Venting a window AC unit

I would like to place a window AC unit in a 14x13 bedroom. There is only one window in a dormer area that faces east and I would prefer not putting it the window. All the other walls are surrounded by attic space. Could I place the unit in one of the walls and vent it into the attic?

Answer from Green Energy Efficient Homes

You should definitely not vent a window AC unit into an attic for two reasons: heat and humidity. Your window AC unit extracts heat and moisture from the room and blows it out the exhaust vent. If you vent your window AC unit to the attic you will be adding humidity to your attic, which could damage the wood structure of the attic as well as dampen any insulation you have there. Damp insulation becomes less effective, and also tends to compress on its own due to the added weight of any moisture, which again makes it less effective, so by running your AC unit to your attic you would be decreasing the R value of attic ceiling insulation, which means more heat flowing down from the attic to the bedroom (and any other rooms immediately below the attic).

The other factor to consider is heat. Since you will also be adding heat to your attic it is possible that venting your window AC unit to the attic may heat the attic up hotter than it normally gets. It is not a given that you'll heat the attic up; if the attic is poorly vented, or you have dark roofing material or your roof has a south or southwestern exposure it's possible your attic already gets quite hot, and the exhaust air from a window AC unit will be cooler than the air already present in the attic. In this case, venting the unit to the attic would lower the temperature in the attic (at least during sunny times when solar energy heats the attic up). But the humidity problems still apply so I would not recommend this at all.

Your other option would be to use a portable air conditioner, which can be moved around and which can vent out a window without taking more than a small amount of window space It works well for sash windows where a small opening can be used to insert the exhaust tube and the trim kit that holds the tube in place. You can also vent such a unit out a hole in the wall if you don't mind adding the hole.

Comments for
Venting a window AC unit

Average Rating

Click here to add your own comments

Aug 12, 2010
Rating

Vent the attic as well
by: Anonymous

If you set up an exhaust fan with a decent CFM (cubic feet per minute, a fancy term for 'amount of air moved') and actively vent the heat from the attic, you should have no trouble from the heat the a/c adds to the space.

While it's true a/c's dehumidify the air in a room, the exhaust coming out the back is incredibly dry and all the extracted water is in solid form, it's condensate that can be collected in a drip tray and emptied manually every day, or pumped away by a condensate pump. So humidity is unlikely to be an issue from the a/c venting into the attic.

That said, a/c's add a *tremendous* amount of heat into enclosed spaces, it's rare to find an attic as hot as a room with even a small a/c venting into it. So imo you will need to actively vent the attic to remove this excess heat. If you do it properly with a gable fan, you will have the benefit of exhausting all the stale air from your entire home as well, so it's a good way to go.

Click here to add your own comments

Join in and write your own page! It's easy to do. How?
Simply click here to return to Home Energy Efficiency Questions

Feel free to link to this page to spread the word about energy efficiency