Measure Guideline: 

Buried and/or Encapsulated 

Ducts

C. Shapiro, W. Zoeller, and P. Mantha 

Consortium for Advanced Residential Buildings

August 2013
NOTICE

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iii

Measure Guideline: Buried and/or Encapsulated Ducts

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, W. Zoeller, and P. Mantha 

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-0-40342-03 

August 2013 
iv

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v

Contents

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

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

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

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

Progression Summary: New Construction............................................................................................... x

Progression Summary: Existing Homes .................................................................................................. x

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

2 Technical Description .......................................................................................................................... 3

2.1 Buried Ducts ........................................................................................................................3

2.2 Encapsulated Ducts ..............................................................................................................5

2.3 Buried and Encapsulated Ducts ...........................................................................................5

2.4 Code Compliance .................................................................................................................7

2.5 Effective R-Values ...............................................................................................................8

2.6 System Interactions ..............................................................................................................9

3 Decision-Making Criteria ................................................................................................................... 11

3.1 Advantages, Disadvantages, and Risk Identification .........................................................11

3.1.1 Ducts Within the Thermal Envelope ......................................................................12

3.1.1.1 Unvented (Cathedralized) Attics ......................................................................13

3.1.1.2 Ducts in Dropped Ceilings or Internal Soffits (Furred-Down Chase) .............14

3.1.1.3 Ducts in Modified Truss (Furred-Up Chase) ...................................................14

3.1.1.4 Ducts Between Floors ......................................................................................14

3.1.2 Buried and Encapsulated Ducts .............................................................................14

3.2 Cost and Performance ........................................................................................................16

3.2.1 Dry Climate Decision-Making Example: Las Vegas, Nevada ..............................19

3.2.2 Humid Climate Decision-Making Example: Atlanta, Georgia ..............................20

4 Measure Implementation ................................................................................................................... 22

4.1 Climate-Specific Factors ....................................................................................................22

4.2 Installation Procedures for Buried and Encapsulated Ducts in New Construction ..........24

4.2.1 Buried Ducts Installed After Ceiling (Dry Climate Only) .....................................26

4.2.2 Buried Ducts Installed Before Ceiling (Dry Climate Only) ..................................27

4.2.3 Buried and Encapsulated Ducts Installed After Ceiling (Any Climate) ................28

4.2.4 Buried and Encapsulated Ducts Installed Before Ceiling (Any Climate) ..............30

4.2.5 Encapsulated Ducts in New Construction (Any Climate) .....................................32

4.3 Installation Procedures for Buried and Encapsulated Ducts in Existing Homes ..............32

4.3.1 Installation and Planning Steps for All Installations..............................................33

5 Verification Procedures and Tests ................................................................................................... 35

References................................................................................................................................................. 36

Appendix A : Installation Procedure Diagrams...................................................................................... 40

Appendix B : Effective R-Value Tables................................................................................................... 47

Appendix C : Energy Savings by Climate and Roof Slope................................................................... 50
vi

List of Figures

Figure 1. Insulated flex duct an existing home........................................................................................ 3

Figure 2. Diagram of buried ducts............................................................................................................. 4

Figure 3. Categorization of buried duct insulation levels; not for moist and marine climates........... 4

Figure 4. Fiberglass duct board ducts in an attic.................................................................................... 5

Figure 5. Sheet metal supply plenum ....................................................................................................... 5

Figure 6. Encapsulated duct in attic ......................................................................................................... 5

Figure 7. Diagram of BEDs......................................................................................................................... 6

Figure 8. Categorization of BED insulation levels................................................................................... 6

Figure 9. Duct wrap..................................................................................................................................... 7

Figure 10. Heat flux magnitude through buried and encapsulated duct. Red and yellow show 

greatest flux. ......................................................................................................................................... 8

Figure 11. How duct leakage can affect building pressures ................................................................ 10

Figure 12. Interior duct insulation options............................................................................................. 12

Figure 13. AERC versus source energy savings compared to improved benchmark for Las Vegas20

Figure 14. AERC versus source energy savings compared to improved benchmark for Atlanta.... 21

Figure 15. IECC climate regions .............................................................................................................. 23

Figure 16. Methods of air sealing ductwork with spray foam to prevent condensation ................... 24

Figure 17. Step-by-step procedure diagram for buried ducts installed after ceiling in new 

construction ........................................................................................................................................ 41

Figure 18. Step-by-step procedure diagram for buried ducts installed before ceiling in new 

construction ........................................................................................................................................ 42

Figure 19. Step-by-step procedure diagram for buried and encapsulated ducts installed after 

ceiling in new construction ............................................................................................................... 43

Figure 20. Step-by-step procedure diagram for buried and encapsulated ducts installed before 

ceiling in new construction ............................................................................................................... 44

Figure 21. Step-by-step procedure diagram for buried ducts installed in existing homes............... 45

Figure 22. Step-by-step procedure diagram for buried and encapsulated ducts installed in existing 

homes .................................................................................................................................................. 46

Unless otherwise noted, all figures were created by CARB.
vii

List of Tables

Table 1. Summary of Effective R-Values for an 8-in. Duct...................................................................... 9

Table 2. Applicability Decision Table (Green Dots = Yes; Red Dots = No) ........................................ 12

Table 3. Insulation for Condensation Control........................................................................................ 13

Table 4. Percentage Total Source Energy Savings by Roof Slope (Atlanta, Georgia) ...................... 17

Table 5. Example Cost for 2,400-ft2 Single-Story House With 6:12 Gable Roof in Climate Zones 

1, 2, or 3. Duct Surface Areas Based on BA Benchmark With Two Returns............................... 18

Table 6. Cost Analysis Assumptions...................................................................................................... 19

Table 7. Fuel Prices and Characteristics for Energy and Cost Analysis ............................................ 19

Table 8. Cost Analysis for Las Vegas Home Compared to Improved Benchmark............................. 19

Table 9. Cost Analysis for Las Vegas Home Compared to Improved Benchmark............................. 20

Table 10. Code Minimum Attic R-Values ................................................................................................ 22

Table 11. Practical Achieved R-Value of Attic........................................................................................ 22

Table 12. Effective R-Values of Round Insulated Flexible Ducts......................................................... 47

Table 13. Effective R-Values of Encapsulated Round Flexible Ducts by Insulation Thickness ....... 48

Table 14. Effective R-Values of Buried Round Ducts............................................................................ 48

Table 15. Effective R-Values of Buried and Encapsulated Round Ducts............................................ 49

Table 16. Duct Leakage Rates (% of AHU Flow) for BEopt Modeling.................................................. 50

Table 17. Jacksonville, Florida (2A) ........................................................................................................ 51

Table 18. Tucson, Arizona (2B)................................................................................................................ 51

Table 19. Atlanta, Georgia (3A)................................................................................................................ 52

Table 20. Las Vegas, NV (3B)................................................................................................................... 52

Table 21. San Francisco, California (3C) ................................................................................................ 53

Table 22. Lexington, Kentucky (4A) ........................................................................................................ 53

Table 23. Albuquerque, New Mexico (4B)............................................................................................... 54

Table 24. Seattle, Washington (4C) ......................................................................................................... 54

Table 25. Boston, Massachusetts (5A) ................................................................................................... 55

Table 26. Denver, Colorado (5B).............................................................................................................. 55

Table 27. Madison, Wisconsin (6A)......................................................................................................... 56

Table 28. Billings, Montana (6B).............................................................................................................. 56

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

Definitions

AHU Air handling unit

BA Building America

BED Buried and/or encapsulated duct

BPI Building Performance Institute

ccSPF Closed cell polyurethane spray foam

HVAC Heating, ventilation, and air-conditioning

IECC International Energy Conservation Code

IRC International Residential Code

XPS Extruded polystyrene

Limits of Liability and Disclaimer of Warranty

Steven Winter Associates, Inc. makes no representations about the suitability of this document 

for all situations. The accuracy and completeness of the information provided by the authors and 

the opinions stated herein are not guaranteed or warranted to produce any particular results. This 

document is provided “as is” without express or implied warranty. Steven Winter Associates, 

Inc. shall not be liable in any event for incidental or consequential damages in connection with, 

or arising out of, the furnishing, performance, or use of this documentation.
ix

Executive Summary

Buried and/or encapsulated ducts (BEDs) are a class of advanced energy efficiency strategies 

intended to address the significant ductwork thermal losses associated with ducts installed in 

unconditioned attics. BEDs are ducts installed in unconditioned attics that are covered in loosefill insulation and/or encapsulated in closed cell polyurethane spray foam (ccSPF) insulation. 

This Building America Measure Guideline covers the technical aspects of BEDs as well as the 

advantages, disadvantages, and risks of BEDs compared to alternative strategies. Detailed 

guidance on installation of BEDs strategies in new and existing homes through step-by-step 

installation procedures is also provided

This guideline synthesizes previously published research on BEDs and provides practical 

information to builders, contractors, homeowners, policy analysts, building professionals, and 

building scientists. Some of the procedures presented here, however, require specialized 

equipment or expertise. In addition, some alterations to duct systems may require a specialized 

license. 

This guideline provides valuable information for a building industry that has struggled to address 

ductwork thermal losses in new and existing homes. As building codes strengthen requirements 

for duct air sealing and insulation, flexibility is needed to address energy efficiency goals. While 

ductwork within the thermal envelope has been promoted as the panacea for addressing 

ductwork thermal losses, BED installations approach—and sometimes exceed—the performance 

of ductwork within the thermal envelope.

Acknowledgments

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

program and its funding and support of development of this technical report as well as research 

that informed it. 
x 

Progression Summary: New Construction

Determine 

System to Install

Determine whether to install typical ductwork, an interior ductwork system, or a BED 

method. If a BED method is going to be installed, proceed to step 1.

Determine 

Construction 

Sequence

Determine whether the ceiling will be installed prior to duct 

installation.

Install ceiling if ductwork will be installed after ceiling installation.

Install ceiling if ductwork was installed before ceiling installation.

Encapsulate ductwork with at least 1.5 in. of ccSPF if BEDs are installed.

Design Duct System

Design a compact, low-profile duct system using industryaccepted best practices. The system should be oriented such 

that the greatest degree of duct burial can be achieved.

1

2

Determine Burial 

Depth

Based on the height of the duct branches and supply trunks 

and the height of the loose-fill insulation, determine the depth 

and category of burial. Increase loose-fill depth if higher 

performance is desired.

3

Install Ductwork

Install compact, low-profile duct system using R-8 insulated 

ducts. Mastic seal all connections, test duct leakage, and air 

seal ceiling plane penetrations prior to encapsulation or 

burial.

Bury Ducts

Bury ductwork with loose-fill insulation. At least 1.5 in. of 

fiberglass must cover all ccSPF not rated for exposed attic 5 installations.

4
xi 

Progression Summary: Existing Homes

ss Inspect Home

Inspect the condition of the existing home and determine if there are health and safety 

issues in the home that prevent safe installation of a BED method.

Do not proceed if:

• The attic has active knob and tube wiring. 

• The attic has asbestos, mold, or other contaminants.

• The attic has structural issues. 

• The attic has insufficient access. 

! !

Install any needed temporary protection and access.

Relocate or reconfigure ducts to accommodate BED design.

Buried (and encapsulated ducts): Bury ductwork with loose-fill insulation. At least 1.5 

in. of fiberglass must cover all ccSPF not rated for exposed attic installations.

Survey Attic

Survey and document the layout and condition of the 

existing ducts and attic insulation.

Create Retrofit Plan

Based on the configuration and condition of the ducts and 

attic, determine whether insulation and ductwork can be 

reused or must be reconfigured or replaced

Remove Insulation

Complete duct and insulation removal as necessary

Air Seal Attic

Air seal attic ceiling penetrations.

3

2

1

(Buried and) encapsulated ducts: Apply at least 1.5 in. of ccSPF insulation to all duct 

surfaces.

4
1 

1 Introduction

Ductwork installed in unconditioned spaces—such as attics, crawlspaces, and garages—can 

contribute significantly to the overall heating and cooling costs of residential buildings. 

Estimated duct thermal losses for single-family residential buildings with ductwork installed in 

unconditioned spaces range from 10%–45% of total cooling and heating loads (Shapiro et al. 

2012). As a result, poorly installed duct systems can account for almost half of the total heating 

and cooling costs of some residential buildings. 

There are three primary methods, listed below, of reducing thermal losses associated with 

ductwork in unconditioned spaces. All of these methods are covered by the Building America 

(BA) Measure Guidelines described below. These methods have various cost, performance, 

safety, and practical tradeoffs that must be considered before implementation, but each can 

greatly improve heating and cooling system efficiencies.

