My house is 112 years old. I was in my attic and soaking my shirt because it was 120° in there (I live in Florida). I thought how nice it’d be to get some fresh air in there. I thought it’d be even better if I could get that air from the (somewhat) cooler air in my crawlspace. Then I noticed the 4" cast iron pipe that was used to be the drain pipe for the house from 1915 through about 2000. The vent for that pipe runs through my attic and straight down to my crawlspace…unused and forgotten. So I cut the pipe and disconnected it from the roof and cut off and closed up any branches it had. I cut it and slapped a screen on it in the crawlspace. In the attic I put on an inline fan (6 watts) and a bit of duct to vaguely direct it around the attic. Early tests say my attic is about 7° cooler. Am I crazy?
It’s from the crawlspace under my house, not a living space at all
Still might be a problem, especially if you don’t have great insulation under your subflooring. You’re pushing cool air from under your house to the attic. That vacuum of cold air is either going to be replaced by cooler air pulled from your house if you don’t have enough insulation, or from the hot air outside.
Pulling in hot air from outside is going to make your AC work harder because that hot air will rise and warm your house. And pulling colder air from your house into the crawl space means your AC is now working harder to cool your house, your crawl space, and your attic.
The space in your attic and your crawlspace are supposed to act as insulated buffer cells between your living space and outside. Connecting the two basically creates a vacuum that vents cool air from the ground to the vent in your roof.
Plus… Depending on how old your home is, you may unintentionally be pumping a dangerous amount of radon into your attic.