Measure Guideline: 

Energy-Efficient Window 

Performance and Selection

John Carmody and Kerry Haglund

NorthernSTAR

November 2012
NOTICE

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iii

Measure Guideline: 

Energy-Efficient Window Performance and Selection

Prepared for:

The National Renewable Energy Laboratory

On behalf of the U.S. Department of Energy’s Building America Program

Office of Energy Efficiency and Renewable Energy

15013 Denver West Parkway

Golden, CO 80401

NREL Contract No. DE-AC36-08GO28308

Prepared by:

John Carmody, Kerry Haglund

NorthernSTAR

University of Minnesota

2004 Folwell Avenue

St. Paul, MN 55108

NREL Technical Monitor: Stacey Rothgeb

Prepared under Subcontract No. KNDJ-0-40338-02

November 2012
iv

Acknowledgments

This report was prepared for the U.S. Department of Energy Building America program. Funding 

for this work was provided by the National Renewable Energy Laboratory under contract KNDJ0-40338-02. Additional funds were provided by the University of Minnesota, the Initiative for 

Renewable Energy and the Environment, and Building Knowledge Inc.

This guide is the product of a collaborative effort and builds on work done through the U.S. 

Department of Energy’s Windows and Glazing Program and related work done by other 

Building America teams. We appreciate the contributions of Jim Larsen (Cardinal Glass 

Industries), Garrett Mosiman (Center for Sustainable Building Research), and Nils Petermann 

(Alliance to Save Energy). We offer special thanks to Ren Anderson, Stacy Rothgeb, and Chuck 

Booten at the National Renewable Energy Laboratory.
v 

Contents

List of Figures ........................................................................................................................................... vii

List of Tables ............................................................................................................................................ viii

Definitions ................................................................................................................................................... ix

Executive Summary.................................................................................................................................... x

Progression Summary............................................................................................................................... xi

1 Introduction ........................................................................................................................................... 1

2 Measuring Window Performance........................................................................................................ 2

2.1 U-Factor ...............................................................................................................................3

2.2 Solar Heat Gain Coefficient .................................................................................................3

2.3 Visible Transmittance ..........................................................................................................4

2.4 Air Leakage ..........................................................................................................................4

2.5 Condensation Resistance .....................................................................................................5

3 Window Technologies.......................................................................................................................... 6

3.1 Glazing Types ......................................................................................................................6

3.1.1 Multiple Layers ........................................................................................................6

3.1.2 Low-Emittance Coatings .........................................................................................7

3.2 Low-Conductance Spacers and Gas Fills ............................................................................9

3.2.1 Low-Conductance Gas Fills .....................................................................................9

3.2.2 Warm Edge Spacers .................................................................................................9

3.3 Frame Types.......................................................................................................................12

3.3.1 Metal Frames .........................................................................................................12

3.3.2 Thermally Broken Metal Frames ...........................................................................12

3.3.3 Nonmetal Frames ...................................................................................................13

4 Window Selection Process................................................................................................................ 16

4.1 Energy Codes .....................................................................................................................16

4.1.1 Windows in the 2009 International Energy Conservation Code ...........................17

4.1.2 Windows in the 2012 International Energy Conservation Code ...........................18

4.2 ENERGY STAR ................................................................................................................18

4.3 Window Selection Tool .....................................................................................................20

4.4 RESFEN .............................................................................................................................21

5 Cost and Performance ....................................................................................................................... 23

5.1 Energy and Cost Savings for New Windows .....................................................................23

5.1.1 Savings for New Windows in Climate Zones 1 and 2 ...........................................24

5.1.2 Savings for New Windows in Climate Zones 3 and 4 ...........................................26

5.1.3 Savings for New Windows in Climate Zones 5–8 .................................................29

5.2 Energy and Cost Savings for Replacement Windows .......................................................33

5.2.1 Savings for Replacement Windows in Climate Zones 1 and 2 ..............................33

5.2.2 Savings for Replacement Windows in Climate Zones 3 and 4 ..............................35

5.2.3 Savings for Replacement Windows in Climate Zones 5–8 ...................................38

5.3 Window Costs (New and Replacement) ............................................................................42

5.4 Life Cycle Cost Analysis ...................................................................................................43

5.4.1 Life Cycle Cost Summary ......................................................................................44

5.5 Other Benefits ....................................................................................................................45

5.5.1 Comfort ..................................................................................................................45

5.5.2 Reduced Peak Demand and Heating, Ventilation, and Air Conditioning Costs ...49
vi 

6 Impact of Design on Performance .................................................................................................... 52

6.1 Orientation .........................................................................................................................52

6.1.1 Orientation in the Northern Zone (Heating Dominated) ........................................52

6.1.2 Orientation in the Central Zones (Heating and Cooling) .......................................53

6.1.3 Orientation in the Southern Zone (Cooling Dominated) .......................................54

6.2 Window Area .....................................................................................................................55

6.2.1 Window Area in the Northern Zone (Heating Dominated) ...................................55

6.2.2 Window Area in the Central Zones (Heating and Cooling) ..................................56

6.2.3 Window Area in the Southern Zone (Cooling Dominated) ...................................57

6.3 Shading ..............................................................................................................................58

6.3.1 Shading in the Northern Zone (Heating Dominated) .............................................58

6.3.2 Shading in the Central Zones (Heating and Cooling) ............................................59

6.3.3 Shading in the Southern Zone (Cooling Dominated) ............................................60

Important Resources ..........................................................................................................61

7 Installation........................................................................................................................................... 61

7.1 General Installation Guidelines ..........................................................................................61

7.2 Watertight Installation .......................................................................................................62

7.2.1 Storage or Mass Wall System ................................................................................62

7.2.2 Perfect Barrier System ...........................................................................................62

7.2.3 Drained Wall System .............................................................................................63

7.3 Replacement Windows and Sashes ....................................................................................63

References................................................................................................................................................. 65
vii

List of Figures

Figure 1. Heat flow through a window ...................................................................................................... 2

Figure 2. Window properties on the NFRC label...................................................................................... 2

Figure 3. Heat loss through a window by conduction, convection, and radiation............................... 3

Figure 4. Solar gain through a window..................................................................................................... 3

Figure 5. Visible light through a window.................................................................................................. 4

Figure 6. Air leakage through window assembly .................................................................................... 4

Figure 7. Condensation on window surface............................................................................................. 5

Figure 8. Advancements to improve energy efficiency .......................................................................... 6

Figure 9. Spectral transmittance curves for glazings with low-e coatings........................................... 8

Figure 10. Surface placement of low-e coatings ..................................................................................... 8

Figure 11. Various metal and nonmetal spacer systems...................................................................... 11

Figure 12. Thermogram of double-glazed clear window with an aluminum spacer (left) and doubleglazed low-e window with an insulating spacer (right). Cold regions in purple and blue 

represent the large amounts of heat flowing through the spacer................................................. 12

Figure 13. Aluminum frame...................................................................................................................... 13

Figure 14. Aluminum frame with thermal break..................................................................................... 13

Figure 15. Wood frame ............................................................................................................................. 13

Figure 16. Wood with clad frame............................................................................................................. 13

Figure 17. Vinyl frame............................................................................................................................... 14

Figure 18. Insulated vinyl frame .............................................................................................................. 14

Figure 19. Hybrid frame............................................................................................................................ 14

Figure 20. Climate zone map referenced in IECC 2006 and later versions......................................... 17

Figure 21. ENERGY STAR zone map ...................................................................................................... 18

Figure 22. Results from the Window Selection Tool ............................................................................. 20

Figure 23. Manufacturers listed for a specific window in the Window Selection Tool...................... 21

Figure 24. Products by a manufacturer listed in the Windows Selection Tool .................................. 21

Figure 25. RESFEN computer simulation data entry screen................................................................ 22

Figure 26. IECC climate zone map with cities used in simulations ..................................................... 23

Figure 27. Comparison of inside glass surface temperature for different glazing types.................. 47

Figure 28. Probability of discomfort near a window in the winter....................................................... 47

Figure 29. Probability of discomfort near a window in the summer.................................................... 48

Figure 30. Peak summer cooling loads in Phoenix, Arizona................................................................ 50

Figure 31. Peak summer cooling loads in Minneapolis, Minnesota .................................................... 50

Figure 32. Annual energy cost by orientation in Boston, Massachusetts.......................................... 52

Figure 33. Annual energy cost by orientation in Sacramento, California........................................... 53

Figure 34. Annual energy cost by orientation in Phoenix, Arizona ..................................................... 54

Figure 35. Annual energy cost by window area in Boston, Massachusetts....................................... 55

Figure 36. Annual energy cost by window area in Sacramento, California........................................ 56

Figure 37. Annual energy cost by window area in Phoenix, Arizona .................................................. 57

Figure 38. Annual energy cost by shading type in Boston, Massachusetts ...................................... 58

Figure 39. Annual energy cost by shading type in Sacramento, California ....................................... 59

Figure 40. Annual energy cost by shading type in Phoenix, Arizona ................................................. 60

Unless otherwise noted, all figures were created by the NorthernSTAR team.
viii

List of Tables

Table 1. Properties of Generic Set of Windows ..................................................................................... 15

Table 2. Prescriptive Window Requirements in the 2009 IECC............................................................ 17

Table 3. Prescriptive Window Requirements in the 2012 IECC............................................................ 18

Table 4. Current ENERGY STAR Performance Requirements ............................................................. 19

Table 5. Proposed 2013 ENERGY STAR Performance Requirements................................................. 19

Table 6. Properties of Windows Used in Climate Zones 1 and 2 ......................................................... 24

Table 7. Savings of New Windows in Miami, Florida............................................................................. 25

Table 8. Savings of New Windows in Houston, Texas .......................................................................... 25

Table 9. Savings of New Windows in Phoenix, Arizona........................................................................ 25

Table 10. Properties of Windows Used in Climate Zones 3 and 4 ....................................................... 26

Table 11. Savings of New Windows in Atlanta, Georgia ....................................................................... 27

Table 12. Savings of New Windows in Las Vegas, Nevada .................................................................. 27

Table 13. Savings of New Windows in San Francisco, California........................................................ 27

Table 14. Savings of New Windows in Washington, D.C. ..................................................................... 28

Table 15. Savings of New Windows in Albuquerque, New Mexico...................................................... 28

Table 16. Savings of New Windows in Seattle, Washington ................................................................ 28

Table 17. Properties of Windows Used in Climate Zones 5–8.............................................................. 29

Table 18. Savings of New Windows in Chicago, Illinois ....................................................................... 30

Table 19. Savings of New Windows in Boston, Massachusetts........................................................... 30

Table 20. Savings of New Windows in Denver, Colorado..................................................................... 30

Table 21. Savings of New Windows in Minneapolis, Minnesota .......................................................... 31

Table 22. Savings of New Windows in Billings, Montana ..................................................................... 31

Table 23. Savings of New Windows in Bismarck, North Dakota.......................................................... 31

Table 24. Savings of New Windows in Fairbanks, Alaska .................................................................... 32

Table 25. Properties of Windows Used in Climate Zones 1 and 2 ....................................................... 33

Table 26. Savings of Replacement Windows in Miami, Florida............................................................ 34

Table 27. Savings of Replacement Windows in Houston, Texas......................................................... 34

Table 28. Savings of Replacement Windows in Phoenix, Arizona....................................................... 34

Table 29. Properties of Windows Used in Climate Zones 3 and 4 ....................................................... 35

Table 30. Savings of Replacement Windows in Atlanta, Georgia........................................................ 36

Table 31. Savings of Replacement Windows in Las Vegas, Nevada ................................................... 36

Table 32. Savings of Replacement Windows in San Francisco, California ........................................ 36

Table 33. Savings of Replacement Windows in Washington, D.C....................................................... 37

Table 34. Savings of Replacement Windows in Albuquerque, New Mexico....................................... 37

Table 35. Savings of Replacement Windows in Seattle, Washington ................................................. 37

Table 36. Properties of Windows Used in Climate Zones 5–8.............................................................. 38

Table 37. Savings of Replacement Windows in Chicago, Illinois ........................................................ 39

Table 38. Savings of Replacement Windows in Boston, Massachusetts ........................................... 39

Table 39. Savings of Replacement Windows in Denver, Colorado...................................................... 39

Table 40. Savings of Replacement Windows in Minneapolis, Minnesota ........................................... 40

Table 41. Savings of Replacement Windows in Billings, Montana...................................................... 40

Table 42. Savings of Replacement Windows in Bismarck, North Dakota........................................... 40

Table 43. Savings of Replacement Windows in Fairbanks, Alaska ..................................................... 41

Table 44. Example of Window Costs....................................................................................................... 42

Table 45. Simple Payback for New Windows ......................................................................................... 42

Table 46. Simple Payback for Replacement Windows.......................................................................... 43

Table 47. Winter and Summer Comfort Index for Typical Windows.................................................... 48

Unless otherwise noted, all figures were created by the NorthernSTAR team.
ix

Definitions

AAMA American Architectural Manufacturers Association

AL Air leakage

ASHRAE American Society of Heating, Refrigerating and Air-Conditioning 

Engineers

ASTM formerly American Society for Testing and Materials

CR Condensation resistance

DOE U.S. Department of Energy

EIA Energy Information Administration

EPA U.S. Environmental Protection Agency

EWC Efficient Windows Collaborative

HVAC Heating, ventilation, and air conditioning

IECC International Energy Conservation Code

IGU Insulating glass unit

LBNL Lawrence Berkley National Laboratory

LCC Life cycle costs

LCCA Life cycle cost analysis

Low-e Low-emittance coating

MBtu Million (106) Btu

NFRC National Fenestration Rating Council

RECA Responsible Energy Codes Alliance

SHGC Solar heat gain coefficient

VT Visible transmittance
x

Executive Summary

This measure guideline helps builders, homeowners, and designers select energy-efficient 

windows in new and existing residential construction in all U.S. climate zones. It includes 

information about window products, attributes, and performance. It provides cost/benefit 

information about window energy savings and about nonenergy benefits such as thermal comfort 

and reduced heating, ventilation, and air conditioning demands. The guideline also provides 

information about the energy impacts of design decisions such as window orientation, total 

glazing area, shading conditions, and about proper window installation. The guideline is intended 

to complement other Building America information and efforts.

Windows are an important component of the energy performance of any house. They are also 

one of the more costly and multifaceted building products with a wide range of technology 

options. Up-to-date information about window products, attributes, and performance is needed in 

the Building America Program to reach the higher levels of overall energy efficiency that are 

targeted. Over the last 20 years, window technology and building codes have advanced to the 

point where low-e windows are commonplace and often required. However, optimizing window 

technology and related design decisions is not well understood.
xi

Progression Summary
1

1 Introduction

Residential buildings consume about 21% of the total energy in the United States. According to 

Lawrence Berkeley National Laboratory (LBNL), windows are responsible for 25%–30% of 

residential heating and cooling energy use (about 2.5 quadrillion Btu). Upgrading the current 

window stock to ENERGY STAR® performance could save more than 1 quadrillion Btu. 