• Proper sealing and insulation of ductwork can reduce thermal losses from ductwork to 

around 10%, but further reduction of duct thermal losses requires more advanced 

techniques. The proper techniques for sealing and insulating ductwork are covered in the 

Building America Measure Guideline Sealing and Insulating Ducts in Existing Homes

(Aldrich and Puttagunta 2011).

• Installing or moving ducts within the thermal envelope can eliminate duct thermal 

losses entirely. Depending on the method employed, however, living space may be 

affected, and the building envelope load may be increased slightly. Installation of ducts 

within the thermal envelope is covered in the BA Measure Guideline, Summary of 

Interior Ducts in New Construction, Including an Efficient, Affordable Method to Install 

Fur-Down Interior Ducts (Beal et al. 2011). 

• Burying and/or encapsulating ductwork in attics can reduce ductwork thermal losses 

to around 3% or less. Under this family of duct insulation approaches, ductwork may be 

encapsulated in closed-cell spray polyurethane foam (ccSPF) insulation and/or buried 

beneath loose-fill insulation. Buried and/or encapsulated ducts (BEDs) reduce duct 

thermal losses without affecting living space or increasing thermal envelope loads. This 

method is the subject of this guideline. 

BEDs have been the subject of building science research for more than a decade, particularly 

through the BA program (CARB 2000; Griffiths et al. 2002; CARB 2003; Griffiths et al. 2004; 

Griffiths and Zuluaga 2004; Vineyard et al. 2004; Zoeller 2009; Shapiro et al. 2012). As a result 

of BA research, BEDs have been incorporated into several energy conservation codes and 

standards. For example, buried ducts are compliant, when properly installed, with Title 24 of the 

California Code of Regulations (CEC 2007, 2008), which governs construction of buildings 

throughout the state of California and stipulates minimum energy conservation levels (CBSC 

2010). Spray foam encapsulated ducts are prescriptively allowed by the 2009 International 

Residential Code (IRC). As an alternative to interior duct installations, the U.S. Department 

of Energy’s Challenge Home National Program Requirements allows ductwork in attics that is 
2 

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

(DOE 2012). 

This guideline synthesizes previously published research on BEDs and provides practical 

information to builders, contractors, homeowners, policy analysts, building professions, and 

building scientists. Both existing homes and new construction are covered by this guideline. This 

guideline is split into three major sections. First, the technical aspects of BEDs are discussed in 

depth, including duct materials, insulation materials, encapsulated ducts, buried ducts, BEDs, 

effective R-values of these methods, and interactions with other systems. Second, the decisionmaking process is outlined through a discussion of the risks of BEDs and other methods of 

reducing duct thermal losses, and a comparison between the cost and performance of BEDs and 

other methods of reducing duct thermal losses. Finally, detailed step-by-step measure 

implementation instructions are provided for all BEDs methods in new and existing homes. 
3 

Duct Materialsa

The three most common duct 

materials are sheet metal, 

fiberglass duct board, and 

flexible duct. A duct 

distribution system may utilize 

a combination of these 

materials, such as sheet metal 

trunks off the AHU and flexduct branches to supply the 

individual registers.

Flexible Duct 

Flex duct is composed of three 

materials: (1) an interior 

polymer tube with diameters 

ranging from 4 in. to 24 in. 

supported by a spiral, metal 

coil; (2) fiberglass insulation 

with thicknesses ranging from 

1 in. to 3 in.; and (3) an outer 

polymer liner, which usually 

incorporates a low-emissivity 

foil facing that provides a 

radiant and vapor barrier. This 

flexible composition means 

that flex duct is relatively 

inexpensive and comes preinsulated. Furthermore, flex 

duct can be run longer 

distances—with several bends 

and turns—without additional 

fittings or connections. With 

fewer transitions and fittings, 

flex duct can be very airtight, 

but as with other duct 

materials, leakage problems

can still occur at connections. 

Figure 1. Insulated flex 

duct an existing home

a Text for some sidebars is 

derived from Aldrich and 

Puttagunta (2011).

2 Technical Description

Conventional heating, ventilation, and air-conditioning 

(HVAC) design manuals dictate that space-conditioning 

air in cooling-dominated climates be discharged from 

ceiling or high wall registers (ACCA 1992). As a result, 

most houses in hot climates with ducted spaceconditioning systems have ductwork, and in some cases 

air handling units (AHUs), in unconditioned attics. 

Energy losses from duct leakage to the outside, which 

commonly vary from as little as 3% to much more than 

20%, are exacerbated by large temperature differentials 

between the conditioned air inside the duct and the air in 

the unconditioned attic. 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 supply air passing 

through attics with 20°F ambient attic air. 

BEDs are a class of solutions that provide simple and 

cost-effective methods for reducing thermal losses from 

ductwork installed in unconditioned attics. There are 

three possible combinations under this strategy: (1) 

buried ducts; (2) encapsulated ducts (with ccSPF); and 

(3) BEDs. The best solution depends on the climate, age 

of the house, and the configuration of the HVAC system 

and attic. Ducts that are only encapsulated without burial 

are not recommended for new construction because 

additional burial requires only minor increases in 

planning and cost over encapsulation alone. Thus, BEDs

yield significantly higher performance with minimal 

additional effort. Buried ducts without encapsulation 

should not be installed in moist or marine climates, 

because there is a risk of condensation on the surface of 

the ductwork. 

2.1 Buried Ducts

Buried ducts involve placing ductwork as close to the 

ceiling as practical—either on top of the gypsum board 

ceiling or over the truss bottom cords—and burying the 

ductwork beneath loose-fill insulation. Any loose-fill 

insulation, such as fiberglass or cellulose, can be used for 

this strategy. When more loose-fill insulation is used to 
4 

boost the duct R-value, there is an added benefit of increasing the ceiling assembly R-value as 

well. See Figure 2 for a diagram showing a buried duct installation. 

Figure 2. Diagram of buried ducts 

For analysis purposes, buried ducts are categorized into three burial classes based on the distance 

from the top of the duct to the top of the loose-fill insulation: (1) partially buried; (2) fully 

buried; and (3) deeply buried. Partially buried ducts are buried to 3.5 in. below the top of the 

duct. Fully buried ducts are buried to the top of the duct. Deeply buried ducts are buried with 

3.5 in. of loose-fill insulation over the top of the ductwork. See Figure 3 for a diagram showing 

the three burial classes. 

Figure 3. Categorization of buried duct insulation levels; not for moist and marine climates 
5 

Fiberglass Duct Board

Duct board is a rigid, dense, fiberglass sheet; the outside typically has a fiberreinforced foil facing which acts as a vapor barrier. Duct board is usually sold 

in large sheets (e.g. 4 ft × 10 ft) with thicknesses of 0.75 in. to 2 in. The 

fiberglass provides thermal insulation and sound attenuation.

Duct board is typically cut, folded, and shaped into rectangular sections of 

duct. Duct board is primarily used for plenums, trunks, and junction boxes; 

smaller branch ducts can be sheet metal or, more commonly, insulated flex 

duct. The most common areas for leakage are at collars, transitions, and 

connections to other materials. Figure 4. Fiberglass duct 

board ducts in an attic

Sheet Metal

Galvanized steel metal is a very common duct material and is required in 

some jurisdictions. Round and rectangular shapes are the most common, but 

ovals and other shapes are sometimes used. There is also a wide array of sheet 

metal fittings that connect sections of duct, such as elbows, transitions, 

takeoffs, and reducers. 

Screws are typically used to connect straight round ducts to collars, elbows, 

Ts, Ys, reductions, boots, and other fittings. The Sheet Metal and Air 

Conditioning Contractors National Association recommends using at least 

three #8 sheet metal screws spaced equidistant (SMACNA 1998). If seams, 

gaps, and connections are not sealed before insulation, substantial leakage can 

result. Figure 5. Sheet metal 

supply plenum

2.2 Encapsulated Ducts

Encapsulated ducts involve spraying ductwork with ccSPF to boost the R-value of the duct 

insulation and reduce air leakage (Figure 6). Although ccSPF may be applied directly to the 

exterior of uninsulated ductwork, insulation of ductwork with fiberglass duct wrap prior to 

encapsulation is a lower cost way to increase the duct R-value. Unlike buried ducts, where 

effective sealing of ductwork prior to burial is a must for proper installation, encapsulated ducts 

do not require the same level of rigorous air 

sealing. Ducts should be sealed to reasonable 

tightness, and ccSPF will act as an additional 

air barrier. In order to be code compliant, the 

ccSPF used must be approved for exposed attic 

installations. (See Section 2.4 below.) 

2.3 Buried and Encapsulated Ducts

With buried and encapsulated, ducts are first 

encapsulated with ccSPF insulation and 

subsequently buried under loose-fill insulation. 

Initially conceived as a way to apply the buried 

ducts concept to humid climates (where 

condensation could occur on the outer surface 

of buried ducts), BEDs can also be used as a 

high-performance duct insulation strategy in all 

Figure 6. Encapsulated duct in attic
6 

climates. BEDs have even higher R-values than buried ducts and include the air sealing benefits 

of encapsulated ducts. Figure 7 shows a diagram of BEDs, and Figure 8 shows the burial levels 

associated with BEDs. 

Figure 7. Diagram of BEDs 

Figure 8. Categorization of BED insulation levels 
7 

Fiberglass Duct Insulation

Most sheet metal ducts are 

uninsulated or wrapped on the 

outside with fiberglass 

insulation blankets, sometimes 

called “duct wrap” (Figure 9). 

These sheets (typically 2–4 ft 

wide) are 1–4 in. thick and 

include a vapor barrier on the 

outside—often with a foil 

radiant barrier. The R-value of 

fiberglass duct wrap is 2.5–4 

h-°F- ft2

/Btu-in., depending on 

the specific product and 

density. Newer sheet metal 

ducts that have interior, 

insulating duct liners are now 

available, but these liners are 

not generally appropriate for 

retrofit applications (Aldrich 

and Puttagunta 2011).

Figure 9. Duct wrap

Insulation Materials

Four insulation products are typically involved in BEDs installations: loose-fill fiberglass, loose-fill cellulose, ccSPF, 

and fiberglass duct wrap. 

Loose-Fill Fiberglass 

Loose-fill fiberglass insulation is 

composed of loosely packed rows of 

glass fibers. Large bales of 

fiberglass are torn apart and blown 

through a hose onto the attic ceiling 

assembly (Krigger and Dorsi 2004). 

R-values for this insulation are 

2.2–2.9/in. (ASHRAE 2009).

Loose-Fill Cellulose

Loose-fill cellulose insulation is 

composed of newspaper and other 

wood waste treated with fire 

retardants (Krigger and Dorsi 2004). 

R-values for this insulation are 

3.2–3.7/in. 

Closed-Cell Spray Polyurethane 

Foam

ccSPF, also known as mediumdensity foam, has R-values of 

6–7/in., qualifies as a class II vapor 

retarder at 1.5 in., and has excellent 

air sealing characteristics (Krigger 

and Dorsi 2004). 

2.4 Code Compliance

While there are no significant obstacles to ensuring that 

buried duct installations are code compliant, installations 

involving ccSPF must be carefully considered. In 

general, the 2009 IRC requires that spray foam 

insulation applied to the exterior of ductwork (Sections 

M1601.3 and M1601.4) in attics (Section R316.5.3) 

meet several requirements:

• The spray foam has a flame spread index no 

greater than 50 and a smoke developed index no 

greater than 450.

• Attic access is required by Section R807.1 

(typically required in low-density residential 

construction).

• The attic is “entered only for purposes of repairs 

or maintenance” (meaning no storage or 

habitation is allowed).

• The spray foam is protected by an ignition 

barrier.

• The spray foam has a maximum water vapor 

permeance of 3 perm/in. (meaning that open cell 

spray foam cannot be used). 

• The spray foam meets the general requirements 

for use in residential buildings (Section R316). 

The 2009 IRC allows exposed installations of ccSPF in 

attics, but the spray foam used must be specifically 

approved for installation without an ignition barrier 
8 

In Detail: Modeling Buried Duct Effective R-values 

The heat transfer between the air inside buried ducts and the attic can be 

calculated using finite element computer modeling programs, such as 

THERM. The complicated geometry, lack of symmetry, and complex 

heat flow of buried ducts make direct calculation of effective R-values 

difficult. Unlike typical hung ductwork, buried ducts involve heat flow 

between three air spaces: the living space below the ceiling, the 

conditioned air inside the duct, and the attic air. Heat flows between 

these areas simultaneously, and the degree of heat flow changes based 

on the conditions of each area. To find the effective R-value, which is 

the R-value of a hung duct with the same performance, the heat flow 

between the duct and the attic must be isolated.

Figure 10. Heat flux magnitude 

through buried and 

encapsulated duct. Red and 

yellow show greatest flux.

