Reducing Thermal Losses and 

Gains With Buried and 

Encapsulated Ducts in 

Hot-Humid Climates

C. Shapiro, A. Magee, and W. Zoeller

Consortium for Advanced Residential Buildings

February 2013 
NOTICE

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iii

Reducing Thermal Losses and Gains with Buried and

Encapsulated Ducts in Hot-Humid Climates

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. Shapiro, A. Magee, and W. Zoeller

Steven Winter Associates, Inc.

of the

Consortium for Advanced Residential Buildings (CARB)

61 Washington Street

Norwalk, CT 06854

NREL Technical Monitor: Cheryn Metzger

Prepared under Subcontract No. KNDJ-0-40342-00

February 2013
iv

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v

Contents

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

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

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

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

Acknowledgments .................................................................................................................................... xii

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

1.1 Ducts in Conditioned Space: Current Practice.....................................................................2

1.2 Buried Ducts: Past Research and Current Practice ..............................................................3

1.3 Reducing Duct Losses in Existing Homes in Hot-Humid Climates....................................5

2 Previous Research ............................................................................................................................... 7

3 Materials and Methods....................................................................................................................... 10

3.1 Monitoring Methodology...................................................................................................10

3.2 Retrofitting Methodologies................................................................................................12

3.2.1 Buried and Encapsulated Ducts.............................................................................12

3.2.2 Encapsulated Ducts................................................................................................14

3.2.3 House 1 Retrofit.....................................................................................................15

3.2.4 House 2 Retrofit.....................................................................................................16

3.2.5 House 3 Retrofit.....................................................................................................19

4 Retrofit Methodology Evaluation ...................................................................................................... 20

4.1 Lessons Learned.................................................................................................................20

4.2 Quality Control ..................................................................................................................23

4.3 Retrofit Evaluation Summary ............................................................................................26

5 Field Monitoring and Performance Testing Results ....................................................................... 27

5.1 Building Specifications......................................................................................................27

5.2 Qualitative Discussion of Thermal Performance...............................................................31

5.3 Qualitative Discussion of Condensation Potential.............................................................36

6 Theoretical Effective and Apparent R-Values of Round Ductwork by Insulation Strategy ........ 41

6.1 Traditionally Insulated Round Ductwork ..........................................................................42

6.2 Encapsulated Ducts............................................................................................................45

6.3 Buried Ducts ......................................................................................................................48

6.4 Buried and Encapsulated Ducts.........................................................................................51

7 Effective Heat Transfer Coefficients of Test Home Ductwork and R-Value Validation............... 56

8 ASHRAE 152 Distribution System Efficiencies ............................................................................... 59

9 Condensation Potential of Encapsulated and Buried Ducts ......................................................... 61

10 Predicted Energy and Cost Savings................................................................................................. 63

11 Conclusion .......................................................................................................................................... 68

11.1 Retrofit Methodology and Lessons Learned..........................................................68

11.2 Field Evaluation.....................................................................................................68

11.3 Effective and Apparent R-Values..........................................................................69

11.4 Effective Heat Transfer Coefficients.....................................................................69

11.5 ASHRAE Standard 152 Distribution System Efficiency.......................................69

11.6 Condensation Potential of Buried and Encapsulated Ducts...................................70

11.7 Predicted Energy and Cost Savings.......................................................................70

11.8 Summary................................................................................................................70

References................................................................................................................................................. 72

Appendix A: Attic Dew Point Temperature............................................................................................. 75
vi

List of Figures

Figure 1. Traditional ductwork hung from attic rafters .......................................................................... ix

Figure 2. Ducts being encapsulated with ccSPF .................................................................................... ix

Figure 3. Detail of buried ducts ................................................................................................................. 3

Figure 4. Detail of buried and encapsulated duct.................................................................................... 4

Figure 5. Categorization of duct insulation levels by burial class......................................................... 7

Figure 6. Heat transfer between duct interior, conditioned space, and attic........................................ 8

Figure 7. Existing ductwork at House 1.................................................................................................. 16

Figure 8. Supply ductwork for House 2 .................................................................................................. 17

Figure 9. Return ductwork for House 2................................................................................................... 17

Figure 10. Supply ductwork jump over abandoned return ductwork.................................................. 17

Figure 11. Duct reconfiguration at House 2 ........................................................................................... 18

Figure 12. Open duct take-off at House 2............................................................................................... 18

Figure 13. Finished encapsulated duct hung from roof deck .............................................................. 19

Figure 14. Baffling used to ensure proper burial of supply duct plenum (post-encapsulation and 

pre-burial)............................................................................................................................................ 22

Figure 15. Baffling used to prevent ccSPF application on flue duct................................................... 22

Figure 16. Electrical, telephone, and other wiring permanently spray foamed to the ceiling and 

ductwork during retrofit..................................................................................................................... 23

Figure 17. Improperly spray-foamed plenum box at House 1 .............................................................. 24

Figure 18. Exposed duct jacket at the underside of duct ..................................................................... 25

Figure 19. Exposed underside of ductwork not adequately covered with ccSPF.............................. 25

Figure 20. Ductwork well-sealed to sheetrock with ccSPF................................................................... 25

Figure 21. Rigid insulation inserted under ductwork to serve as a substrate and provide insulating 

value..................................................................................................................................................... 25

Figure 22. Varying thickness of ccSPF and interference from cross bracing.................................... 25

Figure 23. Varying application thicknesses shown on rectangular (left) and round (right) ducts... 25

Figure 24. AHU at House 2 located in garage with leaky return and supply plenums ...................... 29

Figure 25. Pre- and post-retrofit Manual J building cooling loads compared to installed capacity 30

Figure 26. Pre- and post-retrofit Manual J building heating loads compared to installed capacity 30

Figure 29. Pre-retrofit surface and air temperatures at duct run 2 in House 1................................... 32

Figure 30. Post-retrofit surface and air temperatures at duct run 2 in House 1................................. 32

Figure 31. Pre-retrofit surface and air temperatures at duct run 3 in House 2................................... 33

Figure 32. Post-retrofit surface and air temperatures at duct run 3 in House 2................................. 33

Figure 33. Pre-retrofit surface and air temperatures at duct run 2 in House 3................................... 34

Figure 34. Post-retrofit surface and air temperatures at duct run 2 in House 3................................. 34

Figure 27. Pre-retrofit infrared thermal imaging at House 2................................................................. 35

Figure 28. Infrared thermal imaging of encapsulated ductwork at House 2....................................... 35

Figure 35. Pre-retrofit boot surface temperatures and attic dew point at House 1............................ 38

Figure 36. Post-retrofit ccSPF surface temperatures and attic dew point at House 1....................... 38

Figure 37. Pre-retrofit boot surface temperatures and attic dew point at House 2............................ 39

Figure 38. Post-retrofit ccSPF surface temperatures and attic dew point at House 2....................... 39

Figure 39. Pre-retrofit boot surface temperatures and attic dew point at House 3............................ 40

Figure 40. Post-retrofit ccSPF surface temperatures and attic dew point at House 3....................... 40

Figure 41. Flexible ductwork encapsulated in ccSPF ........................................................................... 45

Figure 42. Diagram of buried duct modeling configuration ................................................................. 48

Figure 43. Diagram of buried and encapsulated ducts......................................................................... 51

Figure 44. Heat flux magnitude through encapsulated and fully buried 8-in. diameter duct............ 51

Figure 45. Definition of mounded-buried ducts..................................................................................... 53

Figure 46. Effective R-values of R-4.2 buried and encapsulated ducts .............................................. 54

Figure 47. Apparent R-values of 6-in. buried ducts encapsulated in ccSPF by attic temperature... 54
vii

Figure 48. R-4.2 duct (4 in.) deeply buried in fiberglass insulation. Isotherms from the steady state 

model (in black) are compared to the dew point of the air at several key locations (in red). .... 62

Figure 49. R-4.2 duct (4 in.) deeply buried in fiberglass insulation and encapsulated in 1.5 in. of 

ccSPF. Isotherms from the steady-state model (in black) are compared to the dew point of the 

air at several key locations (in red). ................................................................................................. 62

Figure 50. Predicted versus actual pre-retrofit energy use.................................................................. 64

Figure 51. Ambient conditions in Jacksonville, Florida, on July 29, 2010.......................................... 75

Figure 52. Pre-retrofit ambient and attic conditions at House 1 .......................................................... 76

Figure 53. Ambient and attic conditions at U.S. Department of Housing and Urban Development 

Study House 4..................................................................................................................................... 76

Unless otherwise noted, all figures were created by CARB.

List of Tables

Table 1. Percentage of Households With Central, Ducted, Forced-Air Space-Conditioning Systems 

by Climate.............................................................................................................................................. 5

Table 2. Effective R-values of Buried Ducts by Insulation Level and Type (h-ft2-°F/Btu).................... 8

Table 3. Equipment Used for Performance Testing and Long-Term Monitoring ............................... 11

Table 4. Building Enclosure and Space-Conditioning Specifications of Monitored Houses............ 28

Table 5. Pre- and Post-Retrofit Duct, Building, and System Specifications....................................... 29

Table 6. Weather Conditions for Days Graphed in This Section.......................................................... 31

Table 7. Inner Surface Film Resistances by Duct Inner Diameter........................................................ 43

Table 8. Effective R-Values of Round Insulated Flexible Ducts........................................................... 43

Table 9. Effective R-Values of ccSPF Encapsulated Round Flexible Ducts by Insulation 

Thickness ............................................................................................................................................ 48

Table 10. Material Conductivities and Thicknesses .............................................................................. 49

Table 11. Boundary Conditions ............................................................................................................... 49

Table 12. Effective R-Values of Buried Round Ducts............................................................................ 50

Table 13. Apparent R-Values of Buried Round Ducts ........................................................................... 50

Table 14. Effective R-Values of Buried and Encapsulated Round Ducts............................................ 52

Table 15. Apparent R-Values of Buried and Encapsulated Round Ducts........................................... 52

Table 16. R-Values of Mounded-Buried R-4.2 Ducts Encapsulated in 1.5 in. of ccSPF..................... 53

Table 17. Summary of Duct Effective R-Values for 8-in. Duct by Insulation Strategy ....................... 55

Table 18. Theoretical UA Values for Monitored Jacksonville Houses................................................. 56

Table 19. Relative Apparent UAs as Measured in Test Houses ........................................................... 58

Table 20. Pre- and Post-Retrofit ASHRAE 152 DE and DSEs for House 1 .......................................... 60

Table 21. Pre- and Post-Retrofit ASHRAE 152 DE and DSEs for House 2 .......................................... 60

Table 22. Pre- and Post-Retrofit ASHRAE 152 DE and DSEs for House 3 .......................................... 60

Table 23. Regression Statistics for Pre-Retrofit Utility Bills................................................................. 63

Table 24. Regression Statistics for Pre-Retrofit Energy Predictions (BEoptE+ 1.2).......................... 64

Table 25. Predicted Energy Savings From BEoptE+ 1.2....................................................................... 65

Table 26. Comparison of Predicted Savings Between BEopt and ASHRAE 152 

(Duct Retrofit Only) ............................................................................................................................ 65

Table 27. Installed Cost of Encapsulated and Buried and Encapsulated Ducts at Houses.............. 65

Table 28. Cost Estimates for Duct Retrofits........................................................................................... 67

Table 29. Annualized Savings by Retrofit Measure and House ($)...................................................... 67

Unless otherwise noted, all tables were created by CARB.
viii

Definitions

ACCA Air Conditioning Contractors of America

ACEEE American Council for an Energy-Efficient Economy

ACH50 Air changes per hour at 50 Pascal

AHU Air-handling unit

Apparent R-value R-value of ductwork including heat transfer between the duct 

and conditioned space

ASHRAE American Society of Heating, Refrigerating and AirConditioning Engineers

CARB Consortium for Advanced Residential Buildings

ccSPF Closed-cell polyurethane spray foam

CDD75 Cooling degree days at base 75°F

CEC California Energy Commission

cfm Cubic foot per minute

DE Delivery effectiveness

DOE U.S. Department of Energy

dof Degrees of freedom

DSE Distribution system efficiency

Effective R-value R-value of ductwork excluding heat transfer between the duct 

and conditioned space

EIA Energy Information Administration

HDD65 Heating degree days at base 65°F

ICC International Code Council

kW Kilowatt

kWh Kilowatt-hour

LBNL Lawrence Berkeley National Laboratory

LMTD Log mean temperature difference

NCDC National Climactic Data Center

NREL National Renewable Energy Laboratory

Pa Pascal
ix

Executive Summary

Thermal losses and gains from ductwork installed in unconditioned spaces—such as attics, crawl

spaces, and garages—can contribute significantly to the overall heating and cooling loads of 

single-family residential buildings, with estimated duct losses ranging from 10%–45 %.

1 The 

impact is particularly severe in hot climates where air conditioning dominates and extensive duct

systems are typically located in hot attics. The duct system inefficiencies are greatest at peak 

demand conditions when the attic is hottest and run times are the highest. The thermal losses and 

gains at the three single-story houses in Florida monitored in this study accounted for 15%–35% 

of the annual pre-retrofit heating and cooling energy use and 6%–25% of the total pre-retrofit 

annual utility bill.

Even though a number of existing methods can effectively move ducts into the conditioned 

space, they are not currently being widely implemented. Physically locating the ducts within the 

conditioned space is most often applicable only to new construction projects and requires 

planning during design. Attics can be insulated at the roof deck with closed-cell polyurethane 

foam (ccSPF) to reduce duct losses and infiltration rates, but this method has high up-front costs 

and the potential to increase loads on the building enclosure. In hot-dry climates, attic duct 

systems can be buried under loose-fill insulation at a significantly lower cost to reduce thermal 

loads. In hot-humid climates, however, condensation on the duct jacket can occur when the 

humidity in the attic is high.

Figure 1. Traditional ductwork hung from 

attic rafters

Figure 2. Ducts being encapsulated with ccSPF

There are almost 3.9 million houses in Florida alone (U.S. Department of Commerce 2003), most 

with ducts located in vented attics (Figure 1). A feasible and cost-effective retrofit strategy is 

needed to reduce duct losses in existing homes in hot-humid climates. Through past research, the 

Consortium for Advanced Residential Buildings has demonstrated the benefits of buried ducts 

and buried and encapsulated ducts in new construction applications. The buried and encapsulated

duct strategy, which utilizes ccSPF to address condensation concerns (Figure 2), was developed 

specifically for hot-humid climates.

Buried and encapsulated ducts have the potential to substantially improve the thermal 

performance of existing ductwork and reduce duct air leakage rates. A second variation of the 

 1 The range cited here is based on an extensive literature review described further in Section 1.
x

concept, just encapsulating the ducts without burying them, evolved during this research in 

response to site constraints. Encapsulating the ducts without burying them beneath loose-fill

insulation results in a lower duct R-value, but the trade-offs are lower up-front costs and 

additional advantages in coordination and installation.

To evaluate the buried duct concept in a retrofit scenario for a hot-humid climate, encapsulated 

and buried and encapsulated ducts were installed and their performance was monitored in three 

Jacksonville homes. Ease of installation was also examined as a key factor in this study. 

Encapsulating ductwork in ccSPF increased the thermal resistance of the existing ductwork and 

decreased duct leakage rates. Burying these encapsulated ducts beneath loose-fill insulation 

further increased the R-value of the encapsulated ductwork. 

A multifaceted approach—field monitoring, computer modeling and simulation, and numerical 

analysis—was used to develop a comprehensive view of encapsulated and buried and 

encapsulated ducts. The primary goals were to quantify the associated energy savings, cost

effectiveness, and condensation potential of this retrofit measure. To calculate energy savings 

and condensation potential, the researchers performed computer modeling and analysis. Data 

from field testing were used to validate these conclusions and determine cost effectiveness. 

This report begins with a comprehensive overview of the buried duct research that has been 

conducted to date, including the analysis approaches used to support previous research

(Sections 1 and 2). In Section 3, detailed information is presented on the retrofit methodology 

used to install and test the three existing duct systems, including short- and long-term data 

collection. Section 4 evaluates the retrofit methodology. 

After the methods used to retrofit the systems are explained, Section 5 summarizes the results of 

the field evaluation. The performance testing results are given, and the thermal performance and 

condensation potential of buried ducts are discussed qualitatively using the data collected during 

the monitoring period. Changes in the mechanical and distribution system configuration and 

operation at each house created challenges for interpreting long-term field monitoring and 

performance testing data. Despite these challenges, the data offered interesting insights into duct 

system performance and served to validate the system performance calculations. 

Building on the qualitative findings from the field demonstration, a combination of analytical 

calculations and modeled analysis was used to determine the potential energy savings and cost 

effectiveness of the retrofit strategy. Section 6 outlines the analytical methodology used to 

develop the effective and apparent R-values for each strategy of duct insulation. Due to added 

system complexity, a finite-element heat transfer analysis was required to calculate these values 

for the buried duct strategy. The finite-element heat transfer model showed that encapsulating the 

ductwork in 1.5 in. of ccSPF (R-6.7 h-ft2-°F/Btu-in.) can improve the existing R-4.2 flexible 

ductwork to values between R-9 and R-13, depending on the size of the duct. Burying the 

encapsulated ductwork (to create buried and encapsulated ducts) will further increase the 

effective R-values to between R-16 and R-31.

Using the R-values calculated in Section 6, the effective and apparent heat transfer coefficients 

(UA values) for the duct systems of the three Jacksonville homes were calculated, and are 

presented in Section 7. This analysis was validated by comparing the results with test data for 
xi

each home. The reduction in apparent UA values, based on field data, correlated well with the 

theoretical UA value reduction as the analysis in Section 6 shows. The values were found to be 

within 10% of each other, which is a reasonable error given that the calculation assumes steadystate operation.

The heat transfer coefficients developed in Sections 6 and 7 were then used to determine the 

efficiency of the thermal distribution systems, based on the ASHRAE Standard 152-2004 

methodology (ASHRAE 2004). This is covered in Section 8. Both delivery effectiveness and 

distribution system efficiency were calculated. The calculations showed that duct leakage has a 

significant impact on delivery effectiveness, as a result of the direct relationship between duct 

leakage and duct losses. 

Duct leakage rates were substantially reduced through encapsulation with ccSPF. For the houses 

with an air-handling unit in the living space, duct leakage to the outdoors was reduced to rates

typical for houses with ducts in the living space. For the house with an air-handling unit in the 

garage, the duct leakage rates were dramatically reduced, but not to levels associated with ducts 

in the living space.

In Section 9, the condensation potential of buried and encapsulated ducts is explored further 

using modeling. For this analysis, the steady-state two-dimensional thermal model developed in 

Section 6 was combined with a one-dimensional, dynamic, hygrothermal model to predict the 

potential for condensation on the surface of the duct. This analysis was conducted for both buried 

and buried and encapsulated ducts using the worst case configurations. The results from the 

analysis predict condensation issues for the buried ducts without additional ccSPF insulation, 

which was observed by Griffiths et al. (2002). The analysis did not predict a condensation issue 

for the buried and encapsulated ducts as specified.

