Heating, Ventilation, and 

Air Conditioning Design 

Strategy for a Hot-Humid 

Production Builder

P. Kerrigan

Building Science Corporation

March 2014
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iii

Heating, Ventilation, and Air Conditioning Design Strategy for a 

Hot-Humid Production Builder

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: 

P. Kerrigan

Building Science Corporation

30 Forest Street

Somerville, MA 02143

NREL Technical Monitor: Cheryn Metzger

Prepared under Subcontract No. KNDJ-0-40337-03

March 2014
iv

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v

Contents

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

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

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

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

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

1.1 Purpose.................................................................................................................................1

1.2 Research Questions..............................................................................................................2

1.3 Previous Research................................................................................................................2

1.4 Whole-House Specifications................................................................................................2

2 Development of Duct Strategy ............................................................................................................ 5

2.1 Initial Design Development .................................................................................................5

3 HVAC Design and Implementation ................................................................................................... 10

3.1 Plan 4127 ...........................................................................................................................10

3.2 Plan 4128 ...........................................................................................................................15

3.3 Plan 4069 ...........................................................................................................................19

4 Testing Results................................................................................................................................... 23

4.1 Overview............................................................................................................................23

4.2 Plan 4127 ...........................................................................................................................23

4.3 Plan 4128 ...........................................................................................................................26

4.4 Plan 4069 ...........................................................................................................................27

4.5 Duct Test Discussion .........................................................................................................30

5 Duct Design Cost Analysis................................................................................................................ 31

5.1 Plan 4127 ...........................................................................................................................31

5.2 Plan 4128 ...........................................................................................................................33

5.3 Plan 4069 ...........................................................................................................................35

5.4 Cost Analysis Discussion...................................................................................................35

6 Code Compliance Analysis ............................................................................................................... 37

7 Conclusions ........................................................................................................................................ 41

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

Appendix.................................................................................................................................................... 45
vi

List of Figures

Figure 1. Plan 4127...................................................................................................................................... 3

Figure 2. Plan 4128...................................................................................................................................... 3

Figure 3. Plan 4069...................................................................................................................................... 3

Figure 4. Duct design strategy 1—Dropped ceilings and mechanical closet....................................... 5

Figure 5. Duct design strategy 2—Buried ducts and mechanical closet .............................................. 6

Figure 6. Duct design strategy 3—Dropped ceiling and mechanical coffer ......................................... 7

Figure 7. Duct design strategy 4—Ductwork and AHU in mechanical coffer....................................... 8

Figure 8. Duct design strategy 5—Unvented cathedralized attic........................................................... 9

Figure 9. Duct layout for plan 4127 ......................................................................................................... 11

Figure 10. Plan 4127 mechanical closet section.................................................................................... 11

Figure 11. Plan 4127 mechanical closet during framing visit............................................................... 12

Figure 12. Plan 4127 constructed duct coffer........................................................................................ 12

Figure 13. Jump duct coffer..................................................................................................................... 13

Figure 14. HVAC runouts in dropped ceiling at plan 4127.................................................................... 14

Figure 15. Plan 4127 mechanical closet in attic..................................................................................... 14

Figure 16. Plan 4127 add-on mechanical closet .................................................................................... 15

Figure 17. Plan 4128 second-floor duct layout ...................................................................................... 16

Figure 18. Plan 4128 mechanical closet building section..................................................................... 17

Figure 19. Plan 4128 mechanical closet ................................................................................................. 17

Figure 20. Plan 4128 return duct in knee wall ........................................................................................ 18

Figure 21. Plan 4128 first-floor duct layout ............................................................................................ 18

Figure 22. Plan 4128 dining room duct coffer........................................................................................ 19

Figure 23. Plan 4069 second-floor duct layout ...................................................................................... 19

Figure 24. Plan 4069 upstairs mechanical closet coffer ....................................................................... 20

Figure 25. Plan 4069 second-floor guest suite coffer ........................................................................... 20

Figure 26. Plan 4069 insulation trough................................................................................................... 21

Figure 27. Plan 4069 first-floor duct layout ............................................................................................ 21

Figure 28. Plan 4069 first floor kitchen “raceway” location ................................................................. 22

Figure 29. Plan 4128 front elevation........................................................................................................ 23

Figure 30. Plan 4127 design register flow versus measured register flow plot ................................. 25

Figure 31. Plan 4128 front elevation........................................................................................................ 26

Figure 32. Plan 4128 design register flow versus measured register flow plot ................................. 27

Figure 33. Plan 4069 front elevation........................................................................................................ 28

Figure 34. Inconsistencies in air barrier at floor joist in attic kneewall space ................................... 28

Figure 35. Plan 4069 design register flow versus measured register flow plot ................................. 29

Figure 36. Plan 4127 BEopt source energy savings versus the BA Benchmark—end use 

breakdown........................................................................................................................................... 46

Figure 37. Plan 4127 BEopt parametric graph ....................................................................................... 46

Figure 38. Plan 4128 BEopt source energy savings versus the BA Benchmark—end use 

breakdown........................................................................................................................................... 47

Figure 39. Plan 4128 BEopt parametric graph ....................................................................................... 47

Figure 40. Plan 4069 BEopt source energy savings versus the BA Benchmark—end use 

breakdown........................................................................................................................................... 48

Figure 41. Plan 4069 BEopt parametric graph duct layouts ................................................................. 48

Unless otherwise indicated, all figures were created by BSC.
vii

List of Tables

Table 1. Floor Plan Dimensions and Areas .............................................................................................. 3

Table 2. Summary of DWH – Houston Energy Efficiency Package Components ................................ 4

Table 3. Plan 4127 Infiltration Testing Results....................................................................................... 24

Table 4. Plan 4127 Duct Testing Results ................................................................................................ 24

Table 5. Plan 4127 HVAC ESPs................................................................................................................ 25

Table 6. Plan 4127 Room Pressure Measurements ............................................................................... 25

Table 7. Plan 4128 Infiltration Testing Results....................................................................................... 26

Table 8. Plan 4128 Duct Testing Results ................................................................................................ 26

Table 9. Plan 4128 HVAC ESPs................................................................................................................ 27

Table 10. Plan 4128 Room Pressure Measurements ............................................................................. 27

Table 11. Plan 4069 Infiltration Testing Results .................................................................................... 29

Table 12. Plan 4069 Duct Testing Results .............................................................................................. 29

Table 13. Plan 4069 HVAC ESPs.............................................................................................................. 30

Table 14. Plan 4069 Room Pressure Measurements ............................................................................. 30

Table 15. Plan 4127 Enclosure Costs...................................................................................................... 32

Table 16. Plan 4127 Mechanical Costs.................................................................................................... 32

Table 17. Plan 4128 Enclosure Costs...................................................................................................... 34

Table 18. Plan 4128 Mechanical Costs.................................................................................................... 34

Table 19. Plan 4069 Enclosure Costs...................................................................................................... 36

Table 20. Plan 4069 Mechanical Costs.................................................................................................... 36

Table 21. Plan 4127 IECC 2006 Compliance Analysis Specifications ................................................. 39

Table 22. Plan 4127 IECC 2006 Energy Cost Compliance Chart .......................................................... 40

Table 23. DWH Cost Data for BEopt Analysis........................................................................................ 45

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

Definitions

AFUE Annual fuel utilization efficiency

AHU Air handling unit

BA Building America Program

BSC Building Science Corporation

CFIS Central fan integrated supply

CFM Cubic feet per minute

DWH David Weekley Homes

EF Energy factor

ESP External static pressure

HERS Home Energy Rating System

HVAC Heating, ventilation, and air conditioning

IECC International Energy Conservation Code

OSB Oriented strand board

SEER Seasonal energy efficiency ratio

SHGC Solar heat gain coefficient

TESP Total external static pressure

WIC Inches water column

XPS Extruded polystyrene
ix

Executive Summary

BSC worked directly with the David Weekley Homes (DWH) – Houston division to redesign

three current floor plans in order to locate the heating, ventilation, and air conditioning (HVAC)

system in conditioned space. The purpose of this project is to develop a cost-effective design for 

moving the HVAC system into conditioned space. In addition, BSC conducted energy analysis to 

calculate the most economical strategy for increasing the energy performance of future 

production houses. This is in preparation for the upcoming code changes in 2015. The builder 

wishes to develop an upgrade package that will allow for a seamless transition to the new code 

mandate.

