Expert Meeting Report: 

Windows Options for 

New and Existing Homes 

C. Ojczyk, J. Carmody, and K. Haglund

NorthernSTAR

May 2013
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iii

Expert Meeting Report: 

Windows Options for New and Existing Homes

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: 

C. Ojczyk, J. Carmody, and K. Haglund 

NorthernSTAR Building America Partnership 

The University of Minnesota 

2004 Folwell Avenue 

St. Paul, MN 55108 

NREL Technical Monitor: Stacey Rothgeb 

Prepared under Subcontract No. KNDJ-0-40338-00 

May 2013 
iv

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v 

Contents

List of Figures ............................................................................................................................................. v

List of Tables ............................................................................................................................................... v

Definitions ................................................................................................................................................... vi

Executive Summary.................................................................................................................................... 1

1 Background ........................................................................................................................................... 2

2 Meeting Information ............................................................................................................................. 3

3 Meeting Objectives and Agenda ......................................................................................................... 4

3.1 Research Questions ..............................................................................................................4

3.2 Agenda .................................................................................................................................4

3.3 Presenter Biographies ..........................................................................................................5

4 Presentation Summaries ..................................................................................................................... 7

4.1 New Homes ..........................................................................................................................7

4.1.1 Charlie Curcija: Overview of Window Research at Lawrence Berkeley National

Laboratory ................................................................................................................7

4.1.2 John Carmody: Guidelines for Energy-Efficient Windows in New Construction 10

4.1.3 Jim Larsen: The Science of Window Comfort ......................................................11

4.2 Existing Homes ..................................................................................................................15

4.2.1 Pat Huelman: Window Challenges for Existing Homes ........................................15

4.2.2 Peter Yost: Examining Window Retrofit Options .................................................15

4.2.3 Peter Baker: Window Repair, Rehabilitation, and Replacement ...........................19

4.2.4 Theresa Weston: Windows Retrofit .......................................................................21

5 Group Discussion to Cover Key Questions and Action Items ...................................................... 23

List of Figures

Figure 1. Windows as energy producers.................................................................................................. 8

Figure 2. U-Factor and SHGC in mixed climates ..................................................................................... 8

Figure 3. Thermal comfort zones ............................................................................................................ 12

Figure 4. Predicted percent dissatisfied................................................................................................. 13

Figure 5. High performance glass and inside glass surface temperature .......................................... 14

Figure 6. Infrared image of single pane window—winter conditions .................................................. 17

Figure 7. One window opening, three window variables...................................................................... 17

Figure 8. Wall performance as a function of window type ................................................................... 20

Figure 9. Flashing detail for a window replacement ............................................................................. 21

Figure 10. Strategic planning for window replacement ........................................................................ 22

Unless otherwise indicated, all figures were created by NorthernSTAR.

List of Tables

Table 1. Expert Meeting Participants ........................................................................................................ 3

This table was created by NorthernSTAR.
vi 

Definitions

DOE U.S. Department of Energy

EWC Efficient Windows Collaborative

HVAC Heating, ventilation, and air conditioning

LBNL Lawrence Berkley National Laboratory

Low-e Low-emittance coating

NREL National Renewable Energy Laboratory

PPD Predicted percent dissatisfied

SHGC Solar heat gain coefficient
1 

Executive Summary

The NorthernSTAR Building America Partnership held an expert meeting on Windows Options 

for New and Existing Homes on November 14, 2011 at the Nolte Building on the University of 

Minnesota campus in Minneapolis. Featured speakers included John Carmody and Pat Huelman 

of the University of Minnesota, Charlie Curcija of Lawrence Berkeley National Laboratory, Jim 

Larson of Cardinal Glass Industries, Peter Yost of Building Green, Peter Baker of Building 

Science Corporation, and Theresa Weston of DuPont Building Innovations. Audience 

participation was actively encouraged during each presentation to uncover needs and promote 

dialog among researchers and industry professionals. Key results from the meeting were:

• A greater understanding of the windows research that Building America partners and 

national laboratories are engaged in to help professionals and consumers choose best 

practice strategies

• A greater understanding of the role of comfort in window performance and satisfaction 

• A need for research to quantify costs, benefits, and risks of window replacement and 

window improvement options for existing homes 

• Definition of future research needs to be investigated. 

Findings from the meeting include: 

• The extensive research already being undertaken by the Building America partners has 

resulted in the development of websites, guides, fact sheets, and other resources to help 

users make more informed decisions about window options for new and existing homes. 

The Building America partners continue to develop resources to help users move beyond 

information gathering to action. 

• There is a gap between reported cost information and true costs for materials and 

installation across all window replacement and retrofit options. There needs to be a 

greater emphasis on engaging the building industry to provide real cost information 

(materials and labor) that can be included in the resources that Building America partners 

are creating for the public. 

• More research is needed to quantify the role of windows and window attachments in 

improving the house as a system. There is a solid understanding of best practices for new 

construction, but the many variables in existing home situations leave gaps in 

understanding proposed replacement and retrofit options and their impacts on energy 

efficiency; heating, ventilation, and air conditioning systems; humidity; durability; and 

comfort. 

The next steps are to define research projects that address the needs uncovered in the windows 

expert meeting. 
2 

1 Background

Windows are critical components of new and existing homes in cost-effectively meeting the 

Building America energy targets. They have very important impacts on heating, cooling, and air 

leakage, and provide many benefits such as comfort, increased durability, and increased home 

value that are harder to quantify than energy consumption. 