Finally, Section 10 presents information on the predicted energy savings associated with this 

retrofit and an assessment of cost effectiveness. Predicted energy savings were based on a 

calibrated Building Energy Optimization model. The modeled energy savings, which range from 

5%–20% of total energy use for the three houses, appear reasonable in comparison to the 

predicted cooling and heating energy savings derived from the ASHRAE 152 delivery system 

efficiencies (ASHRAE 2004). These savings show that nearly all of the thermal losses and gains 

from the pre-retrofit duct systems were mitigated through these strategies. A cost-effectiveness 

analysis determined that the buried and encapsulated duct retrofit achieved $10 in annualized 

savings. The encapsulated-only strategy yields $141 in annualized savings. The higher 

annualized savings of the encapsulated-only strategy is due, in part, to avoiding the material and 

labor requirements associated with duct reconfiguration and blown-in insulation.

Based on this research study, encapsulated and buried and encapsulated ducts were found to 

dramatically improve the distribution efficiency of existing ductwork. Pre- and post-retrofit 

distribution system efficiencies were calculated using ASHRAE Standard 152-2004. The best 

case scenario estimates a post-retrofit distribution system efficiency of 97%–98%. Potential 

energy savings of 8%–20% per year were predicted through simulation. The predicted total 

energy savings are consistent with mitigating the majority of the duct losses from the homes.

Energy savings associated with thermal losses and gains for these houses had a strong correlation 
xii

with duct leakage to the outdoors, resulting in a significant range of potential energy savings.

Encapsulated and buried and encapsulated ducts were found to be cost effective.

This research has been incorporated into the U.S. Department of Energy (DOE) Challenge Home 

National Program Requirements (DOE 2012). Although forced-air ducts are typically required to

be inside the home’s thermal and air barrier boundary, Section 10(c) of DOE (2012) allows an 

exception for buried and encapsulated ducts. Under this exception ductwork must be 

encapsulated with at least 1.5 in. of ccSPF and buried under 2 in. of blown-in insulation.

Acknowledgments

Steven Winter Associates, Inc., acknowledges the U.S. Department of Energy Building America 

Program for funding and supporting the development of this technical report and previous 

research that informed the report. A special thanks to BASF for financial and technical support. 

In addition, the authors thank First Choice Supply, Inc., Rix Mechanical, Inc., and the 

participating homeowners.
1

1 Introduction

Heat gains and losses from ductwork installed in unconditioned spaces—such as attics, crawl

spaces, and garages—can contribute significantly to the overall heating and cooling loads of 

single-family residential buildings. Unfortunately, installing ductwork in unconditioned spaces is 

a common construction practice throughout the United States. In the early 1990s, approximately 

35% of American single-family homes contained ductwork installed in unconditioned spaces 

(Modera 1993). Over the past two decades, the number of homes with ductwork has increased 

considerably. Nearly all new single-family houses have ducted distribution systems; during the 

late 1990s, the percentage was estimated at 96% (National Association of Home Builders 

Research Center 1999). As a result, the percentage of housing units containing central forced-air 

cooling and/or heating systems has increased significantly, from 66% in 1993 (Energy 

Information Administration [EIA] 1993) to 76% in 2009 (EIA 2009).2

Although most studies emphasize the degree of duct losses in unconditioned spaces, estimates of 

these losses differ greatly, ranging from 10% to 45% (EPA 2009; Siegel et al. 2003; Vineyard et 

al. 2003; Jump et al. 1996; Palmiter and Francisco 1994; Modera 1993; Andrews and Modera 

1992).3 Duct leakage rates to the outside, which commonly vary from as little as 3% to more 

than 20%, are compounded by large temperature differentials between the conditioned air inside 

the duct and the air in the unconditioned space. During the cooling season, 55°F conditioned 

supply air can be separated from 120°F ambient attic air by duct insulation with a rated thermal 

resistance as low as R-4.2 (h-ft2

-°F/Btu). During the heating season, this temperature differential 

can be even higher, with 110°F conditioned air passing through an attic with a 20°F ambient 

temperature.

The wide range of duct loss estimates cited here demonstrates that duct losses can be reduced 

substantially for very leaky and underinsulated systems by properly sealing and insulating 

ductwork. (See Aldrich and Puttagunta 2011 for proper techniques.) Even when properly sealed

and insulated, however, the duct losses of traditional duct systems commonly account for 

 2 The authors calculated these percentages using the Public Use Microdata Files furnished by the EIA for the 1993 

and 2009 Residential Energy Consumption Surveys.

3 ASHRAE 152-2004 (See 2004; Section 8) defines duct losses in various ways, depending on the use of the duct 

loss term. Delivery effectiveness (DE) includes both thermal losses and duct leakage losses. Thermal losses 

represent the heat transfer through the duct material and any insulation. Duct leakage occurs when the conditioned 

air escapes through gaps and cracks in the ductwork. The physical location of the ductwork, duct insulation level, 

and duct sealing all affect the magnitude of the DE. Distribution system efficiency (DSE) includes DE, but also 

accounts for system cycling, fan power, thermal regain, equipment capacity, and air infiltration. Each term is 

calculated for heating and cooling design and seasonal conditions. Design conditions represent the efficiency under 

peak loads; seasonal conditions represent the losses over the entire heating or cooling season. The cited studies 

calculate duct losses in various ways and do not necessarily correspond to the definitions given in ASHRAE 152-

2004. Because DE and DSE are typically similar under both design and seasonal conditions, the values given in 

these studies are directly compared to demonstrate the magnitude of the duct losses. In this report, the term duct loss 

is used generically for DE and DSE under design, seasonal, or other conditions. DSE and DE are used when they 

directly correspond to the conditions defined under ASHRAE 152-2004. Design and seasonal conditions are noted if 

applicable. 
2

approximately 10% of the heating and cooling loads.4 As a result, more aggressive strategies are 

needed to further reduce duct thermal losses. The primary methods, which are discussed in 

Sections 1.1 and 1.2, involve placing ducts within the thermal enclosure and burying ducts 

beneath loose-fill insulation. The encapsulated and buried and encapsulated duct strategies seek 

to achieve similar benefits. Although this report focuses on using these strategies as a retrofit 

methodology for existing homes, these methods can be employed with minimal changes in new 

construction.

1.1 Ducts in Conditioned Space: Current Practice

With proper planning and careful attention to detail, the thermal losses associated with ductwork

can be eliminated in new construction by placing ducts within the thermal enclosure. In new 

construction applications, ductwork can be placed within the thermal enclosure using one of four 

methods: (1) expanding the thermal enclosure to incorporate the unconditioned space (e.g.,

insulating attics at the roof deck and insulating basements at the basement walls); (2) installing 

ductwork in a soffit or dropped ceiling; (3) installing ductwork between floors; and (4) using a 

modified truss to create a plenum for ductwork in the attic (Roberts and Winkler 2010; Hendrick 

2003; Consortium for Advanced Residential Buildings [CARB] 2000).

New construction strategies for placing ducts in conditioned space cannot always be applied to 

existing homes. In retrofit applications, Methods 2 through 4 are typically impractical. Method 1, 

incorporating the unconditioned space into the thermal enclosure, is a more common retrofit 

strategy, but it can have significant disadvantages associated with lower energy savings, higher

costs, and increased moisture. For ducts placed in attics, which is the principal subject of this 

report, Method 1 results in insulating the attic at the roof deck and incorporating an unvented 

attic into the design. Insulating the attic at the roof deck increases the thermal enclosure surface 

area, which results in larger space-conditioning loads from the enclosure (Hendrick 2003). 

Although this penalty can be overcome by savings from ductwork thermal losses, the net energy 

savings (duct savings minus increased enclosure loads) might be less than those achieved with 

other methods of placing ducts within the thermal enclosure.

Furthermore, building codes require minimum levels of air-impermeable insulations 

(International Code Council [ICC] 2009), and insulation must be installed so that it does not 

become dislodged from the roof deck assembly. As a result, closed-cell spray polyurethane foam 

(ccSPF) insulation is typically used to insulate buildings at the roof deck. Achieving the 

equivalent R-value at the roof deck using ccSPF is significantly more expensive than installing 

loose-fill insulation along the ceiling plane. The larger surface area of the roof deck results in 

higher costs compared to ceiling insulation, and ccSPF is comparatively more expensive than 

loose-fill insulation. Insulating the building at the roof deck using ccSPF, however, does yield 

greater air sealing benefits than typical ceiling insulation methods.

Additionally, the moisture dynamics of insulation at the roof deck must be carefully addressed to 

prevent serious moisture-related problems. Minimum values of air-impermeable insulation are 

required by Table R806.5 of the International Residential Code for One- and Two-Family 

 4 This figure is an approximate number derived from the data presented in the literature (EPA 2009; Siegel et al. 

2003; Vineyard et al. 2003; Jump et al. 1996; Palmiter and Francisco 1994; Modera 1993; Andrews and Modera 

1992). These might apply to DE and DSE under both design and seasonal conditions generally as an approximate 

value.
3

Dwellings to prevent condensation on the inside surface of the insulation during the heating 

season (ICC 2009). Solar radiation can drive moisture from wetted asphalt shingles into the 

structural sheathing during the cooling season, causing shingle buckling and sheathing 

deterioration (Lstiburek 2006; Hendrick 2003).

1.2 Buried Ducts: Past Research and Current Practice

Given the many barriers associated with retrofitting the building to include existing ducts within 

the thermal enclosure and the resistance of some builders to carefully plan for ducts in 

conditioned space, other methods are needed to address ductwork thermal losses in new and 

existing homes. Burying ductwork beneath a layer of loose-fill insulation (Figure 3) has been 

proposed as a comparable alternative to ducts in conditioned space. This method has received 

significant attention under the Building America Program and has been shown to substantially 

reduce duct thermal losses (Griffiths and Zuluaga 2004; Griffiths et al. 2004; Vineyard et al. 

2004; Griffiths et al. 2002).

Figure 3. Detail of buried ducts

For new construction projects, the ductwork is installed as close to the ceiling plane as possible 

to maximize insulation coverage. The ducts are covered with insulation, which provides thermal 

resistance for the ductwork and boosts the R-value of the existing ceiling insulation. Although 

buried ducts do not eliminate duct losses entirely, the energy savings can be similar to those 

achieved by implementing Method 1 as discussed in Section 1.1, in which the attic is insulated at 

the roof deck. Some thermal losses will still be present with buried ducts, but these losses might 

be less than or equal to the increased thermal loads resulting from the increased enclosure area 

associated with Method 1.
4

The effective R-values of deeply buried ducts are between 25 and 31 (Griffiths et al. 2004). 

These values are greater than or equal to many attic insulation levels in existing homes. Although 

burying ducts deeply is uncommon and requires careful attention, the practice, when combined 

with proper duct sealing techniques, can reduce thermal losses substantially in comparison to 

traditional duct installations. Furthermore, in retrofit applications, the buried duct strategy can be 

implemented at a fraction of the cost associated with insulating the attic at the roof deck because 

the approach uses less expensive loose-fill insulation to cover the existing ductwork.

Buried ducts, however, are not well suited for hot-humid climates. After demonstrating buried 

duct benefits in hot-dry climates, Building America research was conducted to evaluate the

strategy in hot-humid climates. After burial, duct jacket surface temperatures were observed to 

be lower than the dew point of the attic air; this creates a potential for condensation on the 

surface of the duct (Griffiths and Zuluaga 2004; Griffiths et al. 2004; Griffiths et al. 2002). As a 

result, a modified methodology to reduce the condensation potential was sought. Ductwork at a 

test home in Atlanta was encapsulated in a 1-in. layer of ccSPF insulation before being buried

beneath loose-fill insulation. The buried and encapsulated duct strategy (Figure 4) implemented 

at this home successfully eliminated the condensation potential of buried ducts in a hot-humid 

climate (CARB 2003).

Figure 4. Detail of buried and encapsulated duct

The application of ccSPF, which has high R-values per inch and low vapor and air permeability, 

increases the thermal resistance of the duct insulation and reduces duct leakage by filling gaps 

and voids in the ductwork. Building on the buried duct concept, buried and encapsulated ducts 

start by encapsulating the ductwork before burying it beneath a layer of loose-fill mineral fiber 

insulation. In addition to the lower duct leakage rates expected from using ccSPF, installing 
5

additional insulation through the ccSPF and burying the ductwork beneath loose-fill insulation 

are expected to result in significantly higher R-values. Furthermore, the risk of condensation 

associated with buried ducts is expected to be eliminated by elevating the surface temperature of 

the outside vapor-impermeable layer of the duct above the dew point of the surrounding air.

1.3 Reducing Duct Losses in Existing Homes in Hot-Humid Climates

Recognizing that cost-effective and safe methods are needed to address thermal losses from 

ductwork in the current housing stock in hot-humid climates, it was clear that the feasibility of 

applying the buried and encapsulated duct strategy to existing homes warranted further research.

Furthermore, more research was needed to validate the findings by CARB (2003) in hot-humid 

climates and quantify the energy savings potential of this strategy. The vast majority of singlefamily homes in the hot-humid climates in the United States have central forced-air cooling 

systems (see Table 1). Since conventional HVAC design manuals dictate that space-conditioning 

air in cooling-dominated climates be discharged from ceiling or high wall registers (Air 

Conditioning Contractors of America [ACCA] 1992) and living space is at a premium, the vast 

majority of these homes have ductwork installed in attics.

Table 1. Percentage of Households With Central, Ducted, Forced-Air 

Space-Conditioning Systems by Climate

Climate Cooling

(%)

Heating

(%)

Heating and/or 

Cooling (%)

Very Cold/Cold 48.9 68.0 72.7

Hot-Dry/Mixed-Dry 60.6 67.1 73.0

Hot-Humid 82.5 79.4 86.1

Mixed-Humid 71.4 73.4 79.4

Marine 17.8 57.4 58.1

All Climates 61.3 70.9 76.3

Source: EIA (2009).

This report investigates the potential of two ductwork retrofit strategies employing ccSPF, buried 

and encapsulated ducts and encapsulated ducts alone. The encapsulated duct approach is a 

variation of the buried and encapsulated duct strategy that arose during the field retrofit because 

of feasibility limitations. Encapsulated ducts simply enhance the existing insulated ductwork by 

applying a layer of ccSPF. Requiring less modification and fewer contractors and materials, 

encapsulated ducts are expected to have higher R-values and lower duct leakage rates than

existing ductwork. This study sought to explore the trade-offs in energy savings, installation cost, 

and installation ease between the two approaches.

When properly installed, these two duct insulation strategies are both compliant with the 2009 

International Residential Code for One- and Two-Family Dwellings (ICC 2009). In general, the 

code allows spray-foam insulation to be applied to the exterior of ductwork (Section M1601.3) 

as long as the spray foam has a flame spread index no greater than 50 and a smoke developed 

index no greater than 450, is protected by an ignition barrier (Sections R316.5.3 and R316.5.4), 

and meets the general requirements for use in residential buildings (Section R316). As a result, 

buried and encapsulated ducts covered with at least 1.5 in. of mineral fiber insulation and 

installed in attics “entered only for purposes or repairs or maintenance” (meaning no storage or 

habitation allowed) meet these requirements (ICC 2009). Thermal barriers are not required for 
6

this type of attic, and 1.5 in. of fiberglass, which is considered mineral fiber insulation (ASTM 

2011), meets the minimum requirements for ignition barriers. 

Encapsulated ducts or buried and encapsulated ducts covered with less than 1.5 in. of fiberglass 

insulation can also be code compliant if the specific spray foam meets the broader requirements 

of Section M1601.3. Although not all ccSPF materials meet these requirements, the installed 

material has undergone testing to verify performance. Exposed applications of the spray foam 

used in this study are compliant with Section M1601.3, as described in the International Code 

Council Evaluation Service Report (International Code Council Evaluation Service 2012), and 

are therefore code compliant (International Code Council Evaluation Service 2010).

The primary goals of this study are to quantify the energy savings associated with encapsulated 

and buried and encapsulated ducts, along with their cost effectiveness and condensation 

potential. Although the field demonstration supplied valuable information about the installed 

performance of the distribution systems and important insights into the feasibility of this retrofit 

strategy, variations in field conditions necessitated supplemental data analysis. As with any field

testing of occupied homes, quantifying energy savings is difficult because of the dramatic 

changes in occupant behavior and building system operation. Despite these difficulties, field

evaluations are necessary to support other analytical methods with real-life data. In this report, 

computer modeling and analysis were used to calculate energy savings and condensation 

potential. Data from the field evaluation were used to validate these conclusions and determine

cost effectiveness.
7

2 Previous Research

Previous research on buried ducts has focused on determining effective duct R-values, 

investigating the potential for condensation, and estimating associated energy savings in new 

construction applications. Unlike traditional hung ducts, the thermal resistance of buried ducts 

cannot be calculated by knowing the geometry of the duct insulation and its conductivity. To 

make an appropriate comparison with traditionally insulated duct R-values, effective R-values of 

buried ducts must be determined. 

Using Algor FEA (Zoeller 2009), a two-dimensional, steady-state, finite-element heat transfer 

model, Griffiths and Zuluaga (2004) calculated effective R-values of ducts buried under various 

levels of loose-fill insulation. Effective R-values were defined as “the equivalent R-value that 

conventional hung ducts must be wrapped with to achieve the same thermal performance as 

buried ducts” (Griffiths and Zuluaga 2004; 723). Under this definition, effective R-values 

exclude the heat transfer between the conditioned space and the duct that results from burial in 

loose-fill insulation.

The primary driver of effective R-values was found to be the distance from the top of the 

insulation to the top of the duct, and the overall R-value of the attic insulation was found to be 

less important than the height of the insulation. Furthermore, the impact of insulation below the 

duct was minimal; effective R-values were unaffected by placing the ducts on the gypsum board 

ceiling surface, over loose-fill insulation, or over the lower truss cord. As a result, Griffiths and 

Zuluaga (2004) categorized buried duct insulation levels by the distance from the top of the 

insulation to the top of the duct. Ducts were defined as partially buried, fully buried, and deeply 

buried (see Figure 5). 

Figure 5. Categorization of duct insulation levels by burial class

For these levels of burial, effective R-values were calculated for fiberglass and cellulose attic 

insulation (Table 2). Although the effective R-values of ducts were found to be a function of the 

duct inner diameter, the listed R-values are an average of small and large duct simulations. These 

R-values became the basis of Table R3-38 of the Residential Compliance Manual (California 

Energy Commission [CEC] 2007; 2008). 
8

Table 2. Effective R-values of Buried Ducts by Insulation Level and Type (h-ft2-°F/Btu)

Loose-Fill Insulation Type Deeply Buried Fully Buried Partially Buried

Fiberglass R-25 R-13 R-9

Cellulose R-31 R-15 R-9

Validating these theoretical effective R-values is difficult because the effective R-value 

explicitly excludes the nondetrimental heat transfer between the duct and the conditioned space

(see Figure 6 for an image showing heat transfer). Validation has been attempted under 

laboratory conditions and in field testing (Griffiths et al. 2004; Vineyard et al. 2004), but the 

methods used in these validation studies do not exclude heat transfer between the duct and the 

conditioned space, resulting in a different definition of R-value. To address the conflation of 

these two R-value definitions in previous studies, this report distinguishes between effective Rvalues, which exclude heat transfer between the duct and conditioned space, and apparent Rvalues, which include heat transfer between the duct and conditioned space.