The following research questions were addressed by this research project.

1. What is the most cost-effective, best-performing, and most easily replicable method of 

locating ducts inside conditioned space for a hot-humid production home builder that 

constructs one- and two-story single-family detached residences?

2. What is a cost-effective and practical method of achieving 50% source energy savings 

versus the 2006 International Energy Conservation Code for a hot-humid production 

builder?

3. How accurate are the pre-construction whole house cost estimates compared to confirmed 

post-construction actual cost?

BSC and the builder developed a duct design strategy that employs a system of dropped ceilings 

and attic coffers for moving the ductwork from the vented attic to conditioned space. The furnace 

has been moved to either a mechanical closet in the conditioned living space or a coffered space 

in the attic.

The development of a design for duct coffers in the attic space allows DWH to embrace the 

strategy of locating ductwork in conditioned space, in its existing housing stock, without having 

to rely solely on dropped ceilings. The builder does recognize that a full series of dropped 

ceilings, without any coffers, would be the most economical strategy for locating ducts.

However, the builder perceives this design as having a negative impact on aesthetics and could 

not be attractive to its customer base. This is one of the more important developments in this 

research work, as DWH (and its customers) greatly value full ceiling height in the majority of 

spaces and wish to avoid dropped ceilings as much as possible. The main contribution of this 

research was to provide a working alternative for builders who wish to move their HVAC 

systems into conditioned spaces without extended dropped ceilings plus a mechanical closet, or 

converting to a full unvented cathedralized attic.

Relocating the HVAC system to within conditioned space resulted in significant energy savings 

toward the goal of achieving 50% energy savings versus the 2006 International Energy 

Conservation Code. Moving the HVAC system inside conditioned space saves around 4%–5% in 

source energy use and can reduce the Home Energy Rating System Index by around 4 points.

The costs for implementing this duct design strategy on the three research homes was in the 

range of $6,000–$10,000; however, the builder expects that this figure can improve in future 

homes to around $4,000–$6,000. This is more affordable compared to the popular strategy of 
x

constructing an unvented cathedralized attic with spray polyurethane foam, which for these plans 

could cost in the range of $10,000–$15,000.

The additional energy upgrades implemented for meeting the 2015 code provision includes 

improvements to the walls, ceiling, infiltration rate, air conditioner and hot water heater. The 

predicted costs for these improvements were very accurate compared to the post-construction 

confirmed costs, as this production builder is adept and experienced with costing specific 

components.
1

1 Introduction

1.1 Purpose

Through the advanced new construction energy efficiency packages evaluation in Texas, 

Building Science Corporation (BSC) seeks to acquire important information about the 

performance of energy-efficient technology packages designed for a production builder in a hothumid climate. This research addresses several important gaps and barriers:

• Cost-competitive and replicable designs for locating ducts inside conditioned space by a 

production builder

• Complete high performance technology packages that will comply with expected future

code improvements.

Through this work, BSC expects to collect information about:

• Cost and implementation issues with locating the entire heating, ventilation, and air 

conditioning (HVAC) system in conditioned space in production homes.

• Comparisons between the various predicted energy savings methodologies available in 

industry (source energy savings versus the Building America (BA) Benchmark compared 

to site energy savings provided by commonly used residential modeling software)

The technology package proposed for this pilot community project is most appropriate for 

single-family detached production houses. From a building science perspective, the Houston 

package is suitable for other hot-humid production environments. The information gained 

through this research about the implementation of the technology package at a production 

community scale and the longer term performance data from the community of houses will 

support widespread deployment of this package in new housing across the hot-humid climate 

zone.

The most immediate impact of the research project will be to inform the work of David Weekley 

Homes (DWH) – Houston. Lessons learned both in the economics of the variations in design and 

constructability can be applied to the future business model of the production builder.

The adoption of the new and more stringent 2012 International Energy Conservation Code 

(IECC) is greatly reducing the performance gap between code-built homes and those that are 

constructed to meet an energy efficiency standard (Bailes 2012). A major component of the 2012 

IECC is the mandatory inclusion of the entire HVAC system in conditioned space. This mandate 

is forcing builders to establish cost-effective strategies for moving the ductwork inside the 

thermal enclosure. The project also has the potential to impact BA measure guidelines on 

improving the replicability and cost effectiveness of designs that not only meet the current 

energy code, but will meet future proposed building code improvements.

This presents a major opportunity for considering the importance of a cost-effective method of 

locating ducts inside conditioned space in order for production home builders to remain 

competitive in the market. There is also an opportunity to gauge what house designs will meet 

the likely 2015 IECC provisional mandate. DWH sees this as part of a long-term strategy for 
2

maintaining a high performance design that will be compliant with potential future code 

provisions. This work will also provide important design and construction data for other 

production builder in a hot-humid climate as well as for local Home Energy Rating System 

(HERS) raters and architects.

1.2 Research Questions

The following research questions will be answered by this project.

1. What is the most cost-effective, best-performing, and most easily replicable method of 

locating ducts inside conditioned space for a hot-humid production home builder that 

constructs one- and two-story single-family detached residences?

2. What is a cost-effective and practical method of achieving 50% source energy savings 

versus the 2006 IECC for a hot-humid production builder?

3. How accurate are the pre-construction whole house cost estimates compared to confirmed 

post-construction actual cost?

1.3 Previous Research

Moving ducts inside conditioned space offers the highest energy savings compared to other 

energy-related improvements (Lubliner et al. 2008). As energy programs and codes become 

more stringent, this strategy will need to be implemented more throughout the country. 

Previous research has provided guidance and identified risks with duct design strategies, 

specifically buried ducts (Chasar and Withers 2013; Griffiths et al. 2004). General duct design 

strategies have been developed for low-energy homes to inform designers and builders (Burdick 

2011). Previous studies have calculated potential energy losses due to ducts being located in 

unconditioned space as being in the range of 25%–40% (Andrews 2003), thus prioritizing the 

need for developing affordable strategies for moving the HVAC system within the thermal 

enclosure.

Many of the whole-house characteristics for these homes are connected to previous research 

work on advanced framing, the effectiveness of ventilation systems, and other whole-house 

energy efficiency packages for affordable housing as described below.

The enclosure included in the Houston energy efficiency package uses advanced framing (BSC 

2009; Lstiburek and Grin 2010) and insulating sheathing as the primary thermal control layer and 

the drainage plane (Baker 2006; BSC 2007).

This work also draws on whole house energy efficiency research work that has been published 

by BSC the Builder’s Field Guides series (Lstiburek 2005) and in research reports on community 

scale evaluations in hot-humid and other climate zones (BSC 2010).

The package specifies a central fan integrated ventilation supply (CFIS) system that has been 

extensively researched and tested by BSC (Rudd 2008; Hendron et al. 2008).

1.4 Whole-House Specifications

Figure 1 through Figure 3 show the front elevation of the three research houses for this project.
3

Figure 1. Plan 4127 Figure 2. Plan 4128

Figure 3. Plan 4069

Table 1 lists the general dimensions and areas for each of the three floor plans.