(Section R316.6). Approval is determined on the basis of specific fire-related test procedures.

Not all ccSPF materials meet the requirements of Section R316.6. 

If the spray foam used is not approved for exposed attic installations, encapsulated ducts must 

also be buried in loose-fill insulation such that the ccSPF is covered by an ignition barrier. For 

BEDs, 1.5 in. of fiberglass, which is considered mineral fiber insulation (ASTM C554-11), meets 

the minimum requirements for ignition barriers. BEDs using a ccSPF material specifically 

approved under Section R316.6 may be buried under cellulose. All other BED installations 

require 1.5 in. of fiberglass or mineral wool insulation coverage. 

2.5 Effective R-Values

The thermal resistance of duct insulation products is typically listed as a nominal R-value that 

does not account for convective heat transfer and the geometry of the duct system (Palmiter and 

Kruse 2006). Effective R-values, on the other hand, account for these effects and are therefore a 

more precise measure of the amount of energy that is transferred between the ducts and the 

unconditioned space. For low R-values, the difference between the nominal and effective Rvalues is generally negligible, but for larger duct insulation values, effective R-values can be 

considerably smaller than the nominal ratings. 

Unlike traditional duct insulation, buried ducts cannot be described using nominal R-values, and 

effective R-values must be simulated using computer models (Griffiths et al. 2004; Griffiths and 

Zuluaga 2004; Zoeller 2009; Shapiro et al. 2012). To provide an apples-to-apples comparison of 

the various duct insulation methods, effective R-values for an 8 in. flex duct are shown in Table 

1. Other duct sizes will have different effective R-values, and in general, effective R-value 

increases with increasing diameter. (See Appendix B for detailed tables showing effective Rvalues for a wider array of insulation strategies and duct sizes.) 
9 

Table 1. Summary of Effective R-Values for an 8-in. Duct 

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 Beneath Fiberglass 8.1 10.2 12.3

Fully Buried Beneath Fiberglass 12.0 14.1 16.2

Deeply Buried Beneath Fiberglass 20.7 22.1 23.5

Encapsulated (1.5 in.) and Partially Buried Beneath Fiberglass 18.4 19.7 21.0

Encapsulated (1.5 in.) and Fully Buried Beneath Fiberglass 22.6 23.8 25.0

Encapsulated (1.5 in.) and Deeply Buried Beneath Fiberglass 29.6 30.3 31.1

Source: Shapiro et al. (2012)

2.6 System Interactions 

The energy savings associated with BEDs depend heavily on the space-conditioning system 

installed in the house. Counterintuitively, BEDs may reduce space-conditioning energy use by a 

larger percentage when a multispeed or right-sized air-conditioning system is installed. These 

systems operate at lower capacities for longer periods of time than conventional systems, and 

thermal losses from ductwork therefore account for a larger percentage of the total load (Beal et 

al. 2011). For example, a recent study found that switching from an attic duct system to an 

indoor duct system reduced cooling energy use by 17.3% for a multispeed seasonal energy 

efficiency ratio 21 system but only reduced cooling energy use by 11.2% for a single-speed 

seasonal energy efficiency ratio 13 system. The experiment was conducted by alternating the use 

of these systems in a 1,600-ft2

 laboratory test house in Florida (Cummings and Withers 2011). 

BEDs will reduce the heating and cooling loads of the building, and as a result, the spaceconditioning systems may be downsized in some cases. In new construction, equipment 

downsizing will reduce the cost of implementing a BED strategy. In retrofit scenarios, the energy 

savings and cost of a BED strategy will depend on whether a new, downsized space-conditioning 

system is installed. If the space-conditioning system is not downsized, the reduction in load 

associated with BEDs may increase cycling in the space-conditioning equipment, reducing the 

efficiency of the equipment and energy savings. Alternatively, simultaneous replacement of 

space-conditioning equipment and implementation of a BEDs strategy may be a way to reduce 

installation costs and maximize energy savings. 

Ducts installed in unconditioned attics can also cause comfort issues for the occupants. When the 

space-conditioning system is not running, the air inside the ductwork can be heated or cooled 

substantially, depending on the season. When the system begins operation, this hot or cold air is 

pushed into the conditioned space, increasing the space-conditioning load and causing comfort 

problems for the occupants. By increasing the R-value of the ductwork and reducing air losses, 

BEDs can dramatically reduce these effects.
10 

Furthermore, leakage from the duct system can depressurize or pressurize the conditioned space, 

which leads to increased envelope air infiltration and a greater potential for moisture problems. 

Return-side leakage can create positive pressure in the home and result in unconditioned air and 

pollutants being pulled into the airstream. Positive pressure can be an issue during winter months 

in cold climate regions if relatively moist interior air is pushed into the wall assembly and 

condenses. Similarly, attic supply side leakage can create negative pressure in the home and 

result in conditioned air being lost to the outdoors. If there are atmospheric combustion 

appliances in the home, negative pressures can interfere with proper drafting, potentially 

resulting in exhaust gases building up within the home. In hot, humid climate regions, 

depressurization can also lead to high indoor relative humidity levels as outdoor air infiltration is 

exacerbated (Aldrich and Puttagunta 2011). Encapsulating ducts can reduce leakage; in addition 

to substantial energy savings, lower duct leakage can alleviate moisture, comfort, and safety 

concerns. 

Source: Aldrich and Puttagunta (2011) 

Figure 11. How duct leakage can affect building pressures 
11 

3 Decision-Making Criteria

While each combination of building construction, ductwork installation, and space-conditioning 

system poses unique challenges to reducing ductwork thermal losses, there is fortunately a wide 

array of solutions that may meet the challenges of each building. Broadly speaking, there are 

three major categories for reducing ductwork thermal losses: (1) proper duct sealing and 

insulation; (2) placing ducts within the thermal envelope; and (3) burying and/or encapsulating 

ducts in unconditioned attics. Under each duct installation category are multiple implementation 

options, and each option must be carefully considered based on the tradeoffs in cost, energy 

savings, and practicality of each building installation. 

Proper duct sealing and insulation are applicable to the widest array of system types, but 

ductwork thermal losses can only be reduced to around 10% of total space-conditioning loads 

through duct sealing and insulation alone (Shapiro et al. 2012). Furthermore, proper duct sealing 

and insulation may not be practical in all existing buildings without major reconfiguration of the 

duct system. Various duct sealing and insulation methods may be employed for different system 

types; see the BA Measure Guideline, Sealing and Insulating Ducts in Existing Homes (Aldrich 

and Puttagunta 2011) for more information. 

To minimize or eliminate duct thermal losses, however, ducts within the thermal envelope or 

BEDs must be installed. Choosing the most appropriate method of reducing duct thermal losses 

can be a complicated endeavor that relies heavily on the building and space-conditioning 

systems. This guideline attempts to clarify this decision-making process by providing guidance 

on which methods may be appropriate for specific circumstances. First, this guideline provides a 

summary of the advantages, disadvantages, and specific risks associated with each of the 

measures in order to provide guidance on which measures may not be applicable for the 

installation. Second, cost and performance metrics are provided to allow homeowners and 

contractors to select the best of the remaining options that fit the desired performance needs.

3.1 Advantages, Disadvantages, and Risk Identification

Each of the options available for minimizing or eliminating ductwork thermal losses has 

advantages, disadvantages, and risks that will affect the decision-making process. The sections 

below give detailed descriptions of the various methods of reducing ductwork thermal losses, 

including the four major methods of placing ducts within the thermal envelope and the BED 

strategies. Table 2 summarizes which options are available for each building construction type. 

The table shows yes or no values based on building age (new or existing) and climate region (dry 

or humid). For the purposes of this guideline, humid climates are all moist and marine climate 

zones as classified by the International Energy Conservation Code (IECC). This classification is 

discussed in greater detail in Section 4.1. 
12 

Table 2. Applicability Decision Table 

(Green Dots = Yes; Red Dots = No) 

3.1.1 Ducts Within the Thermal Envelope 

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

can be eliminated by placing ducts within the thermal envelope using one of four methods: (1) 

expanding the thermal envelope to incorporate ductwork (e.g., insulating attics at the roof deck 

Climate Humid Dry Humid Dry

Unvented attic

Furred-down chase

Furred-up chase

Encapsulated ductsa

Buried ducts

Buried & encapsulated ducts

New Construction Existing Buildings

a

 Encapsulated ducts can be installed safely in new construction, but it is better to bury and encapsulate

Figure 12. Interior duct insulation options 
13 

and insulating basements at the basement walls); (2) installing ductwork in a soffit or dropped 

ceiling; (3) using a modified truss to create an insulated space within the thermal envelope for 

ductwork in the attic; and (4) installing ductwork between floors (CARB 2000; Hendrick 2003; 

Roberts and Winkler 2010; Beal et al. 2011). (See Figure 12 for a diagram showing these 

methods.) Methods (2) through (4) are the most effective of the interior duct strategies. While 

method (1), expanding the thermal envelope to include ductwork, eliminates ductwork thermal 

losses, the expanded enclosure surface area can result in increased space-conditioning loads. 

Although the increased space-conditioning loads can be smaller than the eliminated ductwork 

losses, the resulting savings from this method may be less than the other methods. 

3.1.1.1 Unvented (Cathedralized) Attics

Moving the thermal boundary to the roof deck creates an added interior volume for placing ducts 

within the thermal envelope. By taking this approach, the surface area of the thermal envelope is 

also increased, which results in increased space-conditioning loads from the enlarged enclosure 

(Hendrick 2003). While this penalty may be overcome by savings from ductwork thermal losses, 

the net energy savings (duct savings minus increased enclosure loads) may be less than savings 

from other methods of placing ducts within the thermal envelope. 

Furthermore, by placing all the bulk water, water vapor, and thermal control boundaries within 

the same assembly, very different moisture control dynamics are created at the roof deck. The 

properties of the various materials used and how they are assembled must be clearly understood 

by the designer. Building codes require minimum levels of air-impermeable insulation for 

condensation control (Table 3), and insulation must be installed such that it does not become 

dislodged from the roof deck assembly. As a result, spray foam is typically used to insulate 

buildings at the roof deck. Achieving the equivalent R-value at the roof deck using spray foam is 

significantly more expensive than installing loose-fill insulation along the ceiling plane. The 

larger surface areas of the roof deck and vertical gable walls, if present, result in higher costs 

compared to ceiling insulation, and spray foam is comparatively more expensive than loose-fill 

insulation per achieved R-value. Insulating the building at the roof deck using spray foam, 

however, provides greater air sealing benefits than typical ceiling insulation methods. Rigid 

insulation may be placed on top of the roof deck, removing the need for ccSPF insulation except 

at the soffits, and as a result, unvented attic construction may be less expensive in new 

construction or at the end of an existing roof’s service life than in most retrofit situations.

 Table 3. Insulation for Condensation Control

Climate Zone Minimum Air-Impermeable Insulation R-Value

2B and 3B with tile roof None

1, 2A, 2B, 3A, 3B, and 3C R-5

4C R-10

4A, 4B R-15

5 R-20

6 R-25

7 R-30

8 R-35

Source: IRC Table R806.4
14 

3.1.1.2 Ducts in Dropped Ceilings or Internal Soffits (Furred-Down Chase)

A furred-down chase is essentially a dropped ceiling or internal soffit strategically placed to 

carry ductwork through the home. Furred-down chases require planning and architectural 

coordination on the part of the building designer to ensure that the dropped ceiling can be placed 

in an inconspicuous location, such as a hallway or tray ceiling, and still allow the ducts to reach 

all the required register locations. Ductwork must be carefully planned and sized to fit inside the 

furred-down chase. Large spaces may be particularly difficult to serve using this method because

register throw distances are limited. Except in modest floor plans, compact distribution may be 

difficult to achieve. Coordination between trades is also further complicated during the 

implementation phase (Hendrick 2003; Beal et al. 2011). This method may be very difficult to 

implement in existing homes. 

3.1.1.3 Ducts in Modified Truss (Furred-Up Chase)

A furred-up chase is similar to a furred-down chase, except a contained volume for the ductwork 

is created above the ceiling plane in the attic. This method typically uses a modified truss system 

to create an insulated space within the thermal envelope for the ductwork. A squared-off plenum 

truss or a modified scissors truss can be used to create space for the duct distribution system. 

Like the furred-down chase described above, the dedicated duct space must be carefully air 

sealed and thermally separated from the attic space. A variety of materials can be used as the air 

barrier, including gypsum board, laminated fiber sheathing, and oriented strand board. This 

strategy tends to work best for compact floor plans where the insulated space can be 

correspondingly compact. As with the previous two methods, this method is typically not 

practical for existing homes.

