Measure Guideline: 

Wood Window Repair, 

Rehabilitation, and Replacement

P. Baker

Building Science Corporation

December 2012
NOTICE

This report was prepared as an account of work sponsored by an agency of the United States 

government. Neither the United States government nor any agency thereof, nor any of their employees,

subcontractors, or affiliated partners makes any warranty, express or implied, or assumes any legal 

liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, 

product, or process disclosed, or represents that its use would not infringe privately owned rights.

Reference herein to any specific commercial product, process, or service by trade name, trademark, 

manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or 

favoring by the United States government or any agency thereof. The views and opinions of authors 

expressed herein do not necessarily state or reflect those of the United States government or any agency 

thereof.

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy

and its contractors, in paper, from:

U.S. Department of Energy

Office of Scientific and Technical Information

P.O. Box 62

Oak Ridge, TN 37831-0062

phone: 865.576.8401

fax: 865.576.5728

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from:

U.S. Department of Commerce

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

phone: 800.553.6847

fax: 703.605.6900

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
iii

Measure Guideline:

Window Repair, Rehabilitation, and Replacement

Prepared for: 

Building America

Building Technologies Program

Office of Energy Efficiency and Renewable Energy 

U.S. Department of Energy 

Prepared by: 

P. Baker, P. Eng.

Building Science Corporation

30 Forest Street

Somerville, MA 02143

NREL Technical Monitor: Cheryn Metzger

Prepared under Subcontract No. KNDJ-0-40337-00

December 2012
iv

[This page left blank]
v

Contents

List of Figures ............................................................................................................................................ vi

List of Tables ............................................................................................................................................. vii

Definitions................................................................................................................................................. viii

Executive Summary................................................................................................................................... ix

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

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

2 Decision-Making Criteria ..................................................................................................................... 4

2.1 Risk Identification................................................................................................................4

2.1.1 Lead and Other Hazardous Materials ......................................................................4

2.1.2 Site Conditions and Project Staging ........................................................................4

2.1.3 Identification of Water Infiltration Concerns ..........................................................4

2.1.4 Identification of Deteriorated or Damaged Materials..............................................6

2.1.5 Identification of User Comfort Concerns ................................................................7

2.2 Cost and Performance ..........................................................................................................7

3 Technical Description............................................................................................................................ 10

3.1 System Interaction .............................................................................................................10

3.1.1 Window Water Management Functions ................................................................11

3.1.2 Window Air Leakage.............................................................................................13

3.1.3 Interstitial Condensation ........................................................................................17

3.1.4 Window Thermal Performance..............................................................................18

3.2 Measure Selection Criteria.................................................................................................21

3.2.1 Measure 1: Window Rehabilitation .......................................................................22

3.2.2 Measure 2: Exterior Storm Windows ....................................................................22

3.2.3 Measure 3: Interior Removable Storm Windows ..................................................23

3.4.4 Measure 4: Interior Permanent Fixed Storm Windows .........................................24

3.2.5 Measure 5: Window Sash Modification ................................................................25

3.2.6 Measure 6: Window Sash Replacement ................................................................26

3.2.7 Measure 7: Insert Replacement Window...............................................................27

3.2.8 Measure 8: Complete Window Replacement ........................................................28

4 Measure Implementation ................................................................................................................... 30

4.1 Measure 1: Window Rehabilitation ...................................................................................30

4.2 Measure 2: Exterior Storm Windows ................................................................................31

4.3 Measure 3: Removable Interior Storm Windows ..............................................................33

4.4 Measure 4: Permanent Interior Storm................................................................................33

4.5 Measure 5: Window Sash Retrofit.....................................................................................34

4.6 Measure 6: Window Sash Replacement ............................................................................35

4.7 Measure 7: Insert Replacement Window...........................................................................36

4.8 Measure 8: Complete window replacement.......................................................................39

4.8.1 Window Replacement as Sole Scope of Work ......................................................39

4.8.2 Window Replacement in Conjunction With a Siding Replacement......................41

4.8.3 Window Replacement in Conjunction With an Exterior Insulation Retrofit.........41

References................................................................................................................................................. 43

Appendix A: Window Repair, Rehabilitation, and Replacement Details ....................................... 44

Appendix B: BEOPT Analysis Graphs .............................................................................................. 79
vi

List of Figures

Figure 1. Glazing type (single, double, triple) by climate region ........................................................... 1

Figure 2. Window replacement history by climate region ...................................................................... 1

Figure 3. Impact of window U-value on effective thermal resistance of complete wall assemblies.. 2

Figure 4. Common window water infiltration pathways.......................................................................... 5

Figure 5. Failed window sill with replacement window installed........................................................... 6

Figure 6: Example of BEopt analysis graph............................................................................................. 8

Figure 7. Window components................................................................................................................ 10

Figure 8. Window head component lapping........................................................................................... 11

Figure 9. Window jamb component lapping........................................................................................... 12

Figure 10. Wood window sill on top of masonry sill ............................................................................. 12

Figure 11. Window sill component lapping............................................................................................ 13

Figure 12. Window air leakage pathways ............................................................................................... 14

Figure 13. Infrared images of window air leakage (depressurization test). Dark colored areas 

indicate cold surface temperatures from cold outdoor air leaking into the building.................. 14

Figure 14. Infrared images of window air leakage (depressurization test). Dark colored areas 

indicate cold surface temperatures from cold outdoor air leaking into the building.................. 15

Figure 15. Window to wall interface air leakage pathways................................................................... 15

Figure 16. Infrared images of window air leakage at window interior apron trim. Dark colored areas 

indicate cold surface temperatures from cold outdoor air leaking into the building.................. 16

Figure 17. Sash weight pockets .............................................................................................................. 16

Figure 18. Abandoned sash weight and insulation installation........................................................... 16

Figure 19. Examples of condensation problems with interior storm window retrofits ..................... 17

Figure 20. Examples of condensation problems with interior storm window retrofits ..................... 18

Figure 21. Window energy transfer components................................................................................... 19

Figure 22. Areas of energy transfer affected by the addition of exterior storms............................... 20

Figure 23. Window U-value and SHGC for various windows systems and exterior storm 

configurations (Windows Energy Savings Estimator (2011) ......................................................... 21

Figure 24. Interior removable storm window sample............................................................................ 22

Figure 25. Interior removable storm window sample............................................................................ 23

Figure 26. Installed interior removable storm window.......................................................................... 23

Figure 28. Interior storm window on double-hung original window.................................................... 24

Figure 29. Interior storm window on double-hung original window.................................................... 24

Figure 30. Interior storm detail ................................................................................................................ 25

Figure 31.Window sash modification...................................................................................................... 25

Figure 32. Sash replacement, showing out of square opening and requirement for sash 

extension............................................................................................................................................. 26

Figure 33. Window sash replacement jamb liners................................................................................. 26

Figure 34. Insert replacement vinyl frame window................................................................................ 27

Figure 35. New window rough opening framed with and extension box to allow for the addition of 

exterior insulation............................................................................................................................... 29

Figure 36. Recommended sealant location as part of window frame rehabilitation.......................... 31

Figure 37. Exterior aluminum storm windows, showing weep channel.............................................. 32

Figure 38. Condensation potential for exterior storm retrofits ............................................................ 32

Figure 39. Condensation potential for interior storm retrofits............................................................. 33

Figure 40. Example sill detail for a permanent interior storm window................................................ 34

Figure 41. Integration of an IGU into a wood sash ................................................................................ 35

Figure 42. Frame preparation for replacement window sashes........................................................... 36

Figure 43. Example jamb detail for a replacement window sash......................................................... 36

Figure 44. Frame preparation for insert replacement window ............................................................. 37

Figure 45. Examples of membrane installation in a wood window frame........................................... 38

Figure 46. Example sill detail of insert replacement window............................................................... 38
vii

Figure 47. Replacement window rough opening preparation .............................................................. 40

Figure 48. Example sill detail for window replacement without siding replacement......................... 40

Figure 49. Example sill detail of replacement window in conjunction with siding replacement...... 41

Figure 50. Example sill detail of "innie" window ................................................................................... 42

Figure 51. Example sill detail of "outie" window ................................................................................... 42

Figure 52. Annualized energy related costs versus average source energy savings for Dallas...... 79

Figure 53. Average source energy savings reduction for Dallas......................................................... 79

Figure 54. Annualized energy related costs versus average source energy savings for 

Kansas City ......................................................................................................................................... 80

Figure 55. Average source energy savings reduction for Kansas City............................................... 80

Figure 56. Annualized energy related costs versus average source energy savings for Boston.... 81

Figure 57. Average source energy savings reduction for Boston....................................................... 81

Figure 58. Annualized energy related costs versus average source energy savings for Duluth..... 82

Figure 59. Average source energy savings reduction for Duluth........................................................ 82

Unless otherwise indicated, all figures were created by BSC.