Upgrading the current window stock to the U.S. Department of Energy’s long-term performance 

goals (U-factor = 0.1 with dynamic solar control) could save more than 2 quadrillion Btu.

This measure guideline provides information to help homeowners, builders, and designers select 

energy-efficient windows. Section 2 provides an overview of measuring window performance. 

Section 3 describes window technologies, including glazing types, low conductance spacers and 

gas fills, and frame types. Section 4 leads the user through a step-by-step window selection 

process involving codes, the ENERGY STAR program, and the use of tools such as the Efficient 

Windows Collaborative’s (EWC) Window Selection Tool and RESFEN. In Section 5, the energy 

savings are shown for several window types in 16 cities representing the full range of U.S.

climate zones. Savings are shown in the context of new and replacement windows. Section 5 also 

includes a discussion of window costs, life cycle cost (LCC) issues, and other benefits. Section 6

addresses the impact of design conditions such as orientation, window area, and shading on 

performance, and Section 7 summarizes key issues in window installation. Resources are 

provided in the References section.
2 

2 Measuring Window Performance

Heat flows through a window assembly in three ways: 

conduction, convection, and radiation. Conduction is heat 

traveling through a solid. Convection is the transfer of heat by 

the movement of gases or liquids. Radiation is the movement of 

heat energy through space; it does not rely on conduction 

through the air or on movement of the air (see Figure 1). 

When these basic mechanisms of heat transfer are applied to 

window performance, they interact in complex ways and are not 

typically discussed and measured separately. Instead, three 

energy performance characteristics of windows are used to 

portray how energy is transferred and form the basis for 

quantifying energy performance. 

• Insulating value. When there is a temperature difference 

between inside and outside, heat is lost or gained through 

the window frame and glazing by the combined effects of conduction, convection, and 

radiation. This is indicated in terms of the U-factor of a window assembly. 

• Heat gain from solar radiation. Regardless of outside temperature, heat can be gained 

through windows by direct or indirect solar radiation. The ability to control this heat gain 

is measured in terms of the solar heat gain coefficient (SHGC). 

• Infiltration. Heat is also lost and gained by air leakage through cracks in the window 

assembly. This effect is measured in terms of the amount of air (cubic feet) that passes 

through a unit area of window under given pressure conditions. In reality, infiltration 

varies slightly with wind- and temperature-driven pressure changes.

The National Fenestration Rating Council (NFRC) is a nonprofit, 

public/private organization. It is composed of manufacturers, 

suppliers, researchers, architects and designers, code officials, 

utilities, and government agencies. The NFRC has developed a 

window energy rating system based on whole product 

performance (NFRC 2005) (see Figure 2). 

The NFRC label provides the only reliable way to determine 

window energy properties. It appears on all products certified to 

the NFRC standards on window, door, and skylight products. At 

this time, NFRC labels on window units give ratings for 

U-factor, SHGC, visible light transmittance (VT), and 

(optionally) air leakage (AL) and condensation resistance 

(CR) ratings. Figure 2. Window properties 

on the NFRC label

(Image courtesy of NFRC) 

Figure 1. Heat flow through 

a window
3 

2.1 U-Factor

A principal energy concern about windows is their ability 

to control heat loss. Heat flows from warmer to cooler 

bodies, thus from the inside face of a window to the outside 

in winter, reversing direction in summer (see Figure 3). 

Overall heat flow from the warmer to the cooler side of a 

window unit is a complex interaction of all three basic heat 

transfer mechanisms—conduction, convection, and longwave radiation. A window assembly’s capacity to resist this 

heat transfer is referred to as its U-factor (U-value). It is 

expressed in units of Btu/h∙ft2

∙°F (U.S.) or W/m

2

∙°K 

(European metric). Essentially, the lower the window’s Ufactor, the greater its resistance to heat flow and the better 

its insulating properties.

NFRC’s U-factor rating method is for the whole window, including glazing, frame, and spacers. 

Center-of-glass U-factor is also sometimes referenced, and describes the performance of the 

glazing alone without the effects of the frame. For most energy-efficient windows, the whole 

window U-factor is higher (worse in performance) than the center-of-glass U-factor.

The U-factor is used to express the insulation value of windows; R-value is used for insulation in 

most other parts of the building envelope (walls, floors, roofs). To compare R-value and 

U-factor, divide 1 by the U-factor number; e.g., a 0.25 U-factor equals a 1/0.25 = 4 R-value. 

Low U-factors are most important in heating-dominated climates, although they are also 

beneficial in cooling-dominated climates. ENERGY STAR provides recommended U-factors for 

all U.S. climates. 

2.2 Solar Heat Gain Coefficient 

The origin of solar heat gain is the direct and diffuse radiation 

coming from the sun and the sky (or reflected from the ground and 

other surfaces). Some radiation is directly transmitted through the 

glazing to the building interior, and some may be absorbed in the 

glazing and indirectly admitted to the inside (see Figure 4). Some 

radiation absorbed by the frame will also contribute to overall 

window solar heat gain factor. Essentially, the lower the window’s 

SHGC, the less solar heat it transmits. 

SHGC is expressed as a dimensionless number from 0 to 1. A high 

coefficient signifies high heat gain; a low coefficient means low 

heat gain. 

The glazing type, number of panes, and any glass coatings 

influence solar heat gain. Solar heat gain of glazing ranges from 

above 80% for uncoated clear glass to less than 20% for highly reflective coatings on tinted 

glass. A typical double-pane insulating glass unit (IGU) has an SHGC of around 0.70. This value 

decreases somewhat by adding a tint and can be decreased substantially by adding a low-solargain low-e coating. 

Figure 3. Heat loss through 

a window by conduction, 

convection, and radiation

Figure 4. Solar gain 

through a window
4 

NFRC’s SHGC rating method is for the whole window, including the effects of the frame. The 

area of a frame has a very low SHGC, so the overall window SHGC is lower than the center-ofglass value. Alternatively, the center-of-glass SHGC, which describes the effect of the glazing 

alone, is sometimes referenced. 

Solar heat gain can provide free heat in the winter but can also lead to overheating in the 

summer. The best balance of solar heat gain with an appropriate SHGC depends on climate, 

orientation, shading conditions, and other factors. ENERGY STAR provides simplified guidance 

on recommended SHGC values for all U.S. climates. 

2.3 Visible Transmittance

The VT is an optical property that indicates the amount of visible 

light transmitted (see Figure 5). Although VT theoretically varies 

between 0 and 1, most values of double- and triple-pane windows 

are 0.30–0.70. 

The glazing type, number of panes, and any glass coatings influence 

VT. This value decreases somewhat when a low-emittance (low-e) 

coating is added and decreases substantially when a tint is added. 

Adding another layer of glass also decreases VT. A higher VT 

allows more light to be transmitted and is desirable to maximize 

daylight. 

NFRC’s VT rating method is a whole window rating and includes 

the impact of the frame that transmits no visible light. VT values for 

the whole window are always lower than center-of-glass values, 

because the VT of the frame is zero.

2.4 Air Leakage

Heat is lost and gained via infiltration through cracks in the window 

assembly. It is indicated by an AL expressed as the equivalent cubic 

feet of air passing through 1 ft2

 of window area (see Figure 6). The 

lower the AL, the less air will pass through cracks in the window 

assembly. Windows with an AL of 0.30 or lower (units are cfm/ft2

) 

should be selected. Air leakage also contributes to summer cooling 

loads in some climates by raising the interior humidity level.

AL is an optional NFRC rating. For code compliance purposes, 

however, air infiltration is often tested in accordance with the North 

American Fenestration Standard, which produces similar results to 

the NFRC AL. Figure 6. Air leakage 

through window assembly

Figure 5. Visible light 

through a window
5 

2.5 Condensation Resistance

CR measures how well a window resists the formation of 

condensation on the inside surface. CR is expressed as a 

number between 1 and 100. The higher the number, the better 

a product is able to resist condensation (see Figure 7). 

CR is meant to compare products and their potential for 

condensation formation. CR is an optional rating on the NFRC 

label. 

Figure 7. Condensation on 

window surface
6 

3 Window Technologies

Glazing technology is combined with a spacer system

and a gas fill between the panes to produce an energyefficient IGU. An IGU is assembled with frame and 

operability options to form the complete window 

assembly. Some integrated technological innovations 

that appear in today’s fenestration products are listed 

below (see also Figure 8). 

• Multiple layers of glass or plastic film

• High performance glazing low-e or solar control 

coatings

• Low-conductance gas fills

• Warm edge spacers

• High performance frames

3.1 Glazing Types

The number of glass layers, various coatings, tints, and 

other glass surface treatments can affect the energy 

properties of windows. 

3.1.1 Multiple Layers

Multiple layers of glass or plastic films improve thermal resistance and reduce the heat loss 

attributed to convection between layers. Double glazing reduces heat loss (as reflected by the 

U-factor) by more than 50% compared to single glazing. Although U-factor is reduced 

significantly, the VT and SHGC for a double-glazed unit with clear glass remain relatively high. 

Adding a third layer of glass reduces the VT and SHGC. Adding a low-e coating to a surface, or 

to multiple surfaces, will increase energy performance. Depending on the type of low-e coating, 

the SHGC and VT will also be affected.

Additional panes of glass increase the weight and thickness of the unit, which makes mounting 

and handling more difficult and transportation more expensive. There are physical and economic 

limits to the number of glass panes that can be added to a window assembly. However, multiplepane units are not limited to glass assemblies, but can be made up of one or more layers of 

suspended film.

3.1.1.1 Suspended Films

The middle layer(s) of glass can be substituted with an inner plastic suspended film. The light 

weight of plastic film is advantageous, and because it is very thin, it represents a much smaller

weight increase than glass. Windows using plastic films decrease the U-factor of the unit 

assembly by dividing the inner air space into multiple chambers. The limited strength and 

durability of the plastic film are overcome and the film is protected from scratching, wear, 

weathering, and visual distortions by the inboard and outboard glass panes. The plastic films are 

specially treated to resist ultraviolet degradation and are heat shrunk so they remain taut and flat.

Figure 8. Advancements to improve 

energy efficiency
7

Like glass, a low-e coating can be bonded to the plastic film to lower the assembly U-factor. The 

plastic film can also be treated with spectrally selective coatings to reduce solar gain without 

significant VT loss. The combination of multiple glass panes and plastic films with low-e

coatings and gas fills can achieve very low U-factors.

3.1.2 Low-Emittance Coatings

All materials, including windows, emit (or radiate) heat in the form of long-wave, far-infrared 

energy depending on their temperature. This emission is one of the important components of 

window heat transfer, so reducing the window’s emittance can greatly improve its insulating 

properties. Coating a glass surface with a low-e material and facing that coating into the gap 

between the glazing layers blocks a significant amount of this radiant heat transfer, lowering the 

total heat flow through the window. When heat or light energy is absorbed by glass, it is either 

convected away by moving air or reradiated by the glass surface. The ability of a material to 

radiate energy is called its emissivity.

Low-e coatings are highly transparent and virtually invisible, but have a high reflectance to longwavelength infrared radiation. This reduces long-wavelength radiative heat transfer between

glazing layers by a factor of 5–10, thereby reducing total heat transfer between two glazing 

layers. Low-e coatings may be applied directly to glass surfaces, or to suspended films between 

the interior and exterior glazing layers.

The solar reflectance of low-e coatings can be manipulated to include specific parts of the visible 

and infrared spectrum. This is the origin of the term spectrally selective coatings, which selects 

specific portions of the energy spectrum so desirable wavelengths of energy are transmitted and 

others specifically reflected. A glazing material can then be designed to optimize energy flows 

for solar heating, daylighting, and cooling.

Standard clear glass has an emittance of 0.84 over the long-wave portion of the spectrum, 

meaning that it emits 84% of the energy possible for an object at its temperature. It also means 

that 84% of the long-wave radiation striking the surface of the glass is absorbed and only 16% is 

reflected. By comparison, low-solar-gain low-e glass coatings can have an emittance as low as 

0.04. Such glazing would emit only 4% of the energy possible at its temperature, and thus reflect 

96% of the incident long-wave, infrared radiation. Window manufacturers’ product information 

may not list emittance ratings. Rather, the effect of the low-e coating is incorporated into the Ufactor and SHGC for the unit or glazing assembly.

3.1.2.1 High-Solar-Gain Low-Emittance Coatings

High-solar-gain low-e coatings typically have an SHGC value greater than 0.40 and are designed 

to reduce heat loss but admit solar gain. High-solar-gain products are best suited to buildings 

located in heating-dominated climates and particularly to south-facing windows in passive solar 

designs. Unless properly shaded, high-solar-gain windows may result in overheating from excess 

solar gain in swing seasons (see Figure 9).
8 

3.1.2.2 Moderate-Solar-Gain Low-Emittance Coatings

Moderate-solar-gain low-e coatings typically have an SHGC value of 0.25–0.40. Such coatings 

reduce heat loss, maintain high light transmittance, allow a reasonable amount of solar gain, and 

are suitable for climates with heating and cooling concerns. 

3.1.2.3 Low-Solar-Gain Low-Emittance Coatings

Low-solar-gain low-e coatings typically have an SHGC value less than 0.25. This type of low-e 

product, using a highly spectrally selective low-e glass, reduces heat loss in winter and reduces 

heat gain in summer. Compared to most tinted and reflective glazings, this low-e glass transmits 

visible light, but blocks a large fraction of the solar infrared 

energy, thus reducing cooling loads. 

3.1.2.4 Coating Placement

The placement of a low-e coating within the air gap of a 

double-glazed window does not affect the U-factor, but it

does influence the SHGC (see Figure 10). Thus, in heatingdominated climates, placing a low-e coating on the #3 

surface (outside surface of the inner pane) is recommended 

to maximize winter passive solar gain at the expense of a 

slight reduction in the ability to control summer heat gain. 

In cooling climates, a coating on the #2 surface (inside 

surface of the outer pane) is generally best to reduce solar 

Figure 9. Spectral transmittance curves for glazings with low-e coatings

Figure 10. Surface placement of 

low-e coatings
9 

heat gain and maximize energy efficiency. Manufacturers sometimes place the coatings on 

surfaces for other reasons, such as minimizing the potential for thermal stress (e.g., #2 surface in 

a heating climate). Multiple low-e coatings are also placed on surfaces within a triple-glazed 

window assembly, or on the inner plastic glazing layers of multipane assemblies, which further 

improves the overall U-factor. 

3.2 Low-Conductance Spacers and Gas Fills 

The various layers of glazing layers are assembled in an IGU. A possible improvement to the 

thermal performance of an IGU is to reduce the conductance of the air space between the layers

by using a gas fill and low-conductance spacers that control the properties of the spaces between 

the layers.