Table 1. Floor Plan Dimensions and Areas

Floor

Plan

#

Floors

Floor Area

(ft2)

Surface Area

(ft2)

Volume

(ft3)

Beds

(ct)

Baths

(ct)

Glazing

Ratio

4127 1 1,757 5,875 17,570 3 2.0 17.0%

4128 2 2,179 5,729 21,090 3 2.5 12.7%

4069 2 4,169 8,833 39,474 5 3.5 13.8%

Table 2 lists the characteristics for the three DWH research homes. Whole-house energy analysis 

was performed on each of the three homes. This includes a prediction of source energy savings 

with the characteristics found in Table 2. This analysis can be found in the appendix.
4

Table 2. Summary of DWH – Houston Energy Efficiency Package Components

Enclosure Specifications

Roof

Description Dark color asphalt shingles on rafter roof – vented attic

Insulation R-50 blown fiberglass

Walls

Description 2 × 6 @ 24 in. o.c. with advanced framing with insulating 

sheathing

Insulation R-19 fiberglass batts with R-3.75 3⁄4-in. XPSa insulating sheathing

Foundation

Description Slab on grade

Insulation Uninsulated

Windows

Description Double pane vinyl framed with LoE3 spectrally selective glazing

U-Value U = 0.29

SHGCb SHGC = 0.22

Infiltration

Specification 0.25 CFM 50/ft2 enclosure @ 50 Pa

Performance Test Average = 0.22 CFM 50/ft2 enclosure @ 50 Pa

Mechanical Systems Specifications

Heating

Description 96% AFUEc natural gas furnace

Manufacturer and Model Lennox

Cooling

Description 16 SEERd two-stage air conditioner

Manufacturer and Model Lennox

Domestic Hot Water

Description Tank gas water heater (EFe = 0.62)

Manufacturer and Model Rheem

Distribution

Description R-6 flex ducts in conditioned space

Leakage maximum 5% duct leakage to outside

Ventilation

Description Central fan integrated supply-only system with fan cycler 

33% Duty cycle: 10 minutes on; 20 minutes off, 50 CFM flow

Manufacturer and Model Aprilaire 8126 Ventilation Control System fan cycler

Return Pathways

Description Mostly jump ducts in bedrooms and study

Some active returns in master suites a Extruded polystyrene

b Solar heat gain coefficient

c Annual fuel utilization efficiency

d Seasonal energy efficiency ratio

e Energy factor
5

2 Development of Duct Strategy

BSC visited DWH – Houston in August 2012 and met with representatives from DWH and 

Davis Air Conditioning (HVAC contractor). An initial meeting was conducted to discuss and

develop a proposed duct design strategy. Three homes currently under construction, representing 

a range of DWH house and HVAC system types, were toured over a 2-day period to be used as 

examples for developing strategies for locating ducts within conditioned space.

2.1 Initial Design Development

A number of strategies exist for locating ducts within conditioned space. Figure 4 shows Duct 

design strategy 1: Dropped ceiling and mechanical closet. In this strategy, the air handling unit

(AHU) is located in a mechanical closet in the living space and all of the ductwork is located in a 

series of strategically placed dropped ceilings. The main benefit to this design is that the attic 

remains a typical vented attic and traditional insulation (e.g., fiberglass or cellulose) can be 

installed. The air barrier remains a two-dimensional plane, without any duct penetrations, which 

is the most simplified air barrier geometry (red line in Figure 4). This strategy is the most 

economical out of all the strategies discussed.

Figure 4. Duct design strategy 1—Dropped ceilings and mechanical closet

Negatives associated with this strategy include: conditioned floor area lost due to mechanical 

closet, reduced architectural aesthetics with lowered ceiling level, and potential noise from the 

mechanical closet.
6

The risk of AHU closet noise can be abated by implementing the following measures:

• Installing a variable-speed AHU or furnace with an electronically commutated motor, as 

these motors are quieter than traditional permanent split capacitor motors. 

• Installing a weather-stripped door at the mechanical closet 

• Designing the duct system to operate at low velocities in the supply and return plenums.

BSC recommends designing the supply plenum for 500–750 ft/min and the return plenum 

at 250–550 ft/min. Also, velocities at the return grille should not exceed 350 ft/min to 

prevent whistling (Rudd 2006).

Despite the drawbacks, this is a very popular strategy with builders and is the best performing 

design from an energy standpoint.

Figure 5 shows Duct design strategy 2: Buried ducts and mechanical closet. In this strategy, the 

air handler is again in a mechanical closet but the ductwork is located in the attic closer to the 

ceiling plane. Traditional insulation (e.g., fiberglass or cellulose) is installed and the ductwork is 

covered in insulation. A benefit to this strategy is that dropped ceilings are no longer required.

Figure 5. Duct design strategy 2—Buried ducts and mechanical closet

Drawbacks associated with this strategy include: conditioned floor area lost due to mechanical 

closet and duct penetrations now exist through the air barrier which require additional air sealing. 

Also, the potential exists for condensation to form on the ductwork, even with insulated flex, as 

the ductwork is buried in an air and vapor-permeable insulation (Chasar and Withers 2013;

Griffiths et al. 2004).
7

Figure 6 shows Duct design strategy 3: Dropped ceiling and mechanical coffer. In this strategy, 

the AHU is located in an air-sealed and insulated coffered closet in the attic space and the 

ductwork is located in strategically placed dropped ceilings. Benefits to this strategy include: the 

removal of the AHU from conditioned living space, no duct penetrations through the air barrier.

Figure 6. Duct design strategy 3—Dropped ceiling and mechanical coffer

Drawbacks to this design include: added complexity for the air barrier, as it is now a threedimensional extrusion, and potential added cost for construction, air sealing, and insulating of the 

mechanical coffer (red line in Figure 6 indicates ceiling air barrier). Also, access to the coffer 

space is required and may be difficult with a complicated attic design.

Figure 7 shows Duct design strategy 4: Ductwork and AHU in mechanical coffer. In this 

strategy, the AHU and all ductwork are located in an air-sealed and insulated coffered closet in 

the attic. Benefits to this strategy include: the removal of the AHU from conditioned living space 

and the elimination of duct penetrations through the air barrier (red line in Figure 7 indicates the 

ceiling air barrier) and also the elimination of dropped ceilings.
8

Figure 7. Duct design strategy 4—Ductwork and AHU in mechanical coffer

A drawback to this strategy is potential increased cost. It is possible to minimize the size of the 

coffer if a small, centralized duct system is implemented; however, in large homes with 

complicated attic geometries, the applicability of this strategy may be limited. Also, as in 

Strategy 4, access to the coffer space is required and may be difficult with a complicated attic 

design. 

Figure 8 shows Duct design strategy 5: Unvented cathedralized attic. In this strategy, the AHU

and all ductwork is located in the attic space, and the air and thermal barrier is moved to the 

underside of the roof sheathing (red line in Figure 8. A major benefit to this strategy is that no 

modifications need to be made to the HVAC system, as the entire system can remain in the attic 

space as it originally was designed. Benefits to this strategy include: the removal of the AHU

from conditioned living space and the elimination of duct penetrations through the air barrier 

(red line in Figure 7 indicates the ceiling air barrier) and also the elimination of dropped ceilings.

Drawbacks to this design include: increased cost due to additional surface area to be insulated 

and the requirement of at least R-5 of air-impermeable insulation (e.g., spray polyurethane foam 

or rigid polystyrene foam) for condensation control (ICC 2012). A builder can choose to 

continue to install spray foam for the remainder of the roof insulation or to install traditional 

cavity insulation in the rafter joists. 
9

Figure 8. Duct design strategy 5—Unvented cathedralized attic

DWH and BSC discussed the pros and cons of these five duct design strategies. It was agreed 

that duct design strategies 1 and 5 are ideal; however, these strategies are implemented very 

often in residential homes. The group was not comfortable with pursuing strategy 2 due to the 

added penetrations and the potential for condensation on the buried ducts in a hot humid climate.

It was determined that either strategy 3 or 4 would be of considerable interest for addressing how 

to locate ducts inside conditioned space in an economic and easily replicable method. The 

location of the AHU and ducts would vary for each plan, as DWH’s plan product line spans a 

wide range of sizes and configurations. This hybrid design represented a design methodology

that is worthwhile of a research effort due to the lack of previous work with this specific strategy.

The group took a tour of three existing homes to discuss how to integrate this combination duct 

design strategy in the product line.

Due to the varying complexity of these three house types, it was expected that the most 

economical and replicable strategy would be a combination of strategies 3 and 4. The scope of 

this duct design combination will range depending on the plan types and geometries. Architects 

from DWH attended the site tours and meetings and added critical feedback such as aesthetic 

limitations that must be considered when developing the duct strategies.
10

3 HVAC Design and Implementation

Three floor plans were chosen for this research work, spanning a range of house sizes and 

complexities. Each floor plan was selected such that different categories of HVAC system design 

can be evaluated:

• Single-story house with one HVAC system—Plan 4127

• Two-story house with one HVAC system—Plan 4128

• Two-story house with two HVAC systems—Plan 4069.