List of Tables

Table 1. Window Retrofit Measures ......................................................................................................... ix

Table 2. Window Performance Improvement Measures ......................................................................... 3

Table 3. Benchmark House Characteristics............................................................................................. 8

Table 4. Parametric Steps and Cost for Climate Zones 4 and Above.................................................... 9

Table 5. Reference Cities............................................................................................................................ 9

Unless otherwise indicated, all tables were created by BSC.
viii

Definitions

ASHRAE American Society of Heating, Refrigerating and AirConditioning Engineers

BA Building America Program

BSC Building Science Corporation

DOE U.S. Department of Energy

IGU Insulated glazing unit

NFRC National Fenestration Rating Council

SHGC Solar heat gain coefficient

U-Value Window Thermal Transmittance Value

UA U-Value multiplied by assembly area

WRB Water resistive barrier
ix

Executive Summary

There is a significant push for energy performance upgrades to existing homes. An important 

target is often the windows. Old single-glazed windows have such low thermal resistance that 

their effect on the overall thermal resistance of the walls can be staggering. Improving the 

performance of the window stock is therefore central to the goal of reducing the energy 

consumption of the existing building stock.

This measure guideline provides information and guidance about rehabilitating, retrofitting, and 

replacing wood window assemblies in residential construction. It is intended primarily to help 

contractors and homeowners understand the options for safely improving the performance of 

their wood windows.

Deciding which window measure will be most appropriate for the retrofit project depends on 

several factors, including current conditions, desired appearance or aesthetic goals, energy 

performance goals, cost, disruption to occupants, durability risks, historic requirements, and any 

other project goals or requirements.

Table 1 lists the f most common approaches to improve the performance of wood windows:

Table 1. Window Retrofit Measures

Measure Description

1 Window 

rehabilitation

Repair/rehabilitation of the old window sashes, leaving the original 

wood window frame in place. Frame and sash rehabilitation to 

improve water management and air infiltration performance. Does 

not improve conductance or solar gain. Cost can be highly variable 

depending on window condition and extent of work needed.

2 Exterior storm 

windows

Recommended installation practice for exterior storm windows, 

including preparation of windows to accommodate storm windows.

Improves the air infiltration performance, conductance, and solar 

gain. Additional benefits can be achieved with hard coat low-e

glazing. Cost is relatively low.

3

Interior 

removable storm 

windows

Recommended installation practice for removable interior storm 

windows, including preparation of windows to accommodate storm

windows. Improves the air infiltration performance, conductance, and 

solar gain. Condensation potential on outer prime window could be a 

durability concern. Cost is relatively low.

4

Interior 

permanent storm 

windows

Recommended installation practice for permanent interior storm 

windows, including preparation of windows to accommodate storm

windows. Improves the air infiltration performance, conductance, and 

solar gain. Condensation potential on outer prime window could be a 

durability concern. Cost is moderate to high compared to other 

measures, depending on options chosen.
x

Measure Description

5 Window sash 

modification

Modification of the old window sashes, leaving the original wood 

window frames in place. Frame preparation to accommodate original 

sashes to accept high performance insulating glass units. Improves 

the air infiltration performance, conductance, and solar gain and 

maintains the original appearance of the windows. Cost can be 

moderate to high compared to other retrofit measures.

6 Window sash 

replacement

Removal of the old window sashes, leaving the original wood 

window frames in place. Frame preparation to accommodate high 

performance replacement sashes and tracks. Improves the air 

infiltration performance, conductance, and solar gain and maintains a 

similar appearance of the windows. Cost can be moderate to high 

compared to other retrofit measures.

7

Insert 

replacement 

window

Removal of the old window sashes and reconfiguration of window 

frame to accommodate a replacement window installed in the wood 

window frame. Improves the air infiltration performance, 

conductance, and solar gain. Work can be done quickly with little 

disruption to the occupants. Cost can be moderate to high compared 

to other retrofit measures, depending on performance of replacement 

window chosen.

8

Complete 

window 

replacement

Removal of the old window, including frame, and reconfiguration of 

the rough opening to accommodate a new high performance window 

following current recommended water management installation 

techniques. Improves the air infiltration performance, conductance, 

and solar gain. Provides the most control over the window size, 

location, placement, and integration with other enclosure retrofit 

measures. Cost is high compared to other retrofit measures and is 

typically a significant disruption to the building occupant.

Complementary information about a variety of energy upgrades can also be found in the National 

Renewable Energy Laboratory’s Standard Work Specifications, currently under development 

through U.S. Department of Energy’s Guidelines for Homes Energy Professionals project.
xi

Progression Summary

ss

WOOD WINDOW

OPTIONS

The following outlines the steps to consider when looking to repair, rehabilitate, or 

replace wood windows in a home.

Do Not Proceed If:

• Lead paint is present or suspected on the window or 

window frame.

• Water is intruding from sources other than the window.

! !

SITE CONDITIONS

Do Not Proceed If:

• Site conditions create unsafe work environments. ! !

EVALUATE 

WINDOWS

Evaluate building conditions. Review the building conditions for 

any hazards or anything that needs to corrected before the work 

starts,

1

REVIEW SITE 

CONDITIONS

Review site conditions and project staging. Examine the 

property to identify impacts and risks associated with performing

the work.

2

SELECT MEASURE

Select the window retrofit measure. Choose the most 

appropriate window retrofit measure to address the identified 4 performance concerns that stays within the budget.

IDENTIFY GOALS

Indentify performance concerns. Review the current 

performance problems (water infiltration, air leakage, energy, etc.) 

and rank them to determine the project goals. 

3
1

1 Introduction

There is a significant push for energy performance upgrades to existing homes. An important 

target is often the windows. Poor window performance can have significant impacts on the 

overall thermal resistance and airtightness of the home. The existing U.S. housing stock has a

wide range of window types (Figure 1), ages, and conditions. All are factors in the overall energy 

performance and comfort performance of the home. Most windows are still original to the 

homes; however, it is not uncommon for windows to be replaced (Figure 2). Possibly more 

common than window replacement are attempts at lower cost window retrofits. These retrofits 

have often been done (with varying degrees of success) by homeowners trying to address 

problems without adequate knowledge about how to safely, effectively, and economically 

address the problems.

Figure 1. Glazing type (single, double, triple) by climate region

(EIA 2009)

Figure 2. Window replacement history by climate region 

(EIA 2009)

0

5

10

15

20

25

30

35

40

45

Millions of US Households

Triple-pane

Double-pane

Single-pane

0

5

10

15

20

25

30

35

40

45

Millions of US Households

None

Some

All
2

Old single-glazed windows have such low thermal resistance that their effect on the overall 

thermal resistance of the walls can be staggering. A simple UA1 analysis that compares the total 

wall effective R-value to the window U-value for opaque wall assemblies of varying effective Rvalues shows how significant this impact can be (Figure 3).

Figure 3. Impact of window U-value on effective thermal resistance of complete wall assemblies

Older windows typically represent a significant portion of the total house air infiltration. The 

combined effect of poor thermal performance and air infiltration results in windows being a 

major component to total energy use. Improving the performance of the window stock is 

therefore central to the goal of reducing the energy consumption of the existing building stock.

Table 2 provides information and guidance about rehabilitating, retrofitting, and replacing wood 

window assemblies in residential construction. The intent is to provide information about means 

and methods to improve the energy and comfort performance of wood window assemblies in a 

way that considers component durability, in-service operation, and long-term performance. This 

guideline focuses on proper detailing of strategies to provide a visual reference for how to 

properly implement these recommendations.

This measure guideline is primarily intended to help contractors and homeowners understand the 

options for safely improving the performance of their wood windows. The details are simple, 

clear, and provide specific information about building condition review, material preparation, 

installation, and other considerations that would not normally be explained in a general retrofit 

recommendation.

 1 The UA analysis multiplies an individual component U-value by its associated area to create an area weighted 

thermal transmittance value.
3

Table 2. Window Performance Improvement Measures

Measure Description

1 Window 

Rehabilitation

Repair/rehabilitation of the old window sashes, leaving the 

original wood window frames in place. Frame and sash 

rehabilitation to improve water management and air infiltration 

performance.

2 Exterior storm 

windows

Recommended installation practice for exterior storm windows, 

including preparation of windows to accommodate storm

windows.

