Being a life long general contractor, i'll try to keep this short winded LOL!!
proper ventilation is key. The attic space should NEVER be anything warmer/cooler than the outside ambient air temperature...that is how you know your venitlation system is working properly. It will also GREATLY extend the life of your roof. Shingles today are mostly fiberglass, they are not nearly as durable as the older asphalt shingles, so anything you can do to help them last longer is a huge financial plus; esspecially with the huge price increases roofing materials have seen in the last 4 yrs.
1st. Attic insulation, do you have the proper amount? Most of it settles over time if they did blow in, dropping the R value. I always shoot for an R60 in attics, R49 is code, but when we blow an R60 I know it's overkill, and I also know if for some reason it settles, it will settle into nothing less than R49
2nd biggest mistake people make is mismatching intake/exhaust systems. They will have gable vents on the end of the house, and then installed ridge vent. They will use ridge vent in addition to "box vents/turtle vents" whatever you want to call them. They will install ridgevent and not realize that the area between the outside wall top plate and bottom side of the roof deck is blocked off with insulation since back in the day the thought was "you dont want any air getting into the attic"...which we now know is false, you was air circulation to try and keep the attic at the same temperature as outside air. SO, you have to pick one ventilation style and run with it, if you do ridgevent...you HAVE to do soffit vents to get the air coming into the attic and force the hot air up and out the ridge vent. Having other forms of venting screws that flow all up and it wont work.
With any venting, you want at LEAST 1sqft of venting per 300sqft of attic space....energy star mandates 1sq/ft of vent per 150sqft of attic space, and that is what we shoot for. In laymans terms, the typical box vent is 1sqft of venting, so if you have a standard 1200sqft ranch, you will need 8 of these roof vents in order to allow enough hot air out.
That is why ridge vent is so popluar because it can easily vent a big attic, but you HAVE to have soffit venting to allow the fresh air in to expell the old stale air out.
3. Are you talking a whole house fan that you can open and shut, these allow you to open your windows at night, turn the whole house fan on and pull cool night air inside. then come morning before you leave for work, you shut the house up, and if it's insulated properly, it will remain nice and cool when you get home....along the same lines as most garage when you walk into them on a hot day, if your the first person in the building it's very cool inside.
4. if your talking JUST an attic fan, I dont even think they make them anymore that are not controlled by an internal thermostat you can set, so that way it monitors and corrects the attic space as needed all by itself. Downfall to attic fans, maintainence. Your introducing an electric motor, which is obviously a wear item, so you may end up replacing the motor a few times in the life of your home....and cost to wire it up, install, etc...
I'm personally a big proponent of allowing physics to vent attic spaces, poly chutes wont ever wear out. There is work involved retro fitting proper venting into older homes, but once it's done it can help reduce utility bills. One customer I remember saved every bill, we did the siding, windows, roof, and addressed a seriously lacking ventilation issue in his house. He called that fall, and doing all that we did literaly dropped his summer utility bills 50%, he was stunned, and thought it was a fluke, but after the summer season, all the small bills, he realized I was'nt just selling him stuff to make money, I was selling him a system to make their home and life better, which reflects good on me
Just like roofs, I will NOT sell a roof unless I can assess and fix any venting issues, and I'm always $1-2000 higher than the typical hack roofers that wanna slap up shingles and go, but I dont beleive in half assed jobs, and I get pissed with these hack roofers nowadays charging what they do for a roof, and then in 5-8 yrs the roof starts to fall apart again, all because they're baking them to death....and, some of it goes back to the homeowner too being cheap, many say they cant afford my bid, and yet, they always find the money to completely redo the roof 8-10yrs later after they used the cheaper roofing company.
