I have increased my airflow to the top floor of my house by sealing ducts,
closing dampers, ect....
I have an open gable vent on each side of the house and also one of those
vented shingle ridge vents on the peak of the roof.
Would an attic fan or a gable fan make much of a difference in the
temperature of the attic, therefore helping the cool air be more efficient
in my 2 upstairs rooms?
Would a gable fan do the job just as well or would the attic fan through the
roof be better?
I don't know why, but I have hesitation about having someone cut a hole in
my new roof. I'm afraid it won't be sealed right and I will get moisture
under the shingles and/or in the attic.
Well that attic fan will help the attic stay cooler, but only by drawing
out the cooler air from the house to do it. A gable fan is not likely to
really help much.
Do you have any under eave vents? How did the venting work before the
ridge vent was added? How much insulation is in the attic floor? Do any of
the supplies or returns for your HVAC go thought the attic?
Note: closing off vents to "force more cool air upstairs" may not be a
good thing to do. It can reduce air flow to the point where not only is
efficiency decreased, but it can damage the system.
If you are having a problem cooling the second floor, there is a design
problem with the current system. You may want to have a professional take a
look at it and do the calculations (manuals).
BTW I have a bit of a problem with the second story as well ,but it is
not bad, but I will have it corrected when it is time to replace my current
If you get snow you want the attic the same temperature as outside. If
the attic is warmer it can melt the snow on the roof. Water flows down
until it gets to the soffit which is not heated. The water then freezes
forming an ice dam. The ice dam can force the water up under the
shingles into the attic which can cause expensive damage.
Please no. You want the attic to be as close to the same temperature as
the outside as possible winter and summer. If you close up the attic
moisture will tend to accumulate and cause damage in the form of rot and
mold. The insulation should be between the heated parts of the home and the
outside, that would be the floor of the attic. It is most efficient there.
The term "attic fan" is used two ways. One refers to roof or gable end
fans that only ventilate the attic. In that case there should be no vents
through the attic floor to the living area. In that case there is still
some small amount of air drawn in from the living area, but not important.
It is also used to refer to whole house attic fan which sucks air out of the
living area and pushes it into the attic area to be released through any
vents that may be there (roof, gable or ridge). It was the second type I
was referring to, but I failed to make that very clear.
My experience has been that a gable fan will cool the attic
considerably, but not result in much net energy savings due
to its own power use. It'll relieve some of the load from
your AC system, though, which is a good thing.
A radiant barrier can be very effective in my experience.
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It was actually pretty easy to seal. The top half easily slid under
the shingles and the bottom half sat on top of them and iirc was
I had the roof replaced a couple years ago, after 20 years that I had
lived here, I saw it when the roof was off. Nothing special around
the fan. No damage, as I expected since the installation is simple
and well designed.
As to the attic, the rain hits the roof, bounces off, then some of it
hits the screeen that came with the fan, and some of that gets past
the screen, and a tiny bit of mist lands on the plywood I covered the
center of the attic with. It's never been enough to more than sit on
the surface, and after 24 years, the unfinished plywood has suffered
no damage from this little bit of water. I guess I wouldn't put an
antique under the fan, but even a grocery store cardboard box would
probably be fine. We have verrrry heavy rains here sometimes.
To prevent the mist, I did look into putting a second (vinyl) screen
inside the circumference of the housing, but A, I'm not sure how this
would have cut down on the amount of water, and B it didn't seem easy
to do. Then I paid attention to the wood and decided there was no
I think Joe must be thinking of a whole house fan when he said it
would draw air from the house. That's why I call these things roof
fans, and not attic fans, which name I think misleads a lot of people.
I don't have a gable fan or vent. I only have about 4 inches of
fiberglasss and an inch or half inch of cellulose under that. I guess
I should have more.
As to the fan as a whole, a lot of people poopoo roof fans, but I
think mine is great. I have a townhouse that was built with a full
width ridge rail, and full width soffitt vents, front and back, about
4 inches deep and as wide as the house is. I almost never use AC and
when I bought the house, when I came home from work in the summer, it
was so hot, I couldn't even go upstairs for a minute. I would make
dinner, work and sleep in the basement, then go upstairs in the
morning to bathe and get new clothes. I put the fan in the first
After the fan, it was 10 or probably 20 degrees cooler. 20 degrees or
maybe 30 cooler in the attic. I stopped sleeping in the basement,
nailed a little fan to the window sill, kept the bed next to the
window, and I only need AC 2 or 3 weeks a year, in Baltimore. I never
thought to or tried to measure how much electricity the fan uses. I
thought, It's a fan. It couldn't use much. But I'm going to look at
the plate and see how many amps.
It runs from 10 or 12 in the morning to 6 to 8 at night, depending on
how sunny and hot it is outside. On hot days it can run from 9am to
10pm or maybe 11. This means it's off by the time I go to sleep.
Plus I put in a swtiches to keep it on or off. I keep it off in the
fall and spring, to use the sun to heat the attic to heat the house.
It works well. I don't take humid showers so there is no need to keep
it on to vent the attic, but it was easy to put in the second switch
when I put in the first. The switch is underneath the fan, in the
second floor hall, a double switch mounted sideways so it still goes
up and down, although iirc up is off and down is on. I get confused
what the normal setting is, so I drew a black line on both that shows
when they are in the "normal" setting, thermostat controlled, no
In restricting airflow you can do damage to your hvac system, one way
is freezing the coil from reduced airflow. Attic fans can increase
cooling costs by pulling out cooled air. No simple answer for you ,
best is a pro to look it over.
I'd consider the radiant barrier solution which was suggested a few
posts back before a powered fan. The radiant barrier is a metal foil
that get stappled to the bottom of the rafters. It blocks most of
the radiant energy so it doesn't heat up the attic as much. I'm
considering one myself, as everything I've seen tends to say they work.
I think the power drawn by the fan would be better spent feeding the
Suppose your fan draws 400W and on a hot day would likely run continuously
from late morning into evening (depending on thermostat settings). Suppose
your central AC draws 4kW total (10 times as much). Running the fan cools
the attic and reduces the heat load on the house a bit (depending on the
insulation above the ceiling). Every hour of attic fan would buy 6 minutes
of central air (one tenth of an hour). This is just armchair
reconnoitering but somehow I think that 6 more minutes of real air
conditioning will help me more than an hour of trying to keep the ceiling a
That's not to say that ample attic ventilation is not a good thing. It
is...as long as you don't have to expend power to do it.
There has also been discussion in this forum in the past about fires caused
by faulty attic fans. I had one lock up because unbeknownst to me it had
sleeve bearings. Motor was sitting there cycling on and off on its own
thermal protection. I vowed I would not put it back but then relented and
did (after polishing and relubing the bearings). For some reason I can't
understand the fan itself slipped down the motor shaft--no idea why, the
set screw was plenty tight--and caught on the supports, locking it from
rotation. Again, saved only by the thermal protector. That was the last
straw and I disconnected it. Makes a nice passive vent.
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