3 Interior removable

storm windows

Recommended installation practice for removable interior storm 

windows, including preparation of windows to accommodate 

storm windows.

4 Interior permanent 

storm windows

Recommended installation practice for permanent interior storm 

windows, including preparation of windows to accommodate 

storm windows.

5 Window sash 

modification

Modification of the old window sashes, leaving the original 

wood window frames in place. Frame preparation to 

accommodate original sashes to accept high performance 

insulating glass units (IGUs).

6 Window sash 

replacement

Removal of the old window sashes, leaving the original wood 

window frames in place. Frame preparation to accommodate 

high performance replacement sashes and tracks.

7 Insert replacement 

window

Removal of the old window sashes and reconfiguration of 

window frames to accommodate replacement windows installed 

in the wood window frames

8 Complete window 

replacement

Removal of the old window, including frame, and 

reconfiguration of the rough opening to accommodate a new 

high performance window, following current recommended 

water management installation techniques.

Do-it-yourself homeowners (as opposed to professional contractors) are likely to use this 

information: therefore, it is important that clear information about means and methods be 

provided, so that retrofits can be executed in a manner which will promote long term durability 

of the building enclosure systems.

Complementary information about a variety of energy upgrades can also be found in the National 

Renewable Energy Laboratory’s (NREL) Standard Work Specifications, currently under 

development through the U.S. Department of Energy’s (DOE) Guidelines for Homes Energy 

Professionals project.
4

2 Decision-Making Criteria

2.1 Risk Identification

Before any retrofit work is conducted, i the 

conditions of the building systems must be 

reviewed.

2.1.1 Lead and Other Hazardous Materials

Lead is commonly located on old wood windows 

and trim. Any work being completed on the 

window systems should follow all state and federal 

laws for handling hazardous materials.

2.1.2 Site Conditions and Project Staging

The home and site should be reviewed to identify impacts and risks associated with completing 

the work.

Scaffolding, lifts, ladders, or other means to access work areas may be needed if the work is to 

be done on the exterior. Work done at height may require fall protection. Proximity to adjacent 

property or vegetation may limit access or create unsafe work areas. Exterior staged work may 

also damage landscaping or vegetation.

If the work will be done from the interior, consideration should be given to disruptions to the 

occupants and clearances for moving equipment and materials into and out of the space. Interior 

work always poses a risk of damage to interior finishes. Planning and protection are required.

2.1.3 Identification of Water Infiltration Concerns

Windows are the most common location of water infiltration issues. It is important to understand 

the various pathways for potential water infiltration, and identify current water leakage problems.

Although the details presented in this guideline are all intended to improve the moisture 

performance of the window assemblies, the guideline is not intended to address all possibilities, 

and is not a replacement for inspection and evaluation of the performance of an individual 

window. Problems should be identified, and the strategy chosen that will be most appropriate to 

address the concern. Window systems water leaks can be grouped in four general categories 

(Figure 4):

• Between the window frame and rough opening

• Through the joints in the window frame

• Between the window frame and the operable sashes

• Through the joints between the glass and the sash frames.

Contractor/Homeowner Safety

U.S. Environmental Protection Agency: 

Lead in Paint, Dust, and Soil: 

Renovation, Repair and Painting 

www.epa.gov/lead/pubs/renovation.htm

Occupational Safety and Health 

Administration: Fall Protection (if window 

work is to be done at height, from the 

exterior) www.osha.gov/

SLTC/fallprotection/index.html
5

Between the frame and rough 

opening

At joints in the frame

Between the sash and frame

Between the glass and sash

Figure 4. Common window water infiltration pathways

Interior and exterior inspection and monitoring of the building conditions should be completed

before any work is started. Water staining, peeling paint or wall paper, and staining on trim or 

floor assemblies below window systems indicate water infiltration and/or condensation. Leakage 

between the sashes and the frame and between the glass and the sash is usually marked by water 

staining on the interior window frame. Condensation on the window frames can also lead to 

staining of the interior finishes. It is important to monitor the questionable area to prevent a false 

diagnosis of the water management problem.

Leakage between the window and the rough opening or through the joints in the window are 

typically contained within the wall assembly and may go unnoticed, or could manifest as staining 

and peeling paint below the window or damaged flooring.

Other problems such as water infiltration at the window head may be indications of failed or 

missing head flashing. However, other problems not associated with the window system may 

cause the water infiltration. Care must be taken to properly diagnose the infiltration pathway.

If it is a known recurring problem, the infiltration problem must be addressed prior to, or in 

conjunction with, the window retrofit work.

If there are no obvious signs of water infiltration problems and the window elements and 

connection wall components are in good condition, no additional work may be needed. However, 

water infiltration problems are often concealed within wall cavities. This becomes a greater 

concern if the window retrofit work is being done in conjunction with the addition of cavity fill 

insulation, because a cavity that may have had sufficient drying ability may now have prolonged 

moisture accumulation that can lead to material deterioration. If leakage is suspected, further 

investigation, including thermal scans, moisture content measurements, and cutting of 
6

investigation holes below window assemblies to look for signs of moisture problems are

recommended.

Unless the problem is obvious, it may be prudent to contact someone with experience diagnosing 

water infiltration problems before proceeding.

2.1.4 Identification of Deteriorated or Damaged Materials

Damaged materials should be removed and replaced as part of the retrofit. Certain elements will 

be more critical to the proper implementation of the chosen strategy.

The window sill is arguably the most important element of the window assembly, as water will 

drain downward by gravity either into the wall (e.g., hole through sill) or directly onto the wall 

(failure of the sill extension). For all proposed measures in this guideline (except complete 

window replacement), the condition of the sill is critical to the performance of the measure. 

Cracked or rotting sills need to be replaced before any work is done (see Figure 5).

Figure 5. Failed window sill with replacement window installed

The window frame, including the exterior casings, is the next most critical element. If the casing 

is deteriorating, its replacement may be warranted. This should not be confused with the exterior 

trim, which is often installed as a decorative element on top of the casing. Deteriorating trim may 

not affect the water management performance of the window; however, it may be an indication 

of other problems and generally creates an aesthetic problem.

Depending on the measure being examined, the condition of the window sashes may or may not 

be a concern to the performance of the measure. For window rehabilitation, sash retrofit, or 

interior storm window retrofit, the condition of the sashes is critical to the performance of the 

window. For exterior storm windows, the sashes are more protected from the elements, and the 

condition is less important from a water management perspective, yet still critical from an energy 

and condensation resistance perspective. For sash replacement, window insert, or full window 

replacement, the condition of the sash is irrelevant, as they will be removed. Thus, windows with 

severely deteriorated sashes may be better candidates for the latter retrofit measures.
7

2.1.5 Identification of User Comfort Concerns

As part of the initial review, comfort concerns related to the window systems should be 

evaluated. Window air leakage is a significant source of occupant comfort problems. Unlike 

other common enclosure leakage pathways, window air leakage is commonly very direct, 

resulting in distinct drafts.

Radiation effects from cool glass surfaces are another common comfort problem. This is more 

difficult to identify, as many people assume that the discomfort felt when near a window is from 

air leakage or drafts. This results in some misdiagnosis of the dominant function. A general 

recommendation is to increase the interior surface temperature of the window system to reduce 

the radiant heat transfer from the occupant to the window. This is commonly done by adding 

additional panes of glass (or films) to create an insulating air (or other gas) space between the 

layers.

2.2 Cost and Performance

The cost effectiveness of a window retrofit will depend on numerous factors, though possibly 

most critical is the existing window performance. Single-glazed wood windows have 

proportionally very poor performance compared to other glazing systems (U = 0.87, solar heat 

gain coefficient [SHGC] = 0.62). If the baseline begins from this point, almost any window 

retrofit strategy will provide a cost justifiable improvement to the current conditions.

A preliminary evaluation was completed looking at the cost versus energy performance of 

several wood window retrofit measures. Cost data for the windows were taken from several 

sources, including averages based on direct quotes from manufacturers for multiple product lines 

and from RS Means Construction Data (2011 Reed Construction Data). Product costs vary 

widely, so estimated averages were used to develop a representative sample. For each project, 

specific cost analysis will be required to ensure the cost effectiveness.

