Homeowners in the United States spend 

one out of every eight dollars 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 purchased to cool a specific 

room and are easy for anyone to install. 

In contrast to these benefits, window 

A/Cs come at a cost—they operate less 

efficiently (using more energy to do the 

same cooling) than most other residential 

A/C systems.

Researchers at the National Renewable 

Energy Laboratory (NREL) studied 

window A/Cs on behalf of the U.S. 

Department of Energy’s Building America 

program, to understand how they perform 

and how they could be improved. 

The study showed that window A/C 

installation resulted in significant air 

leakage—equivalent to having a 5-in2 hole 

in the outside wall. All summer long, hot 

outdoor air flows into the home, as shown 

in the figure to the right, making the A/C 

run longer and use more energy. This 

outdoor air reduces comfort for occupants 

through increased heat and often carries 

humidity into the home.

A portion 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 cool off 

the home and is a secondary waste of 

energy. Also, the researchers verified the 

importance of appliance maintenance and 

cleaning.

Finally, NREL’s team identified simple 

measures to improve both efficiency 

and comfort. Accessories provided by 

manufacturers can be replaced with 

inexpensive hardware store materials 

to improve a window A/C installation, 

increase efficiency, improve comfort, 

and lower utility bills with a payback 

of less than one year.

PROJECT INFORMATION

Building Component: HVAC

Application: Retrofit; single and/or 

multi-family

Year Tested: 2012

Applicable Climate Zone(s): All 

PERFORMANCE DATA

Cost of Window A/C unit: $150–$600

Cost of Materials for Improved Installation: 

$10–$15

Energy Savings: up to 7% cooling savings, 

or up to 280 kWh/year

Electricity Bill Savings: up to $31/year; 

enough to pay for the cost of the unit over 

its lifetime

FOR MORE INFORMATION

Read the full report, Laboratory 

Performance Testing of Residential 

Window Air Conditioners, NREL/TP-5500-

57617, February 2013. www.nrel.gov/docs/

fy13osti/57617.pdf

“Air sealing around the window 

and the air conditioner is critical 

for best performance”

 —Chuck Booten, Ph.D., 

 Senior Engineer, NREL

U.S. Department of Energy

SM

Building America Case Study: 

Technology Solutions for Existing Homes

A Homeowner’s Guide to

Window Air Conditioner Installation 

for Efficiency and Comfort

Typical air leakage pathways increase electricity 

use and decrease comfort. Illustration by Marjorie 

Schott, NREL

BUILDING TECHNOLOGIES OFFICE
Five Easy Steps to Limit 

Window A/C Infiltration

1. Remove accordion panels. Typically, a sliding 

keeper can be removed. Pull the frame out, 

then remove another keeper from the side 

of the A/C. (Do not remove 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 

spaces beside the A/C. Measure the thickness of 

the window sash to determine foam thickness; 

3⁄4–11⁄2 in. 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 outside surface of 

foam for increased durability. If this is done, 

work from bottom to top and overlap tape so 

water will drain appropriately. (Cost of foam: 

$3–$10 for multiple windows.)

3. Foam strips provided by the manufacturer for 

sealing between sashes are prone to air leaks. 

Instead, use backer rod (closed cell cylindrical 

foam) between sashes. Measure gap thicknesses 

to select appropriate size. (Cost of backer 

rod: ~ $4, enough for multiple windows.)

4. No matter what foam is used, it is important to 

also plug the top of the side channels. 

5. Use tape to secure the foam panels and 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. Different 

colors of tape are available. If window frames 

are painted, consider using tape with a less 

aggressive bond to prevent peeling. (Cost of 

one roll of duct tape: ~ $6, enough for multiple 

windows.)

Go Further: Address Cool Air 

Recirculation

To further enhance performance, install a diverter 

between the cool air supply and room air return of 

the air conditioner. This reduces short-circuiting 

of air from the supply to the return and maximizes 

the amount of cool air that goes into the room, 

saving energy and money.

Diverters can be made from 1⁄4-in. medium density 

fiberboard or similar material. (Cost for one sheet 

of fiberboard: ~ $5.)

1

2

3

4

5

Installing a diverter (bottom) helps to maximize cool air 

flow from the A/C unit. Illustration by Marjorie Schott, NREL

BUILDING TECHNOLOGIES OFFICE: WINDOW AIR CONDITIONER INSTALLATION

The Bottom Line

■ Air leakage wastes energy and costs money, but homeowners can reduce this leakage easily.

■ Recirculation of air near the unit lowers efficiency and can be easily reduced.

■ Periodic cleaning of intake and exhaust grills on both the indoor and outdoor portion of the unit 

can help maintain efficient performance.

■ Remove unit from window or seal it up completely on the inside after cooling season is over; 

otherwise, air will leak through the unit itself.

DOE/GO-102013-3920 • June 2013

Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 10% post consumer waste.

For more information, visit:

www.buildingamerica.gov

The U.S. Department of Energy’s Building America 

program is engineering the American home for energy 

performance, durability, quality, affordability, and comfort.