground.

I could see placing hundreds of one-gallon water bottles in this space
and running an intake vent through it.  I would open this vent when I
start the whole-house fan.  As the thermal mass heats up, I suppose
that I would then have to close this vent.  I would continue to draw
air through open windows later into the evening.

I can see two problems with this approach.   First, the space
underneath the house is bare dirt, and smells musty.  So the air might
be cooler, but unpleasant in other ways.  Maybe I can arrange to filter
the intake.  Second, although it doesn't get very cold here, I would
still have to make an effort to avoid freezing and bursting of the
water bottles.

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

Posted by Gary on November 9, 2004, 12:29 am
 John Ladasky wrote:

You might think about a whole house fan.  This is a high capacity fan
that would install in the ceiling of your upper living level.  It
pulls air through open windows in your living space and exhausts it
into the attic.  The fans are typically sized to do a full house air
change in less than 5 minutes.  They are best installed in an area
that connects to all rooms (like a hall ceiling).
When used during the day time, they provide some cooling by just
keeping a breeze going. In the evening, after the outside temperature
cools below the inside temperature, they are very effective in cooling
the house quickly.  They can also be run on a timer during the coolest
part of the night to pre-cool the thermal mass of the house for the
next day.
To use a whole house fan, you need a large vent in the attic for the
air to exit out of (several square feet).
I have had a whole  house fan in my last two houses, and found them
to be very effective -- but, you have to have a climate where nights
are cool for them to work well.
I think the two speed feature is very worthwhile.
They consume much less power than refrigeration style air conditioners.

Since you are replacing windows, you  might think about windows with
lower solar heat gain to try to avoid overheating your upstairs.
This site is pretty good for sorting through all the new window

Another option might be to add overhangs on some of the windows that
gain a lot of unwanted heat.  For South facing windows, an properly
designed overhang can prevent summer gain while still allowing winter
Overhang design tool: http://susdesign.com/overhang/index.html

Solar gain shades might also help?


Posted by John Ladasky on November 9, 2004, 4:53 pm
 Hi, Gary,

Thanks for your reply.

You're the second person to suggest a whole-house fan.   I will
definitely look into this possibility.  San Jose has arid summers, and
thus it does cool off pretty quickly after sunset.  I'm not sure if it
gets cool *enough*, but I'll try to find some data.  Just bringing
down the temperature of the second floor so that it is equal to the
temperature of the first floor would be helpful.

Ah.  This is a strong argument in favor of installing a *ridge vent*,
as opposed to a few eyebrow vents or solar attic fans.  Our roof has a
single, 40-foot ridge.  I estimate we'll get 3-4 square feet of
exhaust ventilation by installing a ridge vent, but under 1 square
foot with the other two choices.

Yes. I'm certainly replacing my windows.  Even if I wasn't interested
in the heating and cooling benefits of low-E glass, our old windows
are cheap and worn out.

Oh yeah, I forgot to mention overhangs!  I intend to install overhangs
above my SSE-facing windows, and a trellis with deciduous vines above
the sliding glass door in that same wall.  I have a single, small
window on the second floor of my WSW-facing wall.  I'm not sure
whether to install an overhang here.  It would look a little weird out
there by itself.  Also, this lone window is high up, near to the eaves
of the house, and thus it already gets a little summer shade.
Finally, I think that my WSW *wall* is conducting a lot more heat than
the window located in that wall -- we can feel that the wall is warm
to the touch on hot summer evenings.  I may be blowing in some wall

I just did a Google search on "solar shades", and they look
interesting.  I'm not sure how much help they would offer, after we
add the double-pane glass.  Besides, we already have highly-reflective
white Venetian blinds in most windows of the house.

Thanks again.

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

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