I recently installed a gable mount attic exhaust fan to help keep the attic
cool. So far it hasn't been much help.
I have the gable vents all around the house, but what appears to be
happening is the attic fan is just sucking hot air from outside into the
attic, and not helping keep the attick temp down.
I am wondering, is it a good idea to cover the gable vents (mine are 5 inch
wide screens running the length of the house) during the hot summer months
with say plastic sheeting? Would this help keep the fan from sucking hotter
air in from the outside? Would it basically ruin the idea behind the attic
It just seems that the fan is a waste when all it does is suck in more hot
air through the gable vents.
Any insights from anyone with similar experiences or ideas will be
I would think that on a hot day, the air in the attic would run 120 to 140
degrees. Assuming the outside air temp is 90 degrees, you will still be way
ahead by pulling in the outside air. Don't block any of the vents with
You'll also need soffit vents. You need to have airflow. I'm going to
put in an attic fan and a few soffit vents (I have eave vents, but I need
another foot or so of intake) as a stop-gap measure until the roof needs
to be done, then I'll have a ridge vent and the rest of the soffit vents
put in. An attic fan and a few soffit vents are about $250, installed by
me. A new roof, ridge vent, and soffit vents is way more than that, even
if I do the work myself.
Bob, everything you say is true, but I can't imagine $250? Gable fans
are cheap at $30 for the smaller size and 60 for much larger, and
soffit vents are about $2 each with 10 needed per 1500 square feet.
So $50 would do it for a 1500 sq-ft house and $100 for a 3000 sq-ft
house. Things must be really expensive in CT.
Bob in CT wrote:
Unless of course you have a tile roof and live in an area that requires
product approval on ventilation products. There are currently no ridge
vents approved for use under tile in the State of Florida.
sucking hot air in? the word hot only has meaning in comparison to
something else. put a thermometer up in the attic and check the temp on a
'hot' day. now turn the attic fan off and see what happens to the temp.
the result is very predictable. your attic is much hotter than the air its
sucking in from outside.
Youre supposed to bring outside air in...thats the whole point of having
an attic ventilator, to cause cooler outside air to become make up air
for the hotter attic air thats being exhausted. ON a 90 degree day with
the sun out, your attic will be at least 130 f and higher without an
attic ventilator .
Attic fans are sized to the size of the home. My old house needed 2.
If you block out the vents where is the air for the fan to exhaust going to
Does your fan have a themostat on it? Where do you have it set? I had mine
set at ~120 degrees. They would generally turn on around noon and run to 7-9
Have you considered metal turbines? They work for free and do quite well.
Can't use the turbines, I have a tile roof and installing them would be a
small nitemare. My attic fan is direct wired, has a thermostat and it is
set to about 95 degrees. It usually kicks on at about 10AM and can run
easily until 10 at nite. I have a thermometer in the attic and the gauge is
inside the house. It shows the average attic temp is about 105 degrees.
When the fan is not running (I also installed a cut off switch) it only
varies, so far, by about ten degrees. When the outside temp is about 90 the
attic is running 110, and does come down to around 95-100 but if the fan
goes off it climbs right back up. It pretty much has to run all day
regardless. Since my air conditioner duct work is in the attic, when the
air goes off it is so hot up there that the ducts heat up and when the air
kicks on, the first thing that comes out into the house is extremely hot air
about about 20 seconds until it starts to cool down.
I feel like I"m robbing peter to pay paul here by trying to decide if it's
worth it to try to keep the attic temp down when it heats up the duct work
anyway and fills the house with heat every time the AC kicks on.
When considering covering up the "soffit" vents, I had considered covering
them all except for the opposite end of the house. My line of think was
that at least it would draw air from one end to the other rather than
getting all it's flow from the vents nearby and doing nothing to move the
attic air on the other end of the house.
Hope that makes sense.