3.2.1 Low-Conductance Gas Fills 

With the use of a low-e coating, heat transfer across a gap is dominated by conduction and 

natural convection. Air is a relatively good insulator, but other gases (such as argon, carbon 

dioxide, krypton, and xenon) have lower thermal conductivities. Using one of these nontoxic 

gases in an IGU can reduce heat transfer between the glazing layers.

In a sealed IGU, air currents between the two panes of glazing carry heat to the top of the unit 

and settle into cold pools at the bottom. The air in the space between the panes can be replaced 

with a less conductive and more viscous (slower moving) gas. This replacement minimizes the 

convection currents in the space, which reduces conduction through the gas and the overall 

transfer of heat between the inside and outside.

Manufacturers generally use argon or krypton gas fills, with measurable improvement in thermal 

performance. Both gases are inert, nontoxic, nonreactive, clear, and odorless. Krypton has better 

thermal performance than argon and is more expensive to produce. The optimal spacing for an 

argon-filled unit is the same as for air, about 1⁄2 inFK. Krypton performs better than argon when the 

space between glazings must be thinner than normally desired (for example, 1⁄4 in.), but it is more 

costly. A mixture of krypton and argon gases is sometimes used as a compromise between 

thermal performance and cost. Argon and krypton occur naturally in the atmosphere, but 

maintaining long-term thermal performance is certainly an issue. Studies have shown less than 

0.5% leakage per year in a well-designed and well-fabricated unit, or a 10% loss in total gas over 

a 20-year period. The overall effect of a 10% gas loss would change the U-factor by only a few 

percentage points. Keeping the gas within the glazing unit depends largely on the quality of the 

design, materials, and, most important, assembly of the glazing unit seals. 

3.2.2 Warm Edge Spacers 

Heat transfer through the metal spacers that are used to separate glazing layers can increase heat 

loss and cause condensation to form at the edge of the window. “Warm edge” spacers use 

improved materials and better designs to reduce this effect.
10

The glass lights in an IGU must be held apart at the appropriate distance by spacers. In addition 

to keeping the glass lights separated, the spacer system must serve a number of functions:

• Accommodate stress induced by thermal expansion and pressure differences.

• Provide a moisture barrier that prevents passage of water or water vapor that would fog 

the unit.

• Provide a gas-tight seal that prevents the loss of any special low-conductance gas in the 

air space.

• Create an insulating barrier that reduces the formation of interior condensation at the 

edge.

The traditional approach for IGUs is to use metal spacers and sealants. These spacers, typically 

aluminum, also contain a desiccant that absorbs residual moisture. The spacer is sealed to the 

glass lights with organic sealants that provide structural support and act as a moisture barrier. 

There are two generic systems for such IGUs: a single-seal spacer and a dual-seal system.

Unfortunately, aluminum is an excellent conductor of heat, and the aluminum spacer used in 

traditional edge systems represents a significant thermal “short circuit” at the edge of the IGU, 

which reduces the benefits of improved glazings. In addition to the increased heat loss, the colder 

edge is more prone to condensation. To overcome these problems, warm edge spacers are now 

used in more than 90% of new windows.

Innovative edge systems have been developed to address these problems, including solutions that 

depend on material substitutions as well as new designs. One approach to reducing heat loss has 

been to replace the aluminum spacer with a metal one that is less conductive (e.g., stainless 

steel), and change its cross-sectional shape. These designs are widely used in windows today.

Another approach is to replace the metal spacer with a design that uses materials that are better 

insulators. The most commonly used design incorporates spacer, sealer, and desiccant in a 

thermoplastic compound that contains a blend of desiccant materials and incorporates a thin, 

fluted metal shim of aluminum or stainless steel. Another approach uses an insulating silicone 

foam spacer that incorporates a desiccant and has a high-strength adhesive at its edges to bond to 

glass. The foam is backed with a secondary sealant. Extruded vinyl and fiberglass spacers have 

also been used in place of metal designs.

Several hybrid designs incorporate thermal breaks in metal spacers or use one or more of the 

elements described above. Some are specifically designed to accommodate three- and four-layer 

glazings or IGUs incorporating stretched plastic films. All are designed to interrupt the heat 

transfer pathway at the glazing edge between two or more glazing layers (see Figure 11).
11 

Warm edge spacers have become increasingly important as manufacturers switch from 

conventional double glazing to high performance glazing. To determine the overall window 

U-factor, the edge spacer has an effect that extends beyond its physical size to a band about 21⁄2 

in. wide. The contribution of this 21⁄2-in.-wide “glass edge” to the total window U-factor depends 

on the size of the window. For a typical residential-size window (3 ft × 4 ft), changing from a 

standard aluminum edge spacer to a good-quality warm edge spacer will reduce the overall 

window U-factor by approximately .02 Btu/h∙ft2∙°F.

A more significant benefit may be the rise in interior surface temperature at the bottom edge of 

the window, which has the highest risk of condensation. With an outside temperature of 0°F, a 

thermally improved spacer could result in temperature increases of 6°–8°F at the window 

sightline—or 4°–6°F at a point 1 in. in from the sightline, which is an important improvement. 

As new highly insulating multiple layer windows are developed, the improved edge spacer 

becomes an even more important element (see Figure 12).

Figure 11. Various metal and nonmetal spacer systems
12 

3.3 Frame Types

3.3.1 Metal Frames

Aluminum is light, strong, durable, and easily extruded into the complex shapes required for 

window parts. Aluminum window frames are available in anodized and factory-baked enamel 

finishes that are extremely durable and low maintenance.

The biggest disadvantage of aluminum as a window frame material is its high thermal 

conductance. It readily conducts heat, greatly raising the overall U-factor of a window unit. In 

cold climates, a simple aluminum frame can easily become cold enough to condense moisture or 

frost on the inside surfaces of window frames. This condensation problem, even more than heat 

loss, has spurred the development of better insulating aluminum frames. In hot climates, where 

solar gain is often more important than conductive heat transfer, using a higher performance 

glazing system can be much more important than improving the insulating value of the frame. 

3.3.2 Thermally Broken Metal Frames

The most common solution to the heat conduction problem of aluminum frames is to provide a 

“thermal break” by splitting the frame components into interior and exterior pieces and use a less 

conductive material to join them (see Figures 13 and 14). Current technology with standard 

thermal breaks has decreased aluminum frame U-factors (heat loss rate) from roughly 2.0 to 

about 1.0 Btu/h∙ft2

∙°F.

Figure 12. Thermogram of double-glazed clear window with an aluminum spacer (left) and 

double-glazed low-e window with an insulating spacer (right). Cold regions in purple and blue 

represent the large amounts of heat flowing through the spacer.

(Image courtesy of LBNL) 
13 

Figure 13. Aluminum frame Figure 14. Aluminum frame with thermal 

break 

3.3.3 Nonmetal Frames 

3.3.3.1 Wood

The traditional window frame material is wood, because of its availability and ease of milling 

into the complex shapes required to make windows. Wood is favored in many residential 

applications because of its appearance and traditional place in house design (see Figure 15). 

From a thermal point of view, wood-framed windows perform well with frame U-factors at 0.3–

0.5 Btu/h∙ft2∙°F. Wood is not intrinsically the most durable window frame material, because of its 

susceptibility to rot, but well-built, well-protected, and well-maintained wood windows can have 

a very long life.

3.3.3.2 Wood Clad

A variation of the wood-framed window is to clad the exterior face of the frame with either vinyl 

or aluminum, creating a permanent weather-resistant surface. Clad frames thus have lower 

maintenance requirements and retain the attractive wood finish on the interior (see Figure 16). 

Although vinyl and enameled metal claddings offer much longer protection to wood frames, they 

are generally available in limited colors.

Figure 15. Wood frame Figure 16. Wood with clad frame
14

3.3.3.3 Vinyl

Vinyl, also known as polyvinyl chloride, is a very versatile plastic with good insulating value. 

Vinyl window frames do not require painting and have good moisture resistance (see Figure 17). 

Because the color goes all the way through, there is no finish coat that can be damaged or 

deteriorate over time—the surface is therefore maintenance free. Some vinyl window 

manufacturers now offer surface treatments such as laminates (wood veneer, paintable/stainable, 

maintenance free) and coatings. These products increase color selection and surface appearance 

options. Recent advances have improved dimensional stability and resistance to degradation 

from sunlight and temperature extremes. The thermal performance of vinyl frames is comparable 

with that of wood, although there are minor differences (depending on the frame construction). 

Small hollow chambers within the frame reduce convection exchange, as does adding an 

insulating material (see Figure 18).

3.3.3.4 Hybrid

Manufacturers are increasingly turning to hybrid frame designs that use two or more frame 

materials to produce a complete window system. The wood industry has long built vinyl- and 

aluminum-clad windows to reduce exterior maintenance (see Figure 19). Vinyl manufacturers 

and others offer interior wood veneers to produce the finish and appearance that many 

homeowners desire. Split-sash designs may have an interior wood element bonded to an exterior 

fiberglass element.

Figure 17. Vinyl frame Figure 18. Insulated vinyl 

frame

Figure 19. Hybrid frame

3.3.3.5 Composite

Wood particles and resins can be compressed to form a strong composite material. Window 

frame and sash members can be manufactured from wood/polymer composites that have been

extruded into a series of lineal shapes. These composites are very stable, and have the same or 

better structural and thermal properties as conventional wood, with better resistance to moisture 

and decay. They can be textured and stained or painted much like wood.

3.3.3.6 Thermally Improved or Insulated Vinyl

Thermally improved windows may include a combination of features resulting in a lower 

U-factor, such as high performance frame design and low conductance spacers in combination 

with high performance glazing. Although the thermal performance of most vinyl frames is

comparable to that of wood, they can be further improved by creating smaller chambers in the

frame, reducing the convection exchange that can occur in large hollow chambers. Often these 

hollow cavities are filled with an insulating material. Usually these high performance frames are 

used with high performance glazings.
15

As with standard vinyl frames, thermally improved or insulated vinyl frames do not require 

painting and have good moisture resistance. Because the color goes all the way through, there is 

no finish coat that can be damaged or that will deteriorate over time. Recent advances have 

improved dimensional stability and resistance to degradation from sunlight and temperature 

extremes (see Table 1).

3.3.3.7 Fiberglass or Engineered Thermoplastics

Window frames can be made of glass-fiber-reinforced polyester (fiberglass) or engineered 

thermoplastics that are pultruded into lineal forms and then assembled into windows. These 

frames are dimensionally stable and have air cavities (similar to vinyl). The frame cavities can be 

filled with insulation or designed with multiple small chambers to reduce convection exchange. 

Because these materials are stronger than vinyl, the frames can have smaller cross-sectional 

shapes and thus less area, and are therefore particularly well suited to hold heavier triple glazing. 

Usually these high performance frames are used with high performance glazings. 

3.3.3.8 Thermally Improved Wood and Composite Frames

Wood-framed windows have frame U-factors of 0.30–0.50 Btu/h∙ft2

∙°F. Although the absence of 

frame cavities limits the options to further boost wood frame insulating value, thermal 

improvements can be achieved through thicker frame design, by avoiding thermal shortcuts 

through metal parts, and with low conductance spacers.

Table 1. Properties of Generic Set of Windows

(EWC 2012a)

ID Glazing Frame U SHGC VT

1 Single, clear Metal 1.29 0.73 0.69

2 Double, clear Metal 0.83 0.65 0.63

3 Double, tint Metal 0.83 0.54 0.47

4 Double, low-e, high SHGC, argon Metal 0.65 0.58 0.61

5 Double, low-e, medium SHGC, argon Metal 0.64 0.38 0.56

6 Double, low-e, low SHGC, argon Metal 0.63 0.26 0.49

7 Double, clear Metal, thermal break 0.60 0.62 0.63

8 Double, tint Metal, thermal break 0.60 0.51 0.47

9 Double, low-e, high SHGC, argon Metal, thermal break 0.42 0.55 0.61

10 Double, low-e, medium SHGC, argon Metal, thermal break 0.42 0.35 0.56

11 Double, low-e, low SHGC, argon Metal, thermal break 0.41 0.23 0.49

12 Single, clear Nonmetal 0.88 0.64 0.65

13 Double, clear Nonmetal 0.52 0.57 0.59

14 Double, tint Nonmetal 0.52 0.47 0.44

15 Double, low-e, high SHGC, argon, improved Improved nonmetal 0.29 0.50 0.57

16 Double, low-e, medium SHGC, argon, improved Improved nonmetal 0.28 0.31 0.52

17 Double, low-e, low SHGC, argon, improved Improved nonmetal 0.27 0.20 0.46

18 Triple, low-e, high SHGC, argon, improved Improved nonmetal 0.20 0.41 0.50

19 Triple, low-e, medium SHGC, argon, improved Improved nonmetal 0.19 0.28 0.45

20 Triple, low-e, low SHGC, argon, improved Improved nonmetal 0.19 0.18 0.37
16

4 Window Selection Process

Windows provide views to the exterior, light, and natural ventilation. Selecting windows takes 

into account many issues such as appearance, cost, and performance. Follow these window 

selection steps to find energy-efficient windows.

1. Meet code. At a minimum, window energy ratings need to comply with the requirements 

of the code applicable to the home’s jurisdiction. Many states and municipalities have 

adopted the International Energy Conservation Code (IECC). The IECC divides the 

country into climate zones with varying window performance requirements, giving 

preference to insulating value in cold climates and to solar heat control in hot climates.

2. Look for the ENERGY STAR label. Not all jurisdictions have adopted up-to-date 

energy codes. The ENERGY STAR label helps ensure that windows have above-average 

energy performance. In most locations, ENERGY STAR criteria exceed energy code 

requirements.

3. Look for performance properties on the NFRC label. How these technologies such as 

low-e coatings, gas fills, and high performance frames affect a window’s energy 

performance depends on the combined effects of the window’s components. The only 

reliable way to determine whole-window energy properties is the ratings certified by the 

NFRC. In most jurisdictions across the United States, building energy codes require that 

windows bear the NFRC label so that the code compliance of their energy ratings can be 

verified.

4. Use the Window Selection Tool (EWC 2012b). This tool offers a quick comparison of 

how generic window types impact the energy use in typical home in 100 cities.

5. Use RESFEN (LBNL 2012). Simulate the conditions of a specific house with specific 

windows. It is possible to take into account the impact of shading, orientation, and

glazing area.

6. Use BEopt (NREL 2012a). BEopt was developed by the National Renewable Energy 

Laboratory for analyzing whole-building cost performance. BEopt allows for integratedsystem optimization so that tradeoffs between windows and other strategies can be made.

