A Guide to Installing Window Air Conditioners

October 31, 2013
November/December 2013
A version of this article appears in the November/December 2013 issue of Home Energy Magazine.
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Homeowners in the United States spend $1 out of every $8 of utility costs on cooling their living space. Window air conditioners (A/Cs) are an inexpensive alternative to central systems and are sold in greater numbers each year than all other residential cooling systems. They are bought to cool a specific room, are inexpensive to purchase, and are easy for anyone to install. But these benefits come at a cost. Window A/Cs operate less efficiently than most other residential cooling systems, and they may not provide equivalent comfort.


Figure 1. During the summer, hot outdoor air flows into the home, which makes the window A/C unit run longer and use more energy. (Marjorie Schott, NREL)


Figure 2. A/C unit with diverter installed.


Step 1: Remove the accordion panels from the sides of the A/C. (Chuck Booten)


Step 2: Cut and install rigid-foam panels to fill the space beside the A/C. (Chuck Booten)


Step 3: Air seal between the sashes using a closed-cell cylindrical foam backer rod. (Chuck Booten)


Step 4: Plug the top of the side channels. (Chuck Booten)


Step 5: Use tape to secure the foam panels and to prevent air leaks around joints. (Chuck Booten)

To better understand how window A/Cs perform and how they could be improved, researchers at the National Renewable Energy Laboratory (NREL), myself included, studied them as part of DOE’s Building America program. Our study found many opportunities for improvement. First, the window A/C installation resulted in significant air leakage—equivalent to having a 5 in2 hole in the outside wall (see Figure 1). All summer long, hot outdoor air flows into the home, making the A/C run longer and use more energy. This hot outdoor air reduces occupant comfort by increasing the heat load and often carries humidity into the home.

Second, a significant amount of the cool air leaving the A/C is recirculated back into the unit, because the outlet and inlet are so close together. Thus, that cool air does not help to cool off the home and results in a secondary waste of energy.

Third, we found that maintaining and cleaning the A/C on a regular schedule, which is not commonly done, could improve performance.

Finally, our team identified simple measures to improve both efficiency and comfort. Accessories provided by manufacturers can be replaced with inexpensive hardware store materials to improve window A/C installation, increase efficiency, improve comfort, and lower utility bills with a payback of less than one year.

Five Easy Steps to Limit Air Leakage

Our team recommends the following five simple steps to limit infiltration around a window A/C.

  1. Remove the accordion panels from the sides of the A/C. Typically, the sliding keeper can also be removed. Pull the frame out, then remove another keeper from the side of the A/C. (Do not remove the top and bottom braces; they hold the unit in the window. Use manufacturer-supplied hardware to secure the window in place after replacing the unit.)

  2. Cut and install rigid-foam panels to fill the space beside the A/C. Measure the thickness of the window sash to determine the thickness of the foam. Foam ¾–1½ inch thick will fit most window frame channels. Some foams have a skin to help protect the foam from weather. Exterior-grade tape can be used to cover the outside surface of foam for increased durability, since ultraviolet radiation and water can both degrade foam. If this is done, work from bottom to top and overlap tape so water will drain appropriately. A 2-foot x 2-foot foam panel is sufficient to seal one 4-foot by 4-foot window, and will cost about $3; a 4-foot x 8-foot foam panel, which will seal 8–10 windows, will cost about $10.

  3. Foam strips provided by the manufacturer for sealing between sashes are prone to air leaks. Use closed-cell cylindrical foam backer rod between sashes instead. Measure the width of the gap between sashes to select the appropriate size. Four dollars’ worth of backer rod will be enough to seal 3–4 windows that are 4 feet wide.

  4. Plug the top of the side channels. It is important to do this no matter what foam was used to seal between sashes because the side channels have a large opening between the window and the frame and provide an easy way for air to leak into or out of the house.

  5. Use tape to secure the foam panels and to prevent air leaks around joints. Tape the foam panels to the window, window frame, and A/C; tape the top and bottom of the A/C to the window sash and the frame respectively. If window frames are painted, consider using tape with a less aggressive bond to prevent the paint from peeling when the tape is removed. One roll of duct tape costs about $6 and will be enough for 5–10 windows, depending on their size.

Go Further: Address Cool-Air Recirculation

To further enhance performance, install a diverter between the cool-air supply and the room air return of the A/C (see Figure 2). This reduces short-circuiting of air from the supply to the return and maximizes the amount of cool air that goes into the room. This can increase the efficiency of the unit by up to 10%, providing better comfort and saving energy and money. Diverters can be made from ¼-inch medium-density fiberboard or similar material. If aesthetics are important, consider choosing a material that complements your room decor, such as fiberboard with melamine facing, or rigid colored plastic. One sheet of plain ¼-inch fiberboard costs about $5, and will be enough for about 8 windows.

learn more

Download the full report, Laboratory Performance Testing of Residential Window Air Conditioners, NREL/TP-550057617, February 2013.

Watch a YouTube video demonstrating the information in this article.

The Bottom Line

Air leakage wastes energy and costs money, but you can help homeowners reduce this leakage by following this simple five-step guide. Recirculation of air near the unit lowers efficiency and is easily reduced by installing a diverter. Periodic cleaning of intake and exhaust grilles on both the indoor and outdoor portions of the unit can help maintain efficient performance. Removing the unit from the window or sealing it up completely on the inside after cooling season is over will reduce or eliminate leakage through the unit itself.

Here’s how much your customers can benefit. A window A/C unit costs $150–600. Materials for improved installation cost $10–15 per unit. Improved installation results in cooling savings of up to 7% or 280 kWh per year in a hot climate; and electricity bill savings of up to $31 per year. Lifetime of the unit is assumed to be 10 years. This means that the homeowner will probably save enough to pay for the cost of the unit over its lifetime.

Dane Christensen and Jon Winkler also contributed to this article.

Chuck Booten, Jon Winkler, and Dane Christensen are senior engineers at the National Renewable Energy Laboratory.

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