Powered Attic Vent Question

[NewGuy2005 53]

Roofers installed a powered vent in my attic, but left it up to me to wire it into the household current, which I've done.

Is there a general guideline regarding what temperature should the thermostat be set?

[1969912 0]

The vent fans that I have installed had fixed internal thermostats. Poking around on the web I found numbers between 95F and 110F. The best setting might depend on typical temperature and humidity where you live. Utility companies usually have a group that helps customers with reducing utility costs, They may have suggestions.

Broan has a FAQ that suggests an initial setting of 110F here:

http://www.broan.com/display/router.asp?docid=109#Q5

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[DJL 235]

110F is probably a good temp. Essentially you're setting it so that it's likely that the temp inside of your attic is significantly hotter than the outside air and therefore the heat transfer through your ceiling exceeds what you see on your walls. Also the temp probably reflects a point at which the condensation would occur within the insulation of your ceiling cavity because of the large temperature difference between the cooled space and the attic. Ideally your vapor barrier is to the outside (as it should be in the South) so that vapor doesn't actually extend into the insulation.

If you actually want to cut down on your cooling bill then plant trees that will shade your house.

"I encourage all awesome dangerous behavior." - Jeffro Fincher

[NewGuy2005 53]

Excellent. Thanks guys.

[Brains 2]

Be sure that you have adequate soffit venting to allow air to circulate through your attic. In humid climates, it is even more important. Attic vents (powered) will suck air from inside your home and exhaust it out. The air will be pulled from every crack, broken/leaking window and door seal, electrical outlets, plumbing fixtures, etc. Air WILL take the path of least resistance. If you have adequate soffit venting, it will pull from there instead. I have seen extreme cases where there was so much moisture being pulled into the house that it was covered with mold and rotting, it started right after the roofing contractor installed two new powered vents. I am an HVAC contractor, not a roofer and i have to deal with this stuff everyday. Feel free to ask any questions about this if you want.

Never look down on someone, unless they are going down on you.

[CygnusX-1 43]

Quote

If you actually want to cut down on your cooling bill then plant trees that will shade your house.

Unless you live in the South when one day a Hurricane will come along and use your shade tree to give you some extra cooling holes in your roof.

[NewGuy2005 53]

Ouch!! My soffit is partially perforated, so hopefully, that will do the job.

[Brains 2]

Quote

Ouch!! My soffit is partially perforated, so hopefully, that will do the job.

If it is perforated all the way around the house, you should be ok. Just make sure your insulation isn't blocking the soffit.

Never look down on someone, unless they are going down on you.

[billeisele 130]

kill your roofer, power vents are BS except as a last resort retrofit product, they use electricity, don't ventilate enough, make noise and break - along with the other real problems mentioned in earlier posts, if you had a new roof installed and there is adequate length on the ridge (at least 50% of the roof line) then they should have put in a ridge vent or a few passive ventilaters

having said that you are probably stuck with what you got

if the attic has an R30 or more insulation, set the Tstat as high as it will go up to about 120 degrees, if the insulation is less than R30 then add more to get an R30 or better yet R38

at 120 degrees the insulation can handle a 40-50 degree delta

Give one city to the thugs so they can all live together. I vote for Chicago where they have strict gun laws.

[Cornholio 0]

Ok someone edumacate me... I'e thought about adding an attic fan myself.

What's "soffit venting" ?? I'm in California, if that makes any diff. I have these 4x10" perforated vents that are along the walls of my house near the rafters - say 1 every 2-3 rafters. Is that enough ventilation? Last year, I just took a box fan and set it upside down in my attic entrance and it helped dramatically. It sucked air in from the garage and then forced it into the attic out those side vents and the roof vents. My electronic thermometer said it cooled the attic down by about 20 degrees on the hot days.

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[monkycndo 0]

The basic rule of thumb for attic venting is that the free air space of the venting shall be 1/300 of the horizontal projected area of the roof. Half to be installed as close to the peak as possible. The other at the eaves. As an example: If your roof is 1200SF, you need 4SF of venting. Two feet at eave, two feet near the peak. All venting products will give you the free air value of the vent, normally in square inches. If you can only vent at the eave, it shall be at 1/150 of projected roof area. I am for passive venting wherever possible. It doesn't break or require electricity. But if a house uses endvents, a moderate breeze will negate the passive venting. A whirlybird ventilator will do the job in this situation. But they are hard to install in a tile roof. Some municipalities don't allow them due to being "unsightly". Sometimes a powered attic vent is the only way to solve a hot attic. I lived in the Mojave desert for 10 years. I've installed a few of them and they worked great. The power draw was more than saved in decreased AC usage. Because it was so dry, moisture issues were nill.