4.1 Energy Codes

Most locations have building energy codes that mandate minimum performance levels for new 

and replacement fenestration products. Building energy codes are set at the state or municipal 

level but are often based on model energy codes. Most jurisdictions rely on model energy codes 

developed by national code writing entities. National code writing entities modify model energy 

codes every few years, and jurisdictions may adopt any version of a model code, whether in 

whole or with modifications. For residential buildings, jurisdictions most often adopt a version of 

the IECC (see Figure 20). The status of a state’s energy code can be found at the Building

Energy Codes Program website (DOE 2012) and at the Responsible Energy Codes Alliance 

website (RECA 2012).
17 

In the 1992 Energy Policy Act, Congress mandated that all states must review and consider 

adopting the national model energy standard (at that time, the 1992 Model Energy Code). Since 

then, new model energy codes have been developed and in 1998 the first IECC was released.

Version 2012 is the most recent version (ICC 2012). 

4.1.1 Windows in the 2009 International Energy Conservation Code

The 2009 IECC builds on the improvements made in the 2000, 2003, and 2006 IECC and each 

supplemental release. The prescriptive window requirements of the 2009 IECC are listed in 

Table 2. 

Table 2. Prescriptive Window Requirements in the 2009 IECC

Climate Zone Window U-Factor Skylight U-Factor Window and Skylight SHGC*

1 1.20 0.75 0.30 

2 0.65** 0.75 0.30 

3 0.50* 0.65 0.30 

4 except Marine 0.35 0.60 No requirement

5 and Marine 4 0.35 0.60 No requirement

6 0.35 0.60 No requirement

7 and 8 0.35 0.60 No requirement

* There are no SHGC requirements in the Marine zone.

** Impact rated fenestration has a maximum U-factor of 0.75 in Zone 2 and 0.65 in Zone 3.

Figure 20. Climate zone map referenced in IECC 2006 and later versions
1

4.1.2 Windows in the 2012 International Energy Conservation Code

On a national basis, 2012 IECC is roughly 30% more energy efficient than the 2006 IECC and 

15% more energy efficient than the 2009 IECC. The 2012 International Residential Code no 

longer includes its own energy-related code provisions. Instead, this code simply references the 

IECC. The prescriptive window requirements of the 2012 IECC are listed in Table 3. 

Table 3. Prescriptive Window Requirements in the 2012 IECC

Climate Zone Window U-Factor Skylight U-Factor Window and Skylight SHGC

1 0.65 0.75 0.25 

2 0.40 0.65 0.25 

3 0.35 0.55 0.25 

4 except Marine 0.35 0.55 0.40 

5 and Marine 4 0.32 0.55 No requirement

6 0.32 0.55 No requirement

7 and 8 0.32 0.55 No requirement

4.2 ENERGY STAR

The U.S. Department of Energy and the U.S. Environmental Protection Agency (EPA) 

developed an ENERGY STAR designation for products meeting certain energy performance 

criteria (EPA 2012a). Energy-efficient performance of windows, doors, and skylights varies by 

climate, so product recommendations are given for four U.S. climate zones (see Figure 21 and 

Table 4). To compare ENERGY STAR products, use the NFRC label (NFRC 2012) or the 

NFRC Certified Products Directory (NFRC 2005). 

Figure 21. ENERGY STAR zone map 

(,PDJHFourtesy of EPA) 
19

Table 4. Current ENERGY STAR Performance Requirements

Climate Zone

Window

U-Factor

Window

SHGC

Skylight

U-Factor

Skylight

SHGC

North ≤0.30 Any ≤0.55 Any

0.31 ≥0.35

0.32 ≥0.40

North Central ≤0.32 ≤0.40 ≤0.55 ≤0.40

South Central ≤0.35 ≤0.30 ≤0.57 ≤0.30

South ≤0.60 ≤0.27 ≤0.70 ≤0.30

In 2011, market share for ENERGY STAR windows was 81% for windows and 99% for 

skylights. According to the EPA, a market share of ENERGY STAR-qualified products in a 

particular category of 50% or higher will prompt consideration for a specification revision (EPA 

2012b). The other factors that weigh into possible specification revision are:

• A change in the federal minimum efficiency standards

• Technological changes with advances in energy efficiency 

• Product availability

• Significant issues with consumers realizing expected energy savings

• Performance, quality, or test procedure issues.

The 2014 proposed criteria revision will include changes in the U-factor and SHGC 

requirements, as well as air leakage (≤0.30 cfm/ft2

) and installation requirements (see Table 5). 

The new criteria are proposed to take effect January 1, 2014 (EPA 2012a).

Table 5. Proposed 2013 ENERGY STAR Performance Requirements

Climate Zone

Window

U-factor

Window

SHGC

Skylight

U-factor

Skylight

SHGC

North

Tradeoff

≤0.27

=0.28

Any

≥0.32 ≤0.45 ≤0.35

North Central ≤0.29 ≤0.40 ≤0.47 ≤0.30

South Central ≤0.31 ≤0.25 ≤0.50 ≤0.25

South ≤0.40 ≤0.25 ≤0.60 ≤0.25
20 

4.3 Window Selection Tool

The basic thermal and optical properties of a 

window (U-factor, SHGC, and VT) can be 

identified if a residential window is properly 

labeled with an NFRC label. However, 

residential consumers often still do not know 

how these basic properties influence annual 

heating and cooling energy use. The EWC’s

Window Selection Tool (see Figure 22) can 

help determine the most energy-efficient 

window selection (EWC 2012b). The annual 

energy use from computer simulations for a 

typical house in 100 U.S. cities can be 

compared for 29 generic window options. The 

tool provides information to: 

• Compare how various window or 

skylight types affect estimated energy 

cost for a typical house in a specific 

location. 

• Find manufacturers who offer windows 

and skylights in the categories shown

(see Figure 23). 

• Learn more about manufacturers’ 

specific product options (see Figure 24). 

These comparisons assume average conditions. 

The effect of windows on a specific home’s 

heating and cooling costs may vary depending 

on glazing area, shading, and orientation, and 

on thermostat set points, equipment efficiency, 

etc.

Figure 22. Results from the Window 

Selection Tool
21 

4.4 RESFEN

Residential consumers are often confused about how to select the most efficient window type—

one that will help lower heating and cooling costs, increase occupant comfort, and minimize 

window condensation issues. The relative importance of efficient window properties (U-factor, 

SHGC, VT) depends on site- and building-specific conditions.

Figure 24. Products by a manufacturer listed in the Windows Selection Tool

Figure 23. Manufacturers listed for a specific window in the Window Selection Tool
22

RESFEN, a computer simulation tool developed at LBNL (2012), helps residential consumers 

make informed decisions about window products. RESFEN calculates heating and cooling 

energy use and associated costs, peak heating demand, and peak cooling demand for defined 

window products (see Figure 25). These defined window products can be the default generic set 

that is provided with the program, or the user can customize products by assigning specific 

thermal and physical properties. A scenario is defined by specifying house type, geographic 

location, orientation, electricity and gas costs, and building construction details (wall type, floor 

type, HVAC system). The user also specifies size, shading, and thermal properties of windows. 

The thermal properties that RESFEN requires are U-factor, SHGC, and AL.

Figure 25. RESFEN computer simulation data entry screen

(Image courtesy of LBNL)
23 

5 Cost and Performance 

In this section, energy savings are calculated for new and replacement windows in 16 cities 

representing a full range of U.S. climate zones (see Figure 26). These cities are:

1A: Miami, Florida

2A: Houston, Texas

2B: Phoenix, Arizona 

3A: Atlanta, Georgia 

3B: Las Vegas, Nevada 

3C: San Francisco, California

4A: Washington, D.C. 

4B: Albuquerque, New Mexico 

4C: Seattle, Washington 

5A: Chicago, Illinois

5A: Boston, Massachusetts

5B: Denver, Colorado 

6A: Minneapolis, Minnesota 

6B: Billings, Montana 

7: Fargo, North Dakota 

8: Fairbanks, Alaska

The energy use is determined using RESFEN for a 2000-ft2 house with 300 ft2 of window area 

equally distributed on four orientations. Performance will vary with orientation, window area, 

and shading conditions (see Section 6). Each climate grouping has its own set of six windows 

appropriate for that region. 

Figure 26. IECC climate zone map with cities used in simulations
24 

 Energy and Cost Savings for New Windows

For new construction, the base case in each climate is a window that meets the 2009 IECC 

(usually a double-glazed low-e window with a specified U-factor and SHGC). The other four 

windows represent improvements over the base case with different low-e coating types, different 

frame materials, more glazing layers, or other thermal improvements. Because the 2009 IECC 

requires a relatively high performance window as a minimum, the additional savings from even 

higher performance windows are often minimal. (All cases in these tables are simulated with no 

shading; thus, solar heat gain is maximized.) In a heating-dominated climate, increased passive 

solar heat gain from no shading is a benefit, but it actually reduces the energy savings from 

higher performance windows (low U-factor). In cooling-dominated climates, the increase in 

unwanted solar gain from no shading increases energy savings from high performance windows 

(low SHGC). 

.1 Savings for New Windows in Climate Zones 1 and 2 

The base case for new windows in this relatively hot region is a low-solar-gain low-e window in 

a metal frame (Window 6: U = 0.63 and SHGC = 0.26) (see Table 6). Windows with a lower 

SHGC produce a modest savings. Better insulating frames result in lower U-factors that increase

savings slightly (Tables 7–9). A double-glazed window with a low-solar-gain low-e coating 

(SHGC = 0.20) and a triple-glazed window with a low-solar-gain low-e coating (SHGC = 0.18) 

perform best in these climate zones.

Table 6. Properties of Windows Used in Climate Zones 1 and 2 

ID Glazing Frame U SHGC VT

6 Double, low-e, low SHGC, argon Metal 0.63 0.26 0.49 

11 Double, low-e, low SHGC, argon Metal, thermal break 0.41 0.23 0.49 

16 Double, low-e, medium SHGC, argon Nonmetal 0.28 0.31 0.52 

17 Double, low-e, low SHGC, argon Nonmetal 0.27 0.20 0.46 

20 Double, low-e, medium SHGC, 

argon, improved Nonmetal, improved 0.19 0.18 0.37 

The annual energy performance figures shown were generated with RESFEN6 by LBNL (windows.lbl.gov). Results 

assume a typical new construction 2000-ft2 house with 300 ft2 of window area. The windows are equally distributed 

on all four sides with no shading. U-factor and SHGC are for the whole window. Costs for lights, appliances, hot 

water, cooking, and other uses are not included in these figures. The mechanical system uses a gas furnace for 

heating and air conditioning for cooling. Natural gas prices (EIA 2012b) and electricity prices (EIA 2012a) used are 

year 2010 averages provided by the Energy Information Administration. 
25

Table 7. Savings of New Windows in Miami, Florida

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

6 0.63 0.26 0.49 X X 0 4610 $530 $–

11 0.41 0.23 0.49 X X X 0 4520 $519 $11

16 0.28 0.31 0.52 0 5075 $581 $(52)

17 0.27 0.20 0.46 X X X X 0 4392 $504 $26

20 0.19 0.18 0.37 X X X X 0 4289 $492 $38

Table 8. Savings of New Windows in Houston, Texas

ID U SHGC VT

2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

6 0.63 0.26 0.49 X 12 2778 $446 $–

11 0.41 0.23 0.49 X X 11 2713 $428 $18

16 0.28 0.31 0.52 8 3088 $447 $(1)

17 0.27 0.20 0.46 X X X X 10 2619 $408 $39

20 0.19 0.18 0.37 X X X X 9 2546 $395 $51

Table 9. Savings of New Windows in Phoenix, Arizona

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

6 0.63 0.26 0.49 X 5 4724 $600 $–

11 0.41 0.23 0.49 X X 5 4574 $575 $25

16 0.28 0.31 0.52 3 5005 $598 $3

17 0.27 0.20 0.46 X X X X 4 4380 $545 $55

20 0.19 0.18 0.37 X X X X 4 4247 $527 $73
26

5.1.2 Savings for New Windows in Climate Zones 3 and 4

The base case for new windows in this mainly warm region is a low-solar-gain low-e window in 

a thermally broken metal frame (Window 11: U = 0.41 and SHGC = 0.23) (see Table 10). 

Improved frames and glazings result in increased annual savings (Tables 11–16). The tripleglazed moderate-solar-gain low-e window produces the most savings in most cities in these 

climate zones.

Table 10. Properties of Windows Used in Climate Zones 3 and 4

ID Glazing Frame U SHGC VT

11 Double, low-e, low SHGC, argon Metal, thermal 

break 0.41 0.23 0.49

15 Double, low-e, high SHGC, argon, 

improved Nonmetal 0.29 0.50 0.57

16 Double, low-e, medium SHGC, argon, 

improved Nonmetal 0.28 0.31 0.52

17 Double, low-e, low SHGC, argon, 

improved

Nonmetal, 

improved 0.27 0.20 0.46

19 Triple, low-e, low SHGC, argon, 

improved

Nonmetal, 

improved 0.19 0.28 0.45

The annual energy performance figures shown were generated with RESFEN6 by LBNL (windows.lbl.gov). Results 

assume a typical new construction 2000-ft2 house with 300 ft2 of window area. The windows are equally distributed 

on all four sides with no shading. U-factor and SHGC are for the whole window. Costs for lights, appliances, hot 

water, cooking, and other uses are not included in these figures. The mechanical system uses a gas furnace for 

heating and air conditioning for cooling. Natural gas prices (EIA 2012b) and electricity prices (EIA 2012a) used are 

year 2010 averages provided by the Energy Information Administration.
27

Table 11. Savings of New Windows in Atlanta, Georgia

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

11 0.41 0.23 0.49 X 29 1372 $595 $–

15 0.29 0.50 0.57 19 2393 $537 $57

16 0.28 0.31 0.52 24 1656 $545 $49

17 0.27 0.20 0.46 X X X X 27 1318 $560 $35

19 0.19 0.28 0.45 X X X 24 1560 $529 $65

Table 12. Savings of New Windows in Las Vegas, Nevada

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

11 0.41 0.23 0.49 X 13 3271 $564 $–

15 0.29 0.50 0.57 6 4673 $648 $(84)

16 0.28 0.31 0.52 X 9 3605 $558 $6

17 0.27 0.20 0.46 X X X X 12 3121 $532 $32

19 0.19 0.28 0.45 X X X 9 3429 $534 $29

Table 13. Savings of New Windows in San Francisco, California

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

11 0.41 0.23 0.49 X 27 23 $259 $–

15 0.29 0.50 0.57 X 13 85 $135 $124

16 0.28 0.31 0.52 X 20 37 $196 $63

17 0.27 0.20 0.46 X X X X 25 21 $242 $17

19 0.19 0.28 0.45 X X X 20 33 $196 $63
28

Table 14. Savings of New Windows in Washington, D.C.

