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Should I run gable fan while AC is on?

I live in a brick duplex with two stories, an attic, and basement (4 total) and the walk-up attic is finished down the center with crawl spaces on either side. There is no duct work in the attic and a gable window at the only outside wall. I don't think there's any insulation either. There was an ancient AC unit that I removed, and I only use it for storage. It gets very hot so I installed a gable fan (high powered meant for barns I think) right against the window. It's thermostatically controller and the window is open with a screen. I try to use it as a whole-house fan, and during cool days in the fall and spring I run it, open the door to the attic, and crack all the windows on the cool side of the house. I think this works pretty well but it is difficult to say if it works better or worse than just opening all the windows.

My question is one of general strategy. In the hot summer days when we run the air, should I leave this fan on or off? If on, at what temp setting? Also if on, should I leave the attic door in the upstairs hallway open or close? Will this help keep the attic, and therefore the whole house cooler while the ac is running, or will it suck all the conditioned air right out of the house? Please help, I get different answers from different AC guys I talk to who I think are just giving me "rule of thumb suggestions." Any advice is appreciated, and please let me know if I am missing some details that would help. Thanks! -KH

Re: Should I run gable fan while AC is on?

Ventilating the attic would be good, insulating it would be better.
Don't run the fan with the attic door open. It will suck out the conditioned air.
Even with the attic door closed it sounds like the fan may be too big for venting just the attic unless you have adequate makeup air from outside, otherwise it may draw conditioned air from the house.

Re: Should I run gable fan while AC is on?

So is the goal to seal it off from the rest of the house, and then vent it somewhere else besides the window so it pulls air in through those vents and pushes it out through the gable fan/window? So you are constantly refreshing it with fresh air, but the conditioned air in the house is never touched? It seems like I would have to replace the door (it is a walk-up attic) with something that does a better job of sealing/insulating, and find any other places where the air passes from the house to the attic and seal them up too.

Insulating seems like it would be tough because I would like to keep it a walk up attic for storage. You are talking about insulating the attic from the house right? Not insulating the attic from the outside world.

Thanks for the response! So my questions after rambling are:
1 - Is the goal to seal it off from the rest of the house > and vent it somewhere else besides the window so so it is constantly refreshing with fresh air?\
2 - how to better seal the door from the upstairs hallway to the attic steps
3 - How to insulate it and keep it a walk-up.

Thanks! -KH

Re: Should I run gable fan while AC is on?


Insulating is a good idea, and it's not hard, it's certainly a good project for the DIY.

First insulate the spaces between the joists (the floor of the attic), as it's not insulated now like you stated.

If you walk a lot in the attic, you can plywood the floor after insulating (1/2" sheets will be good, if your joists run 16" o.c). Fasten the plywood to the joists. Make sure that sheets of plywood always meet on top of a joist.

Now, the other suggested insulation is for exterior walls and roof deck. When insulating under the roof, don't forget to install foam spacers, that will allow air to flow between the roof deck and the insulation and to secure the insulation in place, so that the it doesn't fall.

The door could use some foam weatherstrip to seal better.

Good luck.

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