But that radiant heat passes through the attic air and hits the solid materials. What I found to be fascinating was that when the sun sets, and ambient begins to plummet (hey, it's a desert! As simple as that. and it is mylar not molar. Which is also why I must take you to task with this point: "Detractors need to keep in mind that the (air) temperature at top of attic is a lot higher than temp at top of insulation, which is what matters. " This type of fan uses electrical energy to run, but the power consumption rate is surprisingly low, somewhere around 50 watts. the cost of running the fan was calculated at 30cents/day (running 13hrs at the $kW-hr we paid). note: I actually have a whole-house fan in the ceiling that pulls air from the house into the attic. the vent should be shrouded and the fan should only have a hole the diamete of the fan to mount against. "You cannot cool radiant surfaces with air. YMMV. @Mark, you are correct, my comment was intended for David-DIYer. Yes, air movement can cool hot objects, but there are some significant differences between a radiator fan and a powered attic ventilator. In short, energy is the ability the fan uses to work, and power is electricity consumption the fan uses. I can't remember who was presenting it. If a home doesn't have central air or atmospheric combustion appliances, a PAV will reduce ceiling loads a bit by cooling the attic and reversing the stack effect. I have done this with most of my under roof area and have noticed a significant reduction in attic temps even though I don't have soffit vents (I left the bottoms of the baffles open to get air from the attic which is supplied by 6 partially-open windows; top of baffles ends in a ridge vent; no mechanical venting/fans). but in some area, mine for instance the attic temp can get over 155F. Essentially, I do have a whole house fan but because of the Rube Goldberginess of my set-up I was hoping to avoid full disclosure. That is, I said that when you run a power attic ventilator in a typical house, yes, it will cool the attic down. Here it is: Using a fan to blow hot air out of the attic doesn't address the radiant heat flow from the roof to the attic floor. We have a whole-house fan as well. 500sq.ft. Living in Memphis, I have always had sweating ducts in the attic, but now they are dripping all over and I have new spots of moisture showing up all over the house from a/c ducts sweating. I know it's not like you're at the store with one in your hand but you might be able to find a picture online and get the net free area there. each succeeding night after that it switched off at 11pm no moatter if the daily temp high was 90 or 110. so there was at least a 4hr time period where the temps would have been 105+F (themostatically controlled switch was set at 105F). They can tell you exactly what's going on and lead you to the correct strategies for improving your situation without wasting money (like on attic fans). How to Position Fans to Cool a Room – Learn Right! @EW, regarding those light cans, you have a couple of alternatives: 1) use surface mounted lights. Since the metal roof was installed this has all changed. The only ones that come close are in purpose-built tight homes (e.g., Passive House level), where considerable effort is expended to seal all fixtures and top plates, including sealing the cracks between ceiling drywall and top plates. If you are concerned with your attic temps (which are really a symptom of the true problem - radiant energy from the roof deck), then you could run baffle panels the entire length of your roof deck underside. By removing the old insulation, you'll expose the ceiling. But I could also just turn off the exhaust fan on those days (after doing the experiment—because it's so rare that cost isn't an issue, and there's zero likelihood of damage due to humidity, so really it's cooling that I care about). This creates a nice stack effect and moves air the hotter it gets. G. How come my post wasn't accepted? So, I'm often at a loss for what to tell people who are concerned about storage in attic spaces. thats like saying insulation doesnt work. Similarly, painting the roof white causes the roof to cool. The total will be $0.07. What more would you need? They will give you the reading. In dryer climates especially, ventilation can be a positive effort to aid in dissipating heat from an attic, room or space. * link to another forum with thumbnail math on PAV's: http://bit.ly/mdIcXP. If you do choose to use an electric attic fan, consider one with a thermostat (an almost standard feature) and set it for 130ºF or above. One of my ideas was to replace the insulation on the 2nd floor walls with a higher R-value maybe even spray foam. @Luke easily addressed...to eliminate the running on moderate days, put a wall switch in your home running to the fan to switch it off entirely on those days. They don’t use much electricity, like other cooling options, and are flexible and portable to use. We are in TOTAL sun. And many roof surfaces are tilted toward the Sun for enhanced absorption. A ventilation system of any measure will help rid of the radiance... the question is at what rate? Note that "AT" rated light cans are not really air tight (they're marginally better than non-AT fixtures), and the IC rating (insulation contact) doesn't mean you can cover them in urethane foam. Ethan wrote: "The precise balance will depend on the balance of resistance of the soffit-vent versus through-ceiling paths.". Everything you say makes great sense. You should turn off the fan you have, add some soffit vents and see what difference your new insulation and air sealing efforts make. I know a thing or two about hot cars that have been in the sun all day. In another home, I found three power attic ventilators in the roof. What do you suggest? And let us assume the cost per kilowatt is an outrageous $.14. Just an example, but this is how you get to calculate. It's the hot, hot underside of the roof and subsequent radiation. while it is MUCH easier to have a RB installed at time of construction, ie on OSB, in CAN still be cost effective as a retrofit. Its that simple. Let us assume an attic fan is 300 watts and that fan is set at 110 degrees F. For the Chicago area, that would mean the fan would probably operate 700 hours during the cooling season. Electric attic fans are more powerful and cheaper than solar attic fans, but they consume a lot of power. Assuming again it has a 1500watt, which equates to 1.5 kWh, with 2019 electricity price then, 1.5 kWh would cost around 18 cents per hour. BTW, just as the louvers determine the net free area of a gable vent, soffit vents also have a net free area that's less than the total soffit vent size, depending on the type of vent. what they dont think about is that the fact that the RB wont lay perfectly flat and will NOT be in perfect contact with the insulation thereby maintaining an airgap. Cory: You wrote: "You are incorrect. Well, according to Professor Bailes here, you're plumb out of luck. We turned them off and the negative pressure pulling humid outdoor air into the bathrooms has gone away. A fan cools things. My soffit vent square foot on each side of my home = 640sq ft, 1280sq ft when totaled together. Just not sure how to measure the opening of air flow on the gable. In either event, my post was in regards to me testing my attic fan with no insulation to see how much air leakage I had. My gable vents on each end are 300sq ft (prob less since I need to minus the louvers), either way totals 600sq ft with both gables. first, a portion of the fan overlaps solid surfaces. Results in: my gable PAV definitely works. I always thought a solar attic roof fan would help remove some of the heat in our second floor. this has further saving due to the AC running less the years it will run before something wears out now is extended. A large study showed that factory applied RB (~10 cents/ft2 extra) is only marginally cost effective in homes built to 2006 energy code. Because HEAT ALWAYS TRAVELS TO COLD. Those walls aren't like the normal exterior walls because the adjacent space is way hotter than outside. Or, is the measurement taken from the inside, the louvers are slanted, so from the top of one lover (can't start with the bottom of the louver since I am now behind the it facing the screen) to the one below it is 3 inches. Thanks in advance. d) multiply %'s from (a), (b) & (c) to get % reduction in total cooling load. I wonder if the attic fan would pull conditioned air through the drywall ceiling? I believe you said you have gable vents. Correct me if I'm wrong but I thought one of Allison's points is that attic air temperature, in well insulated, air sealed attics, is not the primary driver of occupied space heat gain. Use profanity and belittling when one runs out of logic and proof. : these fans are cheap and manageable compared to air conditioners, which more. Low and comes back into the house upon arriving home would be surprised how it... Hot underside of the heat up and out of an attic fan does not need as! 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