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

11 0.41 0.23 0.49 36 1058 $648 $–

15 0.29 0.50 0.57 X 25 1812 $601 $46

16 0.28 0.31 0.52 X X X X 30 1268 $599 $48

17 0.27 0.20 0.46 X X X X 33 1020 $606 $42

19 0.19 0.28 0.45 X X X X 29 1197 $577 $70

Table 15. Savings of New Windows in Albuquerque, New Mexico

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

11 0.41 0.23 0.49 33 1043 $431 $–

15 0.29 0.50 0.57 X 19 1929 $381 $50

16 0.28 0.31 0.52 X X X X 26 1282 $388 $44

17 0.27 0.20 0.46 X X X X 31 988 $402 $29

19 0.19 0.28 0.45 X X X X 26 1190 $374 $57

Table 16. Savings of New Windows in Seattle, Washington

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

11 0.41 0.23 0.49 31 118 $386 $–

15 0.29 0.50 0.57 X X X 23 281 $299 $87

16 0.28 0.31 0.52 X X X 26 170 $331 $55

17 0.27 0.20 0.46 X X X X 28 111 $352 $34

19 0.19 0.28 0.45 X X X X 25 152 $316 $70
29

5.1.3 Savings for New Windows in Climate Zones 5–8

In these colder climate zones, the base case for new windows is a moderate-solar-gain low-e

window in a nonmetal frame (Window 16: U = 0.28 and SHGC = 0.31) (see Table 17). Higher 

solar-heat-gain low-e coatings contribute to passive solar heating, but there is a tradeoff in the 

cooling season. Lower U-factor is the predominant contributor to savings (Tables 18–24). The 

triple-glazed high-solar-gain low-e window produces the most savings in all cities in these 

climate zones.

Table 17. Properties of Windows Used in Climate Zones 5–8

ID Glazing Frame U SHGC VT

15 Double, low-e, high SHGC, argon, improved Nonmetal, 

improved 0.29 0.50 0.57

16 Double, low-e, medium SHGC, argon, improved Nonmetal, 

improved 0.28 0.31 0.52

17 Double, low-e, low SHGC, argon, improved Nonmetal, 

improved 0.27 0.20 0.46

18 Triple, low-e, high SHGC, argon, improved Nonmetal, 

improved 0.20 0.41 0.50

19 Triple, low-e, medium SHGC, argon, improved Nonmetal, 

improved 0.19 0.28 0.45

The annual energy performance figures shown were generated with RESFEN6 by LBNL (windows.lbl.gov). Results 

assume a typical new construction 2000-ft2 house with 300 ft2 of window area. The windows are equally distributed 

on all four sides with no shading. U-factor and SHGC are for the whole window. Costs for lights, appliances, hot 

water, cooking, and other uses are not included in these figures. The mechanical system uses a gas furnace for 

heating and air conditioning for cooling. Natural gas prices (EIA 2012b) and electricity prices (EIA 2012a) used are 

year 2010 averages provided by the Energy Information Administration.
30

Table 18. Savings of New Windows in Chicago, Illinois

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

16 0.28 0.31 0.52 X X X 43 896 $504 $–

15 0.29 0.50 0.57 X X X 37 1377 $507 $(3)

17 0.27 0.20 0.46 X X X X 46 685 $507 $(3)

18 0.20 0.41 0.50 X X X X 37 1185 $482 $21

19 0.19 0.28 0.45 X X X X 41 838 $484 $20

Table 19. Savings of New Windows in Boston, Massachusetts

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

16 0.28 0.31 0.52 X X X 38 638 $661 $–

15 0.29 0.50 0.57 X X X 32 956 $611 $50

17 0.27 0.20 0.46 X X X X 42 471 $687 $(26)

18 0.20 0.41 0.50 X X X X 32 825 $597 $64

19 0.19 0.28 0.45 X X X X 37 595 $638 $23

Table 20. Savings of New Windows in Denver, Colorado

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

16 0.28 0.31 0.52 X X X 28 747 $307 $–

15 0.29 0.50 0.57 X X X 21 1206 $306 $1

17 0.27 0.20 0.46 X X X X 31 548 $316 $(9)

18 0.20 0.41 0.50 X X X X 22 1016 $287 $20

19 0.19 0.28 0.45 X X X X 27 688 $293 $14
31

Table 21. Savings of New Windows in Minneapolis, Minnesota

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

16 0.28 0.31 0.52 X X X 53 767 $541 $–

15 0.29 0.50 0.57 X X X 46 1170 $527 $14

17 0.27 0.20 0.46 X X X X 56 574 $550 $(9)

18 0.20 0.41 0.50 X X X X 46 1002 $506 $35

19 0.19 0.28 0.45 X X X X 51 710 $519 $22

Table 22. Savings of New Windows in Billings, Montana

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

16 0.28 0.31 0.52 X X X 43 671 $436 $–

15 0.29 0.50 0.57 X X X 37 1028 $412 $24

17 0.27 0.20 0.46 X X X X 47 517 $453 $(17)

18 0.20 0.41 0.50 X X X X 37 861 $396 $40

19 0.19 0.28 0.45 X X X X 42 616 $418 $18

Table 23. Savings of New Windows in Bismarck, North Dakota

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

16 0.28 0.31 0.52 X X X 57 587 $507 $–

15 0.29 0.50 0.57 X X X 50 978 $483 $24

17 0.27 0.20 0.46 X X X X 61 415 $524 $(17)

18 0.20 0.41 0.50 X X X X 49 808 $465 $42

19 0.19 0.28 0.45 X X X X 55 534 $487 $20
32

Table 24. Savings of New Windows in Fairbanks, Alaska

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

16 0.28 0.31 0.52 X X X 105 68 $943 $–

15 0.29 0.50 0.57 X X X 100 149 $911 $32

17 0.27 0.20 0.46 X X X X 108 39 $968 $(24)

18 0.20 0.41 0.50 X X X X 97 112 $878 $66

19 0.19 0.28 0.45 X X X X 101 59 $907 $36
33

5.2 Energy and Cost Savings for Replacement Windows

For replacement windows in existing construction, the base case is either a single- or doubleglazed clear metal-framed window (depending on climate). Again, the other five window options 

represent improvements over the base case. Because of the relatively poor performance of the 

base case, energy savings from higher performance replacement windows can be significant.

5.2.1 Savings for Replacement Windows in Climate Zones 1 and 2 

The base case for replacement windows in this relatively hot region is a clear single-glazed 

window in a metal frame (Window 1: U = 1.29 and SHGC = 0.73) (see Table 25). Windows with 

a lower SHGC produce significant annual savings (as much as $600) (Tables 26–28). A doubleor triple-glazed window with a very low-solar-gain low-e coating (SHGC ≤0.20) performs best 

in these climate zones.

Table 25. Properties of Windows Used in Climate Zones 1 and 2

ID Glazing Frame U SHGC VT

1 Single clear Metal 1.29 0.73 0.69

7 Double clear Metal, thermal break 0.60 0.62 0.63

11 Double, low-e, low SHGC, argon Metal, thermal break 0.41 0.23 0.49

17 Double, low-e, low SHGC, argon, 

improved Nonmetal, improved 0.27 0.20 0.46

20 Triple, low-e, low SHGC, argon, 

improved Nonmetal, improved 0.19 0.18 0.37

The annual energy performance figures shown were generated with RESFEN6 by LBNL (windows.lbl.gov). Results 

assume an existing 2000-ft2 house with 300 ft2 of window area. The windows are equally distributed on all four 

sides with no shading. U-factor and SHGC are for the whole window. Costs for lights, appliances, hot water, 

cooking, and other uses are not included in these figures. The mechanical system uses a gas furnace for heating and 

air conditioning for cooling. Natural gas prices (EIA 2012b) and electricity prices (EIA 2012a) used are year 2010 

averages provided by the Energy Information Administration.
34

Table 26. Savings of Replacement Windows in Miami, Florida

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

1 1.29 0.73 0.69 1 11907 $1,381 $–

7 0.60 0.62 0.63 1 11014 $1,271 $110

11 0.41 0.23 0.49 X X X 1 7584 $879 $502

17 0.27 0.20 0.46 X X X X 1 7403 $857 $524

20 0.19 0.18 0.37 X X X X 1 7256 $839 $541

Table 27. Savings of Replacement Windows in Houston, Texas

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

1 1.29 0.73 0.69 1 11907 $1,381 $–

7 0.60 0.62 0.63 1 11014 $1,271 $110

11 0.41 0.23 0.49 X X X 1 7584 $879 $502

17 0.27 0.20 0.46 X X X X 1 7403 $857 $524

20 0.19 0.18 0.37 X X X X 1 7256 $839 $541

Table 28. Savings of Replacement Windows in Phoenix, Arizona

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

1 1.29 0.73 0.69 13 13672 $1,705 $–

7 0.60 0.62 0.63 9 12226 $1,486 $219

11 0.41 0.23 0.49 X X 10 8285 $1,073 $632

17 0.27 0.20 0.46 X X X X 10 8017 $1,032 $673

20 0.19 0.18 0.37 X X X X 9 7830 $1,007 $698
35

5.2.2 Savings for Replacement Windows in Climate Zones 3 and 4

The base case for replacement windows in this relatively hot region is a clear single-glazed 

window in a metal frame (Window 1: U = 1.29 and SHGC = 0.73) (see Table 29). Windows with 

a lower SHGC produce significant annual savings (as much as $500) (Tables 30–35). Depending 

on the dominance of the heating or cooling loads, a double-glazed window with a moderatesolar-gain (SHGC = 0.31) or low-solar-gain low-e coating (SHGC = 0.20) performs well in these 

climate zones; the triple-glazed window with a moderate-solar-gain performs best.

Table 29. Properties of Windows Used in Climate Zones 3 and 4

ID Glazing Frame U SHGC VT

1 Single clear Metal 1.29 0.73 0.69

11 Double, low-e, low SHGC, argon Metal, thermal break 0.41 0.23 0.49

16 Double, low-e, medium SHGC, 

argon, improved Nonmetal, improved 0.28 0.31 0.52

17 Double, low-e, low SHGC, argon, 

improved Nonmetal, improved 0.27 0.20 0.46

19 Triple, low-e, low SHGC, argon, 

improved Nonmetal, improved 0.19 0.28 0.45

The annual energy performance figures shown were generated with RESFEN6 by LBNL (windows.lbl.gov). Results 

assume an existing 2000-ft2 house with 300 ft2 of window area. The windows are equally distributed on all four 

sides with no shading. U-factor and SHGC are for the whole window. Costs for lights, appliances, hot water, 

cooking, and other uses are not included in these figures. The mechanical system uses a gas furnace for heating and 

air conditioning for cooling. Natural gas prices (EIA 2012b) and electricity prices (EIA 2012a) used are year 2010 

averages provided by the Energy Information Administration.
36

Table 30. Savings of Replacement Windows in Atlanta, Georgia

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

1 1.29 0.73 0.69 50 5066 $1,287 $–

11 0.41 0.23 0.49 X 42 2861 $941 $346

16 0.28 0.31 0.52 37 3277 $903 $384

17 0.27 0.20 0.46 X X X X 40 2779 $902 $385

19 0.19 0.28 0.45 X X X 36 3137 $882 $405

Table 31. Savings of Replacement Windows in Las Vegas, Nevada

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

1 1.29 0.73 0.69 93 3166 $1,738 $–

11 0.41 0.23 0.49 X 78 1536 $1,306 $432

16 0.28 0.31 0.52 72 1824 $1,254 $484

17 0.27 0.20 0.46 X X X X 75 1474 $1,256 $482

19 0.19 0.28 0.45 X X X 71 1715 $1,226 $512

Table 32. Savings of Replacement Windows in San Francisco, California

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

1 1.29 0.73 0.69 39 273 $407 $–

11 0.41 0.23 0.49 X 36 75 $354 $53

16 0.28 0.31 0.52 X X 29 108 $293 $114

17 0.27 0.20 0.46 X X X X 34 71 $335 $72

19 0.19 0.28 0.45 X X X 29 94 $290 $117
37

Table 33. Savings of Replacement Windows in Washington, D.C.

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

1 1.29 0.73 0.69 93 3166 $1,738 $–

11 0.41 0.23 0.49 78 1536 $1,306 $432

16 0.28 0.31 0.52 X X X X 72 1824 $1,254 $484

17 0.27 0.20 0.46 X X X X 75 1474 $1,256 $482

19 0.19 0.28 0.45 X X X X 71 1715 $1,226 $512

Table 34. Savings of Replacement Windows in Albuquerque, New Mexico

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

1 1.29 0.73 0.69 63 4416 $1,066 $–

11 0.41 0.23 0.49 55 2235 $759 $307

16 0.28 0.31 0.52 X X X X 47 2592 $725 $342

17 0.27 0.20 0.46 X X X X 52 2157 $726 $340

19 0.19 0.28 0.45 X X X X 47 2461 $707 $360

Table 35. Savings of Replacement Windows in Seattle, Washington

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

1 1.29 0.73 0.69 88 592 $1,123 $–

11 0.41 0.23 0.49 70 150 $865 $259

16 0.28 0.31 0.52 X X X 63 217 $793 $330

17 0.27 0.20 0.46 X X X X 66 137 $825 $298

19 0.19 0.28 0.45 X X X X 62 192 $777 $346
38

5.2.3 Savings for Replacement Windows in Climate Zones 5–8

In these colder climate zones, the base case for replacement windows is a clear double-glazed 

window in a nonmetal frame (Window 13: U = 0.52 and SHGC = 0.57) (see Table 36). Higher 

solar-heat-gain low-e coatings contribute to passive solar heating, but there can be a tradeoff in 

the cooling season. Lower U-factor is the predominant contributor to savings (Tables 37–43). 

The triple-glazed high-solar-gain low-e window produces the most savings in all cities in these 

climate zones.

