Do power attic fans help?

[jtgv]

I have about 2000 square feet of attic space. There are 4 hip roofs that join in the center of the house with only about 8' of ridge vent. We have no soffit vents. The south side of the house is exposed to sunlight all day during the summer and the second floor of the house is very warm (2 AC units runs constantly). Will installing soffit vents alone provide sufficient attic ventilation or could I be a candidate for an attic fan?

[Kevin O'Neill]

I have about 2000 square feet of attic space. There are 4 hip roofs that join in the center of the house with only about 8' of ridge vent. We have no soffit vents. The south side of the house is exposed to sunlight all day during the summer and the second floor of the house is very warm (2 AC units runs constantly). Will installing soffit vents alone provide sufficient attic ventilation or could I be a candidate for an attic fan?

I can't say. It depends on;

Where you are and your climate

What color shingles

How high your attic is

how good your ridge vents are

how much soffit vent you add and the net free area of the opening (screens and louvers reduce the effective opening area)

etc etc etc

Generally, powered vents add more problems than they solve.

Get wind turbines and soffit and ridge vents first

Better ceiling insulation on the second floor will help more than vent fans. So will better windows. and once installed, windows & insulation use no electricity.

[pstu]

I have about 2000 square feet of attic space. There are 4 hip roofs that join in the center of the house with only about 8' of ridge vent. We have no soffit vents. The south side of the house is exposed to sunlight all day during the summer and the second floor of the house is very warm (2 AC units runs constantly). Will installing soffit vents alone provide sufficient attic ventilation or could I be a candidate for an attic fan?

While your hip roof prevents you from some solutions, you have more than just those two choices. For one thing, static "air hawk" vents can be placed near the peak of your roof. Almost every thing I hear, informs me that a power attic fan will consume more energy to run, than it can ever save on your AC.

You say your 2nd floor gets very warm, but we could stand to hear more about your problem description. The fact 2 AC units run constantly may not signal a problem, if indeed they cannot satisfy your thermostat setpoint then certainly that IS a problem. What temperature are you aiming for, and can you achieve that?

It never hurts to say what location you are in. I am a homeowner in S.Texas for example. In my state ductwork is commonly located in an unconditioned attic, and leakage presents dual problems: 1) often 10% of the conditioned air never gets into the house, and 2) that same volume must be made up by infiltration (or rarely, other means). But I don't know anything about your house, nor do any of the great professionals who might offer advice.

Best of luck -- Pstu

P.S. You might get better answers if you repeat your original question, starting a new thread all your own. This one has over 200 posts and to some people represents a tired discussion. Few threads grow to this length, and they become unwieldy.

[Shophound]

I have about 2000 square feet of attic space. There are 4 hip roofs that join in the center of the house with only about 8' of ridge vent. We have no soffit vents. The south side of the house is exposed to sunlight all day during the summer and the second floor of the house is very warm (2 AC units runs constantly). Will installing soffit vents alone provide sufficient attic ventilation or could I be a candidate for an attic fan?

Soffit vents can certainly help get more air into the attic, but for whatever amount of soffit venting is added, there should be just as much if not more amount of venting up high to exhaust the air the soffit vents intake. There is more than one way to naturally ventilate a roof if you can't get enough ridge vent installed. A PAV is one option but the caveat is the ceiling below the attic should be airtight to minimize loss of conditioned air from the house when the PAV runs.

Also, soffit vents are worthless if they become blocked by insulation near the edges of the attic. They must have a clear shot to be effective. Typically a good way to assure this happens is to install baffles against the rafters so no insulation can block the soffit opening.

[jtgv]

Thanks for the quick responses to my original post. Some additional information based on your replies: We live in NJ and we don't have any ductwork running through the attic. There is about 8 inches of insulation on the attic floor. At the center point, the attic ceiling is a good 7-8' high (more than enough room to stand up and move around). As far as the AC temperature, we generally try to keep it around 75-77 degrees and the first floor seems to cool much better than the second (could there be an issue with the speed/power of the AC fan, as we don't have issues in the winter with heating the 2nd floor?)