Simulations were run using BEopt simulation software developed by NREL (see Figure 6). The 

analysis examines the cost effectiveness of the window system upgrades by comparing the 

annualized cost of each measure over the analysis period (assumed at 30 years for this analysis) 

compared to the estimated annualized utility savings. The costs displayed in the cost/energy 

graphs (Appendix B) are full annualized utility bills plus incremental annualized values for other 

cash flows such as mortgage/loan payments, replacement costs, and residual values of the 

measure being implemented. A measure is determined to be economically justifiable if its total 

annualized cost is less than the total annualized cost of the baseline (in this case, the original 

wood windows). The cost optimized solutions have the lowest total annualized cost.
8

Figure 6: Example of BEopt analysis graph

An example home was used as the baseline to help demonstrate the benefits of using exterior 

insulation as part of a house energy retrofit (see Table 3). This benchmark home was assumed to 

be a 1950s era two-story slab-on-grade construction.

Table 3. Benchmark House Characteristics

House Characteristics Square Feet

Finished Floor Area 2312

Ceiling Area 1156

Slab Area 1156

Wall Area 2799

Window Area 410 (17.7% glazing ratio)

The window performance was isolated from all other aspects of the home to examine the 

effectiveness of this single strategy. The baseline window was chosen to be a double-hung,

single-glazed wood window. In addition to the performance changes in both the U-value and the 

SHGC, increases in airtightness were also included as part of the analysis. Levels of increased 

airtightness were estimated because measured data relating specifically to window airtightness 

increases were lacking.

The following parametrics were run to see the effectiveness of the various window retrofit 

strategies to the energy performance and utility cost (Table 4).

Total annualized cost of single-glazed 

wood windows

Points below this line indicate an 

economically justifiable retrofit 

measure compared to the baseline 

single-glazed wood window

Lowest point represents the cost 

optimized retrofit measure
9

Table 4. Parametric Steps and Cost for Climate Zones 4 and Above

Parametric Step Cost/ft2

Benchmark = single glazed wood 

(U = 0.87, SHGC = 0.62)

N/A

Single-glazed wood + clear exterior storm 

(U = 0.49, SHGC = 0.60)

$7.81

Single-glazed wood + low-e exterior storm 

(U = 0.40, SHGC = 0.52)

$12.14

Double-glazed low-e ENERGY STAR® window 

(U = 0.30, SHGC = 0.30)

$40.73

Single-glazed wood + interior double-glazed low-e

ENERGY STAR window 

(U = 0.25, SHGC = 0.25)2

$40.73

Triple-glazed low-e window 

(U = 0.20, SHGC = 0.20)

$77.76

Simulations were run for the following cities (Table 5):

Table 5. Reference Cities

City Climate Zone

Dallas, Texas 3A

Kansas City, Missouri 4A

Boston, Massachusetts 5A

Duluth, Minnesota 7A

Results indicated that most strategies chosen in all climate zones were economically justifiable 

when the baseline windows were single-glazed wood. The exterior clear storms and low-e storm

windows provided the cost optimized solutions for colder climates. However, in Dallas a much 

narrower gap between the storm windows and the window replacements was noted. This is partly 

because the overall storm window performance has a lesser effect on the single-glazed wood 

framed window, and partly because Dallas is in a cooling-dominated climate that is affected 

predominantly from SHGC and not U-value. In none of the analyses were triple-glazed windows 

demonstrated to be economically justifiable.

 2 Values were assumed due to unavailability of actual modeled or measured values for this configuration

Based on general assumptions, most measures chosen (other than triple-glazed replacement 

windows) in all climate zones were economically justifiable when the baseline windows were singleglazed wood. However, costs vary widely, so project-specific costs and analysis will be required to 

determine the cost effectiveness of the measure for each project.
10

3 Technical Description

3.1 System Interaction

It is important to understand the system components and connection/interaction details with the 

wall enclosure, as well as the functions of a traditional wood window, to properly apply the 

various measure recommendations (see Figure 7).

Header

Sheathing

Head flashing

Stop

Parting bead

Exterior casing

Interior trim/casing

Exterior trim

Upper sash

Muntin

Meeting rails

Weight pocket

Lower sash

Studs

Stool

Sill

Apron

Sill framing

Sheathing

Siding

Figure 7. Window components
11

3.1.1 Window Water Management Functions

Traditional double-hung wood windows function by shingle lapping the various components of 

the window assembly (including its surrounding trim components).

For wood-framed walls, a metal flashing is common at the window head to shed water from the 

siding out over the face of the trim and exterior window casing. The casing is shingle lapped 

over the upper sash. The upper sash is always installed outboard of the lower sash. The lower 

sash sits on the window sill. The window sill projects out over top of the siding or cladding 

element (Figure 8).

Siding

Head flashing

Head trim

Window casing

Upper sash

Figure 8. Window head component lapping

At the jambs, the water management is generally from a similar concept of overlapping of 

materials. The wall siding overlaps onto the exterior window casing, and the casing overlaps the 

sashes. The lapping at the jambs cannot wholly rely on gravity to prevent water from infiltrating

into the enclosure, so sealants are often used between the siding and the window trim to limit the 

potential for infiltration.
12

Wall sheathing

Window casing

Window trim

The siding and exterior window casing are 

typically installed in an overlapped manner. 

This provides some protection against direct 

water infiltration; however the joint is still 

often reliant on sealant at the trim to siding 

interface.

Siding

Figure 9. Window jamb component lapping

Similar approaches are used in mass masonry walls with a few slight changes to the concept. At 

the window head, the window frame is set back from the face of the masonry (approximately one

layer of brick), so the head flashing is traditionally eliminated. At the jambs, the window casing 

either butts directly up against the brick, or is overlapped by the outer layer of brick with the 

joint caulked (Figure 10). The sill is shingle lapped over a stone, concrete, or brick sill. The 

masonry sill is usually sloped to the exterior.

Figure 10. Wood window sill on top of masonry sill
13

Wood sills are traditionally continuous with the 

jamb framing, casing and trim all installed over top 

of the sill

This configuration promotes drainage to the 

exterior even if there is a failure in a seal at the sill 

to jamb interface or higher up in the window frame

Figure 11. Window sill component lapping

The continuous sloped wood sill is a critical element to the performance of wood windows. The 

sill significantly protects the wall assemblies below. In essence, the wood sill acts similarly to a 

sloped pan flashing that is part of current recommended new construction practice (Figure 11). In 

some cases, a pan flashing may have been installed below the window frame; however, this is by 

no means guaranteed for all construction, and tends to be more common for masonry buildings 

than for wood-framed buildings. Unfortunately, these subsill flashings often deteriorate over 

time and lose their effectiveness.

3.1.2 Window Air Leakage

There are multiple pathways for air leakage through a window frame. The most common are 

between the meeting rails (where the upper and lower sashes interface), and between the sashes 

and the frame (Figure 12 through Figure 14). These are the operable elements of the window, so

they typically cannot be too tight, as this would affect operation. In addition, repetitive operation 

results in a wearing of materials at the interface, increasing dimensional tolerances. In addition to 

the meeting rail and jamb air leakage, leaks can also be found at the head and sill.

The condition of the wood window frame is critical to the water management performance for all 

proposed measures except complete window replacement. Damaged or deteriorating window sills 

must be repaired or replaced.
14

Air infiltration between sash and frame

Air infiltration at the meeting rails

Figure 12. Window air leakage pathways

Figure 13. Infrared images of window air leakage (depressurization test). Dark colored areas 

indicate cold surface temperatures from cold outdoor air leaking into the building.
15

Figure 14. Infrared images of window air leakage (depressurization test). Dark colored areas 

indicate cold surface temperatures from cold outdoor air leaking into the building.

A typical window installation has void spaces between the frame and the rough opening. The 

gaps are also common areas of uncontrolled air leakage; however, the pathway is less direct, and 

therefore less significant from a user comfort perspective (Figure 15 and Figure 16).

Void created by weight pockets at window jambs

Void below window sill due to frame geometry

Figure 15. Window to wall interface air leakage pathways
16

These voids are typically disconnected from the framing cavities of wood-framed walls, and 

therefore would not be filled during common cavity fill insulation retrofits such as blown-in 

cellulose. Specific measures need to be taken to address the air leakage pathway and associated 

concerns of conductance losses.

Figure 16. Infrared images of window air leakage at window interior apron trim. Dark colored areas 

indicate cold surface temperatures from cold outdoor air leaking into the building.

The largest of the voids are typically at the weight pockets created to house the sash 

counterweights (see Figure 17 and Figure 18). The weights are connected to the sashes through 

either ropes or chains that run from the sash to a pulley near the top of the window jamb and into 

the weight pocket. When combined with an interior storm window (both temporary and 

permanent), the pulley weight pockets can create a condensation potential (discussed in Section 

3.1.3), as they are a pathway for movement of interior air into the space between the old window 

and the new interior glazing element. Often, the sash weights can be eliminated by retrofitting 

the windows a spring-loaded tape balance. Abandoned weight pockets permit energy loss from 

conductance and from uncontrolled air leakage. Even when they are insulated, it is often done in 

a “blind” manner (inserting insulation through an access hole in the jamb), resulting in poor 

installation quality.