Table 36. Properties of Windows Used in Climate Zones 5–8

ID Glazing Frame U SHGC VT

13 Double Clear Nonmetal 0.52 0.57 0.59

15 Double, low-e, high SHGC, argon, 

improved Nonmetal, improved 0.29 0.50 0.57

16 Double, low-e, medium SHGC, 

argon, improved Nonmetal, improved 0.28 0.31 0.52

18 Triple, low-e, high SHGC, argon, 

improved Nonmetal, improved 0.20 0.41 0.50

19 Triple, low-e, medium SHGC, argon, 

improved Nonmetal, improved 0.19 0.28 0.45

The annual energy performance figures shown were generated with RESFEN6 by LBNL (windows.lbl.gov). Results 

assume an existing 2000-ft2 house with 300 ft2 of window area. The windows are equally distributed on all four 

sides with no shading. U-factor and SHGC are for the whole window. Costs for lights, appliances, hot water, 

cooking, and other uses are not included in these figures. The mechanical system uses a gas furnace for heating and 

air conditioning for cooling. Natural gas prices (EIA 2012b) and electricity prices (EIA 2012a) used are year 2010 

averages provided by the Energy Information Administration.
39

Table 37. Savings of Replacement Windows in Chicago, Illinois

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

13 0.52 0.57 0.59 99 1907 $1,151 $–

15 0.29 0.50 0.57 X X X 85 1797 $1,007 $144

16 0.28 0.31 0.52 X X X 93 1148 $1,001 $150

18 0.20 0.41 0.50 X X X X 85 1511 $976 $175

19 0.19 0.28 0.45 X X X X 91 1068 $978 $173

Table 38. Savings of Replacement Windows in Boston, Massachusetts

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

13 0.52 0.57 0.59 85 1283 $1,450 $–

15 0.29 0.50 0.57 X X X 72 1208 $1,249 $202

16 0.28 0.31 0.52 X X X 80 744 $1,304 $147

18 0.20 0.41 0.50 X X X X 73 1001 $1,230 $220

19 0.19 0.28 0.45 X X X X 79 681 $1,278 $173

Table 39. Savings of Replacement Windows in Denver, Colorado

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

13 0.52 0.57 0.59 80 1998 $874 $–

15 0.29 0.50 0.57 X X X 68 1844 $757 $116

16 0.28 0.31 0.52 X X X 77 1150 $753 $121

18 0.20 0.41 0.50 X X X X 69 1549 $732 $142

19 0.19 0.28 0.45 X X X X 76 1055 $734 $139
40

Table 40. Savings of Replacement Windows in Minneapolis, Minnesota

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

13 0.52 0.57 0.59 119 1603 $1,204 $–

15 0.29 0.50 0.57 X X X 102 1507 $1,050 $155

16 0.28 0.31 0.52 X X X 111 922 $1,061 $144

18 0.20 0.41 0.50 X X X X 102 1256 $1,023 $181

19 0.19 0.28 0.45 X X X X 109 839 $1,036 $169

Table 41. Savings of Replacement Windows in Billings, Montana

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

13 0.52 0.57 0.59 107 1705 $1,082 $–

15 0.29 0.50 0.57 X X X 91 1581 $937 $145

16 0.28 0.31 0.52 X X X 100 1013 $962 $120

18 0.20 0.41 0.50 X X X X 92 1336 $919 $163

19 0.19 0.28 0.45 X X X X 99 931 $941 $141

Table 42. Savings of Replacement Windows in Bismarck, North Dakota

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

13 0.52 0.57 0.59 130 1390 $1,161 $–

15 0.29 0.50 0.57 X X X 112 1290 $1,007 $155

16 0.28 0.31 0.52 X X X 121 725 $1,035 $127

18 0.20 0.41 0.50 X X X X 112 1046 $987 $175

19 0.19 0.28 0.45 X X X X 119 650 $1,013 $149
41

Table 43. Savings of Replacement Windows in Fairbanks, Alaska

ID U SHGC VT 2009 

IECC

2012 

IECC

2010 

ENERGY 

STAR

2014 

ENERGY 

STAR

Heat 

(MBtu)

Cool 

(kWh)

Total 

Cost

Annual 

Savings

13 0.52 0.57 0.59 237 211 $2,139 $–

15 0.29 0.50 0.57 X X X 209 188 $1,893 $246

16 0.28 0.31 0.52 X X X 218 75 $1,950 $189

18 0.20 0.41 0.50 X X X X 207 142 $1,862 $277

19 0.19 0.28 0.45 X X X X 214 63 $1,913 $226
42

5.3 Window Costs (New and Replacement)

There can be a wide range of costs for purchasing and installing windows. The National 

Residential Efficiency Measures Database (NREL 2012b) suggests the following average costs 

for new and replacement windows (see Table 44). In both cases installation is included, but 

window replacement requires an additional $3/ft2 for demolition.

Table 44. Example of Window Costs

Glazing Type Frame Type Average Costs 

New (ft2)

Average Costs 

Replacement (ft2)

Double, clear Nonmetal $21 $24

Double, low-e Nonmetal $26 $29

Triple, low-e Nonmetal $31 $34

New windows can cost up to $40/ft2 for double-glazed low-e units and nearly $60/ft2 for tripleglazed low-e units. It is thus impossible to establish generally applicable paybacks for energyefficient windows. Instead, tables have been created so the user can find the appropriate payback 

for the energy savings from the tables in the previous section based on the actual incremental 

cost of the windows. All costs and paybacks calculated here are based on the 2000-ft2 house with 

300 ft2 of glazing, but results should be scalable to larger houses.

It is assumed that all new windows must meet the 2009 IECC; compliant windows are used as 

the base case cost. In new construction then, the incremental cost of installing higher 

performance double-glazed low-e windows is assumed to be $5/ft2, and higher performance 

triple-glazed windows $10/ft2

. Table 45 shows simple paybacks for new windows based on these 

low incremental costs.

Table 45. Simple Payback for New Windows

Annual 

Energy 

Savings

Simple Payback (Years) for Incremental Window Cost

(ft2)

$5 $10 $15 $20 $25

$50 30 60 90 120 150

$100 15 30 45 60 75

$150 10 20 30 40 50

$200 8 15 23 30 38

$250 6 12 18 24 30

$300 5 10 15 20 25

$350 4 9 13 17 21

$400 4 8 11 15 19

$450 3 7 10 13 17

$500 3 6 9 12 15

$550 3 5 8 11 14

$600 3 5 8 10 13

Note: This is based on 300 ft2 of window area on a single-family house.
43

There are two ways to look at replacement window costs. If windows are to be replaced for 

maintenance, aesthetic, or other reasons, the incremental cost of more efficient windows may be 

$5–$10/ft2 (similar to new windows) and Table 45 can be used to determine paybacks. If, 

however, the window replacement is primarily for energy savings, the entire cost of the 

demolition, new window purchase, and installation must be included in the analysis. In this case, 

higher performing double-glazed windows are likely to cost $25–$30/ft2 and higher performing 

triple-glazed windows $35/ft2

or more. Table 46 shows simple paybacks for replacement 

windows with these higher incremental costs.

Table 46. Simple Payback for Replacement Windows

Annual 

Energy 

Savings

Simple Payback (Years) for Incremental Window Cost 

(ft2)

$20 $25 $30 $35 $40

$50 120 150 180 210 240

$100 60 75 90 105 120

$150 40 50 60 70 80

$200 30 38 45 53 60

$250 24 30 36 42 48

$300 20 25 30 35 40

$350 17 21 26 30 34

$400 15 19 23 26 30

$450 13 17 20 23 27

$500 12 15 18 21 24

$550 11 14 16 19 22

$600 10 13 15 18 20

Note: This is based on 300 ft2 of window area on a single-family house.

5.4 Life Cycle Cost Analysis

There are many ways to analyze the costs and benefits of more energy-efficient windows. Each

method has its advantages and disadvantages and each is based on several assumptions that may 

change in the future. The conclusions drawn from any such life cycle cost analysis (LCCA) do 

not necessarily take into account all the relevant factors in the decision-making process. Tables 

45 and 46 show the results of simple payback calculations with no energy inflation rate. The 

most basic payback calculation divides the investment (added construction cost) by the annual 

cost savings to obtain the number of years it takes for the cumulative cash flow to reach zero. If 

the simple payback calculation includes an energy inflation rate, payback periods are shorter. 

The payback amount is about one year shorter for a 2% energy inflation rate.

Simple payback analysis has its shortcomings. It does not account for cash flow after payback 

has been achieved and does not measure the long-term value of an investment. It also ignores the 

time value of money—the principle that money received in the future is less valuable than

money received today. Net present value is an analysis tool that accounts for the time value of 
44

money by discounting future cash flows. If the result of a net present value calculation is a 

positive number over the time period specified, it is considered a good investment; if it is a 

negative number, it is not a good investment.

Net monthly cash flow analysis is another way to evaluate an investment in energy efficiency. 

The monthly mortgage payment is calculated for the additional money spent for the energy 

efficiency improvement. This is compared to the monthly energy savings. Any investment with a 

positive net monthly cash flow is attractive to the homeowner. A final method of comparing 

alternative scenarios is 30-year lifetime cost. This represents the total of the initial construction 

cost, the energy costs, and the cost of the mortgage payment on borrowed money over 30 years.

5.4.1 Life Cycle Cost Summary

The preceding LCCA discussion is not intended to provide definitive answers about the cost 

effectiveness of new or replacement windows. Instead, it serves as a means to illustrate the many 

issues and variables that might be considered in such a decision.

It is important to remember that all this analysis is based on using RESFEN for a typical house 

with a number of assumptions about the house characteristics and window options. Changes in 

key assumptions can significantly change the results and conclusions. The cost of the windows is 

one important factor. As the window cost difference is reduced, the LCC are more attractive. 

Similarly, energy price increases or carbon taxes designed to increase fossil fuel energy prices 

would make the analysis more favorable for investing in higher levels of energy efficiency. 

Remember that if windows are to be replaced for maintenance, aesthetic, or other reasons, 

payback calculations should not be based on the full cost of the window replacement but instead 

on the incremental cost of more efficient windows (typically $5–$10/ft2

).

The LCCA presented here excludes several important factors that can favorably influence the 

decision to invest in energy-efficient windows:

• Replacement windows can represent maintenance cost savings as well as energy savings. 

Durability is a key issue in selecting new windows.

• High performance windows generally improve thermal comfort. On a cold day, the 

warmer surface of a new window means less radiant heat loss. This may allow 

homeowners to be comfortable at an air temperature that is 2°F lower. Lowering the 

thermostat setting to reflect this adds to the energy savings attributable to the windows 

(see next section).

• If installed properly, replacement windows will reduce air leakage through the window 

and through the building envelope surrounding the window. This can significantly 

increase energy savings in cold climates. The fact that this analysis does not include the 

improvement in airtightness means that the value of replacement windows is somewhat 

shortchanged. The improved comfort may also save additional energy if thermostats are 

lowered.

• Installing high performance windows in new construction often means the heating, 

ventilation, and air conditioning (HVAC) system loads are reduced. This in turn means 

that smaller heating and cooling units can be installed, resulting in cost savings that offset 
45

the window investment (see next section). In an existing house, this benefit can accrue 

when the window and mechanical system upgrades occur at the same time.

• Other qualitative benefits of new windows are not accounted for: reduced condensation 

that can degrade window materials over time and allow mold and mildew growth, for 

example. These factors affect indoor air quality.

• The cost of replacing windows may be offset by investments in other attachments (storm 

windows, films, shades, and shutters) that may otherwise be required.

One key fact in considering long-term investments is not recognized by typical LCCA methods:

energy efficiency improvements such as new windows actually increase the resale value of the 

home.

5.5 Other Benefits

Selecting windows based on one attribute (such as energy performance) may not always lead to a

completely balanced outcome. Appearance, comfort, reduced condensation, reduced air leakage, 

cost, and possible HVAC equipment downsizing should all be considered. Two benefits (comfort 

and reduced HVAC sizing) are discussed here in more detail because of their potential positive 

impacts on LCC.

5.5.1 Comfort

Although energy-efficient windows can make up for their cost premium by savings on energy 

bills, their improved thermal comfort is an immediate benefit. A window with good energy

performance will generally provide greater thermal comfort than a poorer energy performer.

Thermal comfort is determined by air temperature, humidity, air movement, mean radiant 

temperature, and direct solar radiation. Surface temperatures in a room, which determine the 

mean radiant temperature, can significantly affect thermal comfort. Even when room air is 

maintained at a comfortable temperature, occupants may experience significant discomfort from 

the radiant heat exchange with window surfaces. Window surface temperatures fluctuate much 

more significantly than those from other room surfaces. Generally in winter, the more efficient a 

window is, the less the window surface temperature deviates from the conditioned room air 

temperature and the less discomfort an occupant feels.

5.5.1.1 Sources of Discomfort From Windows

Windows affect human comfort in several ways. On a cold winter night, exterior temperatures 

drive window interior glass surface temperatures below the room air temperature; how low the 

glass temperature drops depends on the insulating quality of the window. If people are exposed 

to the effects of a cold surface, they experience significant radiant heat loss to that cold surface 

and they feel uncomfortable, even when room air temperatures are comfortable. The closer they 

are to a window, the more they feel its influence. This heat loss, called radiant asymmetry,

occurs on one side of the body more than the other, and causes further discomfort. A familiar 

example of radiant asymmetry is the experience of sitting around a campfire on a winter night. 

The side of the body close to the fire is hot, while the other side is cold. With a cold window, a 

person may be cold in warm clothes in a 70°F room air temperature if part of the body is losing 

heat.
46

Drafts near windows are the second major source of winter discomfort. Many people falsely 

attribute drafts to leaky windows when in fact they are the result of cold air patterns initiated by 

cold window surfaces. Air next to the window is cooled and drops to the floor. It is then replaced 

by warmer air from the ceiling, which in turn is cooled. This sets up an air movement pattern that 

feels drafty and accelerates heat loss. Cold temperature-induced drafts occur at the same time as 

radiant discomfort. This emphasizes the need for better insulating windows that maximize 

interior glass surface temperatures under cold environmental conditions.

Drafts can also be caused by windows that leak because of poor installation or ineffective 

weather stripping. Such drafts are directly correlated to air infiltration levels. Radiant heat loss, 

convective currents from cold window surfaces, and drafts from air infiltration leaks all cause 

people to turn up thermostats. However, as explained above, this action improves comfort levels

very little.

Directly transmitted solar radiation has fairly obvious impacts on thermal comfort. During cold 

periods, solar radiation (within limits) can be a pleasant sensation. During warm weather, 

however, it is invariably a significant detractor to comfort. People often close blinds to prevent 

sunlight from entering, even though this means they can no longer enjoy the view from the 

window. Just as people turn up the heat in response to cold windows in winter, they may use air

conditioning to counter the effects of warm window surfaces and sunlight in summer. If air 

conditioners are not sized or installed properly, some areas may become comfortable while 

others will not. This wastes significant energy.

Solar radiation increases the surface temperature of the glass. The increase depends on the 

absorptance of the glass and the environmental conditions. Typical clear glass windows absorb 

too little solar radiation to make a significant difference in their temperature. With tinted glass, 

surface temperature increases can be significant. Although poorly insulated tinted glass can 

actually feel quite comfortable on a cold sunny day, the comfort consequences at night and on 

hot summer days can be disastrous. During the summer, the interior surface temperatures of 

tinted glass and clear glass with tinted film can become as hot as 140°F. These surfaces radiate 

heat to occupants and can create convection drafts of warm air that can cause discomfort.