Source: CARB (2003)

Figure 6. Heat transfer between duct interior, conditioned space, and attic

Although apparent R-values are expected to be lower than effective R-values because they 

include all heat transfer across the surface of the duct, laboratory and field testing has not 

consistently supported this hypothesis. Griffiths et al. (2004) compared the performance of 

buried ducts against that of conventional hung ducts in two identical houses in Elk Grove, 

California. Apparent R-values determined through field testing were slightly higher than the 

effective R-values predicted by thermal modeling. Vineyard et al. (2004) used laboratory testing 

to calculate R-values and found apparent R-values slightly lower than the effective R-values 

found by Griffiths and Zuluaga (2004). The discrepancy between these results might be 

explained by the averaging used to calculate the effective R-values of buried ducts. For small or

large ducts, the effective R-values could differ considerably from those shown in Table 2.

Research in Phoenix, Arizona, and Sacramento, California, successfully demonstrated that buried 

ducts do not have condensation issues in hot-dry climates. A demonstration in Melbourne, 

Florida, however, found a potential for condensation in a hot-humid climate. Even though the 
9

conditions supporting condensation were relatively short-lived and no damage was found, the 

condensation potential was possibly mitigated by leakage from the building enclosure and 

ductwork. In less leaky conditions, condensation could have been a greater problem (Griffiths et 

al. 2004; Griffiths et al. 2002). Chasar and Withers (2012) also observed condensation on the 

surface of buried ducts at a home in Cocoa, Florida. Laboratory testing determined that ducts 

cannot safely be fully buried in insulation when design dew point temperatures approach 80°F 

(Vineyard et al. 2004).

In a side-by-side evaluation of traditional ductwork and buried ducts, Griffiths et al. (2004) saw a 

measured improvement in distribution efficiency from 86% to 90%.5 Under laboratory testing, 

Vineyard et al. (2004) reported seasonal distribution efficiency improvements ranging from 4% 

to 14% and design distribution efficiency improvements ranging from 5% to 31%. Seasonal 

distribution efficiency reductions represent annual energy savings; design distribution efficiency 

reductions represent potential space-conditioning system capacity reductions.

Field monitoring of buried and encapsulated ducts covered with 1 in. of ccSPF insulation saw no 

condensation potential during the monitored summer for a newly constructed home in Atlanta, 

Georgia. Observed ccSPF surface temperatures were safely above the attic dew point

temperature. Although this study demonstrated the feasibility of buried and encapsulated ducts in 

new construction, no energy savings or distribution efficiency calculations to quantify the 

associated energy savings were reported (CARB 2003). 

Most of the studies mentioned here have used temperature and relative humidity sensors to 

measure condensation potential. Using these sensors, the ambient air dew point is compared to 

the surface temperature of the ductwork. If the surface temperature is below the dew point of the 

surrounding air, there is a potential for condensation. Chasar and Withers (2012) used resistive 

moisture measuring strips in addition to temperature and relative humidity sensors. Both 

methods resulted in similar measures of condensation potential, validating both methods of 

measuring condensation potential.

 5 These values were measured over a period of time, and therefore do not correspond to either design or seasonal 

values.
10

3 Materials and Methods

In this study, three houses of similar size and vintage in Jacksonville, Florida, were monitored 

before and after duct retrofits during the summers of 2010 and 2011. These three test houses are 

single-story buildings that were constructed between 1980 and 1991 and built using typical 

Florida construction methods. Ductwork is constructed out of duct board or insulated flex duct 

and placed in vented attics with insulation at the ceiling plane. Air-handling units (AHUs) are

located inside the house or in the garage. 

System evaluations were performed through single-point performance testing at various project 

stages, long-term pre- and post-retrofit field monitoring, and house and system documentation. 

One house was retrofitted with encapsulated ducts, and the other two were retrofitted with buried 

and encapsulated ducts. This section summarizes the methodology used to monitor the pre- and 

post-retrofit performance of the duct systems in these three houses (Section 3.1) and the 

retrofitting methodology used to insulate the existing duct systems (Section 3.2). The 

performance testing and field monitoring methodologies were developed to gather as much 

information as possible about the temperature, relative humidity, and operation of the systems 

before and after duct insulation. Since encapsulated and buried and encapsulated ducts are a 

novel insulation strategy for ductwork in existing buildings, a retrofit methodology was 

developed to achieve the desired attributes of practical applications, to maximize energy savings, 

and to minimize condensation potential. 

3.1 Monitoring Methodology

The performance of pre- and post-retrofit ductwork assemblies was measured using performance 

testing and long-term monitoring. To assess the long-term performance both before and after 

implementing the encapsulated duct and buried and encapsulated duct strategies, data loggers 

were placed at multiple distribution system locations. Measurements were taken at 1- or 2-min

intervals. Operation of the space-conditioning equipment was measured at the AHU and 

condensing unit using state sensors, which have a Boolean output of either “on” or “off.”

Measurements were taken using the equipment listed in Table 3.

During the pre- and post-retrofit assessments, the following measurements were taken through 

performance testing:

• Pressure drop across the AHU and coil (Pa)

• Temperature change across the AHU and coil (°F)

• Total AHU supply and return airflows (cfm)

• Room airflows (cfm)

• Duct leakage—total and leakage to the outside, supply and return (cfm @ 25 Pa)

• Infiltration of building enclosure (ACH50)

• Power consumption of the AHU and condensing unit (W).
11

Table 3. Equipment Used for Performance Testing and Long-Term Monitoring

Measurement Equipment Needed

Pressure Drop Across the AHU and Coil DG-700 Digital Pressure and Flow Gauge

Temperature Change Across the AHU and Coil HOBO U12-006 data logger with 

TMC6-HD Probe

Total AHU Supply and Return Airflows True Flow Air Handler Flow Meter

Room Airflows Alnor Low Flow Balometer

Duct Leakage—Total and Leakage to the 

Outside, Supply and Return

Minneapolis Duct Blaster System with 

DG-700 Digital Gauge

Infiltration of Building Enclosure Minneapolis Blower Door System with 

DG-700 Digital Gauge

Infrared Thermal Imaging Flir B50 Infrared Camera

Temperature of Airstream HOBO U12-006 data logger with 

TMC6-HD Probe

Surface Temperature HOBO U12-006 data logger with 

TMC6-HE Probe

Indoor/Attic Air Temperature and 

Relative Humidity HOBO U12-011 data logger

Outdoor Air Temperature and 

Relative Humidity HOBO U23-002 data logger

Relative Humidity of Airstream or Surface HOBO U23-002 data logger

AHU and Condenser Run Times HOBO U9-001 data logger with 

CSV-A8 Probe

At each of the three homes, temperature (°F) was measured at the following locations:

• Discharge airstream of longest run

• Discharge airstream of three typical runs

• AHU supply

• AHU return

• Surface of duct jacket at three typical runs

• Surface of ccSPF insulation at three typical runs (post-retrofit only)

• Surface of boots at three typical runs (pre-retrofit only for all houses and one boot per 

house post-retrofit)

• Attic ambient air

• Living space ambient air at two locations

• Outdoor ambient air.
12

Relative humidity (%) was measured at the following locations:

• Discharge airstream of longest run

• Discharge airstream of three typical runs

• AHU supply

• AHU return

• Surface of ccSPF insulation at three typical runs (post-retrofit only)

• Surface of boots at three typical runs (pre-retrofit only)

• Attic ambient air

• Living space ambient air at two locations

• Outdoor ambient air.

3.2 Retrofitting Methodologies

The methodologies for implementing the two ductwork retrofit strategies were influenced by 

concerns about maintaining quality installation, minimizing condensation, and streamlining the 

installation, along with practical concerns identified by the entire team. The team included the 

contractor responsible for applying the ccSPF insulation, the ccSPF manufacturer, the HVAC 

contractor, and the Building America researchers. The researchers were primarily concerned 

with ensuring that ductwork was placed as close as possible to the ceiling plane, the ccSPF 

application was consistent and of sufficient thickness to mitigate the possibility of condensation, 

and the installation was code compliant. The insulation contractor was primarily concerned with

preventing leakage of ccSPF through the boots of the ductwork into the home and preventing 

blowback of fiberglass insulation during ccSPF application, which can affect visibility and 

insulation adherence to the ductwork.

Attic accessibility, temperature, and location of existing services (e.g., plumbing, television 

cable, security wires, and fireplace flues) also needed to be taken into account. Based on 

previous experience in Atlanta, a ccSPF thickness of 1.5 in. was selected to ensure that the 

ductwork would be covered in enough insulation to prevent condensation. This insulation level is 

slightly higher than that used in Atlanta because of the differences in climate. 

Sections 3.2.1 and 3.2.2, respectively, describe the general strategy for encapsulated and buried 

and encapsulated duct retrofits. Although these methods serve as a general guideline for 

retrofitting existing ducts, differences between ductwork systems merit slightly different 

treatment, particularly when reconfiguring the existing system layout. The differences between 

the methods used at the three tested houses are discussed in Sections 3.2.3 through 3.2.5.

3.2.1 Buried and Encapsulated Ducts

For buried and encapsulated ducts, a four-phase methodology was employed. The insulation 

contractor removed the existing ceiling insulation, the HVAC contractor reconfigured the ducts, 

a trained installer applied the ccSPF, and the insulation contractor then installed the loose-fill

insulation. This approach requires coordination between an insulation contractor and an HVAC 

contractor.
13

Additional coordination and home access was required for laborers to remove insulation and seal 

the register boots. The ccSPF contractor opted to remove all blown-in insulation and batt 

insulation as a precaution to eliminate any problems during ccSPF application. The following list 

explains and illustrates the steps taken during the buried and encapsulated ducts retrofit.

1. Remove existing insulation from around the 

ductwork by removing batt insulation and/or 

vacuuming out loose-fill insulation. Note:

To minimize the number of site visits, 

laborers might be able to complete this step 

while the ccSPF equipment is being set up.

2. Seal around the boots with polyurethanebased insulating foam to prevent leakage 

into living space during encapsulation.

Note: The blades of the interior diffusers 

were typically closed during ccSPF 

application for added protection.

3. Cut down or remove supports holding 

ductwork and plenum boxes above the truss 

bottom cords or gypsum board.6 Varying 

degrees of reconfiguration could be 

appropriate, depending on the configuration 

of the ductwork and attic.7 This work, 

including extending sections of ductwork, 

was done by an HVAC technician. 

4. For ductwork that passes over soffits or 

other areas that have a large gap between 

the duct and the sheetrock, 1.5-in. rigid 

insulation board can be placed underneath 

the ductwork to ensure that the ductwork is 

entirely encapsulated in an air- and vaporimpermeable layer of insulation. It is 

preferable to seal the ductwork directly to 

the sheetrock but limited access for the 

ccSPF gun and gear must be considered.

 6 Ducts should be supported in compliance with local building codes and manufacturer specifications.

7 Major reconfigurations might require duct sizing calculations using ACCA Manual D (ACCA 2009). For this 

project, duct reconfigurations were minor, and no efforts were made to rigorously size duct reconfigurations using 

Manual D.
14

5. Protect existing services, such as flues and 

pipes, from ccSPF using duct board or rigid 

insulation. The appropriate material will 

depend on the application.

6. Encapsulate ductwork in at least 1.5 in. of 

ccSPF. The minimum thickness must be 

maintained consistently on all sides to 

mitigate condensation concerns. 

7. Bury encapsulated ductwork in loose-fill 

insulation. The insulation must cover the 

ductwork to be considered fully buried.

Code requires a minimum of 1.5 in. of 

coverage over the ducts for ccSPF materials 

that do not meet the flame spread 

requirements. Coverage of 3.5 in. over the 

duct surface achieves a deeply buried rating.

3.2.2 Encapsulated Ducts

For one of the houses in this study, good duct burial could not be achieved even if significant 

duct reconfiguration had been undertaken. In addition, the project team was concerned that 

relocating the ductwork would significantly affect the room airflows. As a result, the team opted 

to evaluate the benefits of the encapsulated duct strategy.

For encapsulated ducts, the methodology is similar. This strategy, however, is simpler and 

requires no duct reconfiguration or loose-fill insulation. This is a single-contractor approach, 

requiring only ccSPF application. Selecting the appropriate method for each house will depend 

on cost, feasibility, ccSPF material code compliance, and energy performance goals. The 

following list describes and illustrates the encapsulated duct strategy.
15

1. Remove existing insulation from around the 

supply and return register boots by moving 

batt insulation or vacuuming out loose-fill 

insulation.

2. Seal ductwork with polyurethane-based 

insulating foam to prevent leakage into living 

space during encapsulation.

3. For ductwork that is not hung above the 

ceiling or does not have sufficient clearance 

below for proper ccSPF application, place 

1.5-in.-thick rigid insulation board 

underneath the ductwork to ensure that the 

ductwork is entirely encapsulated in an airand vapor-impermeable layer of insulation.

4. Encapsulate ductwork in at least 1.5 in. of 

ccSPF using a material that has the necessary 

ratings to demonstrate code compliance.

Leave the ductwork exposed.

3.2.3 House 1 Retrofit

The existing ductwork in House 1 consisted of R-4.2 flexible ductwork configured with a 

primary trunk extending the length of the house and branch take-offs serving the spaces. The 

AHU is located inside the conditioned area and has a louvered door on the return, eliminating the 

need for return ductwork. Branches are connected to the primary supply trunk using plenum 

boxes constructed with R-4 fiberglass duct board, which are placed on plywood platforms. Duct 

runs are hung from the rafters with strapping and connected to sheet metal boots at the ceiling 

plane (Figure 7). This was the only home served by an AHU with a variable-speed fan motor.

Ductwork at House 1 was retrofitted with a buried and encapsulated duct strategy. Of the three, 

this house underwent the most significant duct reconfiguration. During the duct retrofit, the

plywood platforms supporting the plenum boxes were removed to lower the main trunk closer to 
16

the ceiling plane. The straps supporting the ducts were removed, lowering them to the ceiling 

plane. Where possible, ductwork was reconfigured to minimize elbows and eliminate duct 

overlap. The reconfiguration resulted in approximately 21 linear feet of additional ductwork

routed closer to the gypsum board. These modifications maximized duct burial to the greatest 

extent possible, but still primarily used the existing duct system.

Figure 7. Existing ductwork at House 1

3.2.4 House 2 Retrofit

Existing ductwork in House 2 consisted of a main rectangular supply trunk and rectangular 

branch supply take-offs. The AHU is located in the garage, near a wall adjacent to the 

conditioned space. All supply ductwork runs through the garage and into the attic. There are two 

returns, a central return entering the AHU return plenum through the partition wall, and a second 

return serving the far side of the house via a flex duct routed through the entire attic and garage.

The supply ductwork consists of R-4 duct board rectangular ducts (Figure 8), and the return 

ductwork consists of a large R-4.2 flex duct (Figure 9).

House 2 was retrofitted with a buried and encapsulated duct strategy. During the retrofit, the 

majority of the supply ductwork was left in place because it was already resting on the lower 

truss cords. One exception was a supply duct that had been built to cross over the return duct 

(Figure 10). The duct board return had since been abandoned. The supply duct was replaced with 

a small flex duct using a side take-off, enabling this ductwork to drop low against the ceiling 

plane. In conjunction with this modification, the abandoned duct board return was removed and 

the majority of the return ductwork was relocated to the center of the attic and lowered to the 

truss cords (Figure 11). The abandoned return ductwork was removed and discarded.
17

Figure 8. Supply ductwork for House 2

Figure 9. Return ductwork for House 2

Figure 10. Supply ductwork jump over abandoned return ductwork
18

During removal of the ceiling insulation before encapsulation, a large opening in the far end of 

the supply trunk was discovered. This take-off protruded from the bottom of the trunk toward the 

ceiling, and was most likely meant to serve a register that was never installed. The supply air had 

been escaping to the attic since the system was installed. This opening was sealed before 

encapsulation. This problem was not discovered before pre-retrofit monitoring began. As shown 

in Figure 12, the ceiling in this home had a number of dropped areas for ceiling coffers. In these 

locations, the batt and the blown insulation were very deep. The insulation contractor removed 

much of this insulation and used rigid insulation as a substrate for the ccSPF application. Much 

of the batt insulation could have been left in place, but the open duct would not have been 

detected.

Figure 11. Duct reconfiguration at House 2

Figure 12. Open duct take-off at House 2
19

3.2.5 House 3 Retrofit

House 3 was similar to House 1 in ductwork specifications. Existing R-4.2 flexible ductwork was

configured with a large main trunk running the length of the house and flex duct branch takeoffs. The AHU is located inside the conditioned area and served by a louvered door on the 

return, eliminating the need for any return ductwork. Branches were connected to the supply 

trunk using plenum boxes constructed with R-4 fiberglass duct board, which were placed on 

plywood platforms. Duct runs were hung from the rafters and connected to sheet metal boots at 

the ceiling plane.

In this home, the main living room has a cathedral ceiling and the large duct trunk is suspended 

from the center of trusses for a long run (Figure 13). Even if this trunk were lowered, it would be 

resting on the angled cathedral ceiling plane. Under those conditions, burying the large duct 

would have been difficult and might have necessitated baffling at the perimeter to prevent 

insulation from blocking the limited attic vents. In addition, many of the branch ducts were 

resting on high raised platforms.

Lowering the ductwork would have resulted in severe duct bends, making burial challenging.

The potential impact of the duct reconfiguration on room airflows was difficult to predict, but the 

project team opted to forgo duct reconfiguration in this house. As an alternative to burying the 

ductwork, the team considered wrapping the ducts with fiberglass duct wrap insulation or 

draping them with fiberglass batts. Either approach would have provided the minimum 1.5-in.

coverage over the ccSPF and resulted in a minimal additional amount of insulating value.

Ultimately, the manufacturer confirmed that the ccSPF could be left exposed, so no additional 

insulation was installed.

No reconfiguration was necessary at House 3 because it was retrofitted using the encapsulated 

duct strategy (see Figure 13 for finished retrofit). The ccSPF contractor was initially concerned

that the raised ductwork would be difficult to insulate because of the high application pressures, 

but the installer was able to get access to all sides of the ductwork.

Figure 13. Finished encapsulated duct hung from roof deck
20

4 Retrofit Methodology Evaluation

Retrofitting these houses resulted in valuable information about the coordination issues, 

practicality, and difficulties associated with burial and/or encapsulation of the existing ductwork 

at the three test homes. Section 4.1 describes the lessons learned. Section 4.2 outlines some 

quality control issues experienced during the retrofit process.

4.1 Lessons Learned

During the retrofit process, many important details about the encapsulated and buried and 

encapsulated duct retrofit methodologies were uncovered. Even though the research team 

attempted to preemptively solve any problems that might arise, unforeseen issues were inevitably 

discovered during the installation. Furthermore, some of the actions taken to remove obstacles 

during the retrofits were potentially unnecessary. The following bullet points list potential areas 

of improved installation efficiency, as well as the issues that arose during the installation and 

potential solutions. As with any new building methodology, processes will become more 

streamlined as more installations take place.

• Baffling materials might be required to enable mounding of loose-fill insulation to 

achieve proper burial of vertical duct trunks and raised or larger diameter duct runs

(Figure 14).

• Baffling might be necessary to partition off flues, chimneys, or other house components 

that should not be in contact with the ccSPF or loose-fill insulation. Foil-faced duct board

was typically used because it is rated for use in attics, comes in narrow pieces that can be 

easily transferred to the attic, can be cut with a knife, and can be secured with metal tape

(Figure 15).

• Venting for existing ventilation fans cannot be blocked by the added blown-in insulation.

In one home, the fan was unducted and vented to the attic. Existing unducted ventilation 

fans should be ducted to an exterior termination.