Although considerable technological development and increased code requirements for windows 

have occurred in the last 25 years, there is still a need to understand the impacts of window 

design and installation on the house as a system. Whether a new window is installed in a new or 

in an existing opening, or whether an insert window, a sash replacement, a refurbished window, 

or a window attachment is installed, it is necessary to understand impact on the sizing and costs 

of heating, ventilation, and air conditioning (HVAC) systems and to consider the impacts of the 

product and installation on overall construction needs, condensation and water infiltration and 

management, indoor air quality, and window and house durability. 

Only recently has there been greater interest in the ability to measure performance and cost 

effectiveness for refurbishing windows or applying attachments such as shades, blinds, and high 

performance storm windows. 

The question is whether the cost for new technologies and installation processes can be justified 

through improved energy efficiency, decrease utility costs, improved comfort, and improved 

durability of the home. 
3 

2 Meeting Information

The NorthernSTAR Building America Partnership held an expert meeting on Windows Options 

for New and Existing Homes on November 14, 2011 at the Nolte Building on the University of 

Minnesota campus in Minneapolis. The 31attendees included leading researchers, government 

program managers, and industry experts. Sixteen participants attended via webinar. Seven 

presenters provided overviews of technologies and issues associated with new and existing 

windows. Lively discussion and participation occurred during each presentation and a wrap-up 

discussion was held at the end of the day. A list of participants is included in Table 1.

Table 1. Expert Meeting Participants

Name Organization Email Address

Theresa Weston DuPont Building Innovations theresa.a.weston@usa.dupont.com

Kohta Oeno Building Science Corporation bats22@gmail.com

Peter Yost Building Green peter@buildinggreen.com

Kerry Haglund University of Minnesota khaglund@umn.edu

John Carmody University of Minnesota carmo001@umn.edu

Garrett Mosiman University of Minnesota mosi0019@umn.edu

Tom Schirber University of Minnesota schir056@umn.edu

Pat Huelman University of Minnesota phuelman@umn.edu

Tessa Murray University of Minnesota murry009@umn.edu

Charlie Curcija LBNLa dccurcija@lbl.gov

Pat Donohue University of Minnesota pdonahue@nrri.umn.edu

Sam Breidenbach TDS Custom Construction sam@tdscustomconstruction.com

Mark Mikkelson Andersen Corporation mark.mikkelson@andersencorp.com

Steve Johnson Andersen Corporation steve.johnson@andersencorp.com

Laura Millberg Minnesota Pollution Control Agency laura.millberg@state.mn.us

Tom Kuehn University of Minnesota kuehn001@umn.edu

Michael Anschel Verified Green michael@verifiedgreen.org

Kevin Bruce Marvin Windows and Doors KevinB@marvin.com

Jeff Christian Oak Ridge National Laboratory christianJE@ORNL.gov

Robb Aldrich Steven Winter Associates raldrich@swinter.com

Cindy Ojczyk Simply Green Design cindy@simplygreendesign.com

J. Chestnut J Chestnut jchesnut4@gmail.com

Dan Cautley Energy Center of Wisconsin dcautley@ecw.org

Jim Larsen Cardinal Glass Industries jlarsen@cardinalcorp.com

Josh Novacheck Center for Energy and Environment jnovacheck@mncee.org

Chuck Booten NRELb chuck.booten@nrel.gov

Neal Kruis NREL Neal.Kruis@nrel.gov

Omar Espinoza University of Minnesota Espinoza@umn.edu

Brian Lieburn Dow Chemical WBLieburn@dow.com

Gary Nelson Energy Conservatory gnelson@energyconservatory.com

Steve Schirber Cocoon Solutions steve@cocoon-solutions.com

a LBNL Lawrence Berkeley National Laboratory

b NREL National Renewable Energy Laboratory
4 

3 Meeting Objectives and Agenda

The objective of the full-day meeting was to bring together window experts from Building 

America teams and other related programs and research activities to share information, identify 

gaps in research activities, and determine future research issues and needs surrounding window 

technologies, tools, and installatiosn in new and existing homes. The meeting was designed to be 

interactive and to promote questions and the sharing of experiences to inform the experts from 

research and industry. The meeting outcomes will help shape the Building America program and 

the U.S. Department of Energy’s (DOE) Windows Glazing Program.

3.1 Research Questions

The NorthernSTAR Partnership created the following questions as a means to guide the 

presenters and to stimulate conversation and questions from the attendees: 

• What are the life cycle costs and benefits of new and retrofit window options? 

• What are the tradeoffs between sash replacement and refurbishing or attachments in 

retrofit situations?

• What are the available tools and information to make cost-effective window choices?

• Are there any new technologies or strategies that make windows more energy efficient or 

cost effective?

• What are the problems and solutions associated with window installation? 