Figure 17. Sash weight pockets Figure 18. Abandoned sash weight and 

insulation installation
17

The spaces should be filled with spray polyurethane foam to address these concerns. The weights 

should be removed before the void is filled. In addition to the weight pockets, there are typically 

gaps between the window frame and the rough opening at the head and sill. These voids should 

also be filled.

3.1.3 Interstitial Condensation

The addition of interior or exterior glazing elements to the original window creates a potential 

for the formation of condensation between the two glazing elements. The placement of the 

interior or exterior of the window creates an insulating air pocket, resulting in the outer glazing 

element to be at a lower temperature at wintertime conditions. This increases the chance of 

condensation forming, if interior moisture laden air can infiltrate the space.

Figure 19. Examples of condensation problems with interior storm window retrofits

Air leakage associated with window systems is from two general sources: (1) leakage through the 

window system (such as between the frame and sashes); and (2) leakage between the frame and 

the rough opening. Most of the measures only directly impact the leakage through the frame. 

Consideration should be given to the spaces between the frame and the rough opening when 

planning the retrofit. Abandoning and insulating weight pockets can have a significant effect on the 

overall performance.
18

Figure 20. Examples of condensation problems with interior storm window retrofits

Condensation at exterior storm windows is typically not a durability concern, but mostly an 

operational and aesthetic concern, as it disrupts clear vision to the outside. The exterior storm

windows are typically made of moisture-insensitive materials (aluminum extrusions).

Retrofit of interior storm windows can pose a durability risk: if there is air leakage, the 

condensation will occur on the “prime” window, which is often part of the historical fabric of the 

house, and is made of moisture-sensitive materials such as wood (Brown 1997). Therefore, 

airtightness of interior storm windows is crucial in 

cold climate installations.

Research shows that a small amount of ventilation 

of the space with dry air can mitigate condensation 

concerns (Wilson 1960). Therefore, in cold climates 

it is important that the inner glazing element be as 

airtight as possible to reduce air infiltration into the 

void space. Air bypass of the glazing elements must 

also be considered and addressed.

3.1.4 Window Thermal Performance

Energy is transferred across a window assembly by several mechanisms. Conduction, convection 

(air movement such as air leakage), and radiation are all components of performance. With so 

many materials (wood, polymers, glass, metal) being combined to create a window system, the 

interactions between all these elements are not necessarily simple or straightforward (see 

Figure 21).

Airtightness of interior storm windows 

is of vital importance in cold climate 

installations. Condensation that could 

occur on the existing (“prime”) 

window can create a durability risk, 

as the window is made of moisturesensitive materials.
19

Conduction through wall framing

Convection (air leakage) through and within the void

Conduction through window frame

Convection (air leakage) between frame and sash

Conduction through sash

Convection (air leakage) between sash and glass

Conduction through glass

Radiation through glass

Absorption and emittance from glass

Convection (air leakage) between sash and glass

Conduction through sash

Convection (air leakage) between frame and sash

Conduction through window frame

Convection (air leakage) through and within the void

Conduction through wall framing

Figure 21. Window energy transfer components

Figure 21 is a simplification of the energy transfer through a window and its surrounding 

enclosure elements. The actual energy transfer mechanisms are three dimensional, with all of the 

mechanisms interacting and impacting the actual performance. To more closely quantify window 

performance, extensive laboratory testing (or at minimum two-dimensional heat flow computer 

simulation modeling) is generally required.

Knowing the general mechanisms can help a person understand how a certain measure will 

modify the performance. Elements such as interior and exterior storm windows will impact 

mechanisms such as the conductance through the glass and sashes, air leakage between the frame 

and sashes or between the sashes and glass, and the radiation, absorption, and emittance from the 

vision glazing elements (Figure 22).
20

The exterior storm window creates a buffer of 

still air between the original window and the new 

storm window. 

The exterior storm window will reduce the 

radiation transfer, conductance transfer, and air 

leakage of the sashes and sash-to-frame 

interface.

The storm window does not significantly affect 

conductance and air leakage through the 

window frame (though flanking effects will have 

some impact) or at the window-to-wall interface.

Figure 22. Areas of energy transfer affected by the addition of exterior storms

Until recently, information was available only about the energy performance of the window 

system. However, new standards for rating fenestration attachment products have recently been 

introduced by the National Fenestration Rating Council (NFRC 2010a, 2010b), to help quantify 

the effects of various window coverings. The new standards will help to further quantify the 

performance question of various strategies, though it may be some time before a significant 

database of rated products is developed.

In the interim, some information is available to help in the determination. Some work 

coordinated under the DOE’s High Performance Windows Volume Purchase Program assigned 
21

some values to various high performance windows and configurations of existing windows with 

exterior storm windows (Figure 23).

Figure 23. Window U-value and SHGC for various windows systems and exterior storm 

configurations (Windows Energy Savings Estimator (2011)

Although some general ranking of the expected performance improvements can be made, each 

project should make its own evaluation of the expected performance to determine the best 

approach.

3.2 Measure Selection Criteria

Deciding which window measure will be most appropriate for the retrofit project depends on 

several factors. The following is a list of the proposed measures and some key points to consider 

with each.

The energy performance of the window system involves all three modes of energy transfer 

(conduction, convection, and radiation). Understanding the general mechanisms can help 

understanding of how a certain measure will modify the performance. To more closely quantify

window performance, extensive laboratory testing (or at minimum two-dimensional heat flow 

computer simulation modeling) is generally required.
22

3.2.1 Measure 1—Window Rehabilitation

The first measure looks to improve the window performance without significant modification or 

addition. This measure has practically no impact on the appearance of the window as the 

modifications are all concealed and minor. The work focuses around adding gaskets and seals at 

common air infiltration locations to improve the overall airtightness of the window assembly.

Some methods are covered in detail by Davis (2007).

3.2.2 Measure 2—Exterior Storm Windows

A common and long-standing window 

retrofit approach is to add storm windows 

to the exteriors of the frames. The most 

common design is a triple-track window 

that combines a screen with operable upper 

and lower sashes. Traditionally, storm

windows have been single glazed with 

clear glass, but more recently, hard coat 

low-e glass has become available.

Key points:

1. Maintains both the interior and exterior appearance of the building (This is an important 

strategy where historic preservation is required).

2. This work is best done in conjunction with other measure strategies, including interior and 

exterior retrofit approaches (such as the addition of interior and exterior storms).

3. This work will improve the energy performance of the assembly by reducing air infiltration; 

however it will not improve the thermal conductance of the window system.

4. This work will not address any current condensation problems that may be occurring on the 

window system.

5. If sash weights are maintained, other potential energy and durability considerations may not 

be addressed, such as air leakage and thermal conductance losses at weight pockets.

6. Cost can be highly variable depending on the size, complexity, condition, and value of the 

windows.

Figure 24. Interior removable storm window 

sample
23

3.2.3 Measure 3—Interior Removable Storm Windows

An alternative to exterior storm windows is 

removable interior storm windows. These can 

take a wide variety of forms, from films 

adhered to the interior of the window jambs, 

to site-built frame wrapped with films that are 

friction fit into the window opening, to 

premanufactured fixed or operable glass or 

acrylic litesin a thin metal frame. The 

premanufactured versions are often installed 

via a metal angle and magnetic track fastened 

to the window frame.

These systems are typically less permanent, 

and intended to be installed seasonally

(usually during the winter in cold climates), as 

most limit opening and closing.

Figure 25. Interior removable storm window 

sample

Figure 26. Installed interior removable storm 

window

Key points:

1. Maintains interior appearance of the window; however, the exterior appearance will change. The 

addition of exterior storm windows is typically acceptable for most historic preservation projects. 

2. The measure is reversible. This is an important consideration for historic preservation.

3. This work should be done in conjunction with the rehabilitation work set out in Measure 1—Window 

Rehabilitation.

4. This work will improve the energy performance of the assembly by reducing air infiltration, as well as 

thermal conductance through the assembly. Hard coat low-E storm window glazing can provide 

additional thermal benefits.

5. This work will reduce the potential for interior condensation problems on the window system, though it 

adds some risk of interstitial condensation between the original window and the exterior storm 

(generally an aesthetic/operational concern, as opposed to a durability concern).