• Attic venting at the ridge, gables, and perimeter should be assessed to ensure that added 

blown-in material will not block the airflow.

• Electrical, telephone, cable, security, and other wiring could be permanently spray

foamed into place (Figure 16).

• Plumbing routed through the attic, such as the roof-mounted solar pool heating system

installed in one test home, could be covered with foam.

• Access for installers can be difficult because of the cables, plumbing, and other 

equipment located in the attic.

• Can lighting, exhaust fans, and other electrical equipment located in the ceiling plane can 

get covered in foam and/or require baffling for protection during the retrofit.

• During the summer, high attic temperatures can be a safety concern for the ccSPF 

installers. The ccSPF material used in these installations is heated before spraying, further 
21

increasing the attic temperature. In addition, installers must wear protective masks and 

suits. Forced ventilation air was used in the attics during these applications, although it 

was not cooled.

• Attic access for ccSPF installation must be considered. During screening of potential 

installation sites, houses with adequate access for hoses and equipment were selected.

These houses all had attic access stairs located in the garage, eliminating the need to enter 

the house with hoses and ccSPF equipment.

• During the ccSPF installation, the HVAC system should be disabled. For all three houses, 

the AHUs were turned off during the application. The ccSPF contractor raised concerns 

about any odors escaping into the living space.

• Coordinating with occupants is important. If the AHU system will be disabled for a few 

hours, any pets or occupants should leave the house to avoid fumes and maintain comfort.

Every attempt was made to minimize the number of times access to the house was 

required. To install sensors for this research project and conduct the performance testing, 

access to these houses was required more frequently than would be needed in a typical 

application. 

• The need for boot sealing should be further evaluated. It might not be necessary, but in 

these homes it was left to the discretion of the contractor. If future investigations find that 

this is not required, it could result in lower costs.

• The need for insulation removal should be further evaluated. Fully removing the 

insulation might be unnecessary. Cutting it and rolling it back before applying the ccSPF 

might be sufficient. On the other hand, removing existing attic insulation offers an 

excellent opportunity for sealing the ceiling planes, which would result in even greater 

energy and comfort improvements.

• Sequencing of this retrofit application requires coordination to minimize costs and 

expedite the installation. Ideally, the ccSPF can be applied in a few hours in the morning 

and a second installer can follow behind with a truck to install the blown-in loose-fill

insulation. For these houses, the same contractor performed both services, but with two 

separate crews. Additional time must be left for any insulation removal, boot sealing, 

and/or duct reconfiguration.

• Duct reconfiguration should be undertaken with care and evaluated for each home. For 

these retrofits, minimal duct changes were made and the pre- and post-retrofit airflows 

were verified by testing. Although airflow verifications might not be necessary for 

retrofits with minimal reconfigurations, the airflows could be affected if significant 

changes are made to ductwork. In this case, it might be necessary to install flow dampers 

and verify airflows after the retrofit is complete.

• Using an HVAC contractor for the duct reconfiguration is recommended. Typically, 

insulation contractors are not familiar with the impact of duct changes on the system 

airflows. Although the insulation contractor can do minimal duct reconfiguration (cutting 

down strapping, etc.), changes to the distribution system can void any contractor
22

warranties associated with the installed duct system. Local codes can also require a 

licensed HVAC contractor to perform all work related to the HVAC system.

• The buried and encapsulated duct strategy is a multitrade effort. For these homes, 

Building American Program researchers coordinated with the contractors and 

homeowners. It is unlikely that a homeowner would be willing to serve as the point of 

coordination to implement this strategy. To offer this strategy as a package, two or three 

contractors would be required: a ccSPF installer, a blown-in insulation installer, and an 

HVAC installer (could be optional, if code allows). Ideally, a company that offers a full 

range of home retrofit services could furnish this package. As an alternative,

organizations could partner to offer the package, with one contractor serving as a 

coordinator.

• The encapsulated duct strategy is a single-trade effort. It requires significantly less 

coordination, fewer periods of access to the home, only a single insulation material, and 

lower costs. Insulation contractors might need to be trained to recognize major 

distribution flaws that must be corrected by an HVAC contractor, such as the one 

discovered at House 2. An HVAC contractor might be required to consult on a case-bycase basis if concerns arise and the homeowner opts to pursue them.

• Careful attention must be paid to quality control of the ccSPF installation. This is 

discussed further in Section 4.2.

Figure 14. Baffling used to ensure proper burial

of supply duct plenum (post-encapsulation and 

pre-burial)

Figure 15. Baffling used to prevent ccSPF 

application on flue duct
23

Figure 16. Electrical, telephone, and other wiring permanently spray foamed to the 

ceiling and ductwork during retrofit

4.2 Quality Control

The most important observation from the field demonstration was the inadequate application of 

ccSPF at several specific locations of the ductwork and the general difficulty with maintaining 

consistent insulation thickness. This observation is not intended as a critique of the installation 

contractors. The installers were challenged by extremely high attic temperatures, limited access

for spraying, a tight project schedule, varying site conditions, the unique addition of devices and 

wiring for system performance monitoring, and minimal available information on the preferred 

installation details. Overall, all three installations were very successful, but it is important to 

document opportunities for future improvement.

At House 1, the top of the supply plenum box that extends vertically into the attic was not

foamed (Figure 17). Even with baffling, this large vertical riser was difficult to bury beneath a 

deep fiberglass insulation. Although the lack of insulation reduces the R-value of this duct 

section and the overall distribution system efficiency, it is unlikely to create a condensation 

problem. Since this problem was not discovered until after the spray foaming was finished, the 

top of the plenum box was covered with R-4 rigid board and sealed with metal tape (see 

Figure 14). The supply plenum requires particularly close attention. Because it is closest to the 

AHU, the supply plenum has the highest system pressure and distributes the most extreme air 

temperatures.
24

Figure 17. Improperly spray-foamed plenum box at House 1

Furthermore, at several locations at House 1 where the ductwork runs perpendicular across the 

lower truss cords and other locations where the ductwork was slightly elevated, the underside of 

the ductwork was not completely covered in spray foam. The inadequate application of spray

foam at the underside of the ductwork could create the potential for condensation because the 

duct jacket, which might be below the dew point of the surrounding air, was exposed. 

Although somewhat difficult to see, Figure 18 and Figure 19 show the undersides of ductwork

that were left exposed after the ccSPF was applied. Getting the spray nozzle low enough to apply 

foam in these locations was challenging. Two options could be used to solve this problem. The 

installer can cocoon the duct in ccSPF and directly seal it to the sheetrock (Figure 20). 

Alternatively, a piece of rigid insulation can be inserted under the ductwork to act as a substrate 

for the foam and ensure that a minimal insulation thickness is achieved on the underside of the 

ducts (Figure 21).

In addition, the insulation thickness varied considerably for these applications, as shown in 

Figure 20 through Figure 23. Achieving a consistent minimum thickness of ccSPF without 

wasting material is a difficult balance. For this demonstration, the installing contractors were 

asked to err on the side of too much insulation. Given the high cost of the material, this was often 

a counterintuitive request. As a mainstream practice, educating the installers on the importance 

of providing that minimum insulation thickness will be critical to ensure that condensation 

concerns are mitigated. Proper training and familiarity gained through performing more retrofits 

is expected to decrease waste and improve consistency.
25

Figure 18. Exposed duct jacket at the underside 

of duct

Figure 19. Exposed underside of ductwork not 

adequately covered with ccSPF

Figure 20. Ductwork well-sealed to sheetrock 

with ccSPF

Figure 21. Rigid insulation inserted under 

ductwork to serve as a substrate and provide 

insulating value

Figure 22. Varying thickness of ccSPF and 

interference from cross bracing

Figure 23. Varying application thicknesses 

shown on rectangular (left) and round (right) 

ducts
26

4.3 Retrofit Evaluation Summary

As noted in Sections 4.1 and 4.2, the attics in these existing houses were filled with electrical, 

telephone, and other wiring that crossed the ductwork in many locations. As a result, the wiring 

was permanently spray foamed to the ceiling and ductwork during the retrofit. If there is a 

problem with the existing wiring in the house, it might be difficult to service. For buried and 

encapsulated ducts, however, there might be no practical difference for service technicians trying 

to deal with wiring buried beneath fiberglass insulation and wiring spray foamed to ductwork. In 

practice, there was no way to avoid spray foaming over wiring because these wires were so 

haphazardly laid across the ceiling. 

Although the retrofitting methodology included removing ceiling insulation from around the 

ductwork before applying the spray-foam insulation to the surface of the duct work, there 

appeared to be no issues with blowback of batt insulation during spray-foam application. 

Although the airborne fibers associated with blown-in insulation might pose a health and safety 

problem, the protective equipment worn by the installer is likely sufficient to minimize concern.

The authors feel that removing batt insulation before ccSPF application seems unnecessary, 

except around boots and within a few feet of the ductwork. 

A minimal amount of insulation must still be removed around boots to allow access for proper 

spray-foam adherence to the sheetrock. Similarly, batt insulation can be cut parallel to the 

ductwork and rolled back a few feet to open 1 to 2 ft of access adjacent to the ducts for proper

adherence of the ccSPF to the sheetrock. Any fiberglass remaining below the ductwork can serve 

as a backer to minimize the amount of ccSPF required for encapsulation. Removing and/or 

rolling back the fiberglass insulation is unnecessary when a substrate, such as rigid insulation, is 

inserted beneath the ductwork. A vapor-impermeable insulation of the same insulation level as 

the spray foam must be used to prevent condensation underneath the duct.

With these strategies for removing existing insulation, it might be possible to minimize costs and 

site visits by using a two-person crew. While the ccSPF equipment is being set up, a second 

person can cut back the insulation and expose the boots. After the ccSPF application, the second 

technician can roll the insulation back into place and cover the boots. Furthermore, preapplying 

spray-foam insulation around the boots could be unnecessary for preventing leakage into the 

living space. More testing is needed to see if there is indeed a problem with leakage of ccSPF 

into the living space. Regardless, this could be undertaken fairly quickly during the field 

preparation work.
27

5 Field Monitoring and Performance Testing Results

Field monitoring and performance testing yielded valuable data on the performance of the 

distribution systems before and after ductwork retrofits. Building specifications obtained by 

observing the building system and performance testing are discussed and summarized in 

Section 5.1. Long-term monitoring of occupied buildings reveals valuable information about the 

thermal performance, as discussed in Section 5.2, and the condensation potential, as discussed in 

Section 5.3, of the distribution system.

5.1 Building Specifications

All three homes are single-family detached houses of similar vintage with vented attics, slab on 

grade construction, and attached garages. Table 4 shows building enclosure and mechanical 

equipment specifications for the three houses. The specifications in the table did not change over 

the pre- and post-retrofit period. 

Pre- and post-retrofit duct, building, and system specifications for variables that change between 

the pre- and post-retrofit monitoring periods, such as duct leakage and R-value are shown in 

Table 5. Duct R-values were determined using the methodology outlined in Section 6. Total duct 

leakage and duct leakage to the outdoors were significantly reduced, and flows stayed relatively 

constant between the pre- and post-retrofit testing period. Building infiltration was reduced as 

well through duct sealing, air sealing around duct supply registers, and additional attic insulation. 

The post-retrofit supply duct R-value for House 3 is approximately half of the values for the 

other two homes. The encapsulated duct strategy was used in House 3; houses 1 and 2 received 

buried and encapsulated ducts. House 3 has comparable post-retrofit duct leakage rates to those 

observed in House 1. Both have similar duct construction. At House 1, reconfiguration of the 

ductwork led to an increase in total duct surface area because the duct runs were lengthened to 

lower ductwork closer to the ceiling plane. At House 2, reconfiguration of the ductwork led to a 

reduction in supply duct surface area because a circuitous duct board run was replaced with a 

shorter flex duct run. The return duct surface area was increased because the return ductwork

was lengthened to lower ductwork closer to the ceiling plane. All surface areas were measured at 

the inner duct dimensions.

The pre-retrofit duct leakage numbers for House 2 listed in Table 5 are not entirely accurate. The 

open duct take-off shown in Figure 12 caused issues with measuring duct leakage before the 

retrofit. The duct pressure during the total duct blaster test did not reach 25 Pa, and the results 

were extrapolated from the measured pressure of 15.3 Pa. During the duct leakage to the outside 

test, the duct and house pressures could not be equalized because of the disconnected duct. The 

duct leakage to the outside is not reported in Table 5, but for modeling efforts, the duct leakage 

to the outdoors was assumed to equal the total duct leakage.

Houses 1 and 3 have AHUs placed in the living space, but House 2 has leaky supply and return 

plenums connected to an AHU in the garage (see Figure 24). As a result, House 2 did not 

experience a reduction in duct leakage to the outdoors to the same extent as houses 1 and 3. 

Additional spray foaming of the supply and return ductwork in the garage would have further 

reduced duct leakages, but this retrofit was outside the scope of this project.
28

Table 4. Building Enclosure and Space-Conditioning Specifications of Monitored Houses

House 1 House 2 House 3

Year Built 1991 ~1980 1987

Area of Conditioned

Space (ft2) 2,133 2,100 1,876

Volume of Conditioned

Space (ft3) 20,800 18,900 16,289

Bedrooms 4 3 3

Bathrooms 2 2 2

Exterior Finish Pink stucco Light wood siding Light wood siding

Wall Assembly 2 × 4 R-13 2 × 4 R-11 2 × 4 R-13

Ceiling Assembly R-30 fiberglass 

batts

R-19 fiberglass batts + 

1- to 3-in. blown-in

mounds over kitchen

R-19 fiberglass 

batts

Windows Double, clear Single, clear Double, clear

Skylights None Insulated clear, 

uninsulated light shaft None

Doors R-4 R-4 R-4

Main Roof Dark asphalt 5:12 Light brown asphalt, 

5:12

Light brown 

asphalt 5:12

Outdoor Unit Model 

Number

Payne 

PH15NB048-A

Tempstar 

NHP042AKAI

Payne 

PH10JA036-C

Outdoor Tonnage 4 tons 3.5 tons 3 tons

Indoor Unit Model 

Number

Payne 

PF4MNA061 Trane TWE042C1FC PF1MNA036

Indoor Tonnage 5 tons 3.5 tons 3 tons

Rated Energy Efficiency 

Rating/Seasonal Energy 

Efficiency Ratio

12.0/14.5 8.8/10.0 9.05/10.00

Cooling Capacity 

(Btu/h) 47,500 39,500 34,000

High-Temperature

Heating

(Btu/h)

46,500 38,000 34,000

Low Temperature

Heating

(Btu/h)

29,200 22,000 20,600

Number of Compressor 

Speeds 1 1 1

Control Type Thermostatic Orifice Orifice

AHU Fan Type Permanent split 

capacitor

Permanent split 

capacitor

Electronically 

commutated 

motors
29

Table 5. Pre- and Post-Retrofit Duct, Building, and System Specifications

House 1 House 2 House 3

PreRetrofitPostRetrofitPreRetrofitPostRetrofitPreRetrofitPostRetrofit

Supply Duct Surface Area (ft2)

a 339 374 384 369 214 214

Return Duct Surface Area 

(ft2)

b N/A N/A 163 194 N/A N/A

Supply Duct Effective R-Value 

(h-ft2-°F/Btu) 4.7 21.9 6.5 26.1 4.6 11.1

Return Duct Effective R-Value 

(h-ft2-°F/Btu) N/A N/A 4.9 26.2 N/A N/A

Flow at Operating Conditions 

(cfm) 1,562 1,469 1,144 1,138 949 976

Duct Leakage to Outdoors 

(cfm@25 Pa) 152 29 N/Ab 162 88 22

Total Duct Leakage 

(cfm@25 Pa) 527 188 382c 290 227 117

Building Infiltration 

(cfm@50 Pa) 2,672 2,138 4,066 2,905 2,306 2,168

a Duct surface area is measured at the inner duct dimensions.

b Duct leakage to the outdoors could not be measured because of disconnected duct take-off. c Total duct leakage was measured at 15.3 Pa and extrapolated to 25 Pa.

Figure 24. AHU at House 2 located in garage with leaky return and supply plenums
30

Using the data displayed in Table 4 and Table 5, pre- and post-retrofit building loads were 

calculated using ACCA’s Manual J: Residential Load Calculation (2006), as shown in Figure 25

and Figure 26. At House 1, the building loads are similar to the installed capacity, and the HVAC 

system seems to be properly sized. At House 2, the installed capacity is drastically undersized 

compared to the calculated building loads. Monitoring revealed long run times for the equipment 

in House 2, supporting the calculation that the system is undersized. The large loads are 

primarily caused by the poor window system installed in the house. House 3 Manual J 

calculations show that the system is slightly oversized.

Figure 25. Pre- and post-retrofit Manual J building cooling loads compared to installed capacity

Figure 26. Pre- and post-retrofit Manual J building heating loads compared to installed capacity

47,500

41,285 38,212 39,500

61,263

52,585

34,000

25,328 23,672

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

House 1

Installed

Capacity

House 1

Pre

Manual J

House 1

Post

Manual J

House 2

Installed

Capacity

House 2

Pre

Manual J

House 2

Post

Manual J

House 3

Installed

Capacity

House 3

Pre

Manual J

House 3

Post

Manual J

Building Cooling Load (Btu/hr)

Walls Glazing Doors Ceilings Floors Infiltration Ducts Internal Installed

46,500

33,405 30,352

38,000

54,566

46,813

34,000

27,396 25,755

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

House 1

Installed

Capacity

House 1

Pre

Manual J

House 1

Post

Manual J

House 2

Installed

Capacity

House 2

Pre

Manual J

House 2

Post

Manual J

House 3

Installed

Capacity

House 3

Pre

Manual J

House 3

Post

Manual J

Building Heating Load (Btu/hr)

Walls Glazing Doors Ceilings Floors Infiltration Ducts Internal Installed
31

5.2 Qualitative Discussion of Thermal Performance

Changes in the mechanical and distribution system configuration and operation at each house 

created challenges for interpreting long-term field monitoring and performance testing data.

House 1 was unoccupied and the cooling system was set to a higher set-point temperature during 

the post-retrofit period. At House 2, a large hole in the ductwork was discovered during the 

retrofit, which complicated the comparison between pre- and post-retrofit performance. At 

House 3, the outdoor condensing unit was cleaned and recharged with refrigerant between the 

pre- and post-retrofit periods, which increased the system’s efficiency. The ambient temperature 

sensor was placed too close to the condensing unit to measure temperatures accurately. Despite 

these challenges, the data shown in Figure 27 through Figure 32 give interesting insights into the 

benefits of buried ducts.

The graphs each show a 1-day period for a typical duct at each house over a typical summer day.

The day selected starts at 4 a.m. on July 29 of the year during which the home was monitored. 

Two graphs are shown for each house, enabling a comparison of the pre- and post-retrofit 

performance of the same duct run. Periods of air-conditioning operation can be identified in 

these graphs by looking at the discharge airstream temperature (shown as the light blue line). 

During periods of air-conditioning operation, the discharge temperature drops significantly, 

typically to below 60°F. Table 6 lists the weather conditions for the two days graphed in this 

section using data available from the National Climactic Data Center (NCDC).

Table 6. Weather Conditions for Days Graphed in This Section

July 29, 2010 July 29, 2011

High 96°F 94°F

Low 73°F 73°F

Dew Point 72°F 72°F

Clouds Clear/partly cloudy

with afternoon thunderstorms

Clear/partly cloudy

with afternoon thunderstorms

Source: NCDC (2012).