Architects from DWH drafted the duct designs for each floor plan, based off of discussions with 

BSC on the three existing homes included in the site tour. The architects shared their preferences 

for locating dropped ceilings with BSC and this guided the development of the duct designs for 

each of the three floor plans. The list below outlines DWH’s preference for dropped ceilings (in 

order from most preferred to not willing to drop):

1. Hallways

2. Bathrooms

3. Bedrooms and Study

4. Main living areas (i.e., living room, kitchen, breakfast, dining room).

In areas where dropped ceilings are not preferred, the team advocated the construction of duct 

coffers in the attic space. It was agreed that the duct coffers would be constructed of duct board 

and would be installed by the HVAC installer rather than the framer as a cost-saving measure.

Otherwise an extra visit would be required of framing crews after the ductwork was installed to 

build the coffers, and the coffers themselves would be constructed with a more expensive woodbased material. The HVAC contractor was instructed to tape all seams and to spray foam or 

caulk all joints between the coffers and the ceiling plane.

3.1 Plan 4127

Plan 4127 represents a smaller, single-story floor plan (1,757 ft2

); thus, DWH expressed a desire 

to avoid locating a mechanical closet in conditioned space. It was determined that a mechanical 

coffer could be constructed above parts of the utility closet, bedroom 3, and the master walk-in 

closet. The attic hatch could be located in the utility closet. DWH determined that the 4127 duct 

strategy would be located in a system of dropped ceilings along with a single duct coffer for the 

dining and study. The utilization of a coffer in this area was required to avoid dropping ceilings 

in the dining and study, which are designated as critical aesthetic areas.

Figure 9 shows the designed duct layout for plan 4127. The diagonally hatched area designates

the mechanical closet in the attic space. The shaded areas designate dropped ceilings for locating 

the ductwork in conditioned space. The black triangle icons represent the location of wall supply 

registers that are serving the adjoining rooms. The red square represents the location of the 

furnace in the mechanical closet. Rooms with dropped ceilings in them have ceiling supply 

grilles but are not specifically labeled on the drawings. All bedrooms have jump ducts for 

passive return. Please refer to the appendix for the duct layout.
11

Figure 9. Duct layout for plan 4127

Figure 10 shows a building section with the mechanical closet framing above the utility and 

master closet. DWH considered eliminating the right-hand kneewall (circled in red), thus 

effectively extending the mechanical closet space to the exterior wall. However, DWH ultimately 

decided to construct the kneewall in order to limit the extra space conditioning energy use due to 

the addition of unnecessary volume and enclosure area to conditioned space.

Figure 10. Plan 4127 mechanical closet section

Black Arrows = high wall registers

Shaded Areas = dropped ceiling

Diagonal Hatches = mechanical coffer

Red Box = furnace
12

Figure 11 shows a photo of the mechanical closet from a site visit during framing. BSC observed 

a large return duct outside of the mechanical closet. The diagonal mechanical wall was 

subsequently moved forward to enclose the entire return ductwork in conditioned space. The two 

ducts on the right-hand side were located in a coffer above the bottom chord of the attic framing.

All coffers were constructed by the HVAC contractor.

Figure 11. Plan 4127 mechanical closet during framing visit

Figure 12 shows those same two ducts with the coffers now constructed. Duct board is used as 

the coffer material and had to be notched to allow for flush installation when running 

perpendicular to the bottom chord of the attic framing. Caulk and/or spray foam was used to air 

seal the ceiling drywall to the coffers.

Figure 12. Plan 4127 constructed duct coffer
13

Figure 13 shows a jump duct coffer that is typical throughout the three research houses.

Figure 13. Jump duct coffer

It was stated during the design process that all coffers require full levels of ceiling insulation to 

ensure a consistent R-value throughout the ceiling plane. To this end, DWH constructed 

“troughs” that consist of walls of oriented strand board (OSB) or plywood adjacent to the coffers 

to provide a backing for the blown fiberglass. Photos of the finished trough are not available for 

Plans 4127 and 4128 but there are photos of this in the Plan 4069 section.

Plywood was used as the ceiling plane blocking for the dropped ceilings. Figure 14 shows 

runouts located in a dropped ceiling with plywood installed as the air barrier at the ceiling plane.
14

Figure 14. HVAC runouts in dropped ceiling at plan 4127

Figure 15 shows an inside view of the mechanical closet in the attic. This mechanical closet was 

air sealed and insulated from the inside with open cell spray polyurethane foam as the large size 

allowed for an insulation contractor to insulate from the inside.

Figure 15. Plan 4127 mechanical closet in attic

Figure 16 shows the add-on closet that had to be constructed to move the return ductwork inside 

conditioned space. This closet has a short ceiling to minimize additional volume of conditioned 

space.
15

Figure 16. Plan 4127 add-on mechanical closet

3.2 Plan 4128

Plan 4128 is a medium sized (2,179 ft2), two-story floor plan with a single HVAC system. DWH 

decided to utilize a kneewall space on the second floor as a mechanical closet, effectively 

converting that kneewall space to a small unvented cathedralized attic. The red square in Figure 

17 shows the location of the furnace in the kneewall space, and the thick red line indicates the 

main return trunk line that terminates at a return grille at the stair landing. The mechanical closet 

is accessible by a door in the adjacent bedroom. DWH determined that duct system would be 

located in a series of dropped ceiling in both the first and second floors.

Figure 17 shows the designed second-floor duct layout for Plan 4128. The shaded areas 

designate dropped ceilings for locating the ductwork in conditioned space. The black arrows

represent the location of wall supply registers that are serving the adjoining rooms. The red 

square represents the location of the furnace in the mechanical closet. Room with dropped 

ceiling in them have ceiling supply grilles but are not specifically labeled on the drawings. All 

bedrooms have jump ducts for passive return. Please refer to the appendix for the duct layout.
16

Figure 17. Plan 4128 second-floor duct layout

As a note, the option of open web floor trusses was noted both in the initial tour of the existing 

homes and in the design phase of the research homes. However, it was decided that open web 

joists would not simplify the duct layouts. The complexity of the floor framing system includes 

many flush beams that would eliminate the option of routing ductwork in the floor system in key 

locations. However, there are locations in both two-story research homes where ducts are briefly 

routed in the floor system where applicable, but upgrading the floor framing from dimensional 

lumber to open web floor trusses was not justifiable.

Figure 18 shows a building section of the mechanical closet. It is very similar design to the 

mechanical closet at 4127; however, it is adjoining the second-story living space.

Black Arrows = high wall registers

Black Shaded Areas = dropped ceiling

Solid Red Area = duct coffer into vented attic

Red Box = furnace
17

Figure 18. Plan 4128 mechanical closet building section

Figure 19 shows the Plan 4128 mechanical closet with the furnace, return ductwork, and spray 

foam installed.

Figure 19. Plan 4128 mechanical closet

Figure 20 shows the return duct located in the small unvented cathedralized kneewall space. The 

spray foam on the right-hand side is the air and thermal barrier; therefore, the thin XPS with 

taped joints on the left-hand side are not completely necessary. However, XPS does perform just 

as well as any other thin profile sheathing, and a backing material was required in this location as 

A/C

Closet
18

the left-hand wall was insulated for sound attenuation. This return duct terminates at a wall 

return grille on the stair landing.

Figure 20. Plan 4128 return duct in knee wall

Figure 21 shows the first floor dropped ceiling plan. The red line indicates a coffer that extends 

from the second-floor joists to the dining room.

Figure 21. Plan 4128 first-floor duct layout

Figure 22 shows a photo of the duct coffer during framing.

Black Arrows = high wall registers

Black Shaded Areas = dropped ceiling

Solid Red Area = duct coffer into vented attic
19

Figure 22. Plan 4128 dining room duct coffer

3.3 Plan 4069

Plan 4069 is a large (4,169 ft2) two-story floor plan. There are two HVAC systems in the house, 

one dedicated to the upstairs and one downstairs. DWH decided to utilize a bonus room closet as 

a mechanical closet for the second-floor unit. The red square in Figure 23 shows the location of 

the furnace in the closet. The ceiling above the stairwell was dropped to allow for a main supply 

trunk to be installed for routing ducts to the guest suite. The solid red shapes are coffers that 

were constructed in the second-story attic space. These were implemented to avoid dropping the 

ceilings in the hallways.