5.5.1.2 Quantifying Discomfort

Typically, thermal comfort experiments are conducted on a large number of human subjects who 

report their comfort levels under widely varying conditions in a laboratory set up to represent a 

room or office (see Figure 27). Crude guidelines are available for the most basic causes of 

discomfort under static conditions. For example, the American Society of Heating, Refrigerating 

and Air-Conditioning Engineers (ASHRAE) Standard 55 Thermal Environmental Conditions for 

Human Occupancy (the North American comfort standard) suggests that surface temperatures 

lower than 50°F or higher than 90°F will lead to radiant discomfort (ASHRAE 2010). Other 

factors to consider are how many hours per year the user will experience this discomfort, how 

well the individual tolerates discomfort, clothing levels, distance from the window, and the 

ability of the HVAC system to meet peak conditions.
47 

When the outside temperature is cold, the window’s interior surface temperature drops. How far 

it drops depends on the window’s insulating value. The surface temperature of single glazing, for 

example, is only 10°–15°F higher than the outdoor temperature. The interior surface temperature 

of a clear, double-glazed window is much warmer, but is still significantly lower than room air 

temperature. The double-glazed window with a low-e coating and argon gas fill has an interior 

surface temperature that is warmer still. A window with three to four glazing layers, multiple 

low-e coatings, and gas fills has an interior glass surface temperature very close to the indoor air 

temperature. Frames, which can make up 10%–30% of the area of a typical window, also have 

noticeable effects; surface temperatures of insulating frames will be much warmer than those of 

highly conductive frames. 

Another way to quantify discomfort is through the percentage of people who report it for 

standard ASHRAE winter (cold night) and summer (hot sunny day) conditions. In both cases, 

efficient windows significantly reduce the probability of discomfort. However, it is never 

possible for everyone to be comfortable—at best 10% of a group of people will always consider 

themselves uncomfortable (see Figures 28 and 29). 

Figure 27. Comparison of inside glass surface temperature for different glazing types

(Image courtesy of LBNL

Figure 28. Probability of discomfort near a window in the winter

(Image courtesy of LBNL) 
48 

5.5.1.3 Comparing Windows Based on Thermal Comfort

To enable comparisons between windows, the Center for the Built Environment at University of 

California Berkeley developed a method for determining a Winter and Summer Thermal 

Comfort Index (Huizenga et al. 2006). The Winter Comfort Index represents the minimum 

exterior temperature that will provide indoor comfort for a given window. As shown in Table 47, 

the index is nearly 60°F for single glazing (U-factor = 1.02). This means that the window has the 

potential to be uncomfortable at outdoor temperatures below this level. The index for double 

glazing (U-factor = 0.48) is reduced to 44.2°F and clear triple glazing (U-factor = 0.30) is 

reduced to 28.2°F. Double glazing with either high- or moderate-solar-gain low-e coatings 

further reduce the Winter Comfort Index to 20.8°F and 16.7°F. Triple-glazed low-e options 

perform the best with Winter Comfort Indices of -18.4 to -21.5°F, meaning that they remain 

comfortable as long as it is above these subzero temperatures.

Table 47. Winter and Summer Comfort Index for Typical Windows

(Huizenga et al. 2006)

Glazing U SHGC VT

Winter Comfort 

Index Minimum 

Exterior 

Temperature 

(°F)

Summer 

Comfort 

Index 

Diffuse

Summer 

Comfort 

Index 

Direct

Single Clear 1.02 0.82 0.88 59.5 0.99 0.89

Single Bronze 1.02 0.62 0.53 59.5 1.06 0.80

Double Clear 0. 48 0.70 0.79 44.2 1.01 0.83

Double Medium

Gain Low-e

0.32 0.59 0.75 20.8 1.00 0.76 

Double High 

Gain Low-e

0.30 0.36 0.67 16.7 0.53 0.43 

Triple Clear 0.30 0.62 0.70 28.2 1.06 0.80

Triple Medium

Gain Low-e

0.16 0.45 0.65 -18.4 0.82 0.60 

Triple High Gain 

Low-e

0.16 0.31 0.58 -21.5 0.51 0.39 

Figure 29. Probability of discomfort near a window in the summer

(Image courtesy of LBNL)
49

The Summer Comfort Index is effectively a metric of solar gain’s effect on human comfort: a 

lower number indicates improved comfort. The index can be determined via two approaches:

• Include only diffuse solar radiation, assuming a person in direct sunlight would either 

move or adjust the shades in the room.

• Include direct and diffuse solar radiation.

As shown in Table 47, the Summer Comfort Index (Diffuse) is around 1.00 for clear glazings 

whether they are single-, double-, or triple-glazed units (SHGC = 0.60-0.80). Bronze-tinted 

single-glazing (SHGC = 0.62) actually has a worse Summer Comfort Index (1.06) than the clear 

glazings because of its increased heat absorption and elevated surface temperature. Different 

types of low-e coatings perform very differently in terms of summer comfort. Double glazing 

with a high-solar-gain low-e coating (SHGC = 0.59) has a Summer Comfort Index of 1.00; a 

double-glazed unit with a moderate-solar-gain low-e coating (SHGC = 0.36) has a much lower 

Summer Comfort Index of 0.53. In triple-glazed units, the high-solar-gain low-e unit (SHGC = 

0.45) improves to a Summer Comfort Index of 0.82, but is still well above the 0.51 index for 

moderate-solar-gain low-e (SHGC = 0.31).

5.5.2 Reduced Peak Demand and Heating, Ventilation, and 

Air Conditioning Costs

In addition to reducing annual heating and cooling bills, high performance windows also help 

reduce peak heating and cooling loads. The peak load for a building is the maximum requirement 

for heating or cooling at one time. Peak heating and cooling loads determine the size of the 

furnace, heat pump, air conditioner, and fans that must be installed. In any homebuilding or 

remodeling project, it is important to properly size the HVAC system to ensure that the 

equipment runs efficiently and provides the best comfort. When efficient windows are installed, 

peak HVAC loads are often lower than commonly expected.

Reducing peak loads may allow homeowners to install a smaller heating or cooling system. A 

smaller HVAC system costs less and can offset some of the cost of the efficient windows. 

Figures 30 and 31 show HVAC system sizing for an average size home in Phoenix and 

Minneapolis. The peak system sizing figures shown here were generated from simulation results 

provided by LBNL (2008). Results assume a typical new construction 2000-ft2 house with 15% 

window-to-floor area distributed equally on all four sides. U-factor and SHGC are for the total 

window including frame. Conversion between air conditioner electric load and system sizing 

assumes an 11.2 energy efficiency ratio.

In the Phoenix example, using low-solar-gain low-e windows may allow cooling equipment to be 

2 tons smaller than with single-pane windows and at least 1 ton smaller than with clear doublepane windows. Even in the Minneapolis example, window choices can impact cooling equipment 

size by up to 1 ton.
5

Figure 31. Peak summer cooling loads in Minneapolis, Minnesota 

Figure 30. Peak summer cooling loads in Phoenix, Arizona 
51

5.5.2.1 Rightsizing Heating, Ventilation, and Air Conditioning Systems

Properly sized HVAC systems provide a number of benefits (Proctor et al. 1995):

• Health and comfort. By running more constantly, smaller HVAC systems provide the 

best air quality and comfort.

• Mold prevention. In climates with humidity issues, HVAC systems that are more closely 

matched to peak cooling loads achieve better dehumidification.

• First cost savings. Smaller HVAC units cost less. If, for example, downsizing the HVAC 

system by 1⁄2 ton saves $275, the cost premium of energy-efficient windows presents a 

smaller up-front investment.

• Energy savings. Oversized units may have inefficient stop-and-go cycles; the best 

system efficiency is realized when HVAC units are sized appropriately.

5.5.2.2 Heating, Ventilation, and Air Conditioning Sizing Tools

Several computation procedures are available for proper sizing of HVAC equipment. The most 

prominent ones, which are also recommended by the ENERGY STAR Homes program, are Air 

Conditioning Contractors of America Manual J (ACCA 2011) and the ASHRAE Handbook of 

Fundamentals (ASHRAE 2009). Factors to be considered:

• The energy performance of the windows must be considered in load calculations. NFRCcertified window performance values (U-factor and SHGC) significantly increase the 

accuracy of these calculations.

• Window orientation and overhangs must be taken into account. Overhangs are an 

important factor influencing solar gains through windows. Where internal shades and 

blinds will be actively used, these should also be accounted for in load calculations.
5

 Impact of Design on Performance

.1 Orientation

.1.1 Orientation in the Northern Zone (Heating Dominated)

Simply orienting most windows to the south in a heating-dominated climate increases solar gain 

and reduces heating energy use. Figure 32 shows how the orientation of the windows affects 

heating and cooling costs. These results are the annual heating and cooling costs of a typical 

home in Boston, Massachusetts with half the windows facing one direction and the other half 

distributed evenly on the other three sides. The heating season benefits of southern orientation 

are most pronounced with high-solar-gain low-e glazing, whereas east and west orientations 

emphasize the cooling season benefits of low-e glazing with a lower SHGC. The relative 

performance of higher versus lower solar heat gain glazing also varies by local climate, fuel 

costs, and shading conditions. Window orientation in a house is often dictated by views and 

factors other than optimal solar gain. High performance windows at any orientation can increase

energy efficiency. For example, triple-glazed low-e windows in a north-facing orientation lowers 

energy use more than double-glazed windows in a south-facing orientation. 

Figure 32. Annual energy cost by orientation in Boston, Massachusetts
5

.1.2 Orientation in the Central Zones (Heating and Cooling)

In climates with significant heating and cooling seasons, orienting windows to the south will 

result in greater solar gain in winter; overhangs can be designed to reduce summer solar gain. 

East and west windows are difficult to shade and increase cooling loads. The results shown in 

Figure 33 indicate that, as expected, south-oriented windows perform best. These results are the 

annual heating and cooling costs of a typical home with half the windows facing one direction 

and the other half distributed evenly on the other three sides. The difference between orientations 

is most notable when clear-glazed or high-solar-gain low-e windows are used. With these 

windows, western orientation can significantly increase cooling energy use. The impact of 

orientation is diminished when windows with lower SHGCs are used.

High performance windows can increase energy efficiency regardless of orientation. For 

example, when the house has low-solar-gain low-e windows, any window orientation uses less 

annual energy than a south-facing orientation with clear, double-glazed windows. All the cases 

shown have average window area and shading conditions. If there were no shading or greater 

window area, the difference in energy costs between less efficient and more efficient windows 

would be greater. 

Figure 33. Annual energy cost by orientation in Sacramento, California
5

.1.3 Orientation in the Southern Zone (Cooling Dominated)

In predominantly cooling climates, the goal is to face most windows north, where there is little 

direct exposure, or to the south, where they can be designed with overhangs that will keep out 

most of the hot summer sun. Overhangs on the east and west sides are much less effective 

against the lower angles of sun. Therefore, simply reducing the size and number of east and west 

windows can be the best strategy. 

Figure 34 illustrates the impacts of various window orientations on annual energy costs for a 

typical house in Phoenix, Arizona. Because of intense solar heat, orientation has a significant 

impact when windows with a high SHGC are used. When higher performance windows with 

low-solar-gain low-e coatings are used, window orientation has a greatly diminished impact on 

energy use. 

All the cases shown have average window area and shading conditions. If there were no shading

or greater glazing area, the less efficient glazing would perform worse than the low-solar-gain 

low-e windows.

Figure 34. Annual energy cost by orientation in Phoenix, Arizona
5

.2 Window Area 

.2.1 Window Area in the Northern Zone (Heating Dominated)

High performance windows have reduced the need for limiting window areas to control energy 

use. Over a complete winter heating season, highly insulating windows can offset much of their 

heat loss through solar heat gain or even provide net heating benefits if oriented toward the sun. 

Figure 35 shows how window area affects the annual heating and cooling costs for a typical 

home in Boston, Massachusetts, with windows distributed equally on all four sides.

Total glazing area has a significant impact on energy use when conventional windows are used. 

This difference is diminished with low-e windows. With triple-glazed low-e windows, the low 

U-factor limits heat loss to such an extent that, considering solar gains, glazing area may no 

longer be a major factor in heating energy use. Solar gains through larger glazing areas increase 

cooling season energy use, but can be limited by shifting the window area to preferred 

orientations and employing shading strategies.

Figure 35. Annual energy cost by window area in Boston, Massachusetts
5

.2.2 Window Area in the Central Zones (Heating and Cooling)

In climates with significant heating and cooling loads, high performance windows are needed to 

control winter heat loss and summer heat gain. Figure 36 shows how window area affects the 

annual heating and cooling costs for a typical home in Sacramento, California. 

Total glazing area has a significant impact on energy use when conventional windows are used. 

This difference is diminished with low-e glazing. In a climate with heating and cooling loads, 

larger window areas increase the need for solar control. As the Sacramento, California example 

shows, the cooling benefits of using low-solar-gain low-e windows instead of high-solar-gain 

low-e windows become even more significant with increasing window area. On the flip side, 

larger window areas with low-solar-gain windows increase heating demand unless highly

insulating windows are used.

Although increasing glazing area increases energy use in this climate, the impact is much less 

profound with high performance windows. In all cases, energy use for cooling can be further 

reduced by shifting the window area to preferred orientations and employing shading strategies.

Figure 36. Annual energy cost by window area in Sacramento, California
5

.2.3 Window Area in the Southern Zone (Cooling Dominated)

The traditional approach to reduce heat gain is to reduce the total glazing area. However, lowsolar-gain low-e windows in combination with optimum orientation and shading can minimize 

cooling load impacts.

Figure 37 illustrates the impact of window area on annual energy costs for a house in Phoenix, 

Arizona. If windows with a high SHGC are used, increasing the glazing area has a significant 

impact on the cooling load. The annual energy use for a house with low-solar-gain low-e glazing 

still exhibits the same basic pattern, but the differences are not nearly as great in relative or 

absolute terms.

Although increasing glazing area increases energy use in this climate, its impact will be much 

less profound with high performance windows. In all cases, energy use for cooling can be further 

reduced by shifting the window area to preferred orientations and employing shading strategies.

Figure 37. Annual energy cost by window area in Phoenix, Arizona
5

.3 Shading

6.3.1 Shading in the Northern Zone (Heating Dominated)

For a house to receive solar radiation in winter, it must be located so its south façade is not in the 

shadow of other buildings or landscape elements. Locating the house on the north end of the site 

provides a greater assurance of future solar access. Of course, deciduous trees can be located 

within these limits (see Figure 38). 

Moving from no shading to more shaded conditions in a cold climate increases heating costs. 

However, these costs are offset by decreased cooling costs if the house is air conditioned. The 

overall balance between heating and cooling typically favors no shading. In cold climates, 

shading strategies should be tailored to allow solar gain in winter, although shading provides 

glare control and summer thermal comfort benefits. In this example, all shading strategies have a 

cost penalty. However, shading conditions can be optimized relative to the generic conditions 

assumed here to produce energy savings, even in northern climates. Different climates, fuel 

costs, window orientations, and glazing areas may also produce different results.