3.2 Agenda

8:30 Welcome and Meeting Introduction 

John Carmody, NorthernSTAR Partnership 

Chuck Booten, NREL 

9:00 Charlie Curcija: Overview of Window Research at LBNL

9:30 John Carmody: Guidelines for Energy Efficient Windows in New Construction 

10:45 Break

11:00 James Larsen: The Science of Window Comfort

11:45 Group discussion (led by John Carmody 

12:00 Lunch 

1:00 Pat Huelman: Window Challenges for Existing Homes 

1:30 Peter Yost: Examining Window Retrofit Options

2:00 Peter Baker: Window Repair, Rehabilitation, and Replacement 
5 

2:30 Theresa Weston: Windows Retrofit

3:00 Group Discussion To Cover Key Questions and Action Items (led by John Carmody) 

4:00 Adjourn meeting 

3.3 Presenter Biographies

Dr. Charlie Curcija Dr. Charlie Curcija is a scientist at LBNL in the Energy and Environmental 

Technologies Division, leading the research in thermal and optical performance of windows and 

other fenestration systems. He has been working in the area of the performance of buildings and 

building façades for more than 25 years and earned his Ph.D. at the University of Massachusetts 

in the field of thermal performance of windows.

John Carmody is the director of the Center for Sustainable Building Research at the University 

of Minnesota. He has worked in building-related research for 30 years and is the author of 

several books on building design and construction, including Window Systems for HighPerformance Buildings with LBNL, and the new edition of Residential Windows: A Guide to 

New Technologies and Energy Performance. John is a co-leader of the NorthernSTAR Building 

America team. 

James Larsen is director of Technology Marketing at Cardinal Glass Industries. He has 20 years 

experience in glass products R&D. He supports the recognition of energy-efficient windows 

through state and national codes and provides product support. John serves on the board of 

directors for the National Fenestration Rating Council. 

Pat Huelman is an associate professor in Residential Energy and Building Systems with the 

University of Minnesota’s Department of Bioproducts and Biosystems Engineering and serves as 

coordinator of the Cold Climate Housing program with the University of Minnesota Extension. 

He is the lead faculty member for the Residential Building Science and Technology 

undergraduate degree, a principal investigator for hygrothermal testing at the Cloquet Residential 

Research Facility, and directs the new NorthernSTAR Building America team. With more than 

30 years in the field, Pat has extensive experience and expertise in energy-efficient design, 

innovative building systems, residential indoor air quality, mechanical ventilation, and moisture 

management. 

Peter Yost is residential program director at Building Green. He brings more than 25 years 

experience in building, researching, teaching, writing, and consulting on high performance 

homes to his twin roles as director of residential services for BuildingGreen, and technical 

director for Taunton Press’s GreenBuildingAdvisor.com. Peter has been called on to provide this 

building science expertise to the nation’s leading homebuilding programs, including the National 

Association of Home Builders Green Building Standard, the U.S. Green Building Council’s 

Leadership in Energy and Environmental Design for Homes, the U.S. Environmental Protection 

Agency’s WaterSense, andDOE’s Building America program. 

Peter Baker is a senior associate at Building Science Corporation. He provides building 

forensics and design reviews, sets enclosure design standards, and works as project manager for 

the Building America program. He conducts field investigations of commercial and residential 
6 

buildings with assembly or system problems, provides explanations of the identified problems, 

and develops retrofit recommendations.

Dr. Theresa Weston, DuPont Innovations, leads the building science technology group for 

DuPont Building innovations. She participates in industry research programs and in standards 

and codes development. Theresa has more than 20 years experience in materials and fiber 

development and is an inventor with four U.S. patents. Theresa has a Ph.D. in Chemical 

Engineering from the CalTech. Theresa is actively involved in industry organizations, including 

ASHRAE and ASTM, and has chaired technical and standard committees for both. 
7 

4 Presentation Summaries

Seven main presentations were given by industry experts describing current windows research, 

products, and tools for new construction and existing homes. Active discussion was encouraged 

throughout each presentation to uncover questions and concerns expressed by the attendees.

Comments and questions posed by the attendees and presenter responses appear in the 

summaries in italics to help the reader understand the dynamic exchange of information. 

4.1 New Homes

The Building America program has historically focused on improving building technologies, 

construction systems, and energy efficiency of new homes. Research in this area was presented 

first to provide a foundation from which to investigate window issues in existing homes. 

4.1.1 Charlie Curcija: Overview of Window Research at Lawrence Berkeley 

National Laboratory

Mr. Curcija began his presentation with thought-provoking numbers on the amount of overall 

energy consumed in buildings versus the amount consumed by the window component. 

Buildings consume 39% of total energy in the United States or about 40 quads (1 quad equals 1 

quadrillion Btu). The energy attributed to windows is about 4 quads, or 10% of building energy. 

Of the 4 quads, residential windows represent 2.67 quads, whereas commercial windows 

consume 1.48 quads. The knowledge that residential windows account for nearly 7% of total 

building energy consumed presents opportunities for realizing significant total energy savings 

through improved design. 

LBNL is exploring the concept of zero energy building façades where building components 

supply rather than use energy. In this role, windows would move from being net energy losers to 

energy producers. This line of thinking raises the question of how to design windows for heating 

seasons when solar energy balances or exceeds the losses during daylight hours and thermal loss 

is controlled after dark. During cooling seasons, heat gain from solar radiation is controlled. 

One key measurement that indicates potential for a window to become an energy producer is the 

U-factor, or measurement of a window’s ability to resist heat flow. Figure 2 displays the U-factor 

at which a window moves toward being a net energy producer in a cold climate (a 0.1 or lower 

U-factor). 
8 

Insulating Windows Can Become 

Energy Producers in Cold Climates

Double Glaze: U = .5

+ Gain

- Loss

1973 1980 2010 2020

Single Glaze: U = 1.1

1990

Double, Low “e” U = .3 -.4 (Energy Star)

2000

Window U = .1 - .2 (Triple or Vacuum)

Window U < .1

Annual

Heating

Energy

Balance

Figure 1. Windows as energy producers 

U-factor must be considered alongside solar heat gain coefficient (SHGC) and climate. Figure 3 

shows energy graphs of U-factor versus SHGC in Riverside, California. Improved energy 

performance begins to diminish as U-factor is reduced and SHGC is increased. Reducing Ufactor is less important than reducing SHGC in mixed climates.