Figure 23. Plan 4069 second-floor duct layout

Black Arrows = high wall registers

Black Shaded Areas = dropped ceiling

Orange Shaded Area = dropped stairs ceiling

Solid Red Area = duct coffer into vented attic
20

A coffer was constructed above the mechanical closet for the upstairs furnace allow for enough 

space to route the ductwork to the coffer for the left side (bedrooms 4 and 5) side of the house.

Figure 24 shows this coffer and the spray foam that was used to insulate and air seal. The 

mechanical closet can be made shorter in future designs to save costs.

Figure 24. Plan 4069 upstairs mechanical closet coffer

Figure 25 shows the coffer on the second floor that serves the guest suite.

Figure 25. Plan 4069 second-floor guest suite coffer

As mentioned in the previous section, “troughs” constructed of OSB sheets were constructed to 

hold up blown fiberglass such that consistent levels of ceiling insulation can be installed over the 

coffers. Figure 26 shows a picture of the insulation trough that was installed at the guest suite 

coffer.
21

Figure 26. Plan 4069 insulation trough

Figure 27 shows the first floor duct layout for Plan 4069. The furnace for the first floor HVAC 

system is located in a mechanical closet adjacent to the stairs.

Figure 27. Plan 4069 first-floor duct layout

Routing ductwork to the breakfast area proved to be the most difficult challenge out of all three 

homes. Figure 28 shows that the floor joists are oriented perpendicular to the intended location of 

the duct to the breakfast area. DWH did not wish to sacrifice ceiling space in the kitchen.

Black Arrows = high wall registers

Black Shaded Areas = dropped ceiling

Solid Red Area = duct coffer into vented attic

Red Box = furnace
22

Therefore, a “raceway” (red rectangle in Figure 27 and Figure 28) was retrofitted in the floor 

system to allow for the placement of the breakfast duct in the joists. In future designs DWH 

intends to alter the framing in order to avoid this problem or may consider open web floor joists 

in this location despite additional costs. A photo of the raceway after it was retrofitted in the 

floor framing is not available.

Figure 28. Plan 4069 first floor kitchen “raceway” location
23

4 Testing Results

4.1 Overview

BSC visited and performance tested the three DWH research homes January 1–4, 2013. The 

purpose of this testing was to ensure that each house was fully commissioned, which included 

the following performance tests:

• Blower door test to measure the house infiltration rate, using the depressurization method 

ASTM E779 with computerized multipoint measurements (ASTM 2003a)

• Duct blaster test to measure duct leakage (both total duct leakage and duct leakage to 

outside) using the ASTM E1554 depressurization Test Method A (ASTM 2003b).

• Outside air ventilation rate measurement

• Register flow measurement (to ensure proper airflow from each supply register)

• HVAC equipment external static pressures (ESPs)—measured with a static pressure pitot 

tube 

• Bedroom to hallway pressure difference while door is closed (to ensure that transfer 

grilles or jump ducts were sized properly such that room pressurization can be prevented 

when the door is closed).

4.2 Plan 4127

Figure 29 shows the front elevation for the single-story Plan 4127.

Figure 29. Plan 4128 front elevation

The blower door test yielded 1082 CFM 50, which is well under the goal of 1469 CFM. This 

target infiltration rate is equivalent to a leak ratio of 0.25 (CFM 50/ft2 enclosure). This target 

infiltration rate is typical for most BSC projects that are part of BA. Table 3 below lists the 

details on the infiltration testing for Plan 4127.
24

Table 3. Plan 4127 Infiltration Testing Results

Plan CFM 50measured

(CFM @ 50 Pa)

CFM 50goal

(CFM @ 50 Pa)

ACH 50

(CFM 50/vol/h)

Leak Ratio

(CFM 50/ft2)

4127 1082 1469 3.7 0.18

Table 4 lists the details on the duct testing at Plan 4127. The total duct leakage (189 CFM 25) 

constitutes 24% of the total 800 CFM of nominal cooling flow. This is higher than what is 

typically achieved at DWH houses and is also higher than the BSC recommendation of 15% total 

duct leakage. Total duct leakage is a less critical operating characteristic when the entire duct 

system is located in conditioned space, as any leakage is essentially still within the building 

enclosure. In extreme cases, significant duct leakage can negatively impact comfort levels even 

when the ducts are located in conditioned space. However, the builder and HVAC contractor are 

aware of this result and are striving to improve on this in future homes. This increase in total 

duct leakage may be due to the different ducting strategies utilized in these three homes but a full 

diagnosis, and subsequent identification of leakage locations, was unable to be performed due to 

the majority of the ductwork being concealed beneath insulation and in dropped ceilings. The 

critical metric for assessing duct tightness for a system located entirely within conditioned space 

is the duct leak to outside test. BSC requires, for BA projects, that duct leakage to outside be 

limited to 5% of the total cooling flow. Plan 4127 passes with 4% duct leakage to outside. The 

outside airflow was measured with a balometer flow hood at the exterior wall termination.

Table 4. Plan 4127 Duct Testing Results

Plan Duct25-Total

(CFM @ 25 Pa)

Duct25- to Outside

(CFM @ 25 Pa)

Outside Duct Leakage

(5% Goal)

Outside Airflow

(CFM)

4127 189 33 4% of cooling flow 62

The flow hood was also utilized to measure airflow at each of the supply registers. All flows 

were measured with the HVAC system in second stage cooling, as this is the mode the unit will 

be operating in to meet set point during a peak cooling load event. Figure 30 plots the measured 

CFM flow (in second stage cooling) from each register versus the calculated Manual J8 peak 

cooling CFM. Points above the magenta line indicate flows that exceed the Manual J8 peak 

cooling flow, whereas points below indicate measured flows that are lower than the predicted 

peak cooling CFM. The HVAC contractor routinely measures all register flows during the 

commissioning phase and adjusts flows (using the lever control on the supply register) in order 

to be within ~20% of the calculated Manual J8 flows.
25

Figure 30. Plan 4127 design register flow versus measured register flow plot

A digital manometer was utilized to measure ESPs in the supply and return plenums of the 

HVAC system, which includes a 1-in. MERV 8 furnace filter. Table 5 shows the measured static 

pressures at Plan 4127. The resulting total external static pressure (TESP) is 35.1 Pa. This is 

equivalent to 0.14 in. water column (WIC). HVAC manufacturers typically recommend that an 

AHU not operate at more than 0.5 WIC, as this will result in restricted airflow; therefore, the 

duct system at Plan 4127 is not restrictive according to the static pressure measurements. This 

test was performed on a dry cooling coil, as the site visit was in January.

Table 5. Plan 4127 HVAC ESPs

Plenum ESP

Supply +19.4 Pa

Return –15.7 Pa

A digital manometer was also utilized to measure the pressure difference between the bedrooms 

and central hallway with the doors closed and the HVAC system operating in second-stage 

cooling. BSC recommends a pressure difference of no more than 3 Pa during HVAC operation, 

to prevent pressurization and ensure proper airflow to rooms with doors that are typically closed 

for extended periods of time. Table 6 below shows the measured pressure differences at Plan 

4127.

Table 6. Plan 4127 Room Pressure Measurements

Plan Pressure Difference Return Pathway Type

Owner’s Retreat 1.7 Pa Jump Duct

Bedroom 2 0.5 Pa Jump Duct

Bedroom 3 0.9 Pa Jump Duct

Study 2.2 Pa Jump Duct

0

40

80

120

160

200

240

0 40 80 120 160 200 240

Measured Flow (CFM)

Designed Flow (CFM)

Balometer Measurement in 2nd Stage Cooling
26

4.3 Plan 4128

Figure 31 shows the front elevation for the two-story Plan 4128.

Figure 31. Plan 4128 front elevation

The blower door test yielded 1189 CFM 50, which is well under the goal of 1432 CFM, resulting 

in a leak ratio of 0.21 CFM 50/ft2 enclosure. Table 7 below lists the details on the infiltration 

testing for Plan 4128.