3.1.1.4 Ducts Between Floors

In multistory buildings, ducts can be placed within the floor framing cavity between levels. This 

can be an ideal solution, but design coordination between the structure, floor plan, and duct 

layout is critical for success. The concept works best when utilizing open-web floor trusses 

rather than I-joists or sawn lumber joists. This method works well in heating climates where 

floor registers are acceptable for HVAC distribution, but in cooling climates, where ceiling or 

high-wall registers are more effective, additional design and coordination solutions are required. 

Vertical branches serving high wall registers work well, but serving both floor levels through one 

set of ducts in the shared floor cavity can be challenging. If the home has a basement, ducts may 

be placed in both floor cavities, simplifying the design and installation. This method is not viable 

for existing homes.

3.1.2 Buried and Encapsulated Ducts

BEDs have advantages in performance, first cost, and flexibility of application relative to other 

ductwork improvements. From a performance perspective, BEDs can approach the efficiency of 

true inside-conditioned-space ducts and, in some cases, exceed the performance of ductwork 

placed in unvented attic assemblies. Combining best practices in duct sealing, low-profile and 

compact duct design, and deeply buried ducts will result in a very efficient distribution system

Assuming planning and design costs are held constant, the only additional cost directly

attributable to a BEDs solution for new construction is the ccSPF application. Although some 

attics may not be conducive for this strategy, buried ducts—which do not require ccSPF 
15 

encapsulation—are essentially a zero-cost energy efficiency measure with a slight performance 

tradeoff over true inside-conditioned-space duct systems. 

Furthermore, BED strategies are highly flexible, allowing for use in most vented truss roof 

scenarios. Depending on construction sequencing, buried (and encapsulated)1 ducts can be 

installed either before or after ceiling drywall. Compact HVAC design, which is a preferred 

method of reducing ductwork thermal losses and improving space-conditioning system 

efficiency, can be accommodated in BED applications. As codes push the required ceiling 

insulation to higher R-values, the performance of buried (and encapsulated) ducts will improve; 

i.e., ducts can be buried to a greater extent. 

The most significant risks for BED applications are air leakage into the vented attic—with all the 

corresponding pressure balance and indoor air quality concerns—and condensation potential on 

the buried duct surfaces during cooling operation. These risks are essentially quality control and 

design issues that require contractor training and execution quality assurance to mitigate 

potential problems. Proper duct sealing using industry established best practices will reliably 

prevent air leakage, and following proper protocols for encapsulating buried ducts in adequate 

levels of ccSPF will prevent condensation on the surface of the ductwork. 

To ensure that there are no condensation issues with buried ducts, ducts should not be buried 

without ccSPF encapsulation in moist and marine climates. For BED solutions, moist and marine

climates correspond to IECC climate zones 1A, 2A, 3A, 4A, 4C, and 5A, with limited 

application in 6A and 7A (Figure 15). In humid climates, refer to the buried and encapsulated 

ducts sections. In dry climates, ducts can be insulation buried without the condensation control 

layer provided by ccSPF. 

Where structural or other obstructions exist in the attic space, a third method—encapsulated (but 

unburied) ducts—may be employed. (See Section 2.2 for a detailed description.) This application 

subset is useful in new construction where complete burial of a short length of duct is not 

possible. In retrofits, encapsulated ducts may be used when a more comprehensive reworking of 

the existing ducts is impractical. Since the ignition barrier provided through mineral fiber 

insulation is eliminated in this configuration, ccSPF specifically rated for attic exposure or an 

alternative ignition barrier is required. 

Although research has found that the BED strategies are not difficult to implement in new 

construction, BEDs are not common practice and require modifications to standard installation 

techniques (Griffiths et al. 2004; Griffiths and Zuluaga 2004; Zoeller 2009; Shapiro et al. 2012). 

Proper execution relies on initial planning, good communication, and coordination between the 

HVAC contractor and the project manager responsible for quality control. Training and detailed 

specifications will help to communicate project goals to all stakeholders. 

 

1 Buried (and encapsulated) means buried or buried and encapsulated.
16 

Proper HVAC equipment sizing and duct design will enable the use of smaller duct diameters 

and a more compact duct layout, both of which facilitate burying the ducts below an optimal 

level of insulation. An optimized insulation-buried duct HVAC system design includes: 

• Properly sized air handling equipment, including downsized equipment based on the 

reduced thermal losses, when applicable 

• Air handling equipment installed within the thermal envelope will also reduce the 

impacts of duct leakage

• Properly sized duct runs kept as short as possible to facilitate insulation coverage.

• Ductwork placed as low as practical and not hung from the trusses 

• Minimized duct cross-overs that result in lower insulation coverage levels

• Duct runs placed parallel to truss chords and framing, where possible, to allow greater 

burial 

• Side-entry ceiling register boots, rather than of top-entry boots, that prevent vertical duct 

protrusions above loose-fill insulation at connection points. 

Ducts run perpendicular to the trusses can be supported by the bottom chords, and ducts run in 

parallel can be supported by the ceiling drywall. Ducts are typically “roughed-in” before 

installation of gypsum board ceilings, however, leaving no support for ducts that are run low and 

parallel to the truss chords. In cases where ducts are installed prior to the ceiling, temporary 

straps can be used to support duct lengths installed parallel to the truss bottom cords. 

3.2 Cost and Performance

Once the applicable duct thermal protection options have been determined based on the risk 

identification process, the next step is to select the option that meets the desired performance 

outcome cost effectively in a given circumstance. To compare potential performance outcomes, 

energy modeling is helpful. This guideline provides energy savings estimates for a sample 2,400-

ft2

 house built to Building American (BA) Benchmark specifications in each climate zone. This 

building is then outfitted with buried ducts, encapsulated ducts, buried and encapsulated ducts, 

unvented attics, and ducts in the thermal envelope. Building energy simulations were performed 

in BEoptE+ 1.3. 

Table 4 shows an example of this energy savings analysis. Each option in the table is listed in the 

rows, while the roof slope is shown in the columns. Energy savings over the benchmark are 

shown for each of the options. Table cells are colored based on their relative energy savings. The 

highest savings is shown in green, and the lowest savings is shown in red. The benchmark 

includes a well installed duct system with R-8 insulation in a vented attic. In this example the 

non-encapsulated buried ducts, shown in gray, are not simulated because they are not appropriate 

for this example humid climate. Data for other cities and climate zones, as well as the modeling 

assumptions used, are provided in Appendix C. 
17 

Table 4. Percentage Total Source Energy Savings by Roof Slope (Atlanta, Georgia)2 

In general, BEDs are similar in performance to interior ducts and ducts in unvented attics. The 

added ceiling insulation caused by encapsulating and deeply burying ducts outweighs the added 

benefit of interior ducts. BEDs become more effective than unvented attics as the roof pitch 

becomes higher because unvented attics have increased envelope loads as a result of the 

increased enclosure surface area.

Cost estimates for installations in Climate Zones 1, 2, and 3 are shown in Table 5 and were 

derived from three sources (Beal et al. 2011; RSMeans 2011; NREL 2012). These estimates are 

for new construction and include the insulation cost of the attic/roof assembly and ductwork. 

This table assumes that unvented attics are insulated entirely with ccSPF. Other construction 

methods are available for unvented attic assemblies, but outlining all of these methods is outside 

the scope of this guideline. Using ccSPF is probably the most common method of constructing 

unvented attics and was therefore used in this example. Existing building costs will vary widely 

based on the conditions of the ductwork and the degree of reconfiguration necessary. Other 

climate zones have different insulation requirements for attic installations and vaporimpermeable insulation at the roof deck. 

To demonstrate how these cost and energy savings predictions could be used in the decision 

making process, two examples are provided. These examples cover humid and dry climates, and 

use the same models used for Appendix C. For both cases, a roof slope of 6:12 is assumed. The 

best metric for measuring total lifetime cost is annualized energy related costs (AERC), as 

described by Polly et al. (2011), but simple payback period is also given for each measure. 

Assumptions for the cost analysis are given in Table 6 and Table 7. 

 

2 Savings are color coded by savings potential (green = most energy savings; red = least energy savings). Data are

based on the BA Benchmark home in Atlanta, Georgia (Zone 3A) and modeled in BEoptE+ 1.3.

Assembly (Attic R-value) 4:12 6:12 8:12 10:12

Benchmark (R-30)a 0.0% 0.0% 0.0% 0.0%

Improved benchmark (R-30)a,b 7.6% 7.6% 7.6% 7.7%

Partially-buried (R-33) N/A N/A N/A N/A

Fully-buried (R-42) N/A N/A N/A N/A

Deeply-buried (R-51) N/A N/A N/A N/A

Unvented (R-30)a 10.6% 10.3% 9.9% 9.5%

Encapsulated (R-30)a 9.1% 9.2% 9.2% 9.2%

Partially-buried & encapsulated (R-37) 11.1% 11.1% 11.1% 11.1%

Fully-buried & encapsulated (R-46) 12.1% 12.3% 12.2% 12.2%

Deeply-buried & encapsulated (R-54) 12.9% 13.1% 13.0% 13.0%

Interior ducts (R-30)a 11.9% 11.9% 11.9% 11.9%

Roof slope

a

 Benchmark ceiling or roof deck insulation is R-30 in Zone 3A. Ceiling insulation R-values for buried 

 ducts may be higher than the benchmark.

b

 Improved Benchmark includes IECC 2012 requirements for infiltration (3 ACH50) and duct sealing (4 

 cfm per 100 ft2

 conditioned living space).
18 

Table 5. Example Cost for 2,400-ft2 Single-Story House With 6:12 Gable Roof in Climate Zones 1, 2, or 3. 

Duct Surface Areas Based on BA Benchmark With Two Returns

Partially 

Buried

Fully 

Buried

Deeply 

Buried

Unvented

ccSPF

Encapsulated

Partially 

Buried 

and 

Encapsulated

Fully 

Buried 

and 

Encapsulated

Deeply 

Buried 

and 

Encapsulated

Interior 

Ducts

R-30 ccSPF Roof 

Decka $8,363 

Encapsulated 

ductsa,b $1,678 $1,678 $1,678 $1,678 

Partially Buried

(R-33 Fiberglass)c $95 

Fully Buried

(R-42 Fiberglass)c $380 

Deeply Buried

(R-51 Fiberglass)c $665 

Partially Buried and

Encapsulated

(R-37 Fiberglass)c

$222 

Fully Buried and 

Encapsulated

(R-46 Fiberglass)c

$507 

Deeply Buried and 

Encapsulated

(R-54 Fiberglass)c

$760 

Interior Ductsd $1,680

Total Cost $95 $380 $665 $8,363 $1,678 $1,900 $2,185 $2,439 $1,680 a Costs from RSMeans Residential Cost Data (RSMeans 2011).

b BA Benchmark assumes 888 ft2 of ductwork. Actual ductwork surface area ranged from 218 ft2 to 681 ft2 for the three homes monitored by Shapiro et al. 

(2012). Costs for installations may be significantly lower than the cost cited here.

c Costs from the National Residential Efficiency Measures Database (NREL 2012).

d Cost from Beal et al. (2011).
19 

Table 6. Cost Analysis Assumptions

Cost Metric Assumption

Analysis Period 30 yearsa

Measure Lifetimes 30 yearsb

Inflation Rate 3%a

Real Discount Rate 3%a

Real Fuel Escalation Rate 0%a

Mortgage Rate 4.42%c

Mortgage Period 30 yearsc

Marginal Income Tax Rate 28%a

a BEopt defaults (NREL 2012)

b All building components given the same lifetime for simplicity

c Average rate for 2011 (Freddie Mac 2012)

Table 7. Fuel Prices and Characteristics for Energy and Cost Analysis

Energy Source Site to Source Ratioa Costb Energy Content

Electricity 3.365 $0.1172/kWh 3,412 Btu/kWh

Natural Gas 1.092 $1.10/therm 100,000 Btu/therm a Deru and Torcellini (2007)

b Prices obtained from EIA (2012a, 2012b). Electricity and natural gas prices represent average 

national prices for 2011.

3.2.1 Dry Climate Decision-Making Example: Las Vegas, Nevada

To demonstrate how the savings and cost data would be used to choose the best duct and attic 

insulation option in the dry climate of Las Vegas, Nevada, the cost data provided Table 5 and the 

modeled building energy savings provided in Table 20 are used. Source energy savings and 

AERC compared to the improved benchmark are plotted in Figure 13. Deeply buried and 

encapsulated ducts have the highest energy savings, interior ducts have the highest energy 

savings without increasing costs over the improved benchmark, and deeply buried ducts are the 

lowest cost option. Noticeably, unvented attics installed with ccSPF have the highest cost of all 

the measures. Unsurprisingly, simple payback paints a similar picture (Table 12).