Riverside CA - Mixed Climates:

static medium solar, hi-R (U=0.1 Btu/h-ft2-F) can meet ZEH goals

SHGC

U 

Residential Energy 

Use (MBTU/yr) vs 

Window Thermal 

Properties (U, 

SHGC)

Specific windows 

plotted on map of 

iso-energy use

Annual energy use vs. window properties

Figure 2. U-Factor and SHGC in mixed climates
9 

LBNL researchers are investigating three main areas to understand the best means for 

transforming windows into energy producers and balancing U-factor and SHGC: 

1. Highly insulated glazing. This includes triple- and quadruple-insulated glazings with 

suspended films or nonstructural center layers, spacers that improve condensation 

resistance, vacuum glazing with improved metal to glass bonding or innovative pillars, 

and aerogels. 

Question: Will any of the materials or strategies mentioned above be available soon? 

Response: Nonstructural center glazing seems the closest, but currently the cost 

outweighs energy savings. 

Comment: An audience member commented that the technologies presented are 

wonderful, but that other criteria besides energy performance must be considered. These 

include impact resistance, altitude limits, and wind loads. Special designs, such as 

arches, can negatively impact performance. With vacuum glazing, performance is greatly 

impacted by temperature and the product is known to curl in cold weather. 

2. Highly insulated frames. This research is driven by information collected through the 

Passive Haus Institute in Germany and its success with zero energy homes. The current 

highly insulated frames designed to meet Passive Haus standards must be tested against 

U.S. standards. 

Question: Aren’t there measured savings with a better insulated frame?

Response: Yes, but we need to quantify those savings for our market. We can’t just adopt 

Passive Haus standards and expect similar performance in the different climates we have 

in the United States. 

Comment: Perhaps we can we improve the insulation of the frame with a jobsite detail 

rather than build it into the frame. 

3. Dynamic options for solar, control including electrochromic, thermochromic, 

photochromic, and liquid crystal display technologies, attachments, and shading 

between the glass. LBNL is completing the Windows 7/Therm 7 program for complex 

calculations of performance when a window includes enhancements such as shades, 

blinds, screens, and vacuum glazing. It developed and has incorporated the Complex 

Glazing (and shading) Database. 

Mr. Curcija reminded the audience that although all these ideas can help windows move to the 

position of energy producer, research must also focus on understanding the entire window 

system and how the window interacts with the building system and other potential energy 

producers. 
10 

Comment: NREL will be opening a new facility for testing window durability over wider 

temperature ranges. 

Comment: Oak Ridge National Laboratory will also be embarking on new window 

research. 

4.1.2 John Carmody: Guidelines for Energy-Efficient Windows in New 

Construction

Mr. Carmody tied his introduction to the message delivered by Mr. Curcija: If the current 

window stock were upgraded to ENERGY STAR®, there would be an energy savings equivalent 

to 1 quad. Improving the U-factor further to U = 0.1 using advanced window materials and

dynamic controls could yield an energy savings of nearly 2 quads. These are good reasons to 

keep investing in research related to improved window performance. 

Mr. Carmody, Steve Selkowitz, Dariush Arashteh, and Lisa Heschong wrote a book titled, 

Residential Windows: A Guide to New Technologies and Energy Performance. This book 

provides many levels of information about window selection for new and existing homes. 

Mr. Carmody works with LBNL and the Alliance to Save Energy to manage the Efficient 

Windows Collaborative (EWC). The EWC is composed of window industry manufacturers, 

suppliers, and affiliates. The EWC provides information on the benefits of energy-efficient 

windows, how they work, and recommendations for their selection and use. This information is 

provided through publications and websites. The messages in all the EWC work are: 

• Look for the ENERGY STAR label. 

• Look for the energy-efficient window properties on the National Fenestration Rating 

Council label. 

• Compare annual energy costs for a typical house. 

• Estimate and compare annual energy costs for your house (customized program using the 

RESFEN software tool). 

The EWC website (www.efficientwindows.org) is designed to teach the user how to make 

informed decisions about window options and how they impact energy performance. Options 

include window area, shading devices, and frame types. A window selection tool allows the user 

to choose criteria and then find a window product that meets those characteristics. This helps the 

consumer to move from information gathering to purchasing. Further assisting the selection 

process are guideline fact sheets and energy code compliance guides by state. This website 

averages 50,000 visitors per month. 

Question: Does the program address the addition of storm windows on historic homes? 

 

Response: Not currently, but products such as low-e storm windows are addressed in the 

book. 
11 

Mr. Carmody also introduced the current task of the NorthernSTAR Partnership, which is to 

develop windows measure guidelines for the Building America program. The goal is threefold: 

1. Provide guidelines for selecting energy-efficient windows in all U.S. climate zones. 

2. Provide cost/benefit information. 

a. Energy Benefits: 

• Operating energy

• Peak load reduction 

• Mechanical system reduction

b. Nonenergy benefits: 

• Comfort

• Condensation resistance 

3. Provide energy impacts of design decisions such as window orientation, total glazing 

area, and shading devices and conditions. 

