Following the advice from another member I am reposting my question in this section, as it may be more appropriate:
I bought a two-level townhouse in Miami that has no attic fan. As you can imagine, during the day, my AC struggles to keep the temperatures down. I will be adding more insulation in the attic, but I also want to install an attic fan to draw all that hot air out.
Problem is there is no roof vent for an attic fan to be installed, nor are are any vertical gables. There are soffit vents but no ridge vents, or anything else that may let hot air out escape on the top. The only existing vent in the roof is the one for the clothes dryer, which is connected to a 4" duct.
Before I hire a contractor to install a roof vent by cutting a new hole in the roof, I was wonsdering if I could somehow use this vent as an interim attic exhaust by attaching a fan to it. Problem is that I went to home depot and the only attic fans they have there are 15" in diameter.
My question is, can I use a bathroom fan in the attic instead of the larger attic fan? I am aware that the flow will be much less, but any venting at this point will be better than what I have now, which is zero. Can bathroom fans stand the higher temperatures and more continuous use in an attic environment?
Another alternative is a 6" booster fan that I found on ebay, which says can operate in temperatures of up to 266 degrees. I could use this along with a thermostat to vent hot air out of the attic through the existing 4" vent.
I've already been given great advice to seal any openings in the attic that may allow hot air to leak into the living area, and also the suggestion of installing a foil barrier. I do need some kind of vent, however, and that is my first priority.
So will any of the above options (attaching a fan to the 4" duct and vent) work until I save enough money to hire someone to do the job of installing the true attic fan on the roof?
I am sure you will get an answer from someone who is much more qualified than I am (just a DIYer), but from my understanding of attic ventilation, using a bathroom fan in the attic will be a waste of money. Your best bet is to have a ridge vent installed. Soffit vents and a ridge vent seem to be the ideal ventilation system (so long as there is enough of them in terms of intake area). A ridge vent (along with the insulation you plan on adding) should solve your problem.