Table 8. Cost Analysis for Las Vegas Home Compared to Improved Benchmark

Assembly 

(Attic Insulation)

AERC 

($/yr)

Source Energy 

Savings (%)

Simple Payback 

(yr)

Partially Buried (R-33) –16 1.7 4.9

Fully Buried (R-42) –28 3.5 9.2

Deeply Buried (R-51) –35 5 11.4

Unvented (R-30) 258 2.8 256.8

Encapsulated (R-30) 35 2 71.5

Partially Buried and Encapsulated (R-37) 18 4.1 39.3

Fully Buried and Encapsulated (R-46) 12 5.5 34.1

Deeply Buried and Encapsulated (R-54) 9 6.5 32.2

Interior Ducts (R-30) –2 5.2 27.6
20 

Figure 13. AERC versus source energy savings compared to improved benchmark for Las Vegas

3.2.2 Humid Climate Decision-Making Example: Atlanta, Georgia 

A similar methodology was employed for the house placed in Atlanta, Georgia. The only 

difference, however, is that buried ducts without encapsulation were not considered because 

there is a risk of condensation of the surface of the ductwork. Costs are listed in Table 5 and 

energy modeling results are given in Table 4. As with Las Vegas, deeply buried and 

encapsulated ducts have the highest energy savings. Fully buried and encapsulated ducts have the 

highest energy savings without increasing costs over the improved benchmark, and interior ducts 

are the lowest cost option. Noticeably, unvented attics installed with ccSPF have the highest cost 

of all the measures. Unsurprisingly, simple payback paints a similar picture (Table 12).

Table 9. Cost Analysis for Las Vegas Home Compared to Improved Benchmark 

Assembly 

(Attic Insulation)

AERC 

($/yr)

Source Energy 

Savings (%)

Simple Payback 

(yr)

Unvented (R-30) 243 3 174.3

Encapsulated (R-30) 30 1.7 59.7

Partially Buried and Encapsulated (R-37) 4 3.8 30.8

Fully Buried and Encapsulated (R-46) –6 5 26.6

Deeply Buried and Encapsulated (R-54) –11 5.9 25.4

Interior Ducts (R-30) –17 4.6 22.2

0 1 2 3 4 5 6 7

-50

0

50

100

150

200

250

300

Annualized Energy Related Cost Savings

Compared to Improved Benchmark ($/yr)

Source Energy Savings

Compared to Improved Benchmark (mmBtu/yr)

Partially-Buried

Fully-Buried

Deeply-Buried

Unvented Attic

Encapsulated

Partially-Buried & Encapsulated

Fully-Buried & Encapsulated

Deeply-Buried & Encapsulated

Interior Ducts
21 

Figure 14. AERC versus source energy savings compared to improved benchmark for Atlanta

0 1 2 3 4 5 6 7

-50

0

50

100

150

200

250

300

Annualized Energy Related Cost Savings

Compared to Improved Benchmark ($/yr)

Source Energy Savings

Compared to Improved Benchmark (mmBtu/yr)

Unvented Attic

Encapsulated

Partially-Buried & Encapsulated

Fully-Buried & Encapsulated

Deeply-Buried & Encapsulated

Interior Ducts
22 

4 Measure Implementation

Section 4.1 gives a discussion of climate specific factors impacting BEDs and the following 

sections give a detailed, step-by-step summary of the installation procedures for buried, 

encapsulated, and buried and encapsulated ducts in new and existing homes. Each set of 

instructions is accompanied by a diagram in Appendix A showing the proper installation 

procedure for the system. This guideline does not cover HVAC basics and duct design, both of 

which are covered in other comprehensive documents (ACCA 1992, 1995, 1997, 2006, 2007, 

2009; Burdick 2011; Aldrich and Puttagunta 2011). Those engineering and design principles 

should be well understood by the designer and used as the starting point in designing and 

implementing a BED system. This guideline uses as its foundation the information contained in 

those and other HVAC duct design resources, and provides supplemental information useful in 

successfully designing and implementing the BED technique. 

4.1 Climate-Specific Factors

The climate in which a project is located can impact the implementation of a BED strategy. First, 

there are interactions between IECC code requirements for attic insulation and duct burial level. 

Since attic insulation requirements increase with heating degree days, ducts can be buried to 

higher insulation levels without additional insulation over the IECC attic insulation requirements. 

For example, the minimum R-30 attic insulation requirement (Table 10) in climate zones 1–3 

means that even achieving partial burial with loose-fill fiberglass insulation would require 

additional attic insulation in these climate zones (Table 11). In climate zones 6–8, however, the 

R-49 minimum required attic insulation requirement allows fully buried ducts to be installed 

without additional attic insulation. 

Table 10. Code Minimum Attic R-Values

Climate 

Zones

Minimum Attic 

R-Value

1, 2, and 3 30

4 and 5 38

6, 7, and 8 49

Table 11. Practical Achieved R-Value of Attic

Burial 

Level Buried Buried and Encapsulated

Partially 33 37

Fully 42 46

Deeply 51 54

Scope of Work

1. Determine BED strategy to be employed. 

2. Ensure work can proceed safely.

3. Survey conditions and create a duct installation or retrofit plan to accommodate BED 

design. 

4. Test ductwork to ensure performance criteria are met. 

5. Air seal ceiling plane penetrations.

6. Install BED strategy by burying ductwork beneath loose-fill insulation and/or 

encapsulating ductwork with ccSPF insulation.
23 

As previously mentioned, ccSPF encapsulation is required in humid climates to reduce the risk 

of condensation on the outer surface of the duct jacket. As a result, (buried and) encapsulated 

ducts are recommended in all humid climates. In the United States, humid climates correspond to 

IECC climate zones 1A, 2A, 3A, 4A, 4C, and 5A, with limited application in 6A and 7A (Figure 

15). 

Souce: Baechler et al. (2010)

Figure 15. IECC climate regions

When BEDs are used, the ducts must have at least 1.5 in. of ccSPF insulation encapsulating the 

entire surface of the ductwork. An encapsulation level of 1.5 in. was found to be appropriate for 

climate zone 2B by Shapiro et al. (2012). Since detailed analysis of all climate zones has not 

been performed, 1.5 in. of ccSPF is recommended for all climate zones as a conservative 

measure to prevent condensation. Ducts may be sealed to air-impermeable materials—such as 

gypsum board, extruded polystyrene (XPS) insulation, or polyisocyanurate insulation board—but 

not to air-permeable materials—such as fiberglass or cellulose insulation. Air-impermeable 

insulation is classified as having air permanence ≤ 0.02 L/s-m2

 at 75 Pa pressure (ICC 2009). 
24 

Figure 16. Methods of air sealing ductwork with spray foam to prevent condensation 

4.2 Installation Procedures for Buried and Encapsulated Ducts in 

New Construction 

BEDs in new construction have few impediments and are extremely flexible. As with all 

advanced systems, however, the key to optimal implementation lies with the initial planning. To 

accommodate a BED strategy, the designer must also consider how best to incorporate a lowprofile design, where the system layout is specifically designed to place ducts as low as practical 

and allow ductwork to hug the drywall ceiling where possible. This guideline assumes other best 

practice measures, which assist in achieving the desired low-profile layout, will be incorporated 

into the building, including compact HVAC distribution and right-sized HVAC sizing. 

Specifically, smaller ducts (lower duct height) and inboard registers (shorter duct runs) mean that 

there is less ductwork to bury.

Typically, the duct design will consist of one or more main supply trunks and perpendicular duct 

branches serving each of the ceiling registers. If the trunk is perpendicular to the truss bottom 

chords, then the duct branches can be parallel and rest directly on the ceiling. If the main trunk is 

placed parallel to the ceiling supports, then the branches will need to run perpendicular to and 

rest on top of the truss chords. Either configuration will work, giving the designer the flexibility 

to select whichever method works best for a particular circumstance. In every case, a compact, 

low-profile layout should be a primary goal. 

Return trunks and branches could be treated in the same manner; however, to keep the HVAC 

distribution system at a minimum, while simultaneously providing good comfort and proper 

airflow, the use of central returns is recommended. Return air paths from bedrooms and other 

spaces can be accommodated by low-profile jump ducts. Because condensation is not a concern 

on return ducts, encapsulation is not required. However, encapsulation is an excellent air sealing 

strategy, and is therefore recommended on all ductwork. 

When planning for BEDs installations in new construction, the following steps should be taken. 

This decision-making process should take into account the discussion in Section 3.1. 

1. Determine the BED method to be applied. For new construction, the available 

options are buried ducts and BEDs, depending on the climate and level of 

performance desired. In moist and marine climates, BEDs should be employed. 
25 

2. Design a compact, low-profile duct system. Using industry-accepted best practices 

for the design of ducted heating and cooling air distribution systems, develop a duct 

design with the objective of keeping the height of the system as low as reasonably 

practical relative to the gypsum board ceiling. Taking into account the register 

locations for each room, the location of the AHU, and any structural impediments 

presented by framing, the designer should determine the size and placement of supply 

trunks and supply register branches. Although not essential, it is generally 

advantageous for the main supply trunk to run perpendicular to and on top of the 

ceiling framing, and for the branch runs to run parallel to the ceiling framing and 

directly on the ceiling. Utilizing a rectangular—rather than square or round—section 

for the main supply trunk or trunks helps keep the overall height low, especially if 

placed on top of the ceiling framing. The vertical dimension of the trunk cannot be 

smaller than the diameter of the largest branch duct to allow for proper duct 

connections. All branch connections should be side takeoffs, and all ceiling registers 

should utilize side-entry boots. Duct crossovers should be avoided. 

3. Determine the construction sequence to be employed. Installing the ducts either 

prior to or after the ceiling gypsum board is compatible with BED installations. 

Installing the ceiling drywall first has the advantage of allowing the ducts to rest 

directly on the ceiling plane and therefore being approximately 1.5 in. lower than 

when foaming the ducts first. The ducts are also supported by the ceiling during 

rough-in, and the entire system, including ceiling register boots, gets secured into 

place and air sealed when the ccSPF is applied to the ducts. Installing the ceiling first 

may hamper the ability to inspect or access ducts at low spaces at the edge of the 

sloped roof. Typically, the mechanical rough-ins, which include ducts, are performed 

prior to drywall, and access to the attic space may be constricted by the ceiling being 

in place. Where circumstances allow, however, installing the ceiling first provides 

additional benefits worth considering. 

4. Determine the depth of duct burial. Based on the height of the duct branches and 

supply trunks relative to the ceiling plane and the depth of the loose fill insulation 

intended, determine the depth and category of duct burial. If needed, redesign the 

ducts or specify increased insulation depth to achieve the degree of burial desired. 

Remember, if the ducts are to be ccSPF encapsulated, some foams require a 1.5-in. 

minimum mineral fiber covering for a code required ignition barrier. Refer to 2009 

IRC 316.5.3 for specific requirements.
26 

4.2.1 Buried Ducts Installed After Ceiling (Dry Climate Only)

Buried ducts may be installed before or after the ceiling is installed. This step-by-step installation 

detail explains how to install buried ducts if the ceiling is installed before the ductwork is 

installed. This procedure does not require supporting ductwork before the ceiling is installed, but 

ductwork installation may be more difficult in this case. 

1.

Install ceiling gypsum board prior to installing 

buried ducts. 

2.

Install ductwork with a minimum of R-8 duct 

insulation in accordance with low-profile duct 

design. 

3.

Mastic seal all connections, and pull insulation 

jackets fully over joints and connections following 

best practice duct sealing strategies. Tool-tightened 

tension ties must be applied to the inner and outer 

liners. 

4.

Test total duct leakage to ensure performance 

levels are met (total duct leakage < 3 cfm25/100 ft2 of 

conditioned space).
27 

4.2.2 Buried Ducts Installed Before Ceiling (Dry Climate Only)

This step-by-step installation detail explains how to install buried ducts if the ceiling is installed 

after the ductwork is installed. This method is similar to the method described in Section 4.2.1, 

but requires supporting the ductwork before the ceiling is installed. Temporary strapping must be 

removed before burial. 

5.

Air seal ceiling plane penetrations, including 

sealing duct register boots to gypsum board ceiling. 

Spray foam provides the best sealing benefits for this 

application. 

6.

Install loose-fill insulation to specified depth, and 

verify that the ducts are covered to the level of 

design intent. Either loose-fill cellulose or fiberglass 

insulation may be used.

1.

Install ductwork with a minimum of R-8 duct 

insulation in accordance with low-profile duct 

design. Where ducts are running parallel to ceiling 

framing, provide temporary strap supports to hang 

ducts at approximate ceiling plane level. 

2.