One of the greatest concerns is accurate cost data that can be used in modeling programs such as 

BeOPT. Although some data are available, industry has responded that the numbers are 

unrealistically low. 

Comment: Margins and markups are different for high volume new construction builders 

and custom remodelers. The DOE Windows Volume Purchase program is not useful for 

remodeling. 

Comment: Installed price is a large component of total window cost, not just window 

price. Installed price is also affected by the number and variations in size of window 

openings r in any given home. An estimated cost of $1200/opening is too low. The cost to 

tear out, insulate weight pockets, replace with a new window, and retrim the opening is 

closer to $2000 in the Minneapolis market. 

Comment: Another missing piece is the energy performance and durability difference 

between insert units and whole window replacements.

Comment: In new construction, the Building America program for new construction 

could do delta in material cost as a useful delta, but installed cost of retrofits (and the 

variability) is huge. 

Comment: The most valuable tool would be to create technology that could make window 

installation more consistent. Then performance would be more measurable. 

Comment: People replace windows for many other reasons such as age. Cost data should 

not just be attributed to energy savings. If someone plans to replace a window anyway, 

upgrade costs become the right delta. 

4.1.3 Jim Larsen: The Science of Window Comfort
12 

Mr. Larsen’s presentation introduced the concept of comfort by discussing the six factors that 

define the conditions for thermal comfort:

• Metabolic rate

• Clothing insulation (impacted by seasonal change in temperature)

• Air temperature

• Radiant temperature (impacted by window size, occupant’s position relative to window, 

and window surface temperature)

• Air speed

• Humidity. 

Given that predicting the factors of each installed window and the variations in individual 

comfort is impossible, designing a window to address comfort for everyone is nearly impossible. 

The goal is to move in a direction where most people will experience more comfort.

ASHRAE Standard 55 attempts to address comfort through the ASHRAE comfort tool. Yet 

ASHRAE Building Code 90.1 and 90.2 suggest a wintertime comfort setting of 68oF, which falls 

below the comfort zone suggested by the Standard 55 tool (see Figure 4). 

Figure 3. Thermal comfort zones

The graph also shows how thermal comfort for summer and winter months shifts depending on 

clothing, metabolic rate, and radiant temperature. The ASHRAE 90.1 summer comfort setting is 

suggested to be 78oF and falls within the summer comfort zone. Although the standard and the 
13 

building code align for summer thermal comfort, a key consideration is missing: the impact of 

solar radiation on the thermal comfort zone. Future research is needed to understand the impact 

of solar gain on comfort. 

Another method for looking at comfort is the PPD or predicted percent dissatisfied measurement, 

where low values are desired and equate with greater satisfaction. LBNL researchers looked at a 

variety of window types from single pane clear to high performance against a variety of comfort 

indicators such as clothing level. As Figure 5 suggests, as the insulating value of a window 

increases during a winter night, comfort improves. 

Figure 4. Predicted percent dissatisfied

The LBNL data correlate with EWC data from (Figure 6). As high performance attributes are 

added to windows, the room-side glass surface temperature moves toward or into the comfort 

area, even when outside temperatures drop below 0oF. 
14 

Efficient Windows Collaborative

Figure 5. High performance glass and inside glass surface temperature 

Further data showed, however, that winter nighttime comfort conditions are completely different 

than winter sunny conditions and summer sunny conditions. Comfort goes beyond room 

conditions to the amount of sun passing through glass to heat the room and the temperature of 

the glass facing the room. It is also a factor of the thermal storage potential of the materials in the 

building and the ability of temperature to be distributed throughout the spaces. 

For example, a field study done in Fort Wayne, Indiana, measured the PPD of a home 

experiencing a –5°F outside temperature with clear and sunny conditions. The windows were 

triple glazed high solar gain. The reported PPD numbers were high (toward dissatisfied). Even 

though solar gain was achieved, the house had big temperature swings from room to room. 

Question: Did the study look at the relationship between the field study and the modeling 

program? 

Response: Yes. It is believed the programs overestimate thermal mass in buildings. There 

is a disconnect between the models and the measured results, especially in winter. 

Summer months seem to produce a closer agreement. 

Mr. Larsen also presented results from his own informal research that examined multiple 

window options and glazing strategies in multiple geographic locations. As one moves from 

double pane to triple pane and from high solar gain to low solar gain, the hours of discomfort 

decrease even as climate changes from cold to hot.
15 

In window comfort analysis a single thermostat set point cannot be used for all seasons in the 

manner that is used by energy performance modeling. The temperature at which one is 

comfortable varies from season to season, at different times during the day, and with level of 

clothing and metabolic rate.

Comment: Operative thermostats are available. 

Comment: One of four people actually use their setback thermostat. (What data are 

available to prove this statistic?)

Question: Do we have the modeling capability to translate comfort information into 

energy savings?

Response: Every hour of measured discomfort is a call to action to develop strategies to 

improve comfort. 1oF is 3% change in energy. Reducing temperature 2oF is a 6% 

decrease in energy load. Changing comfort changes livability with potential energy 

improvement. Defining comfort through modeling is difficult because there is no single 

model for comfort. One would need to combine results from different programs (for 

example, EnergyPlus will compute long wave infrared but not short wave). 