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[mnealtx 0]

Soffit<--- clicky

A combination of a vented soffit and some sort of ventilation, whether a turbine vent or a ridge vent can greatly reduce the amount of heat in an attic space.

Mike
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[azdiver 0]

look into changing it into a solar powered vent then sett it as low as it goes so it runs all day long

light travels faster than sound, that's why some people appear to be bright until you hear them speak

[billeisele 130]

sounds like you have this type soffit vent
http://www.cornerhardware.com/howto/ht076.html

avoid the fan by installing a few of these passive vents
http://www.airvent.com/homeowner/products/staticVents-roofLouvers.shtml

hot air rises out the roof vents and that action pulls in cooler air thru the soffit vents, the goal is not necessarily to "cool" the whole attic, it will be hotter near the roof deck, what matters is the temperature at the level of the insulation

what is equally important is how much insulation you have

Give one city to the thugs so they can all live together. I vote for Chicago where they have strict gun laws.

[happythoughts 0]

Ridge vent. It allows hot air to rise through it without consuming electricity.

Vented soffit. Allows cooler air to come up through it and rise out the vent.

Powered vents. Consume more electricity than they are worth.

Turbine vents. Eventually rust and do not spin. They become noisier as they rust. Most leak at some point.
A good storm will take them off.

[jm951 0]

Ditto on the ridge/soffit vent combo. It can be very effective without consuming power.

Powered vents are a bandaid approach to poor planning and construction and should only be considered as a last resort.

[happythoughts 0]

Quote

Ditto on the ridge/soffit vent combo. It can be very effective without consuming power.

My previous house had a 10ft wide covered deck that ran the length of the back of the house, 2nd floor.

On the front, 2nd floor, there was a 4ft wide deck that ran the length of the front. It had vented soffit.

The ridge vent ran 50ft. 12inches of blown-in insulation. Ceiling fans in all rooms.

Pocket sliders on both sides. A 12 ft and 8 ft on the back. 3 8 ft on the front.

Interior doors were all louvered.

[azdiver 0]

cooling the entire attic has a plus side too,it cools the roof and will extended the life of the roof, also if the airs duct work is in the attic space you lose less energy to heat transfer, reducing energy cost. in az there are alot of houses built close to one another, reducing the airflow around the houses. so out here a powered vent has significant effects on air movement through the attic. i have worked in attics in the summer out here and ones with powered vents were alot cooler than ones without, it can get up to 180 degrees and higher in the attics out here. constantly replacing the air will keep the temp down to about the temp of the outside air.

light travels faster than sound, that's why some people appear to be bright until you hear them speak

[billeisele 130]

AZ - agree, i was focusing on the Arkansas weather, similar to what we have in South Carolina, climates like yours require different solutions

Give one city to the thugs so they can all live together. I vote for Chicago where they have strict gun laws.

[DJL 235]

Quote

The basic rule of thumb for attic venting is that the free air space of the venting shall be 1/300 of the horizontal projected area of the roof. Half to be installed as close to the peak as possible. The other at the eaves. As an example: If your roof is 1200SF, you need 4SF of venting. Two feet at eave, two feet near the peak. All venting products will give you the free air value of the vent, normally in square inches. If you can only vent at the eave, it shall be at 1/150 of projected roof area. I am for passive venting wherever possible. It doesn't break or require electricity. But if a house uses endvents, a moderate breeze will negate the passive venting. A whirlybird ventilator will do the job in this situation. But they are hard to install in a tile roof. Some municipalities don't allow them due to being "unsightly". Sometimes a powered attic vent is the only way to solve a hot attic. I lived in the Mojave desert for 10 years. I've installed a few of them and they worked great. The power draw was more than saved in decreased AC usage. Because it was so dry, moisture issues were nill.

That's not just a rule of thumb, that's IBC 1203.2:
...a net free venting area no less than 1/150 of the area of the space being vented with that free area broken equally into an upper and lower portion. The exception is that the venting area can be 1/300 of the area if there is a vapor retarder on the warm side of the attic insulation having a transmission rate not exceeding 1 perm in accordance with ASTM E 96.

"I encourage all awesome dangerous behavior." - Jeffro Fincher

[azdiver 0]

agreed different climates have different needs or reasons for doing things. in high moisture areas replacing the air is just as important to keep things dry though. in areas with lots of precipitation and snow/ice a vent through the top of the roof isnt the best option though soffit fans are more effective. having worked in both extremely wet and dry climates i have seen what lack of proper venting can do to buildings wet you get moisture rot and mold. hot and dry you get dry rot and cracking and excessive energy loss.

light travels faster than sound, that's why some people appear to be bright until you hear them speak