On Wed, 02 Jun 2004 18:01:31 GMT, Lost-In-Translation
Actually, with an attic fan, that's what's recommended -- the fan should
be away from the vents. Do you also have a ridge vent? If you could put
a few thermometers up there (like the Radio Shack remote kind), then you
could try covering some of the soffit vents and seeing what happens. If
you have a ridge vent, then I'd like to see your data. I don't have a
ridge vent and no soffit vents and want to install an attic fan with a few
soffit vents placed at the far ends of the house. However, I also want to
add all soffit vents and add a ridge vent, but am unsure as to whether the
attic fan should go.
Insulate the ^&$$ out of those ducts. It will not be 100%, but I'll bet
it will be a lot more bang for your buck.
BTW those turbines seem to cause more problems that good. There are
some areas and situations where they can be useful, but far more times they
are more trouble than they are worth.
Skip the attic fan and invest in some additional insulation around the
duct work and in your ceiling (particularly if the ceiling insulation is
less than about R-30).
Tests continue to show that attic ventilators have little effect on home
cooling costs or indoor comfort if the home has adequate attic
insulation. Studies conducted by the National Bureau of Standards and
Progress Energy uniformly support this conclusion. Some surprising facts
were revealed by Progress Energy research conducted over two consecutive
summers in Florida. The studies involved 30 different homes and outdoor
temperatures ranging between 88 and 95 degrees Fahrenheit. During the
tests, attic temperatures generally did not reach excessive levels. The
highest recorded temperature at the peak of a roof was 134 degrees
Fahrenheit and the average temperature was 127 degrees Fahrenheit.
Temperature immediately above ceiling insulation reached 119 degrees
Fahrenheit, and averaged only 103 degrees Fahrenheit.
With a turbine ventilator operating, attic temperatures fluctuate -
sometimes decreasing about six degrees and, surprisingly, sometimes
rising by six degrees. These relatively small changes in temperature had
little effect on air conditioning energy use since attic insulation
retarded heat flow from the attic into the home.
Most homes already have some type of attic ventilation such as screened
eaves or gable vents since it is required by the state building code.
Such ventilation prevents moisture build-up and minimizes the
possibility of wood decay. Progress Energy research also indicated that
additional ventilation is unnecessary and would not significantly affect
cooling costs if the home is adequately insulated.
That's a nice study, but I wonder about it. My sole source of ventilation
is two gable vents. I think if you have soffit vents and a ridge vent,
along with good insulation, then I think you have no need for an attic
fan. While I plan to add insulation, I haven't yet done so. Moreover,
some of the attic simply won't be able to have enough insulation over it,
if it's used as a storage area (which mine is). I'm going to use some
additional insulation in that area, but not much. I'm also going to
minimize the amount of area.
I've been redoing my bathroom fans (vented into the insulation in the
attic -- ick!), and it's not been that hot outside, but it's absolutely
brutal in the attic. It was about 70-75 outside, and it was so hot in
the attic that I instantly started to sweat when I went in there, and I
lived 15 years in Arizona. If I can use an attic fan and get the
temperature down to close the outside temperature, I think it'll be worth
it. Then, when I save enough, I'll insulate the attic, add soffit vents,
and add a ridge vent and ditch the attic fan. But adding all of that
stuff when the roof doesn't need to be redone is expensive. So, I'll wait
'till the roof does need to be redone and then redo the roof the right way.
Good plan - venting the bathroom fans vented either through the roof, an
outside wall, or the soffit is the first thing to do! I can't imagine
what bad things come of all that humid air being introduced into the
attic. Particularly on those days when the attic temperature is lower
than the indoor temperature. Sounds like mold heaven <grin>.
My experience tells me that a gable-end fan (even a good sized 1/4 hp
one) will only reduce the attic temperature about 10 degrees during the
average day. And that is with soffit vents - with a single gable intake
vent it might be even less, since all you'll get is pull-through air.
Don't expect to get it close to the outside temp during the day and
evening. That roof is going to be one very large radiator. Vents do help,
it would be a lot worse without them, but even the fan is not going to make
Maybe you should consider checking with a construction contractor to see
if they have some suggestions for upgrading the ventilation.
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