Efficacy of roof attic fans?
I'm considering having a couple of powered attic vents installed. Despite a lot of anecdotal evidence, I'm not sure how much difference they'll make to the temperature inside the house. We live in a high desert environment (Carson City, NV); the single-floor house is 11 years old, and is quite well insulated. It has 2x6 exterior walls which are full of fiberglass, and there is about a foot of blown insulation in the attic. What's giving me pause is some measurements I've made with a handheld infrared thermometer. Readings taken at 3 PM and 8 PM show only one-to-two degree difference between interior walls, exterior walls, and the ceiling, even though it does get quite hot in the attic. Will one or more attic fans make a significant difference in the indoor temperature?
I generally dont like attic ventilators because most of them employ a shaded pole motor which has the least efficiency of any electric motor.
Usually they dont last for more than a summer of two.
I would use some type of temp probe "type K" to get your temp readings
The infared hand held thermometers are a joke as far as i am concerned
I just put one of these ventilators in and I'm about to remove it and return it to the big box store to find another. The carton claimed "made in USA". The mounting frame and support are all plastic. Response to email inquiry about country or origin for this "USA made" ventilator was that various parts "imported". They won't answer who made the thermally protected motor. It will be out of here the next trip up the road.
I think it is reckless engineering practice to mount a motor that can seize and burn in plastic, regardless of the overload protection. Plastics have not performed well in Ford intake manifolds where vibration was an issue. A cracked ventilator mount is likely to drop the motor into the attic and lock the motor to burn.
I want to know how "Made in USA" printed on the carton turns into "imported/various" when directly inquired about slovenly engineering practice of a motor that can burn in your attic being supported in plastic.
Shop carefully. Open the box and look at the contents. It will save you an extra trip down the road.
Originally Posted by southerncomfort
you are very welcome OP.
but I have to admit that I lost the link...and some kind soul posted it
...seems we get LOTS of pav issues here.
I can't remember if it was florida solar energy center or building science
that proved that radiant barriers raised shingle temp 2 to 3 degrees.
But while shingle mfg may pay for bundles of shingles...the tearoff and install
comes from ho's pocket.
Not that I have seen any shingle mfg's that stood behind their warranty...
Good thread...I'll have to re-read when I am not so tired. Those attic temps
really can drain me. I like to work in attics from 6 am til around noon.
keep it going folks!
And have a great day tomorrow...weekend is almost upon us.
PS...good to read your inputs, I like the way you investigate.
The cure of the part should not be attempted without the cure of the whole. ~Plato
What Do It Mean?
Is EFFICACY" anything like "EFFICIENCY?"
If you put powered ventilators in the attic, you have to make sure that everything (boots, grills, light fixture boxes, etc) are sealed or you'll pull the cool conditioned air from the building into the attic.
You are better off with good passive ventilation than with a PAV.
Even if your house is sealed well enough to prevent the PAV from pulling conditioned air up out of the house, most homes are not, the PAV costs money to operate, and has ongoing costs associated with replacing motors and or thermostats when they fail.
Just my opinion, but the powered fans will waste your electricity.
If attic heating is a concern, then radiant barrier is a much better idea. That's where the heat is coming from. The plywood on your roof is radiating with an efficiency of 90 to 95% of the heat of the roof into your insulation.
That will fix the cause and not the symptom.
I'd agree about the value of radiant barrier.. we put that in when our home was built a few years ago, big surprise how much it lowered the attic temp in summer. I suspect it's fouling up my cell reception though.
Regarding OP's attic fan question - if ventilation lowers attic temp by quite a bit (pretty safe assumption) and that's where your ductwork is, you could potentially be doing your a/c a big favor. With only 1" fiberglass insulation typical in R=4.2 flex duct, an attic running 130+ is a factor in how cold your supply air will be once it finally gets to the outlet.
Last edited by dougvhvac; 06-25-2008 at 10:08 AM.
Reason: fixed duct statement
radiant barrier & solar fans
My attic temp went approxiamently 20 to 25 degrees on average so far this summer compared to last year without a radiant barrier.
The solar fans help with not only the condensation that accrues from the radiant barrier but also the convective heat transfer out of the attic.
Moreover, my amp draw on my 1.5 ton HP(for a 1400 sf home) is down about 1/3 over a typical 90 degree day with 70% humidity.