Mastic seal all connections, and pull insulation 

jackets fully over joints and connections following 

best-practice duct sealing strategies. Tool-tightened 

tension ties must be applied to the inner and outer 

liners.
28 

4.2.3 Buried and Encapsulated Ducts Installed After Ceiling (Any Climate) 

This step-by-step installation detail explains how to install BEDs if the ceiling is installed before 

the ductwork is installed. This procedure is similar to the method described in Section 4.2.1 for 

buried ducts with the same installation sequence, but includes the encapsulation process. 

3.

Test total duct leakage to ensure performance 

levels are met (total duct leakage < 3 cfm25/100 ft2

of conditioned space). 

4.

Install ceiling gypsum board. 

5.

Air seal ceiling plane penetrations, including 

sealing duct register boots to gypsum board ceiling. 

Spray foam provides the best sealing benefits for 

this application.

6.

Install loose-fill insulation to specified depth, and 

verify that the ducts are covered to the level of 

design intent. Either loose-fill cellulose or 

fiberglass insulation may be used.
29 

1.

Install ceiling gypsum board prior to installing 

buried ducts. 

2.

Install ductwork with a minimum of R-8 duct 

insulation in accordance with low-profile duct 

design.

3.

Mastic seal all connections, and pull insulation 

jackets fully over joints and connections following 

best practice duct sealing strategies. Tool-tightened 

tension ties must be applied to the inner and outer 

liners.

4.

Test total duct leakage to ensure performance 

levels are met (total duct leakage < 3 cfm25/100 ft2

of conditioned space). Testing should be performed 

before encapsulation because it may be difficult to 

correct sealing issues after the application of spray 

foam.

5.

Apply at least 1.5 in. of ccSPF to all duct 

surfaces, including trunks, branches, and register 

boots. Where obstructions make the bottom of 

ducts inaccessible (such as when a wide trunk is 

placed on top of and perpendicular to the ceiling 

framing), ducts may be placed on 1.5-in. thick XPS 

or polyisocyanurate insulation board. Ducts should 

be entirely encapsulated and sealed to the gypsum 

board, or to rigid insulation board. 
30 

4.2.4 Buried and Encapsulated Ducts Installed Before Ceiling (Any Climate)

This step-by-step installation detail explains how to install BEDs if the ceiling is installed after

the ductwork is installed. This procedure is similar to the method described in Section 4.2.2 for 

buried ducts. This method includes encapsulation and requires supporting the ductwork before 

the ceiling is installed. Temporary strapping must be removed before burial. 

6.

Air seal ceiling plane penetrations, including 

sealing duct register boots to gypsum board ceiling. 

Spray foam provides the best sealing benefits for 

this application.

7.

Install loose-fill insulation to specified depth, and 

verify that the ducts are covered to the level of 

design intent. Fiberglass or other mineral fiber 

insulation must cover the ccSPF by at least 1.5 in. 

unless the foam is rated for exposure in attics, or a 

separate ignition barrier is applied.

1.

Install ductwork with a minimum of R-8 duct 

insulation in accordance with low-profile duct 

design. Where ducts are running parallel to ceiling 

framing, provide temporary strap supports to hang 

ducts approximately 2 in. above ceiling plane to 

allow for thickness of spray foam. 

2.

Mastic seal all connections, and pull insulation 

jackets fully over joints and connections following 

best practice duct sealing strategies. Tool-tightened 

tension ties must be applied to the inner and outer 

liners.
31 

3.

Test total duct leakage to ensure performance 

levels are met (total duct leakage < 3 cfm25/100 ft2

of conditioned space). Testing should be 

performed before encapsulation because it may be 

difficult to correct sealing issues after the 

application of spray foam. 

4.

Apply at least 1.5 in. of ccSPF to all duct 

surfaces, including trunks, branches, and register 

boots. Ducts that are lying directly on the ceiling 

should be sealed to the top of the gypsum board. 

5.

Install ceiling gypsum board. 

6.

Air seal ceiling plane penetrations, including 

sealing duct register boots to gypsum board 

ceiling. Spray foam provides the best sealing 

benefits for this application.

7.

Install loose-fill insulation to specified depth, and 

verify that the ducts are covered to the level of 

design intent. Fiberglass or other mineral fiber 

insulation must cover the ccSPF by at least 1.5 in. 

unless the foam is rated for exposure in attics, or a 

separate ignition barrier is applied.
32 

4.2.5 Encapsulated Ducts in New Construction (Any Climate) 

Where necessitated by architectural or structural obstructions, portions of the attic HVAC ducts 

may be encapsulated in ccSPF but not buried in loose-fill insulation. If the duct is covered by 1.5 

in. of ccSPF, leakage will be reduced and a higher effective R-value will be attained. Even when 

an entire duct system cannot be buried, it is likely that some portions of a duct system may be 

buried beneath insulation. See Appendix B for a table listing the effective R-values of BEDs for 

various duct sizes. The exposed foam must either meet the code requirements for attic exposure, 

or a separate ignition barrier must be applied. 

4.3 Installation Procedures for Buried and Encapsulated Ducts in 

Existing Homes

When retrofitting BEDs into existing homes, the same basic principles, risks, and potential 

benefits as new construction apply, but in a substantially different context. Most significantly, 

cost-effectiveness considerations must: (1) take into account the cost of reworking existing ducts 

and ceiling insulation; (2) determine what is worth replacing; and (3) determine what is worth 

keeping. The interactions are numerous and highly variable, but the process should begin with a 

visual survey of the major factors and physical components of the attic and the HVAC ducts. 

First, assess the physical access and working clearance of the attic. Can the attic space be 

reasonably accessed by a crew with materials and equipment needed to complete the job? 

Assuming the proposed retrofit is not a gut rehab, access must be available without damaging the 

interior of the home. If existing attic access is inadequate, the cost of providing improved access 

must be included in the economic evaluation of the project. If working clearance is too limited, 

the home may not be a good candidate for a BED retrofit. 

Second, consider the existing ductwork conditions and placement. What is the physical condition 

of the ducts? Can they be reused, or is replacement needed? Are the ducts composed of sheet 

metal, flex-duct, duct board, or a combination of these materials? How are the ducts configured, 

and how are they secured in place? Are there physical obstacles that would prevent the ducts 

from being relocated on the attic floor and ultimately insulation buried? Generally the ceiling 

registers will remain in place unless a more significant retrofit is intended, so a more compact 

distribution system will not be part of the process. The original ducts were not intended to be low 

profile, so larger diameter, existing ducts may be difficult to bury beneath insulation. 

Finally, as with any duct retrofit work, it is important to understand basic health and safety 

information related to duct installations in general. In existing homes, inspect the home and 

systems to determine if the duct improvement work can be performed safely prior to beginning 

the retrofit. It may be appropriate to refer to standards such as the Building Performance 

Institute’s (BPI) Technical Standards for the Building Analyst Professional (BPI 2012) or other 

protocols. Contractors should refer to appropriate local codes, regulations, professional 

standards, and common sense as the situation warrants. Take note of the following potential 

issues.3

 

3 The following list of issues is derived from Aldrich and Puttagunta (2011).
33 

• Structural issues: If the building and/or the duct system is damaged or structurally 

unsound, duct retrofits should not be pursued until other repairs are completed. 

• Asbestos, mold, and other contaminants: If there is an existing mold problem in the 

building or in the ductwork, or if suspected asbestos-containing material is present, the 

conditions should be documented and assessed to determine if retrofits can be safely 

completed. If retrofits cannot be safely completed, the homeowner should be instructed to 

have the problems addressed by a qualified professional before continuing with duct 

improvements. 

• Health and safety of the occupants: If there are people with severe medical conditions

living in the home, the situation should be evaluated to ensure that the duct testing and 

retrofits will not cause adverse health conditions. This could include stirring up dust, 

blowing cold air into the house, and shutting off systems to complete work during 

extreme weather. 

• Health and safety of the workers: If ductwork is located in an attic or other overhead 

space, caution must be taken to prevent damage to the ceilings in the occupied spaces 

below. Typically, this requires careful navigation along the ceiling structural members, 

either ceiling joists or truss bottom chords. Insulation often makes it difficult to identify 

solid footing, which can lead to damage in ceiling sheetrock below. In most homes, 

roofing nails penetrate the roof sheathing and extend through the underside of the 

decking. Where head clearance is low, protective head gear should be worn to prevent 

cuts or scrapes from the nails. Similarly, workers should wear gloves, goggles, masks, 

and protective clothing to protect themselves from airborne insulation fibers and dust that 

are released when ductwork is disturbed. Work should only be undertaken by people 

comfortable working in constrained spaces and familiar with safe practices for access in 

attics or other overhead spaces. 

• Make note and be aware of other hazards: Look for toxic materials, solvents, exposed 

or knob-and-tube wiring (have an electrician evaluate), non-IC rated can light fixtures, 

among other issues. Proceed with work only if it can be done safely. 

4.3.1 Installation and Planning Steps for All Installations

As with new construction, several steps should be taken to ensure proper installation of BEDs in 

existing homes. This decision-making process should take into account the advantages, 

disadvantages, and risks discussed in Section 3.1 as well as the duct design criteria listed above. 

Many of the steps below reference steps covered in the new construction 

1. Inspect the existing conditions and determine if there are health and safety issues in 

the home. This initial inspection will determine the feasibility of applying a BED retrofit 

to a specific existing home.

2. Survey and document the layout and condition of existing ducts, and of the existing 

attic insulation. In addition to the climate the home is located in, this will determine 

which BED approach may be appropriate, and the level of duct repair, replacement, and 

reconfiguration needed in order to accomplish the retrofit. 
34 

3. Create a retrofit plan for the BED application, based on Steps 1 and 2, including the 

proposed design of the upgraded system, and the logistical steps needed to conduct the 

work. Determine whether existing insulation is to be removed or temporarily relocated, 

how removal will be accomplished, and how the attic ceiling plane air sealing will be 

incorporated. 

4. Install any needed temporary protection and access, and complete the duct and 

insulation removal determined necessary in Step 3. Inspect all work following removal 

to verify condition of materials to remain.

5. Relocate or reconfigure ducts to accommodate BED design. Mastic seal all connections 

and pull insulation jackets fully over joints and connections following best practice duct 

sealing strategies. 

6. Test total duct leakages to ensure performance levels are met.

7. Encapsulated and buried and encapsulated ducts: Apply ccSPF to all duct surfaces, 

including trunks, branches, and register boots. Ducts that are lying directly on the ceiling 

should be sealed to the top of the gypsum board. A minimum of 1.5 in. of ccSPF should 

be applied. Where obstructions cause the bottoms of ducts to be inaccessible for spraying 

ccSPF (such as when a wide trunk is placed on top of and perpendicular to the ceiling 

framing), the duct may be placed on top of 1.5-in. thick XPS or polyisocyanurate 

insulation board, and the spray foam used to encapsulate the duct to the insulation board. 

8. Air seal ceiling plane penetrations.

9. Buried (and encapsulated ducts): Install loose-fill insulation to specified depth 

covering ducts to the design intent. Fiberglass or other mineral fiber insulation must 

cover the ccSPF by at least 1.5 in. unless the foam is rated for exposure in attics, or a 

separate ignition barrier is applied. 
35 

5 Verification Procedures and Tests

The degree to which verification procedures and tests are 

needed with BEDs depends on the amount of duct 

reconfigurations and equipment modifications 

performed. For new construction, all of the typical duct 

and system tests and procedures should be performed. 

Ducts and systems should be installed in compliance with 

ACCA Standard 5 (ACCA 2007). This standard has 

methods for selecting equipment, installing equipment, 

and installing duct distribution systems. The equipment 

selection section governs building heat gain/loss load 

calculations, proper equipment capacity selection, and 

matching air systems. The equipment installation section 

governs airflow across the indoor coil, refrigerant charge, 

electrical requirements, combusting venting systems, and 

system controls. The duct distribution section covers duct 

leakage and airflow balance.

For existing systems, only the duct leakage and airflow 

balancing requirements may be needed. Duct leakage 

testing involves using a duct blower to measure duct 

leakage (ASTM 2007). Duct leakage to outdoors must be 

no more than 8 cfm/100 ft2 of conditioned floor area, or 

total duct leakage must be no more than 12 cfm/100 ft2 of 

conditioned floor area. Duct leakage is tested at 25 Pa 

(ICC 2009). 

Airflow must be balanced such that individual airflows 

are within ± 20% of the design flow rate. For existing 

homes, testing may be done to ensure that airflow rates

do not change considerably after the retrofit, and 

pressures should be tested after installation to verify that 

the static pressures are within the manufacture recommendations. Total airflow volume at the 

evaporator must be ± 15% of the design or pre-retrofit flow rate.

For houses with combusting appliances, combustion safety and carbon monoxide testing should 

be performed. See Aldrich and Puttagunta (2011) for more information. 