Table 7. Plan 4128 Infiltration Testing Results

Plan CFM 50measured

(CFM @ 50 Pa)

CFM 50goal

(CFM @ 50 Pa)

ACH 50

(CFM 50/vol/h)

Leak Ratio

(CFM 50/ft2)

4128 1189 1432 3.4 0.21

Table 8 lists the details on the duct testing at Plan 4128. The total duct leakage (189 CFM 25) 

constitutes 14% of the total 1200 CFM of nominal cooling flow. This meets the BSC 

recommended maximum 15% total duct leakage. The duct leakage to outside was tested at 3% of 

nominal cooling flow and meets the 5% BSC requirement for duct leakage to outside.

Table 8. Plan 4128 Duct Testing Results

Plan Duct25-Total

(CFM @ 25 Pa)

Duct25- to Outside

(CFM @ 25 Pa)

Outside Duct Leakage

(5% Goal)

Outside Airflow

(CFM)

4128 162 25 3% of cooling flow 69

Figure 32 shows the measured flow versus designed flow plot. Flows that are below the specified 

CFM are no less than 15% below the Manual J8 calculated room CFM.
27

Figure 32. Plan 4128 design register flow versus measured register flow plot

Table 9 shows the measured static pressures at Plan 4128. The resulting TESP is 54.8 Pa. This is 

equivalent to 0.22 WIC; therefore, the duct system at Plan 4128 is not restrictive according to the 

static pressure measurements.

Table 9. Plan 4128 HVAC ESPs

Plenum ESP

Supply +32.8 Pa

Return –22.0 Pa

Table 10 below shows the measured room pressure differences at Plan 4128.

Table 10. Plan 4128 Room Pressure Measurements

Plan Pressure Difference Return Pathway Type

Owner’s Retreat 0.2 Pa Active Return

Bedroom 2 0.6 Pa Jump Duct

Bedroom 3 0.7 Pa Jump Duct

Study 0.5 Pa Jump Duct

4.4 Plan 4069

Figure 33 shows the front elevation for the two-story Plan 4069.

0

40

80

120

160

200

240

0 40 80 120 160 200 240

Measured Flow (CFM)

Designed Flow (CFM)

Balometer Measurement in 2nd Stage Cooling
28

Figure 33. Plan 4069 front elevation

The blower door test initially yielded 2883 CFM 50, which is well over the goal of 2208 CFM, 

resulting in a leak ratio of 0.33 CFM 50/ft2 enclosure. BSC worked with a local rater to identify 

air leakage locations. The main sources of air leakage that were found were not related to the 

duct location strategy related to this research work. Rather, air leakage pathways were 

discovered in the first-story attic kneewall at the front of the house. Figure 34 shows 

inconsistencies in the floor joist blocking in the front attic kneewall space. The photo on the left 

shows missing blocking in a section of a joist bay and the photo on the right show a knot in the 

wood that had become dislodged, thus opening up a hole in the blocking.

Figure 34. Inconsistencies in air barrier at floor joist in attic kneewall space

These air leakage pathways were sealed up by the builder and the house was retested by the local 

rating organization. The infiltration rate was able to be reduced to a 0.26 CFM 50/ft2 leak ratio.

Table 11 below lists the details on the infiltration testing for Plan 4069.
29

Table 11. Plan 4069 Infiltration Testing Results

Plan Test 

Description

CFM 50measured

(CFM @ 50 Pa)

CFM 50goal

(CFM @ 50 Pa)

ACH 50

(CFM 50/vol/h)

Leak Ratio

(CFM 50/ft2)

4069 BSC initial test 2883 2208 4.4 0.33

4069 Follow-up 

after repair 2314 2208 3.5 0.26

Table 12 lists the details on the duct testing at Plan 4069. There are two HVAC systems at this 

house, an upstairs and downstairs unit. The total duct leakage for both the downstairs and 

upstairs units (112 and 95 CFM 25 respectively) constitutes 13% of the total 800 CFM of 

nominal cooling flow for both units. This meets the BSC recommended maximum 15% total 

duct leakage. The duct leak to outside was measured at 3 and 4% of nominal cooling flow and 

meets the 5% BSC requirement for duct leak to outside. An outside air duct was only installed on 

the downstairs furnace so no outside airflow measurement was available on the upstairs unit.

Table 12. Plan 4069 Duct Testing Results

Plan Duct25 Total

(CFM @ 25 Pa)

Duct25 to 

Outside

(CFM @ 25 Pa)

Outside Duct 

Leakage

(5% Goal)

Outside 

Airflow

(CFM)

4069 Downstairs Unit 112 25 3% of cooling 

flow 55

4069 Upstairs Unit 95 34 4% of cooling 

flow n/a

Figure 35 shows the measured flow versus designed flow plot. Flows that are below the specified 

CFM are no less than 15% below the Manual J8 calculated room CFM.

Figure 35. Plan 4069 design register flow versus measured register flow plot

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140

Measured Flow (CFM)

Designed Flow (CFM)

1st Floor Balometer Measurement in 2nd Stage Cooling

2nd Floor Balometer Measurement in 2nd Stage Cooling
30

Table 13 shows the measured static pressures at Plan 4069. The resulting TESP is 72.6 Pa

downstairs and 63.4 Pa. This is equivalent to ~ 0.26 WIC; therefore, the duct system at Plan 

4069 is not restrictive according to the static pressure measurements.

Table 13. Plan 4069 HVAC ESPs

Unit Plenum ESP

Downstairs Supply +42.4 Pa

Return –30.2 Pa

Upstairs Supply +38.2 Pa

Return –25.2 Pa

Table 14 below shows the measured room pressure differences at Plan 4069. All are within the 3 

Pa pressure difference specification.

Table 14. Plan 4069 Room Pressure Measurements

Plan Pressure Difference Return Pathway Type

Owner’s Retreat 2.2 Pa Jump Duct

Bedroom 2 0.6 Pa Jump Duct

Bedroom 3 2.0 Pa Jump Duct

Bedroom 4 0.8 Pa Jump Duct

Study 1.1 Pa Jump Duct

4.5 Duct Test Discussion

These HVAC systems are still able to provide adequate airflow despite some total duct leakage.

This can be attributed to the fact that most installed HVAC systems that are slightly oversized 

compared to the calculated Manual J CFM flows. HVAC systems, particularly cooling systems, 

come in limited sizes. The two-stage 16 SEER cooling systems installed at these homes have 

even more limited sizing options; available in 1-ton increments rather than the 1⁄2-ton that is 

typical in most condensers. Therefore, the “right-sized” cooling system is normally slightly 

oversized compared to the actual peak loads of the house.

Duct leakage to outside testing is a performance test that moves the pressure boundary from the 

ductwork to the air barrier of the house. This test is an indication of how well the ducts are sealed 

and a confirmation that the ducts are located in conditioned space. However, measured duct 

leakage to outside is not an indication that the ducts leak air directly to the outside. In reality, the 

ducts will leak into the conditioned space that they are located in. Depending on the air exchange 

rate of that conditioned space to the outside, some of that air may leak outdoors but normally not 

before the interior space is conditioned.
31

5 Duct Design Cost Analysis

The costs for moving the HVAC system into conditioned space can be broken down into two 

main categories: Additional enclosure costs and additional mechanical costs.

Note on air conditioner efficiency: The air conditioners in these homes were upgraded from a 

single-stage 15 SEER unit to a two-stage 16 SEER system in order to improve the energy 

performance of the homes to meet BA targets. This upgrade is not relevant to the primary 

research goal of locating ductwork in conditioned space via an economical methodology; 

therefore, the HVAC costs for this section assume no change in air conditioner efficiency. Note 

that the 96% sealed combustion furnaces at the three research homes are typical for DWH 

construction.

In addition to the air conditioner efficiency upgrade, non-related enclosure upgrades (e.g.,

increasing ceiling insulation from R-38 to R-50) were also not included in this section. 

The duct design cost analysis will show that this strategy is more economical in existing floor 

plans versus the popular strategy of constructing an unvented cathedralized attic. However, it 

does not appear to be as cost effective as the cost of moving ducts to conditioned space by 

developing new plans from scratch that will allow the designer more flexibility in making 

enough room to avoid the costly coffers up into the attic space. This is discussed more in the 

conclusion. 

