Solar powered attic fan

8 years ago

Live in Texas so it gets hot in summer. Do solar powered attic fans actually reduce demand on HVAC. Have read pros and cons and not sure who is correct. Read one article that stated attic heat is due to radiant heat (roof and timbers soak up the heat) and fan does not really dissipate that heat. That article recommends better insulation in attic to keep rest of house more comfortable and no solar powered attic fans. Makes sense to me. My HVAC guy says need to pull out hot air and humidity out of attic which will decrease work load for HVAC thought this made sense until I read the article recommending no solar powered attic fans. House already has 5 attic turbine vents.

I do plan on adding more insulation that the HVAC guy recommended. His company only does blow in fiberglass not foam. I know foam is more expensive, so should I get that or go with the fiberglass. Duct work already has foam insulation .

Before I commit have requested my power company to do an energy assessment. Anything else I need to consider? Appreciate any advice you can provide.

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Austin Air Companie

8 years ago

Adding any powered type fan for attic venting is not a good idea. Many times these are used as a quick fix to treat the symptom of a hot attic without exploring why the attic is hotter than it should be.

Typically these powered fans don't last that long until the motors in them burn up. The bigger problem they cause as a result of their use is putting the house under a negative pressure, because the reason the attic is hot is most often due to the lack of vented air.

Powered fans are for the lack of a better word, stupid. What I mean by this is the fan itself doesn't know anything about where the air it is attempting to blow is coming from. If the fan is going to blow air, this air must come from some where. If the amount of air being blown out of the attic is greater than the volume of air it needs it will suck air from every receptacle in the home to get the air it is trying to blow. It will continue to do this until eventually the motor burns up.

Can lights, wiring receptacles, cracks and crevices thru insulation, other light fixtures in which electrical boxes are not sealed or the holes that are cut to run wires, piping, duct work and so on. All these cracks and crevices are normal in homes as young as 10 years and in some cases homes younger still.

The whirly birds as well as ridge vents are the best at expelling heat thru the top of the roof. The key is to supply enough natural air vents at eaves around the home so the natural flow is not muted. (air in equals air out.)

Once those whirly birds start to spin, they will do the work provided there is adequate air available. That can be the challenge on some older homes as in some cases the way these homes were roofed presents challenges to putting supply vents all the way around the soffits of the home.

Then when you get to re-insulating the attic, make sure these vents are not blocked up with insulation.

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Comments (11)

randy427
8 years ago
You mention your turbines to get the hot air out, but how is your eave venting to get the replacement air into the attic in order to establish circulation?
Sonja Thompson thanked randy427
Sonja Thompson
Original Author
8 years ago
Good question, I was wondering where the inflow to the attic is other than from the 2nd floor and the various areas where I can see daylight in the roof. Just walked outside and noticed lots of vents on the soffit on the side of the house that is only 1 story. Hard to see them since they are painted the same color as the soffits. I assume the rest of the house has the same vents. So I guess the ambient air flows in the soffit vents to the attic; however hot air rises and cold air sinks so if the ambient temperature is lower than the attic temperature I would think that that ambient temperature air will not rise into the attic since it lower than most of the attic. Also I think air flows from a high pressure to a lower pressure so if the attic fan reduces the pressure in the attic then air would flow into the attic more readily. Am I tracking? But just read another article that these powered attic fans really do not help reduce energy cost.

I am mightily confused. Help
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User
8 years ago
Sonja, never assume on the soffit vents, look and measure. Also, remember the size of the cover doesn't mean the opening is that size, I've seen 9" x "6 vents over a 5" circular hole. My current home was woefully under vented, so I hired a guy to add 9" x 3" openings every 5' or so, and cover them with 10" x 4" vent covers. I went from 10 soffit vents to over 30. Attic was dramatically cooler after that...still hot, it is Texas, but not stiffling.

The whirlybirds work good for us (also in Texas), but positioning is key. A pair of badly positioned vents mean that air flows in the slightly lower vent and out the higher one.
Sonja Thompson thanked User
Sonja Thompson
Original Author
8 years ago
Thank you so much. I really appreciate it.
mike_home
8 years ago
If you already have 5 turbine vents then adding a powered vent is that not going to do much in lowering the temperature of the attic in my opinion.

What is the temperature of the attic on a hot sunny day? If it very hot then the soffit vents are not proving enough air intake. Air has to entering the attic has to equal the air leaving.

If you plan to add insulation to the attic floor then you should add either blown in fiberglass or fiberglass batts. You could do foam but that would be expensive and accomplish the same thing as fiberglass. The advantage of the foam is it would seal all penetrations, but that could be done with calking.
Sonja Thompson
Original Author
8 years ago
Thanks Mike. I actually have no idea how hot it gets up in the attic. I just had a spring tune up by the HVAC guy and he suggested increasing insulation with blown in fiberglass, adding float safety switches on the units that do not have them, an attic tent to keep air from escaping through the attic stairs which are located in the upstairs hallway, and a solar powered attic fan. I called the electric company and they will come out and do an assessment. Sounds like I need to get the temperature in attic before I do anything. I do think the insulation is low up there, can see the joists. HVAC company says they charge $2.00 a square foot for blown in fiberglass insulation, not sure if that is a fair price.
mike_home
8 years ago
You should have float safety switches on any HVAC equipment that is above or near a living space. The attic pull down stair is a good suggestion. You can buy one or make one yourself from rigid foam insulation.

The attic insulation should be at least R-30, and ideally up to R-40. You should be able to add more insulation on top of what you have.

You have to be careful with powered attic fans. Then can do more harm than good if not installed properly.
Sonja Thompson
Original Author
8 years ago
Mike, really appreciate the helpful advice. Will forego the powered attic fan.
energy_rater_la
8 years ago
good decision to not install pav's. attic tent is a good idea. caulk it to air

seal once it is installed.

I"d love to see foam "insulated/sealed" ductwork.

this is something I've preached against for years.

be nice to see how it turned out for you.

best of luck
Sonja Thompson
Original Author
8 years ago
Thank you energy rater. I will update when I have blow in insulation and attic vent installed.