Figure 38. Annual energy cost by shading type in Boston, Massachusetts


6.3.2 Shading in the Central Zones (Heating and Cooling)

In climates with significant heating and cooling seasons, increased solar gain in winter and 

decreased solar gain in summer are desirable. Overhangs on the south side and other shading 

devices elsewhere provide these benefits. As shown in Figure 39 for Sacramento, the cooling 

season benefits of shading are notable.

Reliance on any form of shading is less important when windows with a low SHGC are used. 

Using low-solar-gain low-e glazing reduces energy costs significantly for all conditions, even 

with no shading. This is because the glazing controls solar radiation. However, shading provides 

summer comfort and glare control benefits, even with high performance glazing.

Increased west-facing window orientation or greater glazing area further increases the energy 

penalty of not using strategic shading or high performance windows. 

Figure 39. Annual energy cost by shading type in Sacramento, California
6

6.3.3 Shading in the Southern Zone (Cooling Dominated)

The best place to shade a window is on the outside, before the sun strikes the window. This can 

be accomplished with overhangs on the south and vertical elements on the east and west. 

Awnings, solar screens, and landscaping also are effective exterior shading elements. To most 

effectively reduce solar heat gain on the interior, the shade or drapery used to block the sunlight 

should have high reflectance (light color). The best strategy in hot climates is good shade 

management combined with low SHGC windows (see Figure 40). 

Reliance on any form of shading is much less important when windows with a low SHGC are 

used. Using a low-solar-gain low-e coating reduces energy costs significantly for all conditions, 

even with no shading. This is because the glazing controls solar radiation, so these additional 

measures become less important in terms of energy use. However, shading provides summer 

comfort and glare control benefits, even with high performance glazing. More west-facing 

window orientation or greater glazing area further increases the energy penalty of not using 

shading or high performance windows.

Figure 40. Annual energy cost by shading type in Phoenix, Arizona
61

7 Installation

No matter how advanced the glazing and frame 

materials are in a window, the ultimate 

performance also depends on the quality of its 

installation. Improper installation can contribute 

to air leakage, unnecessary heat loss, 

condensation, and water leakage. This may lead 

to diminished energy performance as well as

deterioration of walls, insulation, and the 

window unit.

A properly installed window must maintain 

barriers keeping air and water from penetrating 

the wall and it must restrict vapor flow. It must 

also reduce heat loss and condensation around 

the window unit. In addition, the installation 

must meet several structural and functional 

requirements. Building loads cannot rest on the 

window frame, the installation must allow for 

movement, the window must protect against 

forced entry, and yet it must maintain ease of 

operation.

7.1 General Installation Guidelines

• Always follow the manufacturer’s instructions.

• Meet all codes for energy efficiency, structure, proper egress, safety glass, and grade 

(design pressure).

• Size the rough opening properly to accommodate thermal expansion and movement.

• Install the window unit level, plumb, and square.

• Maintain the continuity of the weather-resistant barrier. In a barrier system, this is 

achieved with sealants on the outermost surface of the wall. In a membrane/drainage 

system, residual water must drain freely on the drainage plane. Use flashings overlapped 

shingle style and drip caps where needed. Avoid trapping water within the wall. 

• Sill pan flashings are always recommended.

• Do not leave thermal bridges between the interior and exterior. Carefully insulate all 

voids left between window and wall, but use only foam insulation that expands at a 

minimum rate.

• Maintain the integrity of air and vapor retarders. The air barrier must be connected 

continuously from the wall assembly to the window; this is typically accomplished using 

low-expansion foam (as per above) or caulk with a backer rod. Note that fibrous 

insulation (e.g., fiberglass) is air permeable, and should not be used in this application.

Important Resources

ASTM E 2112, “Standard Practice for 

Installation of Exterior Windows, Doors 

and Skylights.” 

www.astm.org/Standards/E2112.htm

Water Management Guide, by Joseph 

Lstiburek, is a guide that shows how to 

minimize water intrusion into home. 

This guide has many installation 

diagrams.

InstallationMasters includes a directory 

of certified installers in the United 

States. 

www.installationmastersusa.com

EPA brochure on lead-hazard for 

renovation, repair, and painting. 

www.epa.gov/lead/pubs/renovaterightbr

ochure.pdf

EPA website for lead in paint, dust, and 

soil. www.epa.gov/lead
62

• Avoid using incompatible materials such as certain metal combinations, or asphaltic 

caulks on chemically vulnerable substrates. Apply caulks and sealants that are compatible 

with the substrate.

• If applicable, follow the EPA’s lead-safe practices during renovation, repair, or painting 

projects.

There are important differences in the details of how a window is installed, depending on the 

type of construction (wood versus masonry) or exterior cladding material (wood siding, stucco, 

brick veneer). In addition, each operator type, frame material, and individual manufacturer may 

have its own recommended installation practices. It is important to refer to the appropriate 

manufacturer’s instructions and not to rely solely on general guidelines.

Given the importance of proper installation, there are some guidelines for installation. The 

American Architectural Manufacturers Association has developed an installer training and 

registration program called, Installation Masters (AAMA 2010) and ASTM has developed a 

window installation standard (ASTM 2007). The Water Management Guide (Lstiburek 2006) is 

another excellent resource.

7.2 Watertight Installation

Although there are many wall materials and construction assemblies, there are three fundamental 

approaches to water control—storage or mass wall systems, the perfect barrier system, and the 

membrane/drainage system (Straube 2011). Determining which is used in the wall affects the 

window installation approach.

7.2.1 Storage or Mass Wall System

This approach requires enough storage mass in the wall to absorb all rainwater not otherwise 

drained away. Then the moisture evaporates to the exterior before reaching the interior wall 

surface. Examples include rubble, brick, and masonry walls. Windows placed into a storage or 

mass wall with a surface barrier system rely on a sealant joint between the frame and the opening 

in the wall.

7.2.2 Perfect Barrier System

A wall with a perfect barrier system relies on the outermost surface to be weather resistant. Some 

window frames, metal, and glass curtain wall systems are perfect barriers. Solid walls of 

masonry, concrete, or brick with no cavities are sometimes referred to as surface barrier systems

but are imperfect. Windows placed into a wall with a surface barrier system rely on a sealant 

joint between the frame and opening in the wall. This type of system has only one line of defense 

against water intrusion. It this requires very careful installation. There is no provision for 

draining moisture that enters the wall. Sill pan flashings are recommended in all cases.
63

7.2.3 Drained Wall System

A wall with a drainage system accepts small amounts of water that may penetrate the outermost 

wall surface. The system is designed to control and drain away any residual water that penetrates 

the wall. Typically a weather-resistant barrier such as house wrap or building paper is placed 

behind the exterior cladding material (wood siding, brick veneer, or stucco). This drainage plane 

must be overlapped shingle-style and open at the base so water drains to the exterior. In some 

cases this requires flashing and weep holes at the base of the wall. It is important to use caution 

when sealing a window unit to the exterior cladding of a membrane/drainage wall system so 

water within the drainage plane is not blocked and is allowed to escape. Sill pan flashings are 

recommended in all cases.

In membrane/drainage systems, a window with a mounting flange (nail fin) is typically used to 

attach the frame to the wall. Block frame windows with brick mold may also be used with a 

membrane/drainage system. The integrity of the drainage membrane must be preserved by the 

proper use of flashings and sealants in this type of installation. The placement of flashings must 

follow a careful sequence resulting in the overlapping of all materials in weather-board (shingle 

style) fashion. One common procedure recommends that the flashing beneath the sill is placed 

first, then the jamb flashings, then the window is installed with mounting fins over these 

flashings, and then the head flashing over the top window fin. Some variations and intricate 

procedures involve proper flashing, depending on the exact wall assembly and construction 

sequence.

7.3 Replacement Windows and Sashes

Half of all windows sold are installed as replacements. Installing these windows presents a wide 

range of possible situations and potential problems. Following manufacturers’ instructions as 

well as guidelines and standards noted earlier in this section is essential. Replacement windows 

can be considered in three categories:

• Removal of and complete replacement of the original window

• Placement of a complete new window within the original window frame

• Replacement of the sash only, where the original frame remains in place, and only the 

glazing, operable sash, and jamb liners are new.

Window installation in a remodeling or renovation must address all the considerations discussed 

previously for new windows as well as greater concerns about maintaining drainage planes and 

air/vapor barriers. Insulation and air/water vapor barrier continuity should be maintained all 

around the window. Foam-in-place sealants can be used to fill irregular voids created between 

old and new components; however, as with new installations, it is important to use lowexpansion foams so frames are not distorted. Remodeling is often an opportunity to check the 

interior condition of walls and increase insulation in the opened areas.

In a simpler approach, the old sash and other adjacent trim are removed, leaving the original 

frame in place. The new window is inserted into this framed opening, following prescribed 

installation procedures. Then, appropriate trim is installed on the interior and exterior. This 

provides many of the benefits of a complete window replacement, but at a lower cost. However, 

the net effect is typically to reduce the total glazing area, as a complete window assembly is 
64

basically fitted into the old frame. Caution must be used to avoid diminishing the opening below 

egress code requirements.

Replacement sashes involve less expense and disruption for a household. They are custom sized 

and detailed to fit into original window frames. Only the glazing, operable sash, and jamb liners

are new. This is a good approach for upgrading window energy performance when the original 

window frames are in good shape. At the same time the sash is replaced, new weather stripping 

that is most appropriate to the window type and frame details should be installed. Some of the 

benefits of energy-efficient glazing can be compromised if the new sash is not properly weather

stripped. For a more in-depth discussion of the topic of rehabilitation of windows, see Baker and 

Eng (2012).
65

References

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ACCA. (2011). Manual J Residential Load Calculation, 7th Edition. Arlington, VA: Air

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ASHRAE. (2009). Handbook—Fundamentals. Atlanta, GA: American Society of Heating, 

Refrigerating and Air-Conditioning Engineers. www.ashrae.org/resources--publications/Tableof-Contents-2009-ASHRAE-Handbook-Fundamentals.

ASHRAE. (2010). Standard 55-2010: Thermal Environmental Conditions for Human 

Occupancy. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning 

Engineers. www.techstreet.com/cgi-bin/detail?doc_no=ashrae|55_2010;product_id=1741646.

ASTM. (2007). “E2112 – 07 Standard Practice for Installation of Exterior Windows, Doors and 

Skylights.” West Conshohocken, PA: ASTM. www.astm.org/Standards/E2112.htm.

Baker, P.; Eng, P. (2012). Measure Guideline: Window Repair, Rehabilitation, and Replacement.

Prepared by Building Science Corporation. Golden, CO: National Renewable Energy 

Laboratory. NREL/SR-5500-55219. www.nrel.gov/docs/fy12osti/55219.pdf.

Carmody, J., Selkowitz, S., Arasteh, D., Heschong, L. Residential Windows: A Guide to New 

Technologies and Energy Performance, 3rd Edition. W.W. Norton, 2007.

Carmody, J., Selkowitz, S., Lee, E., Arasteh, D., Willmert, T. Window Systems for Highperformance Commercial Buildings. W.W. Norton, 2004.

DOE. (2012). Status of State Energy Codes. Washington, D.C.: U.S. Department of Energy. 

www.energycodes.gov/states.

EIA. (2012a). “Electric Power Monthly.”

www.eia.gov/electricity/sales_revenue_price/pdf/table4.pdf.

EIA. (2012b). “Natural Gas.” www.eia.gov/dnav/ng/ng_sum_lsum_dcu_nus_m.htm.

EPA. (2012a).“ENERGY STAR Program Requirements Product Specification for Residential 

Windows, Doors, and Skylights: Eligibility Criteria, Draft 1 Version 6.0.” Washington, D.C.: 

U.S. Environmental Protection Agency, July 2012. www.energystar.gov/ia/partners/ 

prod_development/revisions/downloads/windows_doors/Spec_Draft1_V6.pdf?9087-cb4d.

EPA. (2012b).“ENERGY STAR for Windows, Doors, and Skylights: 

Version 6.0 Draft 1 Criteria and Analysis Report.” Washington, D.C.: U.S. Environmental 

Protection Agency July 2012. www.energystar.gov/ia/partners/prod_development/ 

revisions/downloads/windows_doors/Draft6_V1_Criteria_Analysis_Report.pdf?e34a-4eb4.

EWC. (2012a). Efficient Windows Collaborative. Minneapolis, MN: Efficient Windows 

Collaborative. www.efficientwindows.org.
66

EWC. (2012b). Window Selection Tool. Minneapolis, MN: Efficient Windows Collaborative. 

www.efficientwindows.org/selection.cfm.

Huizenga, C.; Zhang, H.; Mattelaer, P.; Yu, T.; Arens, E. (2006). Window Performance for 

Human Thermal Comfort. Final Report to the National Fenestration Rating Council. Center for 

the Built Environment, University of California, Berkeley.

www.cbe.berkeley.edu/research/pdf_files/SR_NFRC2006_FinalReport.pdf.

ICC. (2012). International Energy Conservation Code. Washington, D.C.: International Code 

Council. www.iccsafe.org/Store/Pages/2012I-Codes.aspx?r=2012icodes.

LBNL. (2012). RESFEN 5.0. Berkeley, CA: Lawrence Berkeley National Laboratory. 

windows.lbl.gov/software/resfen/resfen.html.

LBNL. (2008). 2008 Energy STAR Windows Revisions - Technical Analysis. 

windows.lbl.gov/EStar2008.

Lstiburek, Joseph, W. Water Management Guide. Building Science Press, Inc., 2006.

NFRC. (2005). The NFRC Label. Greenbelt, MD: National Fenestration Ratings Council. 

www.nfrc.org/label.aspx.

NFRC. (2012). The NFRC Certified Products Directory. Greenbelt, MD: National Fenestration 

Ratings Council. search.nfrc.org/search/searchDefault.aspx.

NREL. (2012a). BEopt. Golden, CO: National Renewable Energy Laboratory. 

beopt.nrel.gov/download.

NREL. (2012b). National Residential Efficiency Measures Database. Golden, CO: National 

Renewable Energy Laboratory. www.nrel.gov/ap/retrofits/index.cfm.

Proctor, J., Z. Katsnelson, and B. Wilson. “Bigger is Not Better: Sizing Air Conditioners 

Properly.” Article published in Home Energy Magazine, May/June 1995.

RECA. (2012). “IECC Compliance Guides.” reca-codes.org/iecc-compliance-guides.php.

Straube, J. BSD-013: Rain Control in Buildings, 2011. (www.buildingscience.com/

documents/digests/bsd-013-rain-control-in-buildings?topic=doctypes/digests).

Straube, J. BSD-030: Rain Control Theory, 2010. www.buildingscience.com/

documents/digests/bsd030-rain-control-theory?topic=doctypes/digests.

Windows for High-Performance Commercial Buildings. (2012). www.commercialwindows.org.
DOE/GO-102012-3656 ▪ November 2012 

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