6. Cost is low to moderate, depending on system chosen.
24

3.4.4 Measure 4—Interior Permanent Fixed Storm Windows

In certain cases, there is no urgent need for 

seasonal changeover, and a more permanent 

solution is desired. In these cases, permanent 

interior storm windows can be added. There are 

two approaches with this solution: installation of a 

fixed glazing unit and installation of an operable 

unit.

Fixed glazing units meet requirements if the 

window no longer needs to operate. Fixed units 

can typically be made more airtight, and will have 

thinner frame profiles so that the vision area is not 

impacted. Also, they can often be installed to have

minimal impact on the interior appearance of the 

window so that interior aesthetics can be more 

closely maintained. An operational concern with 

this approach is that the windows can no longer be 

cleaned from the interior, and may be difficult 

from the exterior as well. Although the more 

common approach has been to use single-glazed 

lights (often clear acrylic for retrofits by 

homeowners), double- and triple-glazed IGUs can 

be used to increase energy performance.

Key points:

1. Maintain the exterior appearance of the window; however, the interior appearance will 

change. Removable interior storms are also often acceptable for historic preservation 

projects.

2. The measure is reversible. This is an important consideration for historic preservation.

3. This work should be done in conjunction with the rehabilitation work set out in Measure 1—

Window Rehabilitation.

4. This work will improve the energy performance of the assembly by reducing air infiltration

and thermal conductance through the assembly.

5. This work will reduce the potential for interior condensation problems on the window 

system, though it poses a risk of interstitial condensation between the interior storm and 

original window (both an aesthetic and an operational concern, and may pose durability 

risks).

6. Typically a seasonal installation (requires removal and storage during the summer 

months).

7. Could impact egress.

8. Cost is low to moderate depending on system chosen.

Figure 28. Interior storm window on doublehung original window

Figure 27. Interior storm window on doublehung original window
25

Alternatively, if an operable window is required, a new window 

frame and sash could be installed on the interior of the original 

window. The window is not required to address the water 

management needs of the system, so lower standards may be 

acceptable for the frame. The ideal frame would have a thin profile 

frame for minimal disruption to the vision area of the rough 

opening, and have excellent airtightness. These would be the critical 

areas of performance for these windows. This is an area of potential 

product development for window manufacturers. The benefit of this 

approach is that the windows can still be operable (though two 

sashes would be needed), allowing for egress, natural ventilation, 

and easier cleaning. The cost, however, will be roughly the same as 

for replacement windows, and will similarly reduce the vision area.

This option will be limited based on the width of the wall assembly and the depth of the window 

returns. In addition, in warmer climates and/or unshielded solar orientations, heat buildup may 

become a concern, and should be studied in more detail.

3.2.5 Measure 5—Window Sash Modification

Wood window sashes can 

sometimes be retrofitted to 

accept new double-glazed 

sealed units. This is a good 

approach if there is a demand 

to maintain the original 

appearance from both the 

interior and the exterior (i.e., 

historical applications).

Figure 29. Interior 

storm detail

Key points:

1. Maintains the exterior appearance of the window; however, the interior appearance will 

change. The addition of permanent interior storms may be acceptable for historic 

preservation projects.

2. The measure is reversible. This is an important consideration for historic preservation.

3. This work should be done in conjunction with the rehabilitation work set out in Measure 1—

Window Rehabilitation.

4. This work will improve the energy performance of the assembly by reducing air infiltration 

as well as thermal conductance through the assembly. The use of double or triple glazed 

units will work in conjunction with the existing window to provide better performance than 

just the double or triple glazed IGU’s.

5. This work will reduce the potential for interior condensation problems on the window 

system, though it creates a risk of interstitial condensation between the interior storm and 

original window (both an aesthetic/operational concern, as well as potential for durability 

risks).

 

 

 

 

 

Figure 30.Window sash modification
26

3.2.6 Measure 6—Window Sash Replacement

Window sashes may be replaced without replacing the frame. This is a good option if there is a 

desire to maintain the original appearance, but the old sashes cannot accommodate new IGUs, or 

are in sufficiently poor condition to warrant replacement in lieu of rehabilitation. The 

replacement sashes must be custom built for the frame opening, as there is very little opportunity 

for adjustment. Window frame condition and geometry can be a factor. The window jambs,

however, need to be very close to parallel to allow for smooth operation and to prevent unwanted 

air leakage between the frame and the sash. If the replacement sashes are made from wood, the 

windows can be out of square to a certain degree, as the top and bottom of the sashes can be cut 

to match the angles.

Figure 31. Sash replacement, showing out of square opening and requirement for sash extension

Figure 32. Window sash replacement jamb liners
27

3.2.7 Measure 7—Insert Replacement Window

For window sashes that are in poor 

condition and/or the cost of sash 

rehabilitation, retrofit, or 

replacement is too high, a common 

retrofit approach is to install a 

replacement window. The benefit to 

this is the speed of installation (very 

little rehabilitation of the frame is 

needed), low disruption to the 

homeowner (the installation does 

not overly affect interior or exterior 

trim and finishes), and adjustment 

ability (although the windows need 

to be custom ordered to fit the 

frames, the tolerances do not need 

to be as tight or the measurements 

as precise as the replacement sash 

option). The result is a completely 

new window assembly. A downside 

to this approach is that the vision 

area is reduced by an additional

window frame with sashes. For 

Key points:

1. Affects the interior and exterior appearance of the building (though the effect could be 

minimal depending on how closely the replacement sashes can match the existing 

sashes). The replacement of sashes may be acceptable for historic zoned projects.

2. The measure is not reversible.

3. This work should be done in conjunction with a partial rehabilitation (window frame only) as 

set out in Measure 1—Window Rehabilitation.

4. This work will improve the energy performance of the assembly by reducing air infiltration, 

as well as thermal conductance through the assembly. The replacement sashes can 

accommodate up to triple glazed IGUs, allowing for significant improvements in thermal 

performance.

5. This work will reduce the potential for interior condensation problems on the window 

system.

6. Work will likely replace the sash weight and balance system (with a spring-loaded jamb 

liner), allowing for retrofit of the weight pockets and voids around the window frame.

7. Work requires tight tolerances and is workmanship sensitive; significant air leakage can 

result if tolerances are too loose.

8. Expensive option (equal to or more expensive than an equivalent replacement window).

Figure 33. Insert replacement vinyl frame window
28

already small window openings, this may be significant and undesirable. For larger windows, the 

impact will be less noticeable.

3.2.8 Measure 8—Complete Window Replacement

Ultimately the old window can be completely removed and replaced with a new window. The 

benefit of this measure is that the installation details can be brought up to current installation 

standards. This approach is typically most common with more extensive building retrofits, as it 

generally requires disturbance to both interior as well as exterior finishes. When combined with a 

more extensive energy retrofit that includes the addition of exterior insulation, it provides the 

most freedom for designing and placing the windows. The windows can be installed in plane 

with the framing or be pushed toward the exterior to line up more traditionally with the plane of 

the siding.

Key points:

1. Affects the interior and exterior appearance of the building. Typically not an acceptable approach for 

historic zoned projects.

2. The measure is not reversible.

3. This work should be done in conjunction with a partial rehabilitation (window frame only) as set out in 

Measure 1—Window Rehabilitation.

4. This work will improve the energy performance of the assembly by reducing air infiltration, as well as 

thermal conductance through the assembly. The replacement windows can be high performance 

units (e.g., U<0.25, such as triple glazed with high performance pultruded fiberglass frames), allowing 

for significant improvements in thermal performance.

5. This work will reduce the potential for interior condensation problems on the window system.

6. Existing sash weight and balance system will be abandoned, allowing for an air sealing and insulation 

retrofit of the weight pockets and voids around the window frame.

7. Windows must be custom ordered to size; however, they do not require as tight an installation 

tolerance as the replacement sash option.

8. Vision area is reduced.

9. Expensive option.
29

Figure 34. New window rough opening framed with and extension box to allow for the addition of 

exterior insulation

Key points:

1. Affects the interior and exterior appearance of the building. Typically this is not an 

acceptable approach for historic zoned projects.

2. The measure is not reversible.

3. This work requires complete removal of the window sashes and frame and a 

reconstruction of the window rough opening.

4. This work will improve the energy performance of the assembly by reducing air infiltration, 

as well as thermal conductance through the assembly. The replacement windows can be 

triple glazed with high performance pultruded fiberglass frames allowing for significant 

improvements in thermal performance.

5. This work will reduce the potential for interior condensation problems on the window 

system.