4.2 Existing Homes

The Building America program recently expanded its research focus to improve building 

technologies, construction systems, and energy efficiency strategies for the vast stock of existing 

U.S. homes. Given the many variables in construction techniques from home to home and from 

climate to climate, predicting performance of window upgrades for existing homes is far more 

complicated than for new homes. The following presentation summaries and discussions provide 

a look at the latest research and concerns in the industry. 

4.2.1 Pat Huelman: Window Challenges for Existing Homes

Mr. Huelman provided a brief introduction to the afternoon sessions by discussing the 

complicated nature of window options for existing homes. In new construction, a new window 

will go into a new opening. Options always need to be considered for that new window, but the 

choices grow exponentially for existing homes. Choices need to be weighed against conditions of 

the window, the building envelope, and the home’s mechanical systems. Will the whole window 

be replaced or a new insert added to the frame? Can a new sash be added or the window 

repaired? Will an interior or exterior attachment be sufficient to improve comfort? Will the 

current mechanical system be oversized if the new windows and installation create a tighter, 

more energy-efficient building envelope?

Costs and benefits of each of these strategies need to be weighed against installation, ease of use, 

and maintenance. 

4.2.2 Peter Yost: Examining Window Retrofit Options

Mr. Yost began his presentation by outlining the matrix of considerations that needs to be 

thought through when contemplating the best method for improving the impact of windows on 

existing homes. 
16 

Options for where the retrofit will be placed need to be considered first: 

• Interior attachment (includes blinds, drapes, shades, films) 

• Exterior attachment (includes storms, shutters, rollers, screens, landscaping) 

• Window replacement (full unit, insert, sash replacement, upgrade/repair). 

The secondary areas of consideration are the attributes of those options: 

• Thermal (includes insulation, airtightness, solar heat gain, comfort, condensation 

resistance)

• Visual (includes visual transmittance, views, privacy, aesthetics)

• Use (includes ease of operation, cleaning, repair, adjustability, maintenance, and 

durability)

• Economics (product and installation costs, energy savings, service life) 

• Other (availability of product, ease of install, acoustics, security). 

The issue is that little research has been conducted on how best to make decisions about the 

combination of option and attributes that will result in the best outcome for a particular home.

In some ways it is easy to measure energy efficiency (a thermal attribute) of a specific option but 

difficult to model the other attributes alongside energy efficiency. Because research and results

are lacking, terminology is not consistent in the industry or with consumers. The quality of 

information posted by manufacturers varies widely and window and attachment interactions do 

not always produce the desired results. In fact, the orientation of a window and the resulting 

thermal and solar radiation stress may result in the same type of window in one house 

experiencing different performance outcomes based on its orientation to the sun. Yet the industry 

continues to promote the use of one type of window for all window openings in any given home 

when, in fact, different options and attributes for each window may be a better approach. 

To provide professionals and consumers with a better method for choosing window options and 

attributes, Mr. Yost, his company Building Green, and a team from LBNL were engaged in a 

research project supported by DOE. Extensive literature reviews, laboratory tests, field testing, 

and modeling were completed over the course of 18 months. 

A field test was completed in Mr. Yost’s home during the summer and winter to determine the 

impact of solar gain and temperature on several baseline windows (double pane, single pane, 

low-e, argon filled) and various attachments (interior and exterior storm windows, window 

quilts, solar shades, cellular shades, interior and exterior solar shades). Mr. Yost presented a 

number of images to demonstrate the findings; Figure 7 and Figure 8 show several key images 

that highlight the importance of testing to determine the relationships between options and 

attributes. 

Figure 7 shows the resulting infrared image of a single-pane window without an attachment 

during a winter night. 
17 

Single-pane, no attachment

Figure 6. Infrared image of single pane window—winter conditions 

The temperature of the interior wall was measured at 72.3oF; the window glass ranges from 

52.4oF to 35.6oF. The cold emanating from the window can impact the overall energy needs of 

the room as well as the comfort of any occupant located near the cold window. 

Figure 8 shows the same window opening with three window variables measured during a winter 

night. 

interior low-e storm over 

“vintage” single-pane 

wood frame double hung

Interior low-e storm over 

“base case” double low-e 

Ar sash inserts

“base case” dual 

glazed low-e Ar double 

hung sash inserts

storm double storm triple IGU double

Figure 7. One window opening, three window variables
18 

The left image is the single-pane window covered with an interior low-e storm attachment. The 

purple color indicates a cold spot on the lower part of the storm frame; the window glass 

temperature is moving closer to room temperature because of the attachment. 

The center image demonstrates what happens to the thermal performance of the window when 

the window is replaced with a low-e, argon, double-pane sash and then covered with the low-e 

storm attachment. There are no cold spots and most of the window is moving toward room 

temperature. 

The right image uses the same low-e, argon, double-pane sash as in the center image but without 

the storm attachment. The sash frame shows an area of cold temperature; the window glass is 

moving toward room temperature. 

In this example, the addition of the storm window or improving the window from single-pane to 

low-e argon, double-pane sash inserts has improved the thermal performance of the window.

Comment: Putting a low-e storm on the interior of the window may be fine for a winter 

night but may cause excessive solar gain in the room during summer daytime resulting in 

hot temperatures at the glass surface. Will one attachment require another attachment 

such as an awning or overhang? 

Comment: Interior storm window over single pane window runs the risk of condensation 

forming on interior side of window. This may be a long-term durability issue. 