I have about 2000 square feet of attic space. There are 4 hip roofs that join in the center of the house with only about 8' of ridge vent. We have no soffit vents. The south side of the house is exposed to sunlight all day during the summer and the second floor of the house is very warm (2 AC units runs constantly). Will installing soffit vents alone provide sufficient attic ventilation or could I be a candidate for an attic fan?

[lentz]

You have been given some good information, make your choise and let us know how it worked. If possible add as much ridge vent as possible. You will need some place for the air to get into the attic. Soffit or boxing vents, No roof fans. The air will just short cycle.
Add another 12" of blown insulation in the attic. Check all ceiling penatrations and seal.
Could have a pro. check the speed of the second floor AC and see if the fan speed could be increased for AC.

[bill1274]

Hi. I found a lot of useful information on this thread concerning attic ventilation. There does seem to be a lot of agreement that passive ventilation (along with efforts at radiant barriers, insulation, etc.) is preferable to powered attic ventilation. I am now in the process of removing my attic fan (because it broke), and I am considering putting in passive exhaust venting near the peak of my roof.

My attic floor is 1050 sq. ft. and I have a hip roof at a 4:12 pitch. There are abundant soffit vents, so my intake is good. But the ridge line is extremely short, so a ridge vent is not an option for me.

I have looked at some attic vent manufacturer websites and I see that there in addition to small rectangular exhaust vents with 50 square inches of net free area, there are large round exhaust vents that are commercially available with 144 square inches of net free area (I found them on the Air Vent Inc website and the Ventamatic website). Having a large round exhaust vent is appealing to me because I think it will look better on my roof than numerous small roof vents would look (these small ones would need to be bunched close together near the center peak of my roof). Also, since I am removing a fan, I already have a round hole cut in my roof, so the swap (round vent for fan) should be more straightforward of a job for me.

Does anyone know of how these large round vents do as exhaust vents? Do they perform as advertised and produce good exhaust ventilation?

I probably could use more than 144 square inches of net free area for my exhaust ventilation, but maybe this amount will provide a “near adequate” amount of exhaust for my attic.

Thanks for any thoughts you might have!
Bill (in Pennsylvania)

[BPM in VA]

There does seem to be a lot of agreement that passive ventilation (along with efforts at radiant barriers, insulation, etc.) is preferable to powered attic ventilation. I am now in the process of removing my attic fan (because it broke), and I am considering putting in passive exhaust venting near the peak of my roof.

I would suggest paying more attention to the dissenters on this thread. Here’s my experience:

Twenty-some years ago I bought a three-level duplex in Northern Virginia with one gable vent, soffits front and back, and central air. In the summer, the basement was cold, but the upstairs was hot and stuffy, and the attic was unbearable. So I nailed up an inexpensive attic fan to the gable vent and left the attic access cracked open. Problem solved. The upstairs was just as cool as the downstairs, and the basement was less chilly. I don’t remember any change in my cooling bills. They might have gone down, since the system no longer had to work to cool the hot air from upstairs. Even if the bills went up, it was worth it. The extra cost would have just been the price of a perfectly air-conditioned house. I did zero maintenance on that fan, but it was still working six years later when I sold the house.

With that experience, I’ve tried to make sense of the arguments made here against fans, and I can’t do it. Perfect venting might make fans unnecessary, but few homes are perfectly vented, and installing vents is a lot of work. Perfect sealing between house and attic is also very rare, and if you just seal off your attic, you’re still living with an oven on top of your house. Better to vent the hottest air from both the attic and the house, so your system only has to work to keep cool air cool. The best way to do that may involve a fan.

Just my two cents.

[pstu]

What you are describing is a whole house fan, not at all typical of the attic fans discussed at length on this thread. You are deliberately exhausting house air via the attic, that works well in some climates. Back when we did not care about humidity, it was also done in hot climates like Austin TX and humid ones like Houston TX. But the whole approach is different from the "build it tight and ventilate it right" approach to a well controlled conditioned envelope. More in common with the old "open a window" philosophy.