6. Existing sash weights are removed as part of the frame removal, allowing for the thermal 

and air bypass in these locations to be addressed.

7. Windows sizes can be adjusted as necessary/desired.

8. Freedom for window placement in the plane of the wall: if the wall is being made thicker in 

a deep energy retrofit (addition of rigid foam insulation), the window can be detailed at the 

plane of the original wall (“Innie” window) or at the face of the foam (“Outie” window).

9. Expensive option.
30

4 Measure Implementation

This section steps through eight options for improving wood window systems (see Appendix A 

for details). The order of the measures is generally from least improvement to the most. The 

intent is not to determine specific incremental performance improvement; however, the focus is 

on the detailing the implementation of the strategies.

4.1 Measure 1—Window Rehabilitation

The amount of work required to rehabilitate the windows will depend on their starting condition.

The windows should be assessed as outlined in earlier sections.

For windows that are generally good condition (square, with sashes that properly fit to the 

window frame, and no broken or missing lites), the following work would be recommended 

(covered in detail by Davis 2007):

1. Remove the sashes by removing the interior stops and parting bead of the window frame.

2. Clean the frames and sashes of any flaking paint or other coatings that may impede the 

proper installation of gaskets and seals.

3. Caulk and seal the corners and joints in the window frame. This includes all joints 

between the sill and jambs as well as between the casings and frames.

4. Cut grooves into the sashes where new gaskets will be installed.

5. Prime and paint the window frames and sashes

6. Install new gaskets around the perimeter of the sashes. V-groove type gaskets will likely 

work the best at the jambs and meeting rails, while bubble gaskets work well at the head 

and sill interface.

7. Reinstall the sashes, meeting rails, and interior stops.
31

Clean frame and seal in the following locations 

indicated.

Figure 35. Recommended sealant location as part of window frame rehabilitation

As part of the work, if the weight pockets are to be retained, clean and lubricate the pulleys, 

replace the sash cords or chains, and balance the weights as part of the work.

The weight and balance system could also be abandoned and replaced with a spring-loaded tape 

balance. The weight pockets can then be insulated and sealed, improving the overall thermal 

performance of the window frame-to-rough opening interface.

Windows requiring more extensive rehabilitation such as reglazing, replacement of rotten wood, 

or rebuilding of sashes are outside the scope of this guideline. Work of this nature should be 

completed by a qualified restoration contractor. Alternatively, other retrofit measures, including 

sash replacement, window insert replacement, or complete window replacement should be 

considered for severely deteriorated windows.

4.2 Measure 2—Exterior Storm Windows

Adding exterior storm windows to the exterior of the window is a common, commercially 

available, off-the-shelf technology, and is for the most part straightforward. This work should be 

completed in conjunction with the window rehabilitation work outlined in Section 4.1. The 

window sill must be in good condition, as it will still be exposed to the elements (though 

exposure will be significantly reduced).

The exterior storm window is fastened to the outer window casing, or in some cases directly to 

the window trim. The storm windows should be sealed with an exterior grade paintable sealant at 

the jambs and heads, but left unsealed at the sill. Most storm windows have an adjustable bottom 

leg to account for variations in the sill height. This leg should not be caulked, to allow for 
32

drainage at the bottom. Some systems even provide small weep holes or notches in the bottom 

leg to promote drainage. Systems with enhanced drainage are recommended.

Figure 36. Exterior aluminum storm windows, showing weep channel

To minimize the potential for interstitial condensation, the original window must be made as 

airtight as possible (Wilson 1960). Slight ventilation of the exterior storm is typically provided 

by the weep holes provided at the sill. If the interior window is made sufficiently airtight, then 

the slight ventilation of the weep holes of the storm should provide adequate air change to 

prevent condensation. If condensation does form, slightly increasing the gap to allow for 

additional ventilation of the space is recommended.

Inner window must be made as airtight as possible.

Thermal gradient created by the air space between the 

interior window and the exterior storm will result in

warmer surfaces of the original wood window, but colder 

surfaces on interior side of the exterior storm. 

Uncontrolled air leakage into this space increases the 

risk of condensation on the interior side of the exterior 

storm.

Slight increase in ventilation at the sill may remedy the 

problem; however it will also diminish the overall thermal 

performance of the measure.

Figure 37. Condensation potential for exterior storm retrofits
33

4.3 Measure 3—Removable Interior Storm Windows

Interior storm windows are added solely to improve the thermal performance of the window 

system. They do not upgrade the water management of the assembly.

The installation relies on the original window to primarily manage the water. Thus, the original 

window should be rehabilitated to enhance its water management performance.

An interior storm window creates the potential for condensation on the interior surface of the 

original exterior window, so the interface between the interior storm and the window frame must 

be as airtight as possible. Also, the placement of the units is such that other air leakage paths 

(such as through the pulleys for the sash weights) could bypass the storm window, leading to 

interior air infiltration into this space. These bypasses can lead to problems even if the interface 

between the storm window and the frame is perfectly airtight. Condensation potential can be 

reduced by increasing the ventilation of the space to the outside. This needs to be done carefully, 

as increased ventilation to the exterior will result in diminished energy performance of the 

window, and could render the strategy ineffective. The original window should be rehabilitated 

(per Measure 1) along with this strategy. If condensation problems develop, the exterior window 

can be made incrementally slightly leakier until the problem is resolved. This could be done by

slightly gapping the lower sash at the sill with a shim, or removing the lower sash sill gasket (if 

there is one), although this will have a detrimental effect on airtightness in the summer when the 

interior storm is removed.

Interior storm window must be made as tight as 

possible. Air bypass (such as at weight pocket pulleys) 

must be sealed.

Thermal gradient created by the air space between the 

interior storm window and the exterior original window 

will result in colder surfaces of the original wood window. 

Uncontrolled air leakage into this space increases the 

risk of condensation formation on the interior side of the 

exterior storm.

Slight increase in ventilation at the sill may remedy the 

problem; however, it will also affect the overall thermal 

performance of the measure.

Figure 38. Condensation potential for interior storm retrofits

4.4 Measure 4—Permanent Interior Storm Windows

Interior storm windows are added solely to improve the thermal performance of the window 

system. They do not upgrade the water management of the assembly.

The installation relies on the original window to primarily manage the water. Thus, the original 

window should be rehabilitated to enhance its water management performance.
34

An interior storm window creates the potential for condensation on the interior surface of the 

original exterior window, so the interface between the interior storm and the window frame must 

be as airtight as possible. Also, the placement of the units is such that other air leakage paths 

(such as through the pulleys for the sash weights) could bypass the storm window, leading to 

interior air infiltration into this space. These bypasses can lead to problems even if the interface 

between the storm window and the frame is perfectly airtight. Condensation potential can be 

reduced by increasing the ventilation of the space to the outside. This needs to be done carefully, 

as increased ventilation to the exterior will result in diminished energy performance of the 

window, and could render the strategy ineffective. The original window should be rehabilitated 

(per Measure 1) along with this strategy. If condensation problems develop, the exterior window 

can be made incrementally slightly leakier until the problem is resolved. This could be done by 

slightly gapping the lower sash at the sill with a shim, or removing the lower sash sill gasket (if 

there is one).

Figure 39. Example sill detail for a permanent interior storm window

4.5 Measure 5—Window Sash Retrofit

This work must most likely be completed by a specialized window restoration contractor who 

can disassemble the window frames, route the frame elements to create a larger glazing pocket to 

accommodate the thicker IGU, and reconstruct and reglaze the frames.

A key element to ensure if this approach is taken is that the IGU must be installed on setting 

blocks, and the new glazing pocket must be weeped and drained to the exterior. IGU seal failure 

(identified by condensation or “fogging” of the space between the two layers of glass) is 

commonly a result of the IGU seals being in direct contact with water for extended periods.
35

Figure 40. Integration of an IGU into a wood sash

This measure must also incorporate improvements to the airtightness of the sashes in the 

opening, as it will address the conductance only through the glazing and not through the 

surrounding window frame. This is typically accomplished by retrofit sash liners in 

commercially available products gaskets. If this method is not used, methods outlined in 

Measure 1 should be implemented. 

4.6 Measure 6—Window Sash Replacement

In this measure, the window frames are typically retrofitted with new jamb liners that provide the 

tracks for the sashes to ride in, and eliminate the need for sash weights. Before the jamb liners

are installed, the frame should be cleaned and the corner interfaces between the head, jambs and 

sill caulked. The jambs should be coated with a liquid applied waterproof membrane, or at 

minimum, a high-quality paint (such as an elastomeric paint). The membrane should extend 

down onto the sill. Where exposed it can be painted to match the rest of the sill and trim color.