Response: We did not do much condensation risk testing, but that was meant for testing 

during the second year. We will also need to look at condensation as a function of 

window location based on level of home—main floor versus upper floors. 

Additional images highlight the difference between winter versus summer months. Although a 

window should be insulated to reduce the impact of summer heat on conditioned interior air, 

excessive solar gain from low-e sashes, films, and storm windows can cause elevated 

temperatures on the room side of the window glass. This can have a negative impact on energy 

efficiency and comfort during the cooling months. 

The research results were used to create a new website, www.windowattachments.org, to guide 

consumers in their decision-making process. The DOE fact sheet, “Window Retrofit Options: 

Deciding What to Do With Existing Windows” has a chart of retrofit options versus performance 

attributes that are ranked on a scale from potential detriment to greatest benefit. There are 

additional fact sheets on interior, exterior, and other types of attachments as well as links to 

resources for sourcing products and services. 

The goals of future research are to delve deeper into laboratory and field work and to obtain 

better cost numbers to improve decision making. 

Comment: Does the chart or website address embodied energy of new windows and 

attachments—has anyone done numbers? 
19 

Response: A lot of information on windows is old, not accurate for embodied energy 

calculations now. Website does not address embodied energy.

 

Comment: A working group has been established to develop the Product Category Rules 

to be used with Environmental Product Declarations.) The Product Category Rules will 

include rules for calculating life cycle analysis and embodied energy of windows. 

4.2.3 Peter Baker: Window Repair, Rehabilitation, and Replacement

Mr. Baker’s presentation dove deeper into specific retrofits and how to make them more durable 

and energy efficient. He focused on the product installation system and the connection and 

interaction details that prevent water infiltration, air infiltration, and interstitial condensation.

The first area of investigation in a retrofit should always be to assess the conditions to identify 

current water infiltration concerns. There needs to be a determination about whether there is or 

was a leak and where that leak actually occurred before the type of retrofit option is selected. 

Common leakage areas that are easy to determine and more easily addressed include infiltration 

between the glass and the sash and the sash and the frame. The more difficult areas of leakage to 

assess are those caused by joints in the frame or between the frame and the rough opening. These 

types of leakage are more difficult to repair and may indicate a full window replacement is 

needed to better integrate the window into the wall assembly. This is especially true to prevent 

damage to the building envelope if a wall is insulated or there is intent to insulate.

Air infiltration pathways are similar to water pathways and generally occur between the sash and 

the frame and at the meeting rails. Air can also leak at the junction of the window frame and 

rough opening. These areas often include the void caused by weight pockets in older windows or 

under the sill because of frame geometry. Leakage at the sash and frame can usually be 

addressed in many retrofit options. But air leakage at the rough opening may require full window 

replacement with attention to proper air sealing before the installation is complete.

Interstitial condensation is created by uncontrolled air leakage into the space between the 

window and added coverings such as storms (interior and exterior) and blinds or shades. 

Reducing the risk of condensation is a function of the fit of the covering to the opening - the 

tighter the fit the less risk of condensation. Friction-fit storms and window treatments that do not 

follow a track have a greater risk of condensation than attachments that are measured to fit 

tightly and permanently into a specific opening: Anything that moves or is removable risks 

condensation. The interior-most element needs to be as airtight as possible. The exterior-most 

element should also be as tight as possible but may need to be made incrementally leakier to 

allow it to dry. 

Comment: Are there data on the amount of air leakage in different types of existing 

windows?

Response: No. The variables are too great. In one house the same type of window 

experiences different pressures and performance based on solar orientation and 

occupant use. 
20 

Figure 9 shows the impact windows have on the performance of the wall assembly.

Window Repair, Rehabilitation and Replacement 20

Cost and Performance

R-0

R-1

R-2

R-3

R-4

R-5

R-6

R-7

R-8

R-9

R-10

R-11

R-12

R-13

R-14

R-15

R-16

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

R-5

R-10

R-15

R-20

R-25

R-30

Overall Wall Effective R-value

Window U-Value

Impactof window U-value on effective thermal resistance of complete wall assemblies 

(based on 18% glazing ratio compared to total wall area)

Opaque Wall Effective R-value

R-10.5

R-7.5

R-5

 Effect of window conductance on whole wall R-Value

Energy Star 

Window

Vinyl double 

glazed clear

Single glazed 

wood

Figure 8. Wall performance as a function of window type 

Adding windows into a wall negatively impacts the wall’s ability to be an effective insulator.

Even with low U-value ENERGY STAR windows, an R-20 wall is performing at half its value 

because of the window. Any retrofit options need to consider the estimated performance of the 

new element alongside installation procedures (air sealing) that can improve the overall wall 

performance. 

Baseline window rehabilitation (weather stripping or gasket replacement and sealing) should be 

done in all measures except full window replacement to minimize air leakage that would impact 

performance. Existing elements need to be brought up to the same service life as any added 

elements: rotting wood elements need replacement before the retrofit. Liquid applied waterproof 

membrane helps reduce water infiltration on window jambs and at the junction of jamb and sill. 

This is a good option for sash replacements and insert replacement windows.

Comment: Is this product readily available and why wouldn’t you use existing 

waterproofing membranes? 