And just to be sure I understand your situation, you were running air conditioning while doing this?



Best wishes -- Pstu

[Shophound]

I would suggest paying more attention to the dissenters on this thread. Here’s my experience:

Twenty-some years ago I bought a three-level duplex in Northern Virginia with one gable vent, soffits front and back, and central air. In the summer, the basement was cold, but the upstairs was hot and stuffy, and the attic was unbearable. So I nailed up an inexpensive attic fan to the gable vent and left the attic access cracked open. Problem solved. The upstairs was just as cool as the downstairs, and the basement was less chilly. I don’t remember any change in my cooling bills. They might have gone down, since the system no longer had to work to cool the hot air from upstairs. Even if the bills went up, it was worth it. The extra cost would have just been the price of a perfectly air-conditioned house. I did zero maintenance on that fan, but it was still working six years later when I sold the house.

With that experience, I’ve tried to make sense of the arguments made here against fans, and I can’t do it. Perfect venting might make fans unnecessary, but few homes are perfectly vented, and installing vents is a lot of work. Perfect sealing between house and attic is also very rare, and if you just seal off your attic, you’re still living with an oven on top of your house. Better to vent the hottest air from both the attic and the house, so your system only has to work to keep cool air cool. The best way to do that may involve a fan.

Just my two cents.

Heat loading of an attic can be reduced via passive means, such as radiant barrier attached to the attic rafters or as an integral part of new roof decking (the latter pertaining to new construction or major retrofit of existing buildings).

What you devised with your attic fan nailed to a gable vent and cracking open the attic hatch was in essence a whole house ventilator that is sucking air into the house from wherever it can get in, which includes through the walls, basement, etc. If radon is an issue in your area, sucking air up from the basement could result in radon being spread throughout your house, not to mention any humidity issues in the basement. There is also the potential to backdraft water heaters if you have gas heat. Fireplace? Those dampers aren't exactly airtight.

You're paying to air condition your attic. Sure, you say it doesn't make much difference on your electric bill, but how about your indoor air quality? Indoor humidity control? It's more than about just saving money at the electric meter. There are smarter ways to do things.

[jtgv]

Thanks for the quick responses to my original post. Some additional information based on your replies: We live in NJ and we don't have any ductwork running through the attic. There is about 8 inches of insulation on the attic floor. At the center point, the attic ceiling is a good 7-8' high (more than enough room to stand up and move around). As far as the AC temperature, we generally try to keep it around 75-77 degrees and the first floor seems to cool much better than the second (could there be an issue with the speed/power of the AC fan, as we don't have issues in the winter with heating the 2nd floor?)



I have about 2000 square feet of attic space. There are 4 hip roofs that join in the center of the house with only about 8' of ridge vent. We have no soffit vents. The south side of the house is exposed to sunlight all day during the summer and the second floor of the house is very warm (2 AC units runs constantly). Will installing soffit vents alone provide sufficient attic ventilation or could I be a candidate for an attic fan?

I went ahead and installed soffit vents around the house and, contrary to some of the advice her, an attic fan. Have the thermostat set to 105 degrees and the humistat at 70. This past weekend we had our first 90 degree days here in NJ. I noticed that the fan kicked on relatively early in the day, when the ouside temp wasn't much above 80. With the AC set to 78 degrees, the 2nd floor of the house seemed much more comfortable than it had been in the past. We also have a 2 story foyer that would get really hot on a warm day. It seemed much cooler the past couple of days. Once we have a few more hot days, and an electric bill or two, I will let you know how we are doing. Thanks for all the advice.

[webbyguy]

I was trying to calculate the possible savings I could get from an attic fan (if I assumed there was no air flow from the house to the attic, which I can factor in after calculating my maximum possible savings).

So here is the calculations I came up with, and please let me know if these are right:

My roof is about 1500sq ft. My insulation should be around an R value of 30 (I threw out the plaster and some other things because I didn't know how much resistance they provide), and I am assuming the fan would bring down my attic temp 15 degrees F. (I live in AZ and I'm guessing when it is 115 out it is probably at least 130 up there. I'll have to measure this summer.)