Ensuring Success

If major HVAC retrofits are planned 

as part of a project, the following 

resources may be helpful:

• Air Conditioning Contractors of 

America (ACCA) Manual D, 

Manual J, Manual S, Manual T, 

Manual RS, and Standard 5: 

HVAC Quality Installation 

Specification

• EPA document “ENERGY 

STAR Qualified Homes, 

Version 3 (Rev. 02) HVAC 

System Quality Installation 

Contractor Checklist” (EPA 

2012) 

• ASHRAE Handbook of 

Fundamentals 2009

• Sheet Metal and Air 

Conditioning Contractors 

National Association Residential 

Comfort System Installation 

Standards Manual

• ASHRAE Standard 152 - 

Method of Test for Determining 

the Design and Seasonal 

Efficiencies of Residential 

Thermal Distribution Systems

• ASTM E1554-07: Standard 

Test Methods for Determining 

Air Leakage of Air Distribution 

Systems by Fan Pressurization

. 
36 

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High-Velocity Distribution Systems in Attic and Space Locations.” ASHRAE Transactions; 

109(2); pp. 45-51. 

Zoeller, W. (2009). “Still Placing Ducts in the Attic? Consider Burying Them.” Norwalk CT: 

Consortium for Advanced Residential Buildings. 
40 

Appendix A: Installation Procedure Diagrams

The figures below show comprehensive diagrams of the step-by-step installation procedures 

discussed in Section 4.2. These diagrams may be useful as handouts for installers and 

contractors as well as other stakeholders. The diagrams give a concise description of the 

installation procedure and may be helpful as a supplement for the more detailed installation 

procedures given in Section 4.2.
41 

Figure 17. Step-by-step procedure diagram for buried ducts installed after ceiling in new constru
42 

Figure 18. Step-by-step procedure diagram for buried ducts installed before ceiling in new constr
43 

Figure 19. Step-by-step procedure diagram for buried and encapsulated ducts installed after ceiling in new 
44 

Figure 20. Step-by-step procedure diagram for buried and encapsulated ducts installed before ceiling in ne
45 

Figure 21. Step-by-step procedure diagram for buried ducts installed in existing homes 
46 

Figure 22. Step-by-step procedure diagram for buried and encapsulated ducts installed in existing 
47 

Appendix B: Effective R-Value Tables

Thermal resistances for most duct insulation products are typically listed as a rated, nominal Rvalue, which is calculated for the insulation material lying flat. When the materials are wrapped 

around ductwork, however, the “true” R-value of the insulation can be significantly different 

than the nominal values. Furthermore, the nominal R-value excludes the inner and outer surface 

films of air (Palmiter and Kruse 2006).

For the purposes of evaluating various duct systems, thermal resistances of duct insulation may 

be reported using three metrics: 

• Nominal R-values, which are the listed values for the material, do not include the impact 

of duct geometry, surface film resistances, and heat transfer between the air inside the 

duct and conditioned space 

• Apparent R-values account for duct geometry and surface film resistances. Apparent Rvalues do not include heat transfer between the air inside the ducts and the conditioned 

space. As a result, apparent R-values are used for comparing experimental results to 

predicted values and are not useful for efficiency comparisons. 

• Effective R-values can be considered the “true” R-value of the duct insulation. Effective 

R-values include duct geometry effects, surface film resistances, and heat transfer 

between air inside the duct and conditioned space. For traditional hung ductwork and 

encapsulated ducts, effective R-values are identical to apparent R-values. For buried 

ducts, effective R-values can be considerably different than apparent R-values due to the 

significant heat transfer between the ductwork and conditioned space. Effective R-values 

are necessary to calculate energy savings associated with buried (and encapsulated) ducts. 

The tables below list effective R-values for flex-duct of various sizes and insulation levels. These 

tables include traditional ducts, encapsulated ducts, buried ducts, and buried and encapsulated 

ducts. Effective R-values are also compared to nominal R-values in the tables. 

Table 12. Effective R-Values of Round Insulated Flexible Ducts

Duct Inner Diameter 

(in.)

R-4.2 

Flex Duct

R-6 

Flex Duct

R-8 

Flex Duct

4 4.1 5.0 6.0

6 4.4 5.6 6.7

8 4.6 5.9 7.2

10 4.7 6.1 7.5

12 4.9 6.3 7.7

14 4.9 6.4 7.9

16 5.0 6.5 8.0

Source: Shapiro et al. (2012)
48 

Table 13. Effective R-Values of Encapsulated Round Flexible Ducts by Insulation Thickness 

R-4.2 Flex Duct R-6 Flex Duct R-8 Flex Duct

ccSPF Thickness 1.5-in. 2-in. 2.5-in. 1.5-in. 2-in. 2.5-in. 1.5-in. 2-in. 2.5-in.

Nominal R-Value 14.3 17.6 21.0 16.1 19.4 22.8 18.1 21.4 24.8

4-in. diameter 9.0 10.4 11.6 9.4 10.6 11.7 9.9 10.9 11.9

6-in. diameter 10.4 12.0 13.6 11.0 12.5 13.9 11.6 13.0 14.3

8-in. diameter 11.3 13.1 14.9 12.0 13.7 15.4 12.7 14.4 15.9

10-in. diameter 11.9 13.9 15.9 12.7 14.7 16.5 13.6 15.4 17.1

12-in. diameter 12.3 14.5 16.6 13.3 15.4 17.3 14.3 16.2 18.1

14-in. diameter 12.7 15.0 17.2 13.7 15.9 18.0 14.8 16.9 18.9

16-in. diameter 13.0 15.4 17.7 14.1 16.4 18.6 15.2 17.4 19.5

Source: Shapiro et al. (2012)

Table 14. Effective R-Values of Buried Round Ducts

R-4.2 Flex Duct R-6 Flex Duct R-8 Flex Duct

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 

Source: Shapiro et al. (2012)
49 

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

R-4.2 Flex Duct R-6 Flex Duct R-8 Flex Duct

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

Source: Shapiro et al. (2012)
50 

Appendix C: Energy Savings by Climate and Roof Slope

Source energy savings as a percentage of total house energy use were calculated using BEoptE+ 

1.3 for a 2,400-ft2 single-story home built with slab-on-grade construction. Energy savings are 

compared to the Building America Benchmark (Hendron and Engebrecht 2010) and an improved 

benchmark that includes the IECC 2012 requirements for infiltration (5 ACH50) and duct sealing 

(4 cfm/100 ft2

 of conditioned living space). All modeled options are improvements to the 

improved benchmark. All ductwork and AHUs are assumed to be within the attic, which is 

consistent with the benchmark assumptions. Heating appliances are assumed to use natural gas. 

Modeling results by climate and roof slope are shown in Table 17 through Table 28. 

While using ccSPF to create a sealed attic will undoubtedly reduce the infiltration rate of the 

building, there are few quantifiable data about the infiltration reduction resulting from simply 

spray-foaming the roof deck. The lack of quantifiable data makes modeling the impact of roof 

deck spray foam difficult without simply assuming an infiltration reduction. As a result, all 

modeled configurations have the same infiltration rate as the improved benchmark in an effort to 

avoid trying to make assumptions about infiltration without data. 

Modeling ductwork air leakage is somewhat tricky because the IECC and BA Benchmark define 

total duct leakage using different metrics. The IECC sets a maximum leakage in terms of 

cfm/100 ft2

 of conditioned floor area. The BA Benchmark uses leakage as a fraction of AHU

flow. Comparisons can be drawn between the two standards, however, because the BA 

Benchmark is meant to be consistent with the 2009 IECC. The leakage rates for the improved 

benchmark can be determined by looking at the percent reduction in leakage between the 2009 

and 2012 IECC. The IECC goes from a maximum leakage rate of 12 cfm/100 ft2

 in 2009 to 4 

cfm/100 ft2

 in 2012. This 67% reduction is applied to the BA Benchmark, which reduces the 

leakage rate from 15% to 5%. 

Table 16. Duct Leakage Rates (% of AHU Flow) for BEopt Modeling

Supply Return AHU 

Supply

AHU 

Return Total 

Benchmark 9 1 1 4 15

Improved Benchmark 1.5 1.5 1 1 5

Partially Buried 1.5 1.5 1 1 5

Fully Buried 1.5 1.5 1 1 5

Deeply Buried 1.5 1.5 1 1 5

Unvented 1.5 1.5 1 1 5

Encapsulated 0 0 1 1 2

Partially Buried and Encapsulated 0 0 1 1 2

Fully Buried and Encapsulated 0 0 1 1 2

Deeply Buried and Encapsulated 0 0 1 1 2

Interior Ducts 1.5 1.5 1 1 5

Splitting up the specific leakage rates for the supply, return, AHU supply, and AHU return was 

not as straightforward as taking the BA Benchmark and reducing each component by two thirds. 
51 

Since the ccSPF will eliminate duct leakage, the supply and return duct leakage rates for the 

encapsulated cases will be zero. BEopt does not allow unbalanced duct leakage, however, so the 

portion of the duct leakage attributed to each component of the system had to be reapportioned. 

The AHU leakage was assumed to be 2%, which is the maximum stated rate in the 2012 IECC. 

The remaining leakage was equally apportioned to the supply and return sides. The resulting 

leakage rates for modeling are shown in Table 16. 

Table 17. Jacksonville, Florida (2A)

Table 18. Tucson, Arizona (2B)

Assembly (Attic R-value) 4:12 6:12 8:12 10:12

Benchmark (R-30)a 0.0% 0.0% 0.0% 0.0%

Improved benchmark (R-30)a,b 5.9% 5.9% 5.9% 5.9%

Partially-buried (R-33) N/A N/A N/A N/A

Fully-buried (R-42) N/A N/A N/A N/A

Deeply-buried (R-51) N/A N/A N/A N/A

Unvented (R-30)a 7.5% 7.3% 7.1% 6.8%

Encapsulated (R-30)a 7.1% 7.0% 7.1% 7.0%

Partially-buried & encapsulated (R-37) 8.3% 8.3% 8.4% 8.3%

Fully-buried & encapsulated (R-46) 9.1% 9.1% 9.1% 9.1%

Deeply-buried & encapsulated (R-54) 9.6% 9.7% 9.7% 9.6%

Interior ducts (R-30)a 8.9% 8.8% 8.9% 8.8%

Roof slope

a

 Benchmark ceiling or roof deck insulation is R-30 in Zone 2A. Ceiling insulation R-values for buried 

 ducts may be higher than the benchmark.

b

 Improved Benchmark includes IECC 2012 requirements for infiltration (5 ACH50) and duct sealing (4 

 cfm per 100 ft2

 conditioned living space).

Assembly (Attic R-value) 4:12 6:12 8:12 10:12

Benchmark (R-30)a 0.0% 0.0% 0.0% 0.0%

Improved benchmark (R-30)a,b 5.1% 5.1% 5.1% 5.1%

Partially-buried (R-33) 6.5% 6.5% 6.5% 6.5%

Fully-buried (R-42) 8.1% 8.1% 8.1% 8.1%

Deeply-buried (R-51) 9.3% 9.4% 9.3% 9.3%

Unvented (R-30)a 7.9% 7.5% 7.1% 6.7%

Encapsulated (R-30)a 6.9% 6.8% 6.8% 6.8%

Partially-buried & encapsulated (R-37) 8.7% 8.6% 8.6% 8.6%

Fully-buried & encapsulated (R-46) 9.8% 9.8% 9.7% 9.7%

Deeply-buried & encapsulated (R-54) 10.5% 10.6% 10.6% 10.5%

Interior ducts (R-30)a 9.6% 9.5% 9.5% 9.4%

Roof slope

a

 Benchmark ceiling or roof deck insulation is R-30 in Zone 2B. Ceiling insulation R-values for buried 

 ducts may be higher than the benchmark.

b

 Improved Benchmark includes IECC 2012 requirements for infiltration (5 ACH50) and duct sealing (4 

 cfm per 100 ft2

 conditioned living space).
52 

Table 19. Atlanta, Georgia (3A)

Table 20. Las Vegas, NV (3B)

Assembly (Attic R-value) 4:12 6:12 8:12 10:12

Benchmark (R-30)a 0.0% 0.0% 0.0% 0.0%

Improved benchmark (R-30)a,b 7.6% 7.6% 7.6% 7.7%

Partially-buried (R-33) N/A N/A N/A N/A

Fully-buried (R-42) N/A N/A N/A N/A

Deeply-buried (R-51) N/A N/A N/A N/A

Unvented (R-30)a 10.6% 10.3% 9.9% 9.5%

Encapsulated (R-30)a 9.1% 9.2% 9.2% 9.2%

Partially-buried & encapsulated (R-37) 11.1% 11.1% 11.1% 11.1%

Fully-buried & encapsulated (R-46) 12.1% 12.3% 12.2% 12.2%

Deeply-buried & encapsulated (R-54) 12.9% 13.1% 13.0% 13.0%

Interior ducts (R-30)a 11.9% 11.9% 11.9% 11.9%

Roof slope

a

 Benchmark ceiling or roof deck insulation is R-30 in Zone 3A. Ceiling insulation R-values for buried 

 ducts may be higher than the benchmark.

b

 Improved Benchmark includes IECC 2012 requirements for infiltration (3 ACH50) and duct sealing (4 

 cfm per 100 ft2

 conditioned living space).