Response: Many manufacturers make this product. It adds little dimension to the system 

as opposed to a peel and stick that might make it more difficult to operate the window 

with added thickness at the jamb and sill. 
21 

In full window replacement, bulk water shedding to the outside of the building and the siding 

must be addressed. Figure 9 shows a pan flashing detail that Building Science Corporation uses

to shed water. The sheet metal layered under the membrane was custom bent on site to fit over 

the siding. 

Window Repair, Rehabilitation and Replacement 42

Measure Implementation

Complete Window Replacement

Figure 9. Flashing detail for a window replacement 

These detailed slides on best practices per retrofit measure are currently in draft form for a 

Window Measure Guideline that will soon be available through the Building America program. 

4.2.4 Theresa Weston: Windows Retrofit

Dr. Weston presented the work that DuPont Building Innovations, as part of the Building 

America Retrofit Alliance, has been conducting with Pacific Northwest National Laboratory to 

develop material for a resource tool to teach building professionals about the best practices for 

window retrofits. It also teaches the homeowner about the need to hire skilled and experienced 

installers who understand building science, and how best to integrate new options into existing 

conditions to maintain or improve occupant safety and building durability during energy 

upgrades. 

The resource tool uses an “arrow” of converging variables to emphasize the custom nature of 

retrofits. The arrow process is to be used for each window in the home, as different windows 

may experience different conditions that need special consideration; for example, type of 

cladding, presence of mold, nonsquare openings, damage or defects in construction area, 

precipitation level, and actual exposure of the window to precipitation. The arrow engages the 

user in the practice of creating a best practices plan for improvement. Figure 10 demonstrates 

this process.
22 

© 2011, E. I. du Pont de Nemours and Company. All rights reserved. Use only with written permission of DuPont. 

retrofit plan

Window

special

Damage &

items

Other retrofit

Climate

Air& water management

Cladding

Windows

Replace full window

Replace sash

Retrofit in place

Cladding type

Over-clad

Replace

Leave in place

New

Existing

Precipitation level

Climate zone

Ventillation

HVAC replacement

Air sealing

Asbestos siding

Lead paint

Moisture damage

openings

Non-square

Doubl

e-hung, casement, awning, etc.

Flange or non-fl

ange

Window s hape

R-Value?

insulation?

Adding exterior

EIF

Brick

S

Stucco

SidingFiber-cement

Wood Sidi

ng

Vinyl Siding

Sealants

Air barrier

Flashing

barrierWater-resistive

Sealants

Air barrier

Flas hing

barrier

Water-resistive

M old

Condensation

Water intrusion

Window Retrofit Variables

Figure 10. Strategic planning for window replacement

Further into the resource tool are tables of retrofit scenarios that help guide the user in the 

decision-making process. Will the window retrofit be a full window replacement? If so, is the 

existing cladding staying or is it to be replaced? The answer leads the user to further information 

on how best to proceed. A homeowner who is replacing cladding is directed to follow new 

construction guidelines for window installation. 

Dr. Weston used the window retrofit variables arrow tool to show the decision-making process in 

a Building America Retrofit Alliance demonstration home. The home is brick, has a waterresistive barrier in place, is located in climate zone 2, and all the windows will undergo full 

replacement with nonflanged windows. The photographs document the steps from development 

of the retrofit plan to window removal to proposed steps for installation that will occur in the 

coming weeks. 

Comment: It often costs more to do a window retrofit than to do full replacement. 

Comment: This is where a greater understanding of installation costs per retrofit activity 

is important to have so that the industry is comparing and selling apples to apples. Full 

window replacements are very expensive given the cost of the product and all of the 

installation processes that need to happen, including the repair of interior and exterior 

elements such as sheet rock, trim, and siding. 
23 

5 Group Discussion to Cover Key Questions and Action Items

The primary purpose of the discussion session at the end of the presentations was to determine 

key issues that could be framed for future research. Eighteen items were discussed:

1. Case studies are needed to prove that simple payback should not be used alone but 

combined with value transfer to a new owner. This requires education and coordination 

with the appraisal and real estate communities.

2. Research is needed to determine how long window retrofits last and how long 

improvements maintain their value. Also, the necessary maintenance needs to be 

determined to maintain the value of the retrofit. 

3. Research is needed to determine the durability of interior attachments and the impact of 

condensation. 

4. There is a lack of field validation of retrofits and an understanding of why they fail. 

5. How can windows that meet Passive Haus standards become more cost effective?

6. What is the life expectancy of windows?

7. Product and installation costs continually dominate the discussion, yet data on true 

industry costs are lacking. Professionals practicing in the industry need to be encouraged 

to submit cost and lifetime data to the NREL database.

8. Airflow from windows needs to be quantified. Thermal performance and air leakage need 

to be included and discussions and investigations need to be held on how window 

operation impacts air leakage.

9. Partner with industry to provide better cost estimates

10. What are the benefits of following Passive Haus details?

11. An industry guide is needed with expertise on the hierarchy and sequence for building 

upgrades. 

12. How does window replacement impact ventilation? 

13. Is there a way to “modularize” window frame design to easily and cost-effectively 

accommodate future retrofits?

14. The industry needs easy-to-discuss reasons why houses should be tight and ventilated to 

help remodelers educate homeowners on best practices. 

15. We need to communicate that not all remodelers are qualified and they need to learn 

through experience. 

16. Can college curricula be developed to teach window fenestration design, because no 

school teaches it?

17. Have the benefits and risks of all retrofit options been explored? 

18. How can passive solar be done effectively?
DOE/GO-102013-3829 ▪ May 2013

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