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
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
Why not just go with a solar fan in the attic? It would help remove the heat and you wouldn't need to waste electricity to run it.
I am getting a new system installed tomorrow, but one of the quoting contractors I did not go with told me that my gable fan was not doing anything and recommended a powered roof mounted attic fan.
I have never gotten the temps in the attic, but even when it is 70 degrees outside it is easily 90+ in the attic and in the summer you can "feel" the heat coming down from the ceiling on the top floor. I've added two layers of rolled fiberglass insulation since moving in 9 years ago and it seemed to make little difference.
What amount of insulation (rolled I guess as my attic does not look like my other house that had blown) should I have in Northern VA?
What is a radiant barrier and is this something a homeowner can install?
Should I just turn off the gable fan? "Fix" the gable fan? The reported problems with the existing fan are: "gable vent not appropriate as an exhaust for a fan", "fan installed too far away from vent".
Install an attic fan? Attic is ~1250 square feet, ~3125 cubic feet, but also has ~1000 cubic feet of storage dormers one floor down that vent into the attic.
Just say no to PAVs.
Originally Posted by Llama
Links are good 'uns. Thanks.
I took a hybrid approach since my PAV was already installed on my new construction home before I found this board.
I paid a electrician XX bucks to install a kill switch on the PAV. Now, my PAV is off all the time. The ONLY time I light it up is when the AC is on and its really hot outside. I also have the thermostat set for 120 (I figure let it work only during the 'peak' heat and let the passive ventilation do its thing for everything else).
When the AC gets turned off (usually at night or any day when I can get away with it), I just kill the power to the PAV. This way its not running endlessly and the passive air flow does its thing.
I figure by running it this way I will extend the life of the motor by years, and any benefit I MAY get from the PAV is concentrated into the times when I absolutly need it the most.
If the motor goes, I'll just leave the switch on 'off' and that will be the end of it.
Last edited by Jopopsy; 06-25-2008 at 12:04 PM.
Reason: Took out pricing.