Assembly (Attic R-value) 4:12 6:12 8:12 10:12

Benchmark (R-30)a 0.0% 0.0% 0.0% 0.0%

Improved benchmark (R-30)a,b 7.7% 7.6% 7.6% 7.7%

Partially-buried (R-33) 9.2% 9.2% 9.2% 9.2%

Fully-buried (R-42) 10.9% 10.9% 10.9% 10.9%

Deeply-buried (R-51) 12.2% 12.2% 12.2% 12.2%

Unvented (R-30)a 9.3% 10.2% 9.7% 9.3%

Encapsulated (R-30)a 9.5% 9.5% 9.5% 9.5%

Partially-buried & encapsulated (R-37) 11.4% 11.5% 11.4% 11.4%

Fully-buried & encapsulated (R-46) 12.6% 12.7% 12.7% 12.6%

Deeply-buried & encapsulated (R-54) 13.5% 13.6% 13.6% 13.5%

Interior ducts (R-30)a 12.4% 12.5% 12.4% 12.4%

Roof slope

a

 Benchmark ceiling or roof deck insulation is R-30 in Zone 3B. Ceiling insulation R-values for buried 

 ducts may be higher than the benchmark.

b

 Improved Benchmark includes IECC 2012 requirements for infiltration (3 ACH50) and duct sealing (4 

 cfm per 100 ft2

 conditioned living space).
53 

Table 21. San Francisco, California (3C)

Table 22. Lexington, Kentucky (4A)

Assembly (Attic R-value) 4:12 6:12 8:12 10:12

Benchmark (R-30)a 0.0% 0.0% 0.0% 0.0%

Improved benchmark (R-30)a,b 7.4% 7.4% 7.4% 7.4%

Partially-buried (R-33) N/A N/A N/A N/A

Fully-buried (R-42) N/A N/A N/A N/A

Deeply-buried (R-51) N/A N/A N/A N/A

Unvented (R-30)a 11.9% 11.7% 11.4% 11.0%

Encapsulated (R-30)a 8.5% 8.5% 8.5% 8.5%

Partially-buried & encapsulated (R-37) 10.0% 10.1% 10.1% 10.1%

Fully-buried & encapsulated (R-46) 10.9% 11.1% 11.1% 11.1%

Deeply-buried & encapsulated (R-54) 11.6% 11.8% 11.8% 11.8%

Interior ducts (R-30)a 10.5% 10.5% 10.5% 10.5%

Roof slope

a

 Benchmark ceiling or roof deck insulation is R-30 in Zone 3C. Ceiling insulation R-values for buried 

 ducts may be higher than the benchmark.

b

 Improved Benchmark includes IECC 2012 requirements for infiltration (3 ACH50) and duct sealing (4 

 cfm per 100 ft2

 conditioned living space).

Assembly (Attic R-value) 4:12 6:12 8:12 10:12

Benchmark (R-38)a 0.0% 0.0% 0.0% 0.0%

Improved benchmark (R-38)a,b 10.0% 10.0% 10.0% 10.1%

Partially-buried (R-38) N/A N/A N/A N/A

Fully-buried (R-42) N/A N/A N/A N/A

Deeply-buried (R-51) N/A N/A N/A N/A

Unvented (R-38)a 13.4% 13.0% 12.6% 12.1%

Encapsulated (R-38)a 11.8% 11.7% 11.8% 11.8%

Partially-buried & encapsulated (R-38) 12.7% 12.6% 12.6% 12.7%

Fully-buried & encapsulated (R-46) 13.7% 13.8% 13.8% 13.8%

Deeply-buried & encapsulated (R-54) 14.6% 14.7% 14.8% 14.8%

Interior ducts (R-38)a 14.8% 14.8% 14.8% 14.8%

Roof slope

a

 Benchmark ceiling or roof deck insulation is R-38 in Zone 4A. Ceiling insulation R-values for buried 

 ducts may be higher than the benchmark.

b

 Improved Benchmark includes IECC 2012 requirements for infiltration (3 ACH50) and duct sealing (4 

 cfm per 100 ft2

 conditioned living space).
54 

Table 23. Albuquerque, New Mexico (4B)

Table 24. Seattle, Washington (4C)

Assembly (Attic R-value) 4:12 6:12 8:12 10:12

Benchmark (R-38)a 0.0% 0.0% 0.0% 0.0%

Improved benchmark (R-38)a,b 8.0% 8.0% 8.1% 8.1%

Partially-buried (R-38) 8.9% 9.0% 9.0% 9.0%

Fully-buried (R-42) 10.0% 10.0% 10.1% 10.0%

Deeply-buried (R-51) 11.3% 11.5% 11.5% 11.5%

Unvented (R-38)a 11.3% 10.9% 10.4% 10.0%

Encapsulated (R-38)a 9.7% 9.7% 9.7% 9.7%

Partially-buried & encapsulated (R-38) 10.5% 10.5% 10.6% 10.6%

Fully-buried & encapsulated (R-46) 11.6% 11.7% 11.8% 11.7%

Deeply-buried & encapsulated (R-54) 12.6% 12.8% 12.8% 12.8%

Interior ducts (R-38)a 12.6% 12.6% 12.6% 12.6%

Roof slope

a

 Benchmark ceiling or roof deck insulation is R-38 in Zone 4B. Ceiling insulation R-values for buried 

 ducts may be higher than the benchmark.

b

 Improved Benchmark includes IECC 2012 requirements for infiltration (3 ACH50) and duct sealing (4 

 cfm per 100 ft2

 conditioned living space).

Assembly (Attic R-value) 4:12 6:12 8:12 10:12

Benchmark (R-38)a 0.0% 0.0% 0.0% 0.0%

Improved benchmark (R-38)a,b 8.7% 8.8% 8.8% 8.8%

Partially-buried (R-38) N/A N/A N/A N/A

Fully-buried (R-42) N/A N/A N/A N/A

Deeply-buried (R-51) N/A N/A N/A N/A

Unvented (R-38)a 13.2% 12.8% 12.4% 11.9%

Encapsulated (R-38)a 10.8% 10.9% 10.9% 10.9%

Partially-buried & encapsulated (R-38) 11.8% 11.8% 11.9% 11.9%

Fully-buried & encapsulated (R-46) 12.9% 13.0% 13.0% 13.0%

Deeply-buried & encapsulated (R-54) 13.8% 14.0% 14.0% 14.0%

Interior ducts (R-38)a 14.0% 14.0% 14.0% 14.0%

Roof slope

a

 Benchmark ceiling or roof deck insulation is R-38 in Zone 4C. Ceiling insulation R-values for buried 

 ducts may be higher than the benchmark.

b

 Improved Benchmark includes IECC 2012 requirements for infiltration (3 ACH50) and duct sealing (4 

 cfm per 100 ft2

 conditioned living space).
55 

Table 25. Boston, Massachusetts (5A)

Table 26. Denver, Colorado (5B)

Assembly (Attic R-value) 4:12 6:12 8:12 10:12

Benchmark (R-38)a 0.0% 0.0% 0.0% 0.0%

Improved benchmark (R-38)a,b 13.0% 13.0% 13.0% 13.0%

Partially-buried (R-38) N/A N/A N/A N/A

Fully-buried (R-42) N/A N/A N/A N/A

Deeply-buried (R-51) N/A N/A N/A N/A

Unvented (R-38)a 16.5% 16.1% 15.7% 15.1%

Encapsulated (R-38)a 14.9% 14.9% 14.9% 14.9%

Partially-buried & encapsulated (R-38) 15.9% 15.9% 15.9% 16.0%

Fully-buried & encapsulated (R-46) 17.0% 17.1% 17.1% 17.1%

Deeply-buried & encapsulated (R-54) 17.9% 18.1% 18.1% 18.1%

Interior ducts (R-38)a 18.1% 18.1% 18.1% 18.1%

Roof slope

a

 Benchmark ceiling or roof deck insulation is R-38 in Zone 5A. Ceiling insulation R-values for buried 

 ducts may be higher than the benchmark.

b

 Improved Benchmark includes IECC 2012 requirements for infiltration (3 ACH50) and duct sealing (4 

 cfm per 100 ft2

 conditioned living space).

Assembly (Attic R-value) 4:12 6:12 8:12 10:12

Benchmark (R-38)a 0.0% 0.0% 0.0% 0.0%

Improved benchmark (R-38)a,b 9.9% 10.0% 10.0% 10.0%

Partially-buried (R-38) 11.1% 11.1% 11.2% 11.2%

Fully-buried (R-42) 12.3% 12.4% 12.4% 12.4%

Deeply-buried (R-51) 13.8% 14.1% 14.1% 14.1%

Unvented (R-38)a 13.8% 13.3% 12.8% 12.2%

Encapsulated (R-38)a 11.9% 11.9% 12.0% 12.0%

Partially-buried & encapsulated (R-38) 12.9% 13.0% 13.0% 13.0%

Fully-buried & encapsulated (R-46) 14.2% 14.3% 14.4% 14.4%

Deeply-buried & encapsulated (R-54) 15.3% 15.5% 15.5% 15.6%

Interior ducts (R-38)a 15.4% 15.5% 15.5% 15.5%

Roof slope

a

 Benchmark ceiling or roof deck insulation is R-38 in Zone 5B. Ceiling insulation R-values for buried 

 ducts may be higher than the benchmark.

b

 Improved Benchmark includes IECC 2012 requirements for infiltration (3 ACH50) and duct sealing (4 

 cfm per 100 ft2

 conditioned living space).
56 

Table 27. Madison, Wisconsin (6A)

Table 28. Billings, Montana (6B)

Assembly (Attic R-value) 4:12 6:12 8:12 10:12

Benchmark (R-49)a 0.0% 0.0% 0.0% 0.0%

Improved benchmark (R-49)a,b 14.3% 14.2% 14.3% 14.3%

Partially-buried (R-49) N/A N/A N/A N/A

Fully-buried (R-49) N/A N/A N/A N/A

Deeply-buried (R-51) N/A N/A N/A N/A

Unvented (R-49)a 18.2% 17.6% 17.2% 16.6%

Encapsulated (R-49)a 16.3% 16.3% 16.3% 16.3%

Partially-buried & encapsulated (R-49) 17.3% 17.3% 17.4% 17.4%

Fully-buried & encapsulated (R-49) 17.6% 17.6% 17.6% 17.7%

Deeply-buried & encapsulated (R-54) 18.3% 18.4% 18.5% 18.5%

Interior ducts (R-49)a 19.9% 20.0% 20.0% 20.0%

Roof slope

a

 Benchmark ceiling or roof deck insulation is R-49 in Zone 6A. Ceiling insulation R-values for buried 

 ducts may be higher than the benchmark.

b

 Improved Benchmark includes IECC 2012 requirements for infiltration (3 ACH50) and duct sealing (4 

 cfm per 100 ft2

 conditioned living space).

Assembly (Attic R-value) 4:12 6:12 8:12 10:12

Benchmark (R-49)a 0.0% 0.0% 0.0% 0.0%

Improved benchmark (R-49)a,b 13.4% 13.5% 13.5% 13.5%

Partially-buried (R-49) 14.9% 15.0% 15.0% 15.0%

Fully-buried (R-49) 15.7% 15.8% 15.8% 15.8%

Deeply-buried (R-51) 16.7% 16.7% 16.8% 16.8%

Unvented (R-49)a 18.4% 17.9% 17.5% 16.9%

Encapsulated (R-49)a 15.8% 15.8% 15.9% 15.9%

Partially-buried & encapsulated (R-49) 17.1% 17.1% 17.1% 17.2%

Fully-buried & encapsulated (R-49) 17.5% 17.5% 17.5% 17.5%

Deeply-buried & encapsulated (R-54) 18.3% 18.4% 18.4% 18.4%

Interior ducts (R-49)a 20.2% 20.2% 20.3% 20.3%

Roof slope

a

 Benchmark ceiling or roof deck insulation is R-49 in Zone 6B. Ceiling insulation R-values for buried 

 ducts may be higher than the benchmark.

b

 Improved Benchmark includes IECC 2012 requirements for infiltration (3 ACH50) and duct sealing (4 

 cfm per 100 ft2

 conditioned living space).
DOE/GO-102013-3893 ▪ August 2013

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