The pre-retrofit graphs confirm that the equipment in houses 1 and 3 can satisfy the cooling loads 

because the systems cycle on and off. House 2, however, has an undersized system, and as a 

result, the cooling system runs constantly between 10:00 a.m. and 8:00 p.m. Post-retrofit run

time cannot be assessed at House 1 because of a change in occupancy. The run time at House 3, 

however, shows an increase in cycling caused by a reduction in load corresponding to the 

retrofit. This increase in cycling, though, could be partially caused by the cleaning and 

recharging of the mechanical system that took place before the post-retrofit monitoring started.

At House 2 with its undersized system, no reduction in load is observable through system run

time.

As expected, the additional insulation applied to the ductwork results in a significant reduction in 

the temperature of the duct jacket surface. The temperature of the duct jacket surface during the 

pre-retrofit period decreased substantially when the air-conditioning system was running. By 

contrast, the surface temperature of the ccSPF surface during the post-retrofit period fluctuates 

only slightly during air-conditioner operation. This vast difference in temperature sensitivity of 

the ductwork surface points to a large increase in the R-value of the ductwork.
32

Figure 27. Pre-retrofit surface and air temperatures at duct run 2 in House 1

Figure 28. Post-retrofit surface and air temperatures at duct run 2 in House 1

6:00 a.m. 9:00 a.m. 12:00 p.m. 3:00 p.m. 6:00 p.m. 9:00 p.m. 12:00 a.m. 3:00 a.m.

40

50

60

70

80

90

100

110

120

130

140

Temperature (°F)

Attic

Duct Jacket Surface

Boot Surface

Discharge Airstream

Living Space 1

Living Space 2

6:00 a.m. 9:00 a.m. 12:00 p.m. 3:00 p.m. 6:00 p.m. 9:00 p.m. 12:00 a.m. 3:00 a.m.

40

50

60

70

80

90

100

110

120

130

140

Temperature (°F)

Attic

ccspf Surface

Duct Jacket Surface

Discharge Airstream

Living Space 1

Living Space 2
33

Figure 29. Pre-retrofit surface and air temperatures at duct run 3 in House 2

Figure 30. Post-retrofit surface and air temperatures at duct run 3 in House 2

6:00 a.m. 9:00 a.m. 12:00 p.m. 3:00 p.m. 6:00 p.m. 9:00 p.m. 12:00 a.m. 3:00 a.m.

40

50

60

70

80

90

100

110

120

130

140

Temperature (°F)

Attic

Duct Jacket Surface

Boot Surface

Discharge Airstream

Living Space 1

Living Space 2

6:00 a.m. 9:00 a.m. 12:00 p.m. 3:00 p.m. 6:00 p.m. 9:00 p.m. 12:00 a.m. 3:00 a.m.

40

50

60

70

80

90

100

110

120

130

140

Temperature (°F)

Attic

ccspf Surface

Duct Jacket Surface

Discharge Airstream

Living Space 1

Living Space 2
34

Figure 31. Pre-retrofit surface and air temperatures at duct run 2 in House 3

Figure 32. Post-retrofit surface and air temperatures at duct run 2 in House 3

6:00 a.m. 9:00 a.m. 12:00 p.m. 3:00 p.m. 6:00 p.m. 9:00 p.m. 12:00 a.m. 3:00 a.m.

40

50

60

70

80

90

100

110

120

130

140

Temperature (°F)

Attic

Duct Jacket Surface

Boot Surface

Discharge Airstream

Living Space 1

Living Space 2

6:00 a.m. 9:00 a.m. 12:00 p.m. 3:00 p.m. 6:00 p.m. 9:00 p.m. 12:00 a.m. 3:00 a.m.

40

50

60

70

80

90

100

110

120

130

140

Temperature (°F)

Attic

ccspf Surface

Duct Jacket Surface

Discharge Airstream

Living Space 1

Living Space 2
35

For House 3, the encapsulated ducts were not buried. Comparing the post-retrofit duct surfaces in 

House 3 to those of similar construction in House 1, duct jacket and ccSPF surface temperatures 

are much higher for the unburied case than for the buried case. Unlike the other two houses, the 

temperature of the duct jacket surface in House 1 remains coupled with the temperature of the 

attic. The pre- and post-retrofit comparison for House 1, however, shows a significant decrease 

in the duct jacket surface temperature. The ccSPF surface temperature profile is similar to that of 

the pre-retrofit duct surface but with far less variation related to system cycling. 

Infrared thermal imaging of the pre- and post-retrofit ductwork further validates the large 

increases in duct R-values and reductions in duct leakage. Figure 33 and Figure 34 compare 

thermal images of the pre-retrofit ductwork at House 2 with the same ductwork after 

encapsulation but before burial. Large thermal losses occur in both cases. 

With the AHU operating in cooling mode, the surface temperature of the ductwork before the 

retrofit was significantly lower than that of both the surrounding air and the encapsulated 

ductwork. The temperature of the exterior surface of the duct board was approximately 65°F in 

Figure 33. After encapsulation, the surface of the ccSPF is just under 100°F in Figure 34. As 

demonstrated by the previous graphs, the thermal imaging confirms the duct surface temperature 

has become nearly completely decoupled from the airstream temperature and more closely aligns 

with the ambient attic temperature.

Figure 33. Pre-retrofit infrared thermal imaging 

at House 2

Figure 34. Infrared thermal imaging of 

encapsulated ductwork at House 2

This conclusion is further supported by less direct observations of the field monitoring data. 

During the pre-retrofit monitoring, the discharge airstream temperatures were observed to rise 

above the temperature of the living space during periods when the air conditioner was not 

running. As a consequence, when the air conditioner turned on, the volume of hot air inside the 

ductwork was pushed back into the living space, resulting in an increased load on the house. 
36

Following the duct retrofits, the measured discharge air temperature was observed to stay at or 

below the living space temperature, eliminating this problem.

Pushing this volume of hot, unconditioned air into the living space undoubtedly increases the 

cooling load of the building, but quantifying the degree to which the cooling load is increased is 

difficult and cannot be accomplished using the data collected in this field study. Although 

monitoring was not conducted during the heating season, this retrofit is expected to also mitigate 

a similar heating load penalty. In new construction homes that use buried and encapsulated ducts, 

this peak load reduction benefit is typically accounted for by downsizing the equipment.

5.3 Qualitative Discussion of Condensation Potential 

Graphs of duct surface temperatures and attic dew points reveal some interesting information 

about the condensation potential of the pre- and post-retrofit ductwork (Figure 35 through 

Figure 40). When the surface temperature of the duct falls below the dew point of the 

surrounding air, there is a potential for condensation on the duct. In practice, it is often difficult 

to accurately measure the relative humidity of the surrounding air, and the attic ambient air can 

be used as a proxy for the surrounding air. In reality, however, the dense insulation retards 

moisture transfer from the attic air to the duct surface. Although Chasar and Withers (2012) 

partially validated the use of attic dew point temperatures and duct surface temperatures to 

measure condensation potential, more detailed modeling of the hygrothermal performance of 

buried ducts was performed in this study to accurately evaluate the condensation potential of a 

buried duct installation (see Section 9). 

The graphs in Figure 35 through Figure 40 reveal the change in condensation potential between 

the pre- and post-retrofit cases. The graphs show a 24-h period for each house for a typical 

summer day. These periods are identical to those shown in the graphs in Section 5.2. Two graphs 

are shown for each house, comparing the pre- and post-retrofit performance. In the pre-retrofit 

case, the attic dry bulb and dew point temperatures are plotted on each graph. This is overlaid 

with three boot surface temperature measurements on the left axis (solid lines) and three 

corresponding boot surface relative humidity measurements on the right axis (dashed lines). In 

the post-retrofit case, the surface temperatures and relative humidities are reported for the top 

surface of the ccSPF. The difference in graphed locations is caused by the selection of 

monitoring locations during the pre- and post-retrofit periods. The potential for condensation 

occurs when any of the three boot surface or ccSPF temperatures fall below the black line that 

represents the attic dew point temperature.

At first glance, the attic dew point temperatures fluctuate significantly compared to the outdoor 

dew point, raising questions about the validity of the attic dew point temperature measurements.

Although the mean ambient dew point temperature corresponding to the 1% design temperature 

at Jacksonville Naval Air Station is 70°F (ASHRAE 2009), the dew points in these attics reach

into the upper 80s. Furthermore, even though the ambient dew point is relatively constant 

throughout the day, the attic dew point in these houses drops to around 60°F during the night and 

rises rapidly into the upper 80s during the day. Although the behavior of the attic air is intriguing 

and cannot be easily explained, the attic dew point temperatures measured in this study do not 

seem to be inaccurate. The relative humidity sensors used in this study are accurate in the hot 

attic conditions, and the dew point calculations used in data postprocessing have been rigorously 

verified. Furthermore, a long-term monitoring study that included 20 houses in Florida (Arena et 
37

al. 2010) and a study of an early 1990s vintage home in Florida (Chasar and Wither 2012) found 

similar trends in attic dew point temperatures. Appendix A contains a more comprehensive 

discussion of this issue.

The pre-retrofit cases at all three houses showed a potential for condensation at the boot surface. 

Since significant amounts of condensation would likely stain the ceiling gypsum board, it was 

assumed that any accumulated condensation was minimal and/or able to dry out. Uncontrolled 

air leakage at the boot, which allows conditioned air to escape into the attic through unsealed 

gaps around the boot, could have resulted in the unintentional benefit of mitigating the 

condensation issues, similar to the results observed by Griffiths et al. (2002). The high potential 

of condensation observed in the pre-retrofit case is validated by the high relative humidities 

observed at the surface of the ductwork. These relative humidities approach saturation and the 

relative humidity sensors lose accuracy at these levels. 

As anticipated, the post-retrofit case at houses 1 and 3 showed no condensation potential at the 

surface of the ccSPF applied directly over the entire boot. The surface temperatures were 

significantly higher than the attic dew point. The temperature sensors in the pre-retrofit case 

were placed on the boot and therefore represent the worst-case scenario. In the post-retrofit case, 

however, the sensors were usually placed at the top of the ductwork on the surface of the ccSPF, 

which raises some questions about the condensation potential at other points along the ductwork 

profile. 

Surprisingly, the encapsulation and burial of the ductwork at House 2 did not mitigate the 

existing condensation potential of the pre-retrofit ductwork. As in the pre-retrofit case, it was 

assumed that any accumulated condensation was minimal and/or able to dry out because 

significant amounts of condensation would likely stain the ceiling gypsum board. Although the

exact cause of the post-retrofit condensation potential is unknown, it is possible that the long run

time of the undersized system resulted in lower surface temperatures.

Based on previous research, the 1.5 in. of ccSPF was applied to the ducts specifically to mitigate 

the condensation potential, regardless of the additional R-value provided by any additional 

insulation. The post-retrofit results for House 3, in which the ducts were not buried, can be 

contrasted with the results from the buried ducts in House 1. At House 3, the encapsulated duct 

surface temperature profiles aligned closely with the ambient attic conditions. At the boot 

locations shown in the figures, all measurements were taken beneath the layer of existing attic 

insulation. The temperature differential between the boot surface and attic ambient was larger in 

House 1, where blown-in insulation was added. 

Before the retrofit, the low temperatures and high relative humidities at the boot surfaces in 

House 1 showed a high condensation potential. These issues appear to have been mitigated with 

the buried and encapsulated duct approach. At House 2, the boot surface temperatures were 

lower than the dew point of the attic air in the pre-retrofit case. The relative humidity of the air at 

the surface of the duct, however, was significantly below saturation, and as a result, the 

condensation potential was low. In the post-retrofit case, the condensation potential was not 

worsened by the further burial of the ductwork.
38

Figure 35. Pre-retrofit boot surface temperatures and attic dew point at House 1

Figure 36. Post-retrofit ccSPF surface temperatures and attic dew point at House 1

4:00 a.m. 7:00 a.m. 10:00 a.m. 1:00 p.m. 4:00 p.m. 7:00 p.m. 10:00 p.m. 1:00 a.m. 4:00 a.m.

40

50

60

70

80

90

100

110

120

130

140

Temperature (°F)

4:00 a.m. 7:00 a.m. 10:00 a.m. 1:00 p.m. 4:00 p.m. 7:00 p.m. 10:00 p.m. 1:00 a.m. 4:00 a.m.

0

10

20

30

40

50

60

70

80

90

100

Relative Humidity (%)

Attic Temperature

Attic Dew Point

Boot Surface 1 Temp

Boot Surface 2 Temp

Boot Surface 3 Temp

Boot Surface 1 RH

Boot Surface 2 RH

Boot Surface 3 RH

4:00 a.m. 7:00 a.m. 10:00 a.m. 1:00 p.m. 4:00 p.m. 7:00 p.m. 10:00 p.m. 1:00 a.m. 4:00 a.m.

40

50

60

70

80

90

100

110

120

130

140

Temperature (°F)

4:00 a.m. 7:00 a.m. 10:00 a.m. 1:00 p.m. 4:00 p.m. 7:00 p.m. 10:00 p.m. 1:00 a.m. 4:00 a.m.

0

10

20

30

40

50

60

70

80

90

100

Relative Humidity (%)

Attic Temperature

Attic Dew Point

ccSPF Surface 1 Temp

ccSPF Surface 2 Temp

ccSPF Surface 3 Temp

ccSPF Surface 1 RH

ccSPF Surface 2 RH

ccSPF Surface 3 RH
39

Figure 37. Pre-retrofit boot surface temperatures and attic dew point at House 2

Figure 38. Post-retrofit ccSPF surface temperatures and attic dew point at House 2

4:00 a.m. 7:00 a.m. 10:00 a.m. 1:00 p.m. 4:00 p.m. 7:00 p.m. 10:00 p.m. 1:00 a.m. 4:00 a.m.

40

50

60

70

80

90

100

110

120

130

140

Temperature (°F)

4:00 a.m. 7:00 a.m. 10:00 a.m. 1:00 p.m. 4:00 p.m. 7:00 p.m. 10:00 p.m. 1:00 a.m. 4:00 a.m.

0

10

20

30

40

50

60

70

80

90

100

Relative Humidity (%)

Attic Temperature

Attic Dew Point

Boot Surface 1 Temp

Boot Surface 2 Temp

Boot Surface 3 Temp

Boot Surface 1 RH

Boot Surface 2 RH

Boot Surface 3 RH

4:00 a.m. 7:00 a.m. 10:00 a.m. 1:00 p.m. 4:00 p.m. 7:00 p.m. 10:00 p.m. 1:00 a.m. 4:00 a.m.

40

50

60

70

80

90

100

110

120

130

140

Temperature (°F)

4:00 a.m. 7:00 a.m. 10:00 a.m. 1:00 p.m. 4:00 p.m. 7:00 p.m. 10:00 p.m. 1:00 a.m. 4:00 a.m.

0

10

20

30

40

50

60

70

80

90

100

Relative Humidity (%)

Attic Temperature

Attic Dew Point

ccSPF Surface 1 Temp

ccSPF Surface 2 Temp

ccSPF Surface 3 Temp

ccSPF Surface 1 RH

ccSPF Surface 2 RH

ccSPF Surface 3 RH
40

Figure 39. Pre-retrofit boot surface temperatures and attic dew point at House 3

Figure 40. Post-retrofit ccSPF surface temperatures and attic dew point at House 3

4:00 a.m. 7:00 a.m. 10:00 a.m. 1:00 p.m. 4:00 p.m. 7:00 p.m. 10:00 p.m. 1:00 a.m. 4:00 a.m.

40

50

60

70

80

90

100

110

120

130

140

Temperature (°F)

4:00 a.m. 7:00 a.m. 10:00 a.m. 1:00 p.m. 4:00 p.m. 7:00 p.m. 10:00 p.m. 1:00 a.m. 4:00 a.m.

0

10

20

30

40

50

60

70

80

90

100

Relative Humidity (%)

Attic Temperature

Attic Dew Point

Boot Surface 1 Temp

Boot Surface 2 Temp

Boot Surface 3 Temp

Boot Surface 1 RH

Boot Surface 2 RH

Boot Surface 3 RH

4:00 a.m. 7:00 a.m. 10:00 a.m. 1:00 p.m. 4:00 p.m. 7:00 p.m. 10:00 p.m. 1:00 a.m. 4:00 a.m.

40

50

60

70

80

90

100

110

120

130

140

Temperature (°F)

4:00 a.m. 7:00 a.m. 10:00 a.m. 1:00 p.m. 4:00 p.m. 7:00 p.m. 10:00 p.m. 1:00 a.m. 4:00 a.m.

0

10

20

30

40

50

60

70

80

90

100

Relative Humidity (%)

Attic Temperature

Attic Dew Point

ccSPF Surface 1 Temp

ccSPF Surface 2 Temp

ccSPF Surface 3 Temp

ccSPF Surface 1 RH

ccSPF Surface 2 RH

ccSPF Surface 3 RH
41

6 Theoretical Effective and Apparent R-Values of Round 

Ductwork by Insulation Strategy

Observations of the field demonstration gave the team valuable insight into the feasibility of this 

retrofit strategy. As part of the field work, researchers identified barriers, collected performance 

data, and assessed concerns associated with condensation potential. Installed retrofit costs were 

also estimated based on the contractor costs for these retrofits (Section 10). The next critical step 

was to determine the potential energy savings and, ultimately, the cost effectiveness associated 

with the retrofit strategy. This was done through a combination of analytical calculations and 

modeled analysis.

Knowing effective R-values, which exclude heat transfer between the ductwork and the 

conditioned space, is necessary to calculate energy savings associated with encapsulated and 

buried and encapsulated ducts. Apparent R-values, which include heat transfer between the 

ductwork and the conditioned space, are also useful for comparing experimental results to 

theoretical calculations. Effective and apparent R-values can be calculated using Equations 1

and 2, respectively 8

duct

i

effective

r R

→

∆ = Qattic

2π T

(1)

duct interior duct

i

apparent Q

r R

→ + →

∆ = Qattic

2π T

(2)

where

Qattic→duct = the heat transfer between the attic and duct

Qinterior→duct = the heat transfer between the interior space and duct

ri = duct inner radius

ΔT = temperature difference between duct and attic.

Both effective and apparent R-values of round ductwork were calculated using either analytical 

or computational methods for four insulation strategies:

• Traditionally insulated ducts are insulated with traditional fiberglass duct wrap with 

rated R-values of 4.2, 6, and 8.

• Encapsulated ducts are traditionally insulated ducts encapsulated in a layer of ccSPF

insulation. These ducts have higher R-values and lower leakage rates than traditionally 

insulated ducts.

• Buried ducts are traditionally insulated ducts buried beneath loose-fill insulation at the 

ceiling plane. These ducts have higher R-values, but only slightly lower leakage rates,

than traditionally insulated ducts.

 8 In this report, all thermal resistance values R are given as totals of the entire depth of the application in units of hft2

-°F/Btu. Conductivities k are given as the reciprocal of the R-values per inch in units of Btu-in./h-ft2-°F. As a 

result, R-values should be considered the total of the application, and conductivities are the values per inch of 

application.
42

• Buried and encapsulated ducts are traditionally insulated ducts encapsulated in a layer 

of ccSPF insulation and buried beneath loose-fill insulation at the ceiling plane.