These measures are water control improvements, which increase the water resistance of the 

sloped wood sill, and allow any incidental leakage to drain to the exterior.

The space between the jamb liners and window frame should be sealed at the jambs and head on 

both the interior and exterior for air barrier continuity. Where the jamb liners interface with the 

sill, they should be left unsealed to allow the space to weep out at the bottom.
36

Paint window jamb with liquid applied 

waterproof membrane

Caulk sides of jamb liner to outer casing and 

interior stop

Extend liquid applied water proof membrane 

down onto window sill (jamb liner not shown 

for clarity). Do not caulk bottom edge of 

jamb liner to sill.

Figure 41. Frame preparation for replacement window sashes

Figure 42. Example jamb detail for a replacement window sash

4.7 Measure 7—Insert Replacement Window

In this measure, the wood window frame should be considered to be the new window rough 

opening and treated in a similar manner following current industry recommended practice for 

waterproofing and draining. It should be cleaned of any dirt and loose paint. The parting beads 
37

and interior stops at the head and jambs should be removed. The weight pocket pulleys should be 

removed and the openings left by the pulleys in-filled. All corners between head, jambs, sill, and 

exterior casings should be sealed with caulking. This is recommended for water management and

airtightness. The rough opening should then be coated on all sides with a liquid applied 

waterproof membrane. A membrane product is recommended in this application, because the 

area will no longer be accessible for maintenance once the replacement window is installed.

Window sashes, parting beads, and interior 

stops removed. All gaps and holes in jambs 

patched and filled.

Liquid applied membrane or self-adhered 

membrane waterproofing installed aound rough 

opening.

Use stool as support for the membrane back 

dam.

Figure 43. Frame preparation for insert replacement window 
38

Figure 44. Examples of membrane installation in a wood window frame

The replacement window is typically installed from the interior: it is normally set on the sill, 

tilted up into the rough opening, and pushed up against the exterior casing of the window. An 

interior backer rod and sealant joint should be installed around the interior perimeter of the rough 

opening. New interior stops are then installed (or the originals could be reinstalled if usable). The 

joint between the window and stops is caulked for aesthetics.

On the exterior the window should be caulked to the casing on the head and jambs. The sill,

however, should be left to drain to the exterior.

Figure 45. Example sill detail of insert replacement window
39

4.8 Measure 8—Complete Window Replacement

As discussed earlier, the highest performance but most intrusive and expensive option is a 

complete window replacement. This measure includes removing the entire window frame (back 

to the rough framing). This often includes the interior and exterior casing and trim.

A complete window replacement would most commonly be done in conjunction with a more 

extensive renovation project. Other work such as cladding replacement, insulation upgrades, or 

other work would most likely be completed at the same time.

4.8.1 Window Replacement as Sole Scope of Work

If this work is being done as the sole scope of work (no other enclosure retrofit work being done 

in conjunction with the window replacement), a termination location for the work must be 

chosen. The details provided here are designed so that a future cladding replacement or exterior 

insulation upgrade can be integrated with the window replacement with only minimal disruption 

to the window installation details.

Often the rough opening size is not the desired final dimension and additional blocking is added 

to frame it out to the correct dimension. The new rough opening is lined on all sides with a liquid 

applied or self adhered waterproof membrane. The membrane should extend out a minimum of 4

in. onto the plane of the wall. The membrane will ultimately be covered with exterior window 

trim. Should the wall be upgraded later, only the window trim would need to be removed to 

allow for a tie-in between a new water resistive barrier (WRB) and potentially an air barrier.

Typically, some siding will need to be removed to accommodate the window. The siding should 

be trimmed back with care so that damage to underlying elements such as building paper can be 

avoided if possible. Connecting the window membrane flashings to a building paper or some 

other WRB is recommended. The wall may not have a WRB to connect to. In this case, sealing 

the membrane flashing directly to the wood sheathing, and providing a metal flashing below the 

sill to shed water back out over the cladding is recommended; however, the homeowner must 

accept some risk, because the remainder of the wall may not conform to recommended enclosure 

water management design. In these situations, examining a full cladding replacement at the same 

time may be warranted to retrofit the house with a proper WRB.
40

Remove sashes and window frame back to wall 

framing

New blocking as needed

Cut back siding as required to account for new 

replacement window dimensions

Wrap rough opening with liquid applied or selfadhered membrane flashing

Lap bottom edge of membrane pan flashing over 

sheet metal flashing to shed water back out over 

the siding

Figure 46. Replacement window rough opening preparation

Figure 47. Example sill detail for window replacement without siding replacement
41

4.8.2 Window Replacement in Conjunction With a Siding Replacement

If the siding is replaced and a new wall WRB is installed at the same time as the windows, the 

reinstallation details would be the same as current recommended new construction practice. The 

details would change slightly in that a complete wrapping of the rough opening with a membrane 

would no longer be needed. Instead, the wall WRB could be wrapped into the rough opening on 

the sill and jambs, a pan flashing installed at the sill. After the window is installed, the jambs and 

head need to be sealed with a self-adhered membrane flashing and the WRB would need to be 

shingle lapped over top of the membrane head flashings.

Figure 48. Example sill detail of replacement window in conjunction with siding replacement

4.8.3 Window Replacement in Conjunction With an Exterior Insulation Retrofit

Window replacement in conjunction with an exterior insulation retrofit requires some additional 

consideration for the window installation details. The window can be placed either inboard of the 

insulation layer (“innie” window) or at the front face of the insulation (“outie” window). The 

choice of the window placement is often governed by the location of the WRB (or the placement 

of the WRB may be governed by the desired window placement location).

“Innie” windows are most easily integrated with a WRB that is placed behind the insulation at 

the plane of the exterior wall sheathing. By contrast, “outie” windows are most easily integrated 

with a WRB that is placed at the face of the exterior insulation. In either case, the integration of 

the window flashing with the WRB remains the same.
42

Figure 49. Example sill detail of "innie" window

Figure 50. Example sill detail of "outie" window
43

References

Brown, W.C. (1997). NRC-IRC (Institute for Research in Construction) Construction

Technology Update No. 5. “Window Condensation in Historic Buildings That Have Been 

Adapted for New Uses.”

[EIA] 2009 Energy Information Administration – EIA Residential Energy Consumption Survey 

2009.

Highly Insulating or Low-E Storm Windows Energy Savings Estimator (2011), Retrieved 

August 8, 2011, from 

http://www.windowsvolumepurchase.org/docs/Window_Savings_Estimator_Manual.pdf

Davis, J.M. (December 2007) “New Life for Old Double Hung Windows.” Fine Homebuilding 

Magazine. Newtown, CT: Taunton Press.

NFRC (2010a). NFRC 100A-2010 Procedure for Determining Fenestration Attachment Product 

U-factors, Greenbelt, MD: National Fenestration Rating Council.

NFRC (2010b). NFRC 200A-2010 Procedure for Determining Fenestration Attachment Product 

Solar Heat Gain Coefficient and Visible Transmittance at Normal Incidence, Greenbelt, MD: 

National Fenestration Rating Council.

Wilson, A.G. (1960). NRC-IRC (Institute for Research in Construction) Canadian Building 

Digest CBD 5. “Condensation Between Panes of Double Windows.” Ottawa, ON: National 

Research Council of Canada.
44

Appendix A: Window Repair, Rehabilitation, and Replacement 

Details
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79

Appendix B: BEOPT Analysis Graphs

Dallas, Texas

Utility Rates: $0.13/kWh

$1.09/therm

Figure 51. Annualized energy related costs versus average source energy savings for Dallas 

Figure 52. Average source energy savings reduction for Dallas 
80

Kansas City, Missouri

Utility Rates: $0.08/kWh

$1.23/therm

Figure 53. Annualized energy related costs versus average source energy savings for Kansas City 

Figure 54. Average source energy savings reduction for Kansas City 
81

Boston, Massachusetts

Utility Rates: $0.18/kWh

$1.70/therm

Figure 55. Annualized energy related costs versus average source energy savings for Boston

Figure 56. Average source energy savings reduction for Boston
82

Duluth, Minnesota

Utility Rates: $0.10/kWh

$0.87/therm

Figure 57. Annualized energy related costs versus average source energy savings for Duluth

Figure 58. Average source energy savings reduction for Duluth
DOE/GO-102012-3633 ▪ December 2012

Printed with a renewable-source ink on paper containing at 

least 50% wastepaper, including 10% post-consumer waste.