An R value of 30 means 30 h*ft^2*F/Btu.

Thus, I could save 1500ft^2*15F/(30 h *ft^2*F/Btu) = 750 Btu/hr by adding an attic fan.

My AC unit is about 10SEER, and somewhere online I found that is about 9 EER for an air conditioner. 9 EER is 9 Btu/(W*hr).

Thus, the energy savings I could reap are: (750 Btu/hr)/(9 Btu/W/hr) = 83Watts. This isn't much. Is my calculation wrong?

If the calculation is right, then there are probably better things to spend my money on to save energy. Also this doesn't yet factor in energy loss due to leakage between the house and the attic when an attic fan is added or the energy usage of the fan.

What have I left out? All of my ductwork is in the attic, and I think that has a lower R value. Maybe I should factor that in?

[energy_rater_La]

have you read this thread in its entirety?

[webbyguy]

have you read this thread in its entirety?

If it is a question for me, I read to about page 8, and skipped ahead to page 18. Is there a post that already does those calculations? Could you refer me to it?

[Shophound]

I was trying to calculate the possible savings I could get from an attic fan (if I assumed there was no air flow from the house to the attic, which I can factor in after calculating my maximum possible savings).

So here is the calculations I came up with, and please let me know if these are right:

My roof is about 1500sq ft. My insulation should be around an R value of 30 (I threw out the plaster and some other things because I didn't know how much resistance they provide), and I am assuming the fan would bring down my attic temp 15 degrees F. (I live in AZ and I'm guessing when it is 115 out it is probably at least 130 up there. I'll have to measure this summer.)

An R value of 30 means 30 h*ft^2*F/Btu.

Thus, I could save 1500ft^2*15F/(30 h *ft^2*F/Btu) = 750 Btu/hr by adding an attic fan.

My AC unit is about 10SEER, and somewhere online I found that is about 9 EER for an air conditioner. 9 EER is 9 Btu/(W*hr).

Thus, the energy savings I could reap are: (750 Btu/hr)/(9 Btu/W/hr) = 83Watts. This isn't much. Is my calculation wrong?

If the calculation is right, then there are probably better things to spend my money on to save energy. Also this doesn't yet factor in energy loss due to leakage between the house and the attic when an attic fan is added or the energy usage of the fan.

What have I left out? All of my ductwork is in the attic, and I think that has a lower R value. Maybe I should factor that in?

Aside from crunching your numbers myself, from the surface your result seems realistic.

Duct insulation in an attic can range from R4 -R8. If the outer jacket is foil or foil-like mylar, this acts as a radiant barrier and helps to reduce heat transfer from attic air and surfaces to duct. If you have the old gray jacketed flex up there, you don't have much R value...that jacket deteriorates in hot attics and allows the insulation to decay.

Short of spray foaming the roof deck, the best combination for ventilated attics in hot climates is foil based radiant barrier (either staple-up or impregnated on the roof decking material) and thick insulation on the attic floor, combined with sealing the obvious penetrations between the ceiling below the attic and the attic itself. This will outperform any natural or forced ventilation tactic, as it reduces total heat transfer into the dwelling, slows the rate of heat transfer, and does it without using one watt of electricity.

[mark beiser]

I went ahead and installed soffit vents around the house and, contrary to some of the advice her, an attic fan.

Did you also seal every penetration, including electrical boxes for light fictures and holes in the tops of walls for wiring and plumbing, and assorting building cavities?

If not, you are sucking conditioned air out of the house, and could be causing some walls to be at a negative pressure. Both are the usual unintended consequences of PAV's, and can cause a whole host of problems.

If there are very few penetrations, you may not have any problems.

In a home built like they are in my area, where all of the ductwork and wiring runs through the attic, recessed lighting is extremely common, and all of the combustion appliances get their combustion air from the ventilated attic, PAV's are one of the dumbest things you can put on a house.