5.1 Plan 4127

Table 15 shows the additional enclosure costs related to moving the ductwork inside conditioned 

space. The Initial Cost column indicates the actual cost to implement each measure at each 

house. The Projected Costs correspond to estimated future costs for each measure and include 

improvements in the implementation of said measure. DWH expects a 41% savings in enclosure 

related costs should this duct design strategy be implemented in a future Plan 4127.

Table 16 below shows the additional mechanical costs. A reduction in air conditioner tonnage 

was possible due to the relocation of the entire HVAC system into conditioned space; therefore 

this reduction in cost is included here. Overall, the HVAC contractor expects a 43% reduction in 

savings should this duct design strategy be implemented in a future Plan 4127.

The total projected cost for both the enclosure and mechanical measures at Plan 4127 is $3911. 

BEopt predicts $92 savings in annual utility costs when moving the ductwork into conditioned 

space (assuming R-8 ducts in the vented attic space with 15% total duct leakage). This results in 

a payback period of around 42 years for this measure. The other two plans yield very similar 

paybacks.
32

Table 15. Plan 4127 Enclosure Costs

Measure Initial 

Cost

Projected

Cost Comments

Framing Labor $1,000 $250 Initial framing costs includes various corrective measures

Additional Beams $250 $250 One additional beam only

OSB $100 $100 Coffer material, insulation “trough,” dropped ceiling air barrier

Thin Profile Sheathing $600 $180 Would install OSB in lieu of XPS for mechanical closet

Drywall $0 $0 None, no extra drywall required for this work, mechanical closet in attic

Canned Spray Foam $510 $340 Expected improvement in installation methods

Mechanical Coffer Spray Foam $1,975 $1,975 Low density open cell spray foam

Additional Construction Days $1,530 $425 18 days of additional construction expected to be reduced to 5 days

Additional Framing for 

Dropped Ceilings $125 $125 Dropped ceilings not difficult or expensive according to framers

Total $6,090 $3,645 Projected 41% savings

Table 16. Plan 4127 Mechanical Costs

Measure Initial 

Cost

Projected 

Cost Comments

Downsizing of HVAC system –$165 –$165 Modest savings for downsizing the HVAC system from 3 to 2 tons

Ductwork Material Cost –$12 –$12 Reduction in ductwork due to tonnage savings and more centralized duct 

layout

Ductwork Labor Cost $643 $443 Expected improvement in installation methods

Total $466 $266 Projected 43% savings
33

5.2 Plan 4128

Table 17 shows the enclosure related costs for Plan 4128.

Table 18 shows the additional mechanical costs for Plan 4128. The projected savings for Plan 

4128 (63%) is quite higher than Plan 4127 (43%), and this is primarily due to Plan 4128 having a 

single HVAC system that serves both the first and second floors. The complexities associated 

with ducting both floors to a single furnace led to a higher initial cost than the other houses.
34

Table 17. Plan 4128 Enclosure Costs

Measure Initial 

Cost

Projected 

Cost Comments

Framing Labor $1,000 $250 Initial framing costs includes various corrective measures

Additional Beams $250 $250 One additional beam only

OSB $100 $100 Coffer material, insulation “trough,” dropped ceiling air barrier

Thin Profile Sheathing $1,000 $300 Would install OSB in lieu of XPS for mechanical closet

Drywall $0 $0 None, no extra drywall required for this work

Canned Spray Foam $480 $240 Expected improvement in installation methods

Mechanical Coffer Spray Foam $1,771 $1,771 Low density open cell spray foam

Additional Construction Days $1,530 $425 18 days of additional construction expected to be reduced to 5 days

Additional Framing for Dropped 

Ceilings $125 $125 Dropped ceilings not difficult or expensive according to framers

Total $6,256 $3,461 Projected 45% savings

Table 18. Plan 4128 Mechanical Costs

Measure

Initial 

Cost

Projected 

Cost

Comments

Downsizing of HVAC system –$135 –$135 Modest savings for downsizing the HVAC system from 3 to 2.5 tons

Ductwork Material Cost –$109 –$109 Reduction in ductwork due to tonnage savings and more centralized duct 

layout

Ductwork Labor Cost $1,224 $663 Expected improvement in installation methods

Total $980 $419 Projected 63% savings
35

5.3 Plan 4069

Table 19 shows the enclosure related costs for Plan 4069. One item of note is that additional 

drywall was required for the second mechanical closet (due to the change from one to two 

HVAC systems).

Table 20 shows the additional mechanical costs for Plan 4069. Included in these costs is the shift 

from one zoned HVAC system to two separate HVAC systems. The projected savings for Plan 

4069 (42%) is very similar to Plan 4127 (43%). The two HVAC systems at Plan 4069 essentially 

create two single-story floor plans, with separate HVAC systems, stacked on one another.

5.4 Cost Analysis Discussion

The mechanical coffer constitutes a major portion of the costs for moving the HVAC system 

inside conditioned space however, note that the builder believes that the homeowner greatly 

values the living space that is saved by not having the furnace in a closet in the main living 

space.

The payback for these homes is high however, ducts inside conditioned space yield significant 

predicted improvements in efficiency through other metrics. For example, BEopt predicts an 

overall annual source energy improvement of 5%–6% when moving ductwork inside conditioned 

space. Additionally, REM/Rate predicts a reduction of around 4–5 HERS Index points for the 

same measure. These are very significant improvements in predicted performance, as most 

upgrades typically impact the HERS Index in the range of 1–3 points. Please refer to Section 7 

for a parametric HERS Index analysis on one of the floor plans. Also, energy codes such as the 

2012 IECC and efficiently programs such as the DOE Challenge Home program require ducts in 

conditioned space, regardless of the economics. Therefore, moving ducts inside conditioned 

space is not an elective upgrade for this builder and any other affected by these increasingly 

stringent codes and rating programs. 
36

Table 19. Plan 4069 Enclosure Costs

Measure Initial 

Cost

Projected 

Cost Comments

Framing Labor $1,500 $500 Initial framing costs includes various corrective measures

Additional Beams (2) $400 $400 Required for raceway construction

OSB $380 $380 Coffer material, insulation “trough,” dropped ceiling air barrier

Thin Profile Sheathing $48 $128 Thicker sheets recommended for future work

Drywall $300 $225 HVAC contractor will install drywall for mechanical closet, save 

extra trip by drywall crew

Canned Spray Foam $480 $240 Expected improvement in installation methods

Mechanical Coffer Spray Foam $1,475 $1,475 Low density open cell spray foam

Additional Construction Days $2,160 $600 18 days of additional construction expected to be reduced to 5 days

Additional Framing for Dropped 

Ceilings $50 $50 Dropped ceilings not difficult or expensive according to framers

Total $6,793 $3,998 Projected 61% savings

Table 20. Plan 4069 Mechanical Costs

Measure Initial 

Cost

Projected 

Cost Comments

Zoned Single Furnace to Two 

Furnaces $1,500 $1,500 Cost of added furnace minus the zoning controls

Downsizing of HVAC System –$195 –$195 Modest savings for downsizing the HVAC system from 5 to 4 tons

Ductwork Material Cost –$211 –$211 Reduction in ductwork due to tonnage savings and more centralized 

duct layout

Ductwork Labor Cost $1,805 $884 Expected improvement in installation methods

Total $2,899 $1,978 Projected 42% savings
37

6 Code Compliance Analysis

DWH expressed a desire to better understand the implications of the 2015 IECC code changes 

with respect to energy performance. BSC speculates that the energy performance threshold for 

the 2015 IECC will be around 50% of the 2006 IECC provisional mandate. Note that IECC 

compliance is currently calculated for heating, cooling, and hot water only. Therefore, this 

analysis is to show how a DWH floor plan can save 50% in heating, cooling, and hot water 

energy use compared to the IECC 2006. The HERS Index is included in this analysis as an

additional metric, as it is useful to raters and builders.

Plan 4127 was chosen for this analysis as it is the smallest floor plan of the three research houses 

and therefore is the most conservative. REM/Rate version 14.0 was utilized for this work. This 

software is produced by Architectural Energy Corporation and is one of the most recognizable 

residential modeling software packages in the industry. The HERS rating organization for DWH

– Houston division uses REM/Rate; therefore, this work and methodology are easily 

transferrable to DWH and its contractors.