R-values of traditionally insulated ducts were calculated to accurately compare pre- and postretrofit performance using similar assumptions. R-values of buried ducts were calculated to 

compare the methodology used in this study to that of previous studies on buried ducts. The 

methodology used to calculate the traditional and buried duct insulation strategies also served as

the basis for the R-value calculations of encapsulated and buried and encapsulated ducts.

Since traditionally unburied ducts do not experience heat transfer with the conditioned space, 

apparent and effective R-values are equivalent (i.e., the heat transfer between the duct interior 

and the attic will equal heat losses measured under laboratory or field testing). These R-values 

can be calculated analytically in a closed-form equation. Buried ducts have different effective 

and apparent R-values, which must be computed using numerical methods. The next two 

sections outline the analytical calculations of traditional and encapsulated ductwork, followed by 

two sections that outline the modeled analysis for buried and buried and encapsulated ducts.

6.1 Traditionally Insulated Round Ductwork

When installed over round ductwork, the effective R-value of the insulation application does not 

equal the nominal R-value of the insulation. R-values per inch are rated for a material lying flat, 

and when accounting for the cylindrical installation of the geometry, effective R-values can be 

significantly different from the nominal values. Furthermore, the nominal R-value excludes the 

inner and outer surface films of air (Palmiter and Kruse 2006). The effective R-value of the 

ductwork Reffective in relation to the inner area of the ductwork is the sum of the inner surface film 

resistance Rinner, the actual R-value of the cylindrical insulation Rinsulatio n, and the outer surface 

film resistance Router, corrected for the ratio of the outer diameter do to the inner diameter di.

outer

o

i

effective inner insulation R

d

d R = R + R + (3)

Palmiter and Kruse (2006) found that the inner surface film resistance is dependent on duct inner 

diameter and the exterior surface film is independent of inner diameter. For this analysis, 

Palmiter and Kruse’s inner surface film resistances (see Table 7) and a slightly different outer 

film heat transfer coefficient of 1.76 Btu/h-ft2-°F, to be consistent with previous buried duct 

research, was used. Palmiter and Kruse further determined that the actual R-value of the 

ductwork can be found as a function of the rated R-value Rnominal, the inner diameter of the duct 

di, and the outer diameter of the duct do.

















− =

i

o

nominal

o i

i

insulation d

d R

d d

d R ln (4) 

Based on the modified outer surface film resistance, the effective R-values of round insulated 

ducts found for various insulation levels are shown in Table 8.
43

Table 7. Inner Surface Film Resistances by Duct Inner Diameter

Duct Inner Diameter (in.) Inner Surface Film Heat Transfer Coefficient 

(Btu/h-ft2-°F)

4 2.22

6 2.04

8 1.92

10 1.85

12 1.79

14 1.72

16 1.69

Table 8. Effective R-Values of Round Insulated Flexible Ducts 

Duct Inner Diameter 

(in.)

R-4.2 R-6.0 R-8.0

Thickness = 1.25 in. Thickness = 1.79 in. Thickness = 2.38 in.

Rinsulation Reffective Rinsulation Reffective Rinsulation Reffective

4 3.3 4.1 4.3 5.0 5.3 6.0

6 3.5 4.4 4.7 5.6 5.9 6.7

8 3.7 4.6 5.0 5.9 6.3 7.2

10 3.8 4.7 5.1 6.1 6.5 7.5

12 3.8 4.9 5.3 6.3 6.7 7.7

14 3.9 4.9 5.3 6.4 6.9 7.9

16 3.9 5.0 5.4 6.5 7.0 8.0

Source: Adapted from Palmiter and Kruse (2006).

A derivation of Equation 4 is given in Equation 5 because it is useful for deriving an equation for 

the thermal resistance of encapsulated ducts in Section 6.2. Assuming a homogenous material 

with cylindrical geometry, Fourier’s law of conduction can be stated as

(5)

where

A = the surface area at radius r (ft2)

k = the conductivity of the material (Btu-in./h-ft2-°F)

= the heat transfer rate (Btu/h)

= the rate of change of temperature with radius (°F/in.).

Since the area of heat flow at radius r is equal to the circumference of the circle 2πr times the 

length of the cylinder l, Equation 5 becomes

2 . dr

dT Q = πrlk  (6) 
44

The heat flow between any two radii, r1 and r2, of a homogenous material can be found by 

rearranging Equation 6 and integrating from r1 to r2.

(7) 

Performing the integration and solving for , Equation 7 becomes

( )

,

ln

2

1

2

1 2

















− =

r

r

lk T T Q π 

(8)

which is the general form for heat transfer between two radii of a homogenous material with

cylindrical geometry. 

To apply Equation 8 to the case of round insulated flexible ductwork, let the heat transfer flow 

from the outer radius ro to the inner radius ri. This is equivalent to replacing r1 to ro and r2 with 

ri. Recognizing that the definition of thermal resistance of the insulation Rinsulation is equal to the 

surface area at radius ri, multiplied by the temperature difference across the material, and 

divided by the heat transfer rate 

( )

, 2

Q

lr T T

R i o i

insulation 

− = π

(9)

Equation 9 becomes

.

ln

k

r

r

r

R o

i

i

insulation

















= (10)

The conductivity k is equal to the nominal thermal resistance divided by the thickness of the 

material 

. 2 nominal

o i

R

d d k − = (11)

Replacing the radii terms with diameters and replacing the conductivity term with Equation 11, 

Equation 10 yields 

ln ,

















− =

o

i

nominal

o i

i

insulation d

d R

d d

d R (12) 

which is Equation 4.
45

6.2 Encapsulated Ducts

If the ccSPF is installed over existing fiberglass insulation, Equation 3 can be used to calculate 

the effective R-value of the insulation. Equation 4, however, is not valid because of the different 

conductivities of the materials. For a ccSPF-encapsulated insulated flexible duct, the heat flows 

through two concentric cylinders of homogenous insulations with different conductivities 

(Figure 41). 

Figure 41. Flexible ductwork encapsulated in ccSPF

Since these materials are homogenous, the heat transfer from ri to rj can be found using 

Equation 8

( )

,

ln

2

















− =

i

j

f i j

r

r

lk T T

Q

π 

(13) 

where

kf = conductivity of fiberglass (Btu-in./h-ft2

-°F)

ri = inner radius of the duct (in.)

rj = radius of the junction point between the fiberglass and ccSPF (in.)

Ti = temperature at ri (°F)

Tj = temperature at rj (°F).

Similarly, the heat transfer from rj to ro is 

( )

,

ln

2

















− =

j

o

c j o

r

r

lk T T

Q

π 

(14) 

where

kc = conductivity of ccSPF (Btu-in./h-ft2

-°F)

ro = outer radius of the duct with insulation (in.)
46

To = temperature at ro (°F).

Since steady state conduction, the heat transfer from ri to rj is equal to the heat transfer from rj to

ro. Therefore, the heat transfer terms in Equations 13 and 14 are equivalent. Since the actual Rvalue of the duct is equal to 

( )

, Q

A T T

R i i o

insulation 

− = (15)

where Ai is the area of the duct at ri. Rinsulation can be found by solving Equations 13 and 14 for 

the right-hand term of Equation 15. By solving Equation 14 for Tj

, 2

ln

o

c

j

o

j T

lk

r

r Q

T +

















= π



(16) 

replacing Tj in Equation 13 with the right-hand term of Equation 16

, (17)

and grouping like terms, Equation 17 becomes

. (18) 

Since the area of the duct at the inner radius Ai is equal to 

, (19) 

2πl in Equation 18 can be replaced with Ai/ri

. (20) 

Rearranging Equation 20 to solve for the right-hand term of Equation 15

( )

f c

j

o

f

i

j

i c

i i o

k k

r

r k

r

r

r k

Q

A T T 































+















= −

ln ln

 ,

(21) 
47

and replacing the left-hand term for Rinsulation, the equation for Rinsulation as a function of the 

conductivities and radii of the materials is

f c

j

o

f

i

j

i c

insulation k k

r

r k

r

r

r k

R 































+















=

ln ln

.

(22) 

To write Equation 22 in terms of the R-values of the insulations, the conductivity of fiberglass 

can be replaced with

f,nominal

j i

f R

r r

k − = , (23)

and the conductivity of ccSPF can be replaced with

c,nominal

o j

c R

r r

k − = , (24) 

where Rf,nominal, and Rc,nominal represent the nominal R-values of the insulation application 

typically reported. Rewriting Equation 22 in terms of the nominal R-values yields

































− +















− =

j

o

o j

c,nominal

i

j

j i

f,nominal

insulation i

r

r

r r

R

r

r

r r

R

R r ln ln . (25)

Table 9 lists results for R-4.2, R-6, and R-8 round insulated ductwork encapsulated in 1- to 2.5-in 

of ccSPF. R-values in this table are given in h-ft2

-°F/Btu, where the surface area is based on the 

inner diameter of the duct. These results assume the same internal and external film resistances

and fiberglass R-value (3.36/in.) from the previous section. The results also assume that closedcell spray foam has an R-value of R-6.7/in.
48

Table 9. Effective R-Values of ccSPF Encapsulated Round Flexible Ducts by Insulation Thickness

R-4.2 Flex Duct R-6.0 Flex Duct R-8.0 Flex Duct

ccSPF

Thickness

(in.)

1 1.5 2 2.5 1 1.5 2 2.5 1 1.5 2 2.5

Nominal 

R-Value 10.9 14.3 17.6 21.0 12.7 16.1 19.4 22.8 14.7 18.1 21.4 24.8

4-in. 

Diameter

7.6 9.0 10.4 11.6 8.1 9.4 10.6 11.7 8.7 9.9 10.9 11.9

6-in. 

Diameter

8.6 10.4 12.0 13.6 9.3 11.0 12.5 13.9 10.1 11.6 13.0 14.3

8-in. 

Diameter

9.2 11.3 13.1 14.9 10.1 12.0 13.7 15.4 11.0 12.7 14.4 15.9

10-in. 

Diameter

9.7 11.9 13.9 15.9 10.6 12.7 14.7 16.5 11.7 13.6 15.4 17.1

12-in. 

Diameter

10.0 12.3 14.5 16.6 11.1 13.3 15.4 17.3 12.2 14.3 16.2 18.1

14-in. 

Diameter

10.2 12.7 15.0 17.2 11.4 13.7 15.9 18.0 12.6 14.8 16.9 18.9

16-in. 

Diameter

10.4 13.0 15.4 17.7 11.6 14.1 16.4 18.6 12.9 15.2 17.4 19.5

6.3 Buried Ducts

Unlike insulated and encapsulated round ductwork, the thermal resistance of buried ducts cannot 

be found analytically. Instead, a finite-element heat transfer model must be used to determine the 

effective R-value of buried ductwork. For this analysis, THERM 6.3, which is a two-dimensional 

heat-transfer modeling program developed by the Lawrence Berkeley National Laboratory

(LBNL), was used to calculate effective and apparent R-values. A diagram of the buried duct 

modeling configuration is shown in Figure 42.

Figure 42. Diagram of buried duct modeling configuration

The conductivities and thicknesses of the materials used in this model (see Figure 43 for diagram 

of buried duct configuration) are shown in Table 10. All emissivities were set to 0.9, which is the 
49

default for all materials in the THERM library. Simulations were ended with a heat transfer error 

less than 3%.

Table 10. Material Conductivities and Thicknesses

Material Conductivity k

(Btu-in./h-ft

2

-°F)

Thickness

(in.)

Loose-Fill Fiberglass 0.4

Depends on configuration (see 

Figure 5 for depth of burial 

classes)

Insulating Duct Wrap 0.298 Depends on duct insulation

16-in. on Center Joists With Loose-Fill

Fiberglass Insulation (Weighted 

Average of Framing and Insulation)

0.464 3.5

Gypsum Board 1.1 0.5

Table 11 gives boundary conditions, which include temperature and surface film resistance, for 

all surfaces. The film resistances for the different duct diameters are taken from Palmiter and 

Kruse (2006), and the attic and conditioned space film resistances are identical to those found in

previous studies (CARB 2003; Griffiths and Zuluaga 2004), on buried ducts. Boundary 

conditions were implemented using THERM’s simplified convection/linearized radiation model.

Table 11. Boundary Conditions

Boundary condition Temperature

(°F) 

Surface film resistance

(Btu/h-ft2-°F)

Conditioned Space 75 1.76

Attic Air 120 1.76

4-in. Duct Interior 55 2.22

6-in. Duct Interior 55 2.04

8-in. Duct Interior 55 1.92

10-in. Duct Interior 55 1.85

12-in. Duct Interior 55 1.79

14-in. Duct Interior 55 1.72

16-in. Duct Interior 55 1.69

Apparent and effective R-values for buried round ductwork are shown in Table 12 and Table 13, 

respectively. Effective R-values for R-4.2 ducts are similar to those calculated by Griffiths and 

Zuluaga (2004). Differences between these numbers might be attributable to different simulation 

programs and the error levels of the simulation. Apparent R-values are similar to effective Rvalues for smaller, partially buried ducts, but become significantly smaller for larger, deeply

buried ducts.
50

Table 12. Effective R-Values of Buried Round Ducts

R-4.2 Ducts R-6 Ducts R-8 Ducts

Burial Level Partially Fully Deeply Partially Fully Deeply Partially Fully Deeply

4-in. Diameter 5.6 8.4 14.3 7.1 9.9 15.2 8.5 11.2 16.1

6-in. Diameter 6.9 10.4 17.8 8.7 12.2 19.0 9.3 13.9 20.1

8-in. Diameter 8.1 12.0 20.7 10.2 14.1 22.1 12.3 16.2 23.5

10-in. Diameter 9.0 13.4 23.1 11.4 15.8 24.7 13.7 18.1 26.3

12-in. Diameter 9.9 14.7 25.2 12.5 17.2 27.0 15.0 19.7 28.8

14-in. Diameter 10.7 15.8 27.1 13.4 18.5 29.0 16.2 21.2 31.1

16-in. Diameter 11.5 16.8 28.9 14.3 19.8 31.0 17.3 22.6 33.1

Table 13. Apparent R-Values of Buried Round Ducts

R-4.2 Ducts R-6 Ducts R-8 Ducts

Burial Level Partially Fully Deeply Partially Fully Deeply Partially Fully Deeply

4-in. Diameter 5.6 8.3 13.3 7.1 9.7 14.3 8.5 11.0 15.3

6-in. Diameter 6.8 9.9 16.0 8.6 11.7 17.3 9.3 13.3 18.4

8-in. Diameter 7.8 11.3 18.1 9.9 13.2 19.6 11.8 15.1 21.0

10-in. Diameter 8.7 12.4 19.9 10.9 14.6 21.5 13.0 16.7 23.1

12-in. Diameter 9.4 13.4 13.4 11.8 15.7 23.2 14.1 18.0 24.9

14-in. Diameter 10.1 14.3 22.8 12.6 16.7 24.7 15.1 19.2 26.6

16-in. Diameter 10.7 15.1 24.1 13.3 17.8 26.1 16.0 20.3 28.1
51

6.4 Buried and Encapsulated Ducts

Effective and apparent R-values of buried and encapsulated ducts were similarly calculated using 

THERM. All boundary materials and conductivities were identical to the previous simulations, 

and the ccSPF insulation had a conductivity of 0.149 Btu-in./h-ft2

-°F and a thickness of 1.5 in. 

Figure 43 is a diagram of the buried and encapsulated duct configuration used in the modeling.

Effective and apparent R-values for buried and encapsulated ducts are shown in Table 14 and 

Table 15, respectively. Partially, fully, and deeply buried ducts are defined in the same manner 

as in previous research, as shown in Figure 5 (Griffiths and Zuluaga 2004; Griffiths et al. 2004; 

Griffiths et al. 2002).

Figure 43. Diagram of buried and encapsulated ducts

As previously mentioned, the differences between the apparent and effective R-values are caused 

by heat transfer between the air inside the duct and the interior building air. As suggested by the 

relatively small differences between effective and apparent R-values, the vast majority of the 

heat transferred to the duct comes from the attic at design conditions, as shown in Figure 44. The 

color gradient in Figure 44 indicates magnitude, not direction. The area directly under the duct 

shows heat flux from the interior space to the duct. The areas to the far right and left of the duct 

show heat flux from the attic to the interior space, and the areas in between have small heat 

magnitudes as the heat flux direction changes.

Figure 44. Heat flux magnitude through encapsulated and fully buried 8-in. diameter duct
52

Table 14. Effective R-Values of Buried and Encapsulated Round Ducts

R-4.2 Ducts R-6 Ducts R-8 Ducts

Burial Level Partially Fully Deeply Partially Fully Deeply Partially Fully Deeply

4-in. Diameter 12.8 15.7 20.4 13.6 16.3 20.7 14.4 17.0 21.1

6-in. Diameter 15.8 19.5 25.5 16.9 20.4 26.0 18.0 21.5 26.6

8-in. Diameter 18.4 22.6 29.6 19.7 23.8 30.3 21.0 25.0 31.1

10-in. Diameter 20.6 25.3 33.0 22.0 26.6 34.0 23.6 28.0 35.0

12-in. Diameter 22.5 27.5 36.0 24.1 29.0 37.1 25.8 30.6 38.3

14-in. Diameter 24.2 29.5 38.7 26.0 31.3 39.9 27.9 33.0 41.3

16-in. Diameter 25.8 31.4 41.1 27.7 33.2 42.5 29.7 35.2 44.0

Table 15. Apparent R-Values of Buried and Encapsulated Round Ducts

R-4.2 Ducts R-6 Ducts R-8 Ducts

Burial Level Partially Fully Deeply Partially Fully Deeply Partially Fully Deeply

4-in. Diameter 12.8 15.6 19.7 13.6 16.2 20.1 14.4 16.8 20.4

6-in. Diameter 15.7 18.9 23.9 16.7 19.8 24.5 17.7 20.8 25.1

8-in. Diameter 17.8 21.4 27.1 19.1 22.5 27.9 20.3 23.6 28.7

10-in. Diameter 19.7 23.5 29.7 21.0 24.8 30.7 22.5 26.1 31.7

12-in. Diameter 21.2 25.3 31.9 22.7 26.7 33.0 24.3 28.2 34.2

14-in. Diameter 22.6 26.9 33.9 24.3 28.5 35.2 26.0 30.1 36.5

16-in. Diameter 23.8 28.3 35.6 25.7 30.0 37.0 27.5 31.8 38.5
53

Since the ductwork of the homes tested in this report were buried under a mound of loose-fill 

insulation, not buried under a plane of insulation, effective and apparent R-values of 

encapsulated ducts buried under a mound of insulation were also computed. Mounded-buried 

ducts are defined for the purpose of modeling as bounded by an arc that peaks 1 in. above the 

duct and meets the line tangent to the bottom with a distance of three times the duct’s outer 

diameter, 3do, between the edges of the fiberglass (Figure 45).

Figure 45. Definition of mounded-buried ducts

Table 16 contains effective and apparent R-values for mounded-buried encapsulated ducts. These 

R-values are surprisingly similar to fully buried encapsulated duct R-values. The slight reduction 

in R-value for mounded ducts is attributable to the increased heat transfer through the sides of 

the duct. A large amount of this heat transfer increase is offset by an increase in the amount of 

insulation above the duct (1 in. of insulation was assumed to cover the top of the duct for the 

mounded case, and no insulation was assumed for the fully buried case).