[energy_rater_La]

and when you have an upstairs area that shares walls with the attic,
unless these walls are insulated and sealed with a foam sheathing board
on the attic side that is well nailed and sealed..continous from top plate to bottom plate..and sealed to floor between joists or trusses..the conditioned air is pulled out
of the upstairs area in to the attic. these rooms are surrounded on all sides shared with
attic and below the floors where it is also open to the attic..this is why these
areas are so difficult to condition. hard to bring the temp from 130 to 75 degrees
when the room is surrounded. its either fix the problem or keep paying a higher
utility bill..and life is full of choices.
but hey...like I said years ago...folks either believe the proven facts or disreguard them.

best of luck.

[jscott22]

So, here's a scenario.
***********************************************
2 story house in NC - built in 2003, will dual zone trane system (huge 1 for 1st floor - small 1 for 2nd floor), functioning fine.

Family room on first floor opens up to catwalk/hallway on 2nd floor. 2nd floor hallway has vent on 1 side and the thermostat and ceiling return on the other.

2nd floor bedrooms stay cool - hallway stays hot. on hot days, thermostat can not reach setpoint - and it runs constantly.

Attic: very well insulated w air handler in attic. proper soffiting and ridge vents...no other vents or attic fans.
************************************************

So, I am trying to find a way to reduce the temp on 2nd floor (i know virtually nothing about this subject by the way). I've come to conclusion that the reality is that it gets hot in NC - and an open 2nd story is going to get hot, period.

The one thing that concerns me is that the thermostat is in the hot hallway...and the bedrooms stay cooler. this tranlates to the unit constantly running. as it's not a "true" representation of air temp (2 extremes).

I started invetigiating attic fans - and luckily, came across this site. I hadnt factored in the radiant heat and other variables....

Anyway - I guess I am stuck with a hot 2nd floor.....any other suggestions...?

[Shophound]

So, here's a scenario.
***********************************************

Family room on first floor opens up to catwalk/hallway on 2nd floor.

There's the culprit.

The upper regions of the family room is likely nothing but dead air space. No supply vents up high to turn over that air, so it just heats up, and is drawn toward the second level catwalk/hallway when the upstairs system is running.

This is assuming there are no supply (a/c air) registers in the ceiling or high in the walls of the large space over the family room that is shared by the catwalk/hallway. You did mention one in the hallway adjacent to the bedrooms, which can't do much if the entire adjacent dead air space is doing nothing but gaining heat from the structure all day long.

You might consider having the thermostat relocated to the largest bedroom. Do the bedrooms tend to overcool? Do you leave the bedroom doors closed when the upstairs a/c is running?

[pstu]

So, here's a scenario.
***********************************************
2 story house in NC - built in 2003, will dual zone trane system (huge 1 for 1st floor - small 1 for 2nd floor), functioning fine.

Family room on first floor opens up to catwalk/hallway on 2nd floor. 2nd floor hallway has vent on 1 side and the thermostat and ceiling return on the other.

2nd floor bedrooms stay cool - hallway stays hot. on hot days, thermostat can not reach setpoint - and it runs constantly.

Attic: very well insulated w air handler in attic. proper soffiting and ridge vents...no other vents or attic fans.
************************************************

So, I am trying to find a way to reduce the temp on 2nd floor (i know virtually nothing about this subject by the way). I've come to conclusion that the reality is that it gets hot in NC - and an open 2nd story is going to get hot, period.

The one thing that concerns me is that the thermostat is in the hot hallway...and the bedrooms stay cooler. this tranlates to the unit constantly running. as it's not a "true" representation of air temp (2 extremes).

I started invetigiating attic fans - and luckily, came across this site. I hadnt factored in the radiant heat and other variables....

Anyway - I guess I am stuck with a hot 2nd floor.....any other suggestions...?

JScott, I recommend you start a new thread with the same information so you can get people focusing on your problem. I predict you will receive lots of useful professional opinion that way.

Best of luck -- Pstu