Does attic ventilation = more efficient air conditioning?

June 22, 2009 8:50 AM

Is leaving the windows in my attic cracked beneficial for keeping my air conditioning bill low?

I'm curious if my present strategy has any benefit at all or if I'm misguided in my approach:

My home was built in the 1930s. The attic is half finished, half storage with windows on both ends of the space. There are no heating and cooling vents in the attic.

The door to the attic has weather stripping and a sturdy draft guard at the bottom.

During the summer the attic gets roastingly hot. It's better now that I installed a ridge vent instead of the cruddy little can vents, but it's still quite hot up there.

I've been opening the windows on each end to get a cross breeze which lowers the attic temp. easily 10-15 degrees. For what it's worth I live in a very windy area and there is a steady cross breeze almost 24/7.

Assuming that my floors and walls are properly insulated (the floor between the attic and the living space and the walls under the attic), which I believe they are based on my inspection... is leaving the windows open to provide natural ventilation (no powered fans that would create a negative pressure space) benefiting or hurting my energy efficiency with the air conditioner on?

It seems that lowering the attic temperature can only benefit me, but I'm not heating and cooling specialist and may be overlooking something outside my scope of knowledge.

Thanks!

posted by JFitzpatrick to home & garden (4 comments total)

Lowering the temperature in the attic, under the conditions you describe, will reduce your cooling bill for sure. As long as there isn't significant air exchange between the living space and the attic (and based on what you've described, there shouldn't be) then the key thing is to minimize the amount of heat coming from the attic down to the living space. Cooling off the attic will do that.

How much of a difference this makes will depend on how well insulated the living space ceiling/attic floor is. The better the insulation, the less it matters how hot the attic is.

This can only be counterproductive when there's air flow between the living space and attic--either allowing hot air from the attic to infiltrate the living space, or cool air from the living space to exit through the attic (to be replaced by hotter outside air coming in somewhere).
posted by FishBike at 9:01 AM on June 22

My understanding is that attic fans that pull air through the attic, from outside and venting to outside, are good even if you have central AC. They keep the attic cooler, and that means less heat radiating through your ceiling into your living space. This is effectively what your windows/cross-breeze are doing, without any fans.

The ones that aren't considered good anymore, if you have central AC, are the fans that pull air from the living space (typically through a big louvered vent in the second-floor ceiling somewhere, often in a hallway) and exhaust it outside via the attic. Those were a good idea before central AC because they enhance a natural "chimney effect" and cool upstairs rooms by pulling cooler air up from ground level or a basement, but if you have working AC you're just exhausting expensively-cooled air.

I've recently been looking at houses and have seen older homes from the 50s and 60s with both kinds of setups.
posted by Kadin2048 at 9:28 AM on June 22

Thanks for the replies! I figured my basic natural ventilation setup couldn't hurt but sometimes you're surprised by unconventional things when dealing with home ownership.
posted by JFitzpatrick at 1:04 PM on June 22

Thinking about the three ways heat travels (convention, conduction and radiation), you'd want to try to keep the air in the attic as close to ambient air temp as possible. Because the hotter it is up there, the bigger the differential in temperature between the cooled space and the attic. While also attempting to keep any of your cooled air from leaking into the attic. In the cooling season, this probably is less likely to happen, as heat rises. (There's a possibility you'd even want some of your hot indoor air to seep up and out, so you'd wouldn't have to cool it off again.)

Also consider the costs of an attic fan- if it doesn't work exactly right, you might spend more running the fan than you would just running the AC a little harder.

But yeah, I'd open the windows. If you have double hung, I'd open the higher window. You want the attic space to be a buffer between the hot-ass roof and your cool interior.
posted by gjc at 7:37 PM on June 22