Table 16. R-Values of Mounded-Buried R-4.2 Ducts Encapsulated in 1.5 in. of ccSPF

Duct Inner Diameter (in.) Reffective Rapparent

4 15.2 15.2

6 18.7 18.5

8 21.6 20.9

10 24.1 22.9

12 26.2 24.6

14 28.1 26.2

16 29.8 27.5

R-values of buried and encapsulated ducts increase dramatically with increased inner diameter 

(Figure 46). The direct relationship between effective R-value and duct diameter could be caused 

by two properties of the insulation geometry. First, increasing the duct size also increases the 

total depth of the fiberglass insulation, which reduces the heat transfer from the bottom of the 

duct toward the attic. Second, just as the R-values of traditionally insulated ducts change with 

duct inner diameter because of the cylindrical geometry of round ductwork, the R-values of 

buried and encapsulated round ductwork will be affected by inner diameter. Since the impact of 

inner diameter is greater for buried and encapsulated ducts than for traditionally insulated round 

ductwork, the increased R-values are likely caused by a combination of these factors.
54

Figure 46. Effective R-values of R-4.2 buried and encapsulated ducts

Figure 47. Apparent R-values of 6-in. buried ducts encapsulated in ccSPF by attic temperature

Finite-element modeling showed that effective R-values are independent of boundary condition 

temperatures, but apparent R-values are highly dependent on boundary condition temperature. 

0

5

10

15

20

25

30

35

40

45

2 4 6 8 10 12 14 16 18

Effective R-value (hr-ft2-°F/Btu)

Inner Duct Diameter (in)

Partially Mounded Fully Deeply

0

5

10

15

20

25

30

70 80 90 100 110 120 130 140 150

Apparent R-value of Duct Interior (hr-ft2-°F/Btu)

Attic Temperature ( F)

Deeply Fully Mounded Partially
55

The large insulation levels, coupled with the location of the ducts in relation to the interior space, 

mean that heat gains or losses are dependent on both the interior temperature and the attic 

temperature. As a result, changes in the attic temperature, duct interior temperature, or interior 

temperature will result in a change in the heat flow through the cross-section and a change in 

apparent R-value. Figure 47 shows the effect of attic temperature on the R-value of a 6-in. buried 

and encapsulated duct. The temperature change causes a large change in apparent R-value, 

particularly for deeply buried ducts. In this case, the apparent R-value dropped by almost 40% 

when the temperature changed from 120°F to 75°F. Table 17 summarizes effective R-values of 

an 8-in. duct by insulation strategy.

Table 17. Summary of Duct Effective R-Values for 8-in. Duct by Insulation Strategy

Duct Configuration R-4.2 Ducts R-6 Ducts R-8 Ducts

Traditional Hung Ducts 4.6 5.9 7.2

Hung Ducts Encapsulated in 1.5 in. of ccSPF 11.3 12.0 12.7

Partially Buried 8.1 10.2 12.3

Fully Buried 12.0 14.1 16.2

Deeply Buried 20.7 22.1 23.5

Encapsulated in 1.5 in. of ccSPF and Partially Buried 18.4 19.7 21.0

Encapsulated in 1.5 in. of ccSPF and Fully Buried 22.6 23.8 25.0

Encapsulated in 1.5 in. of ccSPF and Deeply Buried 29.6 30.3 31.1
56

7 Effective Heat Transfer Coefficients of Test Home Ductwork 

and R-Value Validation

Using the R-values calculated in Section 6, the overall effective and apparent UA values of the 

ductwork in the three Jacksonville houses were calculated and are shown in Table 18. For 

plenum boxes used to connect trunks and branches together, the R-value cannot be modeled in 

THERM because of the three-dimensional nature of the geometry. Instead, these boxes were 

assigned an R-value equal to the R-value of a round duct of a similar size. Although not exact, 

this method should give an approximate result for these boxes. For House 2, which has 

rectangular ductwork, each of the duct sizes were modeled in THERM. The existing 

configuration was already partially buried as a result of the configuration of the ductwork and the

fiberglass blown-in insulation (see Figure 8 and Figure 10). The fiberglass insulation was 

assumed to be installed at a level 3 in. above the bottom of the duct. Post-retrofit ducts were 

assumed to have mounded burial with a configuration similar to that shown in Figure 45, and the 

dimension do equals the outer width of the duct parallel to the ceiling.

Table 18. Theoretical UA Values for Monitored Jacksonville Houses

Pre-Retrofit Post-Retrofit Reduction 

in 

Apparent

UA

(%)

Reduction 

in Effective 

UA

(%)

Area 

(ft2

)

Apparent 

UA

(Btu/h-°F)

Effective 

UA 

(Btu/h-°F)

Area

(ft2)

Apparent 

UA 

(Btu/h-°F)

Effective 

UA 

(Btu/h-°F)

House 

1 339.3 72.1 72.1 374.4 17.4 17.1 75.9 76.3

House 

2 546.1 91.9 90.3 563.1 24.5 21.6 73.3 76.1

House 

3 214.4 46.2 46.2 214.4 19.3 19.3 58.2 58.2

The values shown in Table 18 can be validated using test data collected during field monitoring. 

When the space-conditioning system is running in steady-state operation, the heat transfer 

between the conditioned air inside the duct and the attic temperature can be simplified as a 

heat exchanger. Using this simplification, the heat transfer between the duct and the attic is 

governed by 

LMTD

Q UA  = (26)

where

Q = the heat transfer rate between the duct and attic (Btu/h), as defined

by Equation 28

U = the heat transfer coefficient (Btu/h-ft2

-°F)

A = interior surface area of duct (ft2), and

LMTD = log mean temperature difference (°F), as defined by Equation 27.

The LMTD is the logarithmic mean between two ends, A and B, of a heat exchanger.
57

B

A

A B

T

T

T T LMTD

∆

∆

∆ − ∆ =

ln (27)

In this application, stream A is the supply temperature of the duct, where 

∆TA = Tsupply −Tattic ,

and stream B is the discharge temperature, where

∆TB = Tdischarge −Tattic .

The heat transfer rate is 

( ) Q V Tsupply Tdisch arg e = 1.08×  × −  , (28)

where V is the volumetric flow rate (ft3/min).

For typical trunk and branch distribution systems, however, the overall heat transfer coefficient 

of the system will not equal the sum of each discharge because multiple discharges will share the 

same branch. Instead, each heat transfer rate and LMTD must be calculated independently. The 

LMTDs must be weighted as a fraction of the total airflow. The overall UA is the ratio of the 

summed heat transfer rates and weight LMTDs. This equation is defined as

∑

∑ =

i i

i

w LMTD

Q UA



, (29) 

where i represents the index of the discharge register and wi is the airflow fraction V

i V for 

discharge i.

To ensure that these calculations are valid, periods of operation that appear to be largely steady 

state were isolated from the monitored data. Since apparent R-values vary with temperature, 

periods with attic temperature near 120°F, which is the boundary condition used in the finite 

element modeling, were used. The entire UA of the system cannot be effectively calculated 

because only four duct discharge temperatures were measured. Instead, the percent change in UA 

after the retrofit can be calculated from the relative apparent UAs calculated using the method 

described previously. 

Table 19 lists mean apparent UA values and standard deviations from the steady-state period 

identified in the data. The removal of the large hole in the ductwork at House 2 led to changes in 

the airflows of this system, which resulted in difficulties in measuring UA. The post-retrofit UAs 

for House 2 were significantly higher than the pre-retrofit UAs and the House 2 data was deemed 

useless for this analysis. 
58

Table 19. Relative Apparent UAs as Measured in Test Houses

House

Pre-Retrofit Post-Retrofit

Reduction in 

Apparent UA

(%)

Apparent 

UA Tattic

Apparent 

UA Tattic

N Mean Std. Mean Std. N Mean Std. Mean Std.

1 8 13.4 0.6 120.8 4.0 3 2.51 0.72 120.7 5.4 81.3

3 6 34.1 0.01 120.6 2.4 4 11.2 0.4 121.3 3.7 67.2

The apparent UA reductions listed in Table 18 and Table 19 are relatively close, with UA 

reductions varying by no more than 10%. Given that the UAs calculated theoretically cannot 

account for plenum boxes and the issues with measuring apparent UA using a method that 

assumes steady-state operation, the team concluded that these values reflect a reasonable level of 

agreement.
59

8 ASHRAE 152 Distribution System Efficiencies

The heat transfer coefficients calculated in Sections 6 and 7 are necessary for calculating the 

efficiency of the thermal distribution systems. The efficiency of thermal distribution systems can 

be estimated using ASHRAE Standard 152, which determines “the efficiency of space heating 

and/or cooling thermal distribution systems under seasonal and design conditions” (ASHRAE 

2004; 2). Standard 152 provides a methodology for calculating the delivery effectiveness (DE) 

and distribution system efficiency (DSE) of a thermal distribution system.

The DE is defined as “the ratio of the thermal energy transferred to or from the conditioned 

space to the thermal energy transferred at the equipment distribution system heat exchanger”

(ASHRAE 2004; 2). The DSE includes losses calculated in the DE and adds losses associated 

with the impact of unbalanced leakage on building infiltration, energy recovery from losses to 

buffer zones, and losses associated with space-conditioning system cycling. The DSE is

(1 ), DSE = DEcorrFequipFload − Fcycloss (30) 

where

Fequip = equipment factor

Fload = infiltration factor

Fcycloss = equipment cycling factor

DEcorr = DEs corrected for regain.

All DE and DSE calculations were performed in compliance with ASHRAE Standard 152-2004, 

except where explicitly noted. A spreadsheet developed by LBNL in this analysis (LBNL 2003).

To calculate the corrected DE, regain factors for supply and return ducts are needed. Airflow 

between the attic and conditioned space was not measured, so these values cannot be calculated 

explicitly. Instead, the default regain factors of 0.1 used in LBNL’s ASHRAE 152 spreadsheet 

were used in this analysis. 

Calculation of the infiltration factor requires effective leakage area Ln. Blower door tests were 

performed at 50 Pa, and Ln was calculated using an approximately equivalent expression for 

single-story homes (Sherman 1986), which is shown in Equation 31. See ASTM (1987), 

ASHRAE (1993), and ASHRAE (1998) for detailed information about calculating Ln exactly.

20

ACH50 Ln ≈ (31) 

Input values used in the ASHRAE 152 calculation are taken from Table 4 and Table 5. ASHRAE 

152 DEs, corrected DEs, and DSEs are summarized in Table 20 through Table 22. As the 

summary tables show, the retrofit resulted in a dramatic increase, typically 12% or greater, in DE

in all three homes. Similarly, the retrofit resulted in DSE increases. The cooling seasonal DSE 

increased by 11% in House 1, by 25% in House 2, and by 10% in House 3. 

The similar DE improvements in House 1, in which the ducts were encapsulated and buried, and 

House 3, in which the ducts were only encapsulated, could be explained by several factors. First, 

the improvements in duct leakage will have a larger impact in R-value because duct leakage has 
60

a one-to-one direct relationship with duct losses. Second, the ductwork was lengthened in House 

1, which will reduce the impact of the R-value increase. The remaining DE improvements are 

associated with the increased R-value.

Table 20. Pre- and Post-Retrofit ASHRAE 152 DE and DSEs for House 1

Pre-Retrofit Post-Retrofit

DE

(%)

Corrected DE

(%)

DSE

(%)

DE

(%)

Corrected DE

(%)

DSE

(%)

Heating Design 83 85 84 96 96 98

Heating Seasonal 84 86 85 96 97 97

Cooling Design 80 82 82 95 96 97

Cooling Seasonal 85 86 86 97 97 97

Table 21. Pre- and Post-Retrofit ASHRAE 152 DE and DSEs for House 2

Pre-Retrofit Post-Retrofit

DE

(%)

Corrected DE

(%)

DSE

(%)

DE

(%)

Corrected DE

(%)

DSE

(%)

Heating Design 61 64 58 84 85 84

Heating Seasonal 66 69 65 86 87 86

Cooling Design 43 47 39 77 78 73

Cooling Seasonal 59 62 54 83 84 79

Table 22. Pre- and Post-Retrofit ASHRAE 152 DE and DSEs for House 3

Pre-Retrofit Post-Retrofit

DE

(%)

Corrected DE

(%)

DSE

(%)

DE

(%)

Corrected DE

(%)

DSE

(%)

Heating Design 84 85 85 95 95 95

Heating Seasonal 85 86 85 95 96 95

Cooling Design 82 84 79 94 94 90

Cooling Seasonal 86 87 81 95 96 91
61

9 Condensation Potential of Encapsulated and Buried Ducts

As noted in Section 5.3, condensation will occur on the surface of the duct if the surface 

temperature is below the dew point of the surrounding air. Although the attic air temperature and 

relative humidity were monitored during the field testing, the dew point of the air surrounding 

specific points on the duct profile cannot be determined because of the complex process of vapor 

transport. The steady-state, two-dimensional, thermal modeling outlined in previous sections was 

combined with one-dimensional, dynamic, hygrothermal modeling to predict the potential for 

condensation on the surface of the duct. 

The hygrothermal modeling program WUFI Pro 5 was used to simulate the movement of water 

vapor through the attic assembly. The hygrothermal modeling was used to determine the dew

point of the air through the depth of the attic insulation assembly. These dew points were then 

compared to the surface temperatures of the duct at each depth, as calculated using twodimensional steady-state modeling.

This analysis was conducted for buried ducts and buried and encapsulated ducts using the worst 

case of the modeling configurations described previously, a 4-in. R-4.2 duct deeply buried 

beneath attic insulation. Attic temperatures and relative humidities from one of the houses 

monitored in the research were used to simulate the conditions of an attic in a hot-humid climate. 

A constant temperature of 75°F and a relative humidity of 50% were applied to the interior 

conditions. The supply duct air was not explicitly modeled because of the geometric limitations 

of WUFI Pro 5. The maximum dew points along the depth of the duct corresponded to the 

highest attic temperatures monitored in this study. The resulting air dew points were compared to 

the steady-state surface temperatures predicted by THERM for the boundary attic condition 

of 130°F.

The results from the analysis (Figure 48) predict condensation issues for the buried ducts without 

additional ccSPF insulation, which was observed by Griffiths et al. (2002). Figure 48 shows the 

potential for condensation across the entire surface area of the duct under these conditions. The 

surface temperature of the duct, marked as black isotherms, is lower than the dew point of the 

surrounding air, which is marked in red. This case is more severe than the conditions observed 

by Griffiths et al. (2002) because it ignores the effects of leakage from the interior and the duct. 

As a result, this case can be viewed as the worst case potential for condensation. 

A similar analysis of a buried and encapsulated duct shows no potential for condensation 

(Figure 49). The surface temperature is lower than the dew point of the air across the entire depth 

of the insulation. These results validate that the potential for condensation can be mitigated by 

encapsulating buried ducts in a layer of 1.5-in. ccSPF insulation. The closeness of duct surface 

temperature and the dew point of the surrounding air at the bottom of the duct emphasizes that 

1.5 in. of ccSPF should be the minimum insulation level applied to R-4.2 ducts. For ductwork 

with higher existing R-values, lower ccSPF thicknesses might be possible, although consistently 

applying lower thicknesses might not be possible.
62

Figure 48. R-4.2 duct (4 in.) deeply buried in fiberglass insulation. Isotherms from the steady state

model (in black) are compared to the dew point of the air at several key locations (in red).

Figure 49. R-4.2 duct (4 in.) deeply buried in fiberglass insulation and encapsulated in 1.5 in. of 

ccSPF. Isotherms from the steady-state model (in black) are compared to the dew point of the air 

at several key locations (in red).
63

10 Predicted Energy and Cost Savings

Energy savings were predicted in Building Energy Optimization (BEopt)E+ 1.2 using the 

specifications given in Table 4 and Table 5. The pre-retrofit BEopt models were calibrated using 

utility bill data collected for 20 months before the start of the retrofit. Heating and cooling setpoint temperatures and miscellaneous electric load multipliers were modified to match the 

observed utility bills. A utility bill analysis was used to match modeled and observed utility bills 

and account for differences between the observed temperatures and the typical meteorological 

year. The utility analysis was performed using a multivariate, linear, least-squares regression of 

the form

E = β1Ndays + β 2CDD75 + β 3HDD65 , (32)

where

E = electricity use during the billing period (kWh)

Ndays = number of days in the billing period

CDD75 = cooling degree days at base 75°F

HDD65 = heating degree days at base 65°F

β1, β2, and β2 = regression coefficients.

This methodology is similar to that used by the PRISM model, but employs a fixed base for the 

CDD and HDD calculations (Fels 1986). Table 23 shows the regression statistics—including the 

coefficient value, t-statistic, and p-value—for the pre-retrofit utility bills collected for 20 months 

before the retrofits began. The resulting regression coefficients were statistically significant at 

the 95% confidence level. At House 3, the initial regression found a negative coefficient for the 

HDD variable that was statistically insignificant at the 95% confidence level. The resulting 

coefficient was thus forced to zero and ignored in this analysis. 

Table 23. Regression Statistics for Pre-Retrofit Utility Bills

House 1

R2 = 0.974

N = 20, dofa = 17

House 2

R2 = 0.985

N = 20, dof = 17

House 3

R2 = 0.989

N = 20, dof = 18

coef. t p coef. t p coef. t p

Ndays 16.3 5.15 7.97 × 10–5 50.8 11.8 1.28 × 10–9 54.25 30.0 8.14 × 10–5

CDD75 4.02 4.95 1.21 × 10–4 7.40 6.86 2.77 × 10–6 4.52 7.34 8.12 × 10–5

HDD65 2.56 8.91 8.20 × 10–8 1.06 2.70 1.54 × 10–2 N/A N/A N/A a

Degrees of freedom

Table 24 shows identical statistics for the weather data used in and the energy predictions by 

BEopt. The coefficients are all statistically significant at the 95% confidence level and match the 

coefficients in Table 23 far better than typical energy models. Because these retrofits were 

completed relatively recently, insufficient data are available to use utility bill analysis to compare 

pre- and post-retrofit energy savings. To visualize these results, the resulting pre-retrofit 

predicted utility bills are compared in Figure 50 to a 1-year period as observed at the 

Jacksonville houses.
64

Table 24. Regression Statistics for Pre-Retrofit Energy Predictions (BEoptE+ 1.2)

House 1

R2= 0.999

N = 12, dof = 9

House 2

R2 = 0.999

N = 12, dof = 9

House 3

R2 = 0.997

N = 12, dof = 10

coef. t p coef. t p coef. t p

Ndays 25.0 24.7 1.38 × 10–9 52.5 25.9 9.34 × 10–10 49.8 39.7 2.43 × 10–12

CDD75 2.79 11.5 1.11 × 10–6 8.11 16.6 4.61 × 10–8 4.41 11.5 4.37 × 10–7

HDD65 1.63 10.1 3.81 × 10–6 1.14 3.53 6.42 × 10–3 N/A N/A N/A

For the houses with buried and encapsulated ducts, two scenarios were investigated: adding 

blown-in insulation only around the ductwork, and adding blown-in insulation over the entire 

attic plane. In the first scenario, the additional insulation was ignored and the attic insulation was 

modeled at the same level as the pre-retrofit case. For the second scenario, 12 in. of additional 

blown-in attic insulation was assumed for an additional insulation level of R-30. At House 2,

where the ducts were already partially buried, the additional insulation raises the R-value only 

from the existing R-24 to R-25.

The energy savings are relatively robust at 8% to 20% of total annual energy. These scenarios 

are compared to the alternative method of reducing duct losses by converting the attic to an 

unvented attic. House 2 was not modeled using this method because the location of the AHU 

precludes an accurate estimate of the duct losses. Without actually installing this configuration, 

duct leakage cannot be estimated.

Figure 50. Predicted versus actual pre-retrofit energy use

0

500

1,000

1,500

2,000

2,500

3,000

3,500

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Monthly Site Energy Use (kWh)

House 1 - Actual House 2 - Actual House 3 - Actual

House 1 - Predicted House 2 - Predicted House 3 - Predicted
65

These savings are reasonable in comparison to the predicted cooling and heating energy savings 

derived from the ASHRAE 152 seasonal delivery system efficiencies (Table 26). The predicted 

energy savings level from BEopt is consistently higher than that predicted by ASHRAE 152. 

Since these methods are both analytical estimates of the savings, it is difficult to determine which 

is more accurate.

Table 25. Predicted Energy Savings From BEoptE+ 1.2

House 1 House 2 House 3

Encapsulated + 

Mounded 

Buried

Encapsulated 

+ Fully 

Buried

R-30 

Unvented 

Attic

Encapsulated + 

Mounded 

Buried

Encapsulated 

+ Fully 

Buried

Encapsulated

R-30 

Unvented 

Attic

Annual Site Energy 

Savings (kWh) 1,013 1,365 777 4,566 5,203 1,032 1,190

% Savings Over 

Existing 8 10 6 17 20 5 5

Annual Utility Bill 

Savings Over 

Existing ($)

127 171 97 571 650 129 149

Table 26. Comparison of Predicted Savings Between BEopt and ASHRAE 152 (Duct Retrofit Only)

Percent Predicted Energy Savings 

from ASHRAE 152 (Seasonal DSE)

Percent Predicted Energy 

Savings from BEopt 

Cooling Heating Cooling Heating

House 1 10.99 10.53 14.21 11.88

House 2 21.52 19.77 23.34 26.36

House 3 11.34 12.37 12.43 17.07 a This includes the added duct R-value of burying ducts at houses 1 and 2, but it excludes the additional R-value of 

the ceiling assembly after burial.

Table 27 shows the installed costs of the retrofits. These costs were derived from the HVAC and 

insulation contractor invoices for labor and materials, with the exception of the ccSPF insulation. 

The ccSPF costs are based on material and equipment data found in RSMeans (2011). The ccSPF 

was donated by the manufacturer, a partner in this project. As a result, actual ccSPF costs were 

not available but data on ccSPF quantities were used.

Table 27. Installed Cost of Encapsulated and Buried and Encapsulated Ducts at Houses

House 1 House 2 House 3

Ductwork Area (ft2) 483 719 280

Attic Insulation Area (ft2) 521 521 N/A

Installed Cost ($) 2,990 3,806 956
66

The costs given in Table 27 represent the costs associated with this demonstration project. Not 

having undertaken this retrofit before, the contractor pricing is assumed to reflect time needed 

for training, setup, and coordination. These costs, however, do not include time required for 

coordination among the contractors and with the homeowners, which was carried out by the 

Building America team.

For the cost analysis, the researchers assumed that the installed costs will decrease as this 

becomes a widespread practice. The cost estimates for the measures shown in Table 28 assume 

that this retrofit is a mature building practice and processes are streamlined. The cost estimates 

are derived from the National Renewable Energy Laboratory’s (NREL’s) National Residential 

Efficiency Measures Database, RSMeans (2011), and the invoices from the contractors that 

performed the installation for this project. Again, no costs associated with coordination were 

included in these estimates.

Costs for the duct spray foam were taken from RSMeans and multiplied by 1.75 to account for 

the increased difficulty of the installation, material waste, and greater inconsistency of the 

application. Contractor invoices were used for comparison, but the cost estimates reflect average 

pricing and assume that the process has been streamlined. The projected costs, rather than the 

actual, were used for the cost-effectiveness analysis calculations.

Annualized returns were calculated assuming a 30-year analysis period, a 3% inflation rate, a 3% 

real discount rate, and a 7% 5-year loan. The lifetimes of the ducts and insulation were assumed 

to be 30 years. The cost savings are shown in Table 29. Encapsulated and buried and 

encapsulated ducts were shown to be cost effective; converting the attic to an unvented attic, 

though, was not cost effective. Generally, the added benefit of completely covering the attic 

plane with loose-fill insulation was not more cost effective than mounded burial of the ducts.

Houses 1 and 3 have no return ducting, which is common for Florida houses. Many Florida 

houses have AHUs in mechanical rooms with louvered doors with the bottom of the AHU open. 

Another alternative is that the AHU sits on top of a return plenum with a return grille directly 

below a closet door. As a result, the total costs and benefits of the measure are reduced for this 

case. House 2 presents the alternative where there is return ducting, leading to greater energy 

savings at a higher cost.
67

Table 28. Cost Estimates for Duct Retrofits

House 1 House 2 House 3

Encapsulated 

+ Mounded 

Buried

Encapsulated 

+ Fully 

Buried

R-30 

Unvented 

Attic

Encapsulated 

+ Mounded 

Buried

Encapsulated 

+ Fully 

Buried

Encapsulated

R-30 

Unvented 

Attic

Ductwork Area (ft2) 483 483 483 719 719 280 280

Attic Insulation Area (ft2) 521 2,097 2,541 521 2,086 N/A 2,315

Duct Spray Foam ($1.89/ft2)

(RSMeans × 1.75) $912 $912 N/A $1,358 $1,358 $530 N/A

Blown-in Insulation 

($0.63/ft2) (NREL 2012) $328 $1,321 N/A $328 $1,314 N/A N/A

Roof Deck Spray Foam 

($3.58/ft2) (NREL 2012) N/A N/A $9,096 N/A N/A N/A $8,287

Remove Blown-in Insulation 

(Contractor Cost) N/A N/A N/A $756 $756 N/A N/A

Seal Boots and Place Rigid 

Insulation (Contractor Cost) $63 $63 N/A $84 $84 $63 N/A

Duct Reconfiguration

(Contractor Cost) $426 $426 N/A $864 $864 N/A N/A

Total Cost ($) $1,730 $2,722 $9,096 $3,391 $4,376 $530 $8,287

Table 29. Annualized Savings by Retrofit Measure and House ($)

House 1 House 2 House 3

Encapsulated 

+ Mounded 

Buried

Encapsulated + 

Fully Buried

R-30 

Unvented 

Attic

Encapsulated + 

Mounded 

Buried

Encapsulated + 

Fully Buried Encapsulated

R-30 

Unvented 

Attic

Total Cost 1,730 2,722 9,096 3,391 4,376 530 8,287

Utility Bill Savings 127 171 97 571 650 129 149

Annualized 

Savings 10 –28 –821 513 209 141 –655
68

11 Conclusion

This report takes a multifaceted approach, including field testing and analytical methods, to 

evaluate the cost effectiveness, energy savings potential, and condensation potential of 

encapsulated and buried and encapsulated ducts. Although field testing yielded valuable 

information about the installed system performance, variations in field conditions necessitated 

further analysis. As with any field testing of occupied homes, quantifying energy savings was 

difficult because of the dramatic changes in occupant behavior and building system performance. 

Despite these difficulties, field testing was necessary to establish data-driven support for other 

analytical methods. The following sections summarize the conclusions for each of the key 

sections of the report.

11.1 Retrofit Methodology and Lessons Learned

The retrofitting methodology used in this study was evaluated in terms of effectiveness, 

feasibility, and safety. Although the implemented methodology was effective, several issues 

must be more carefully considered in future installations, such as protecting existing materials, 

ensuring proper spray foam coverage, and coordinating among trades. Section 4 covers these 

topics in greater detail.

First, can lighting, exhaust fans, flues, security wire, plumbing, and other services located in or 

along the ceiling plane can get covered in foam and/or require baffling for protection during the

retrofit. Second, training is required to emphasize the importance of ensuring that the minimum 

application thickness of the ccSPF (1.5 in. at R-6.7 h-ft2-°F/Btu-in.) is consistently achieved to 

mitigate the potential for condensation. In areas where applying ccSPF is difficult, such as the 

undersides of ductwork too low to reach with the spray nozzle, ensuring proper installation is 

important. Two options can be used to solve this problem. The installer can cocoon the duct in 

ccSPF and directly seal it to the sheetrock. Alternatively, a piece of rigid insulation can be 

inserted under the ductwork to serve as a substrate for the foam and ensure that the minimal 

insulation thickness is achieved on the underside of the ducts.

Finally, coordination between trades must be considered. The buried and encapsulated duct 

strategy is a multitrade effort. To offer this strategy as a package, two or three contractors would 

be required: a ccSPF installer, a blown-in insulation installer, and an HVAC installer. (The latter 

might be optional depending on local codes). This strategy requires a significant amount of 

coordination, and this role should be assigned early in the process. The encapsulated duct 

strategy, on the other hand, is a single-trade effort, requiring significantly less coordination and

fewer periods of access to the home. In addition, this strategy uses a single insulation material 

and costs less. 

11.2 Field Evaluation

Changes in the mechanical and distribution system configuration and operation at each house 

created challenges for interpreting long-term field monitoring and performance testing data. 

Despite these challenges, the data gave interesting insights into duct system performance and 

validated the calculated system performance. These topics are discussed in greater detail in 

Section 5.
69

Field testing and monitoring showed promising improvements in system performance. Duct 

leakage rates were reduced considerably. At the same time, airflows remained relatively constant 

in both pre- and post-retrofit testing. The R-value of the encapsulated-only ductwork was 

approximately half that of the buried and encapsulated ductwork. A qualitative analysis of the 

monitoring data showed a large improvement in duct R-values and no worsening of any existing 

condensation potential.

11.3 Effective and Apparent R-Values

In order to calculate heat transfer gains and losses from ductwork in unconditioned attics, 

effective and apparent R-values were calculated using several techniques. Section 6 provides 

greater detail on these calculations. Analytical calculations were used to determine the effective 

and apparent R-values of traditional ductwork and encapsulated ducts. Because buried and 

buried and encapsulated ducts are more complex than traditional insulation systems, a finiteelement heat transfer analysis was required to calculate these values for the buried duct strategy.

The resulting R-values from both methods matched existing literature well and showed large 

improvements in R-values for encapsulated and buried and encapsulated ducts. The calculations 

found that, by encapsulating the ductwork in 1.5 in. of ccSPF (R-6.7 h-ft2-°F/Btu-in.), the 

existing R-4.2 flexible ductwork can be improved to values between R-9 and R-13, depending on 

the size of the duct. Comparable buried and encapsulated ducts will have a significantly higher

increase in R-values, ranging between R-16 and R-31.

11.4 Effective Heat Transfer Coefficients

The calculated R-values listed in Section 6 were validated using field monitoring data in 

Section 7. The reduction in apparent UA values, based on field data, correlated well with the 

theoretical UA value reduction that was developed based on the analysis in Section 6. The values 

were found to be within 10% of one another, which is a reasonable alignment given that the 

calculation assumes steady-state operation.

11.5 ASHRAE Standard 152 Distribution System Efficiency

Based on the field monitoring and analytical tools used in Section 6 and validated in Section 7, 

both DE and DSE were calculated for the three test homes. The calculations show that duct 

leakage has a significant impact on DE, resulting from the direct relationship between duct 

leakage and duct losses. Duct leakage rates were substantially reduced through encapsulation 

with ccSPF. Duct leakage to the outdoors was reduced to minimal rates typical for houses with 

AHUs in the living space. For the house with an AHU in the garage, the duct leakage rates were 

dramatically reduced, but still significant.

The heating and cooling seasonal DSEs for the best-performing buried and encapsulated duct 

home were 97% post-retrofit, compared to pre-retrofit percentages of 85% and 86%, 

respectively. The heating and cooling seasonal DSEs for the home with the encapsulated-only 

system were 95% and 91%, respectively, after the retrofit. These values were compared to 85%

and 81% pre-retrofit, respectively. The pre-retrofit seasonal DSEs for the two homes with 

flexible ductwork were similar (85%–87%). Post-retrofit, the heating seasonal DSE was 2% 

higher when the ducts were buried. Similarly, the cooling seasonal distribution system efficiency 

was 6% greater in the home where the ducts are buried.
70

The pre-retrofit seasonal DSE for the home with ductwork constructed primarily of duct board

was 65% for heating and 54% for cooling. This is roughly 20% lower than the flexible duct 

systems. This is reasonable, based on the condition of the existing ductwork, the presence of an 

unsealed duct opening, and the high duct leakage rates. Post-retrofit, the heating seasonal DSE

was 21% higher when the duct board system was encapsulated and buried. Similarly, the cooling 

seasonal DSE was 25% greater after the retrofit and duct repair.

11.6 Condensation Potential of Buried and Encapsulated Ducts

In Section 9, the steady-state two-dimensional thermal model developed in Section 6 was 

combined with one-dimensional dynamic hygrothermal model to predict the potential for 

condensation on the surface of the duct. This analysis was conducted for buried ducts and buried 

and encapsulated ducts using the worst case configurations. The results from the analysis predict 

condensation issues for the buried ducts without additional ccSPF insulation. Griffiths et al. 

(2002) observed these issues as well. The closeness of duct surface temperature and the dew

point of the surrounding air at the bottom of the duct confirmed the need for a minimum 

insulation level of 1.5 in. of ccSPF on ducts with R-4.2 insulation.

11.7 Predicted Energy and Cost Savings

In Section 10, predicted energy savings were based on a calibrated BEopt model. The BEopt 

energy savings, which ranged from 5%–20% of total energy use for the three houses, appear

reasonable in comparison to the predicted cooling and heating energy savings derived from the

ASHRAE 152 DEs (ASHRAE 2004). A cost-effectiveness analysis determined that the buried 

and encapsulated duct retrofit achieved $10 in annualized savings, making it cost effective 

(greater than zero). The encapsulated-only strategy has $141 in annualized savings. The higher 

cost effectiveness of the encapsulated-only strategy is due, in part, to avoiding the material and 

labor requirements associated with duct reconfiguration and blown-in insulation. Although the 

encapsulated duct strategy was the most cost effective, the buried and encapsulated duct strategy 

has the largest amount of predicted energy savings.

11.8 Summary

Based on this research study, encapsulated and buried and encapsulated ducts were found to 

dramatically improve the DSE of existing ductwork. Pre- and post-retrofit DSEs were calculated 

using ASHRAE 152. The best case scenario estimates a DSE range of 97%–98%. Potential 

energy savings ranging from 5% to 20% per year were predicted through simulation. Both 

encapsulated and buried and encapsulated ducts were found to be cost effective.

Encapsulated ducts were found to be more cost effective than buried and encapsulated ducts to 

reduce energy costs associated with ductwork delivery losses. The encapsulated method is less 

expensive to install than buried and encapsulated ducts because it does not require an HVAC 

contractor to cut down and reconfigure the ductwork, nor does it require labor and material 

associated with loose-fill insulation for duct burial. Eliminating the need to reconfigure the 

ductwork also mitigates concerns about affecting the airflows in the home.

As a single-trade method, encapsulation requires fewer visits to the home and no coordination 

with other contractors. This is significant because a multitrade method requires sales 

coordination among differing trades to market the service, as well as field coordination for 

implementation. Encapsulated ducts, though, do require an appropriately fire-rated ccSPF to be 
71

code compliant, and the predicted energy savings are slightly lower. For new construction 

projects and gut rehab projects that include duct replacement, a buried and encapsulated duct 

strategy can result in additional energy savings with minimal additional effort. In these scenarios, 

the incremental installation costs are lower and more trades will already be involved in the 

project. 

This research has been incorporated into the U.S. Department of Energy (DOE) Challenge Home 

National Program Requirements (DOE 2012). Under Section 10(c), buried and encapsulated

ducts are exempt from the requirement that forced-air ducts be inside the home’s thermal and air 

barrier boundary. Under this exception ductwork must be encapsulated with at least 1.5 in. of 

ccSPF and buried under 2 in. of blown-in insulation.
72

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Appendix A: Attic Dew Point Temperature

This study yielded some intriguing insights into the moisture performance of attics in hot-humid 

climates. The dew point temperatures of the attics do not seem to track to outdoor dew points.

Although the mean ambient dew point temperature corresponding to the 1% design temperature 

at Jacksonville Naval Air Station is 70°F (ASHRAE 2009), the attic dew point in these houses 

reached into the upper 80s. Furthermore, the ambient dew point was relatively constant 

throughout the day, but the attic dew point in these houses dropped to around 60°F during the 

night. Figure 51 shows the ambient conditions in Jacksonville, Florida, on July 29, 2010, using 

data from NCDC. On that day the dew point temperature remained relatively constant between 

70°F and 75°F. The attic dew points, though, fluctuated at a much larger interval, 50°F to 90°F, 

as shown in Figure 52 through Figure 53

Source: NCDC (2012)

Figure 51. Ambient conditions in Jacksonville, Florida, on July 29, 2010

This might be surprising at first glance, but the attic dew point temperatures measured in this 

study do not seem inaccurate. The relative humidity sensors used in the study have a rated 

accuracy of ±2.5% in an operating range of –40°F to 158°F. Although dew point is more 

sensitive to errors in relative humidity measurement at higher dry bulb temperatures, the rated 

inaccuracy does not account for the dew point trends. A long-term monitoring study conducted 

for the U.S. Department of Housing and Urban Development that included 20 houses in Florida 

found similar trends in attic dew point temperatures (Arena et al. 2010). Explaining the cause of 

the changing dew point temperatures in the attics of these houses is outside the scope of this 

study, but possible explanations include solar moisture drive, along with moisture storage and 

evaporation in the building components, especially wood. Figure 52 and Figure 53 show the 

12:00 AM 4:00 AM 8:00 AM 12:00 PM 4:00 PM 8:00 PM 12:00 AM

70

75

80

85

90

95

100

105

Temperature (°F)

30

40

50

60

70

80

90

100

Relative Humidity (%)

Drybulb Dew Point Relative Humidity
76

temperature and relative humidity of the attic and ambient air at one house monitored in this 

study and one house monitored by Arena et al. (2010), respectively.

Figure 52. Pre-retrofit ambient and attic conditions at House 1

Figure 53. Ambient and attic conditions at U.S. Department of Housing and Urban Development

Study House 4

07/16/10 07/17/10 07/18/10 07/19/10 07/20/10 07/21/10 07/22/10 07/23/10

50

60

70

80

90

100

110

120

130

140

150

Temperature (°F)

0

10

20

30

40

50

60

70

80

90

100

Relative Humidity (%)

Attic Temperature

Attic Dew Point

Attic Relative Humidity

Ambient Temperature

Ambient Dew Point

Ambient Relative Humidity

07/16/08 07/17/08 07/18/08 07/19/08 07/20/08 07/21/08 07/22/08 07/23/08

50

60

70

80

90

100

110

120

130

140

150

Temperature (°F)

0

10

20

30

40

50

60

70

80

90

100

Relative Humidity (%)

Attic Temperature

Attic Dew Point

Attic Relative Humidity

Ambient Temperature

Ambient Dew Point

Ambient Relative Humidity
DOE/GO-102013-3719 ▪ February 2013

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