The upgrades considered for this analysis were selected in a collaborative effort with BSC and 

DWH, as DWH best understands what upgrades are easily integrated into its production

environment versus others. Therefore, this is not a full parametric analysis in the sense that all 

possible upgrades are considered. Rather, many upgrades options were eliminated through direct 

cooperation with the builder such that the end result represents the most economical upgrade 

package for DWH and the easiest to integrate into current production.

The starting point for the analysis is to begin with Plan 4127 with DWH typical characteristics.

This includes:

• R-38 ceiling

• 2 × 6 walls with R-3 XPS

• Vinyl windows U=~0.29, SHGC = ~0.22

• Infiltration rate = 0.25 leak ratio (CFM 50/ft2 enclosure)

• 96% AFUE furnace and R-6 ductwork in vented attic

• 15 SEER air conditioner

• Gas tank water heater EF = 0.62

• CFIS ventilation with 33% duty cycle fan cycling control.

Plan 4127 with typical building characteristics saves 39% versus the 2006 IECC and scores 66 

on the HERS Index.

Table 21 outlines the upgrades implemented for each compliance run in this analysis. Table 22

shows the pathway for Plan 4127 to meet at least 50% energy savings versus the 2006 IECC 

using the current methodology of assessing only heating, cooling and hot water energy use. Note 

that each run is cumulative. The organization of this chart has been kept like the output from the 

REM/Rate report module for the sake of consistency.
38

Referring to Table 22 the single largest contributor to energy savings is Run 1: Moving the 

furnace and ductwork inside conditioned space, which results in 44% energy savings versus the 

2006 IECC and a HERS Index of 62. Runs 2–8 apply upgrades that DWH prefers to consider for 

achieving 50% savings versus the 2006 IECC. Run 8 (100% compact fluorescent and ENERGY 

STAR® appliances) is an unnecessary step for achieving 50% energy savings versus the 2006 

IECC, as only heating, cooling and hot water energy is considered. However, the impact of this 

improvement on the HERS Index was of interest to the group as DWH estimated the 2015 IECC 

code mandate to be equivalent to a HERS Index in the low 50s. It is also expected that a similar 

HERS score must be achieved to qualify for the DOE Challenge Home criteria.
39

Table 21. Plan 4127 IECC 2006 Compliance Analysis Specifications

Run Description of Specification Upgrade

Increase in

% Savings Versus

IECC 2006

Reduction 

in HERS 

Index

Run 1 Furnace and ductwork moved from the vented attic to conditioned space 5 4

Run 2 Infiltration reduced from 0.25 to 0.20 CFM 50/ft2 enclosure 2 1

Run 3 Wall insulating sheathing R-value increased from R-3 to R-5 1 0

Run 4 Ceiling insulation increased from R-38 to R-50 0 1

Run 5 Air conditioner upgraded from 15 SEER single-stage to 16 SEER two-stage 

condenser 2 1

Run 6 Water heater upgraded from 0.62 EF tank to 0.82 EF tankless water heater 4 3

Run 7 Fluorescent lights increased from 60% to 100% and appliances upgraded to 

ENERGY STAR –1 5
40

Table 22. Plan 4127 IECC 2006 Energy Cost Compliance Chart

Typical DWH Design HERS 66 Run 1: HVAC Inside HERS 62

Energy Cost Compliance 2006 IECC As Designed % 2006 IECC As Designed %

Heating 228 97 57% 230 93 60%

Cooling 382 239 37% 381 209 45%

Water Heating 110 103 6% 110 103 6%

Sub Total—Used for Compliance 720 439 39% 721 405 44%

Run 2: 0.20 Leak Ratio HERS 61 Run 3: 2x6 OVE + R-5 HERS 61

Energy Cost Compliance 2006 IECC As Designed % 2006 IECC As Designed %

Heating 230 84 63% 230 79 66%

Cooling 381 205 46% 381 202 47%

Water Heating 110 103 6% 110 103 6%

Sub Total—Used for Compliance 721 392 46% 721 384 47%

Run 4: R-50 attic HERS 60 Run 5: 16 SEER HERS 59

Heating 230 77 67% 230 75 67%

Cooling 381 201 47% 381 191 50%

Water Heating 110 103 6% 110 103 6%

Sub Total—Used for Compliance 721 381 47% 721 369 49%

Run 6: 0.82 EF Hot Water HERS 56 Run 7: 100% CFL/ES Apps HERS 51

Heating 230 75 67% 237 80 66%

Cooling 381 191 50% 374 184 51%

Water Heating 108 75 31% 115 81 30%

Sub Total—Used for Compliance 719 341 53% 726 345 52%
41

7 Conclusions

This work with DWH served to answer the following research questions.

1. What is the most cost-effective, best-performing, and most easily replicable method of 

locating ducts inside conditioned space for a hot-humid production home builder that 

constructs one- and two-story single-family detached residences?

DWH believes that this developed strategy for locating ductwork in conditioned space has 

potential as an economical method for increasing the energy efficiency of its existing production

homes. However, the builder does believe that developing new floor plans from scratch will 

allow for a more economical integration of ductwork inside conditioned space. Most notably, 

this systems engineering approach could preclude the need for mechanical coffers into the attic 

space and could allow for the sole use of dropped ceilings for locating ductwork.

DWH has provided positive feedback on the duct designs that were implemented at these three 

research homes. The development of a design for duct coffers in the attic space allows DWH to 

embrace the strategy of locating ductwork in conditioned space in its existing housing stock, 

without having to rely solely on dropped ceilings. The builder does recognize that a full series of 

dropped ceilings, without any coffers, would be the most economical strategy for locating ducts.

However, the builder perceives this design as having a negative impact on aesthetics and could 

not be attractive to its customer base. This is one of the more important developments in this 

research work, as DWH (and its customers) greatly value full ceiling height in the majority of 

spaces and wish to avoid dropped ceilings as much as possible. The main contribution of this 

research was to provide a working alternative for builders who wish to move their HVAC 

systems into conditioned spaces without extended dropped ceilings plus a mechanical closet, or

converting to a full unvented cathedralized attic.

DWH is confident that the costs for moving ducts into conditioned space can be reduced in 

future homes, as shown in the cost analysis section. However, this methodology is still 

considered a “retrofit” in that existing floor plans have to be altered to allow for the 

implementation of this duct strategy. DWH has expressed interest in working to develop new 

floor plans that more seamlessly integrate the HVAC system as a part of conditioned space. This 

work would require more upfront collaboration between the architect and HVAC contractor but 

could yield a more cost-effective design. It is possible that this design may not require coffers 

but rather the ducts can be completely located in dropped ceilings in sensible locations. Cost 

estimates are not available at this time for this integrative approach, and more research is needed 

to fully quantify the cost of moving ducts into conditioned space by developing new plans from 

scratch with the architect and HVAC contractor openly collaborating on an efficient duct layout.

It is important to note that the changes in code requirements and the criteria of various energy 

efficiency programs are forcing DWH to quickly integrate ducts into conditioned space in its 

existing floor plan portfolio. This research work identified the most economical strategy to move 

ductwork into conditioned space for existing floor plans. DWH does not have the resources to 

develop new floor plans en masse with ducts inside conditioned space, although this design 

strategy does represent the most cost-effective approach to moving the HVAC system within 

conditioned space.
42

Each house was commissioned by BSC and was found to conform to BA specifications. The 

average infiltration rate for these homes was slightly lower than what is typically constructed by 

DWH, and DWH maintains that this can be improved on in future homes. No degradation in 

performance was observed either in the enclosure or mechanical systems during commissioning.

All ductwork is installed correctly and proper airflow is being supplied to all the rooms via the 

mechanical system. Duct leaks to outside measurements have improved dramatically compared 

to the previous strategy of ducts in the vented attic.

The duct design strategy employed resulted in significant energy savings toward the goal of 

achieving 50% energy savings versus the 2006 IECC. Moving the HVAC system inside 

conditioned space saves around 4%–5% in source energy use and can reduce the HERS Index by 

around 4 points. The costs for implementing this duct design strategy on the three research 

homes was in the range of $6,000–$10,000; however, the builder expects that this figure can 

improve in future homes to around $4,000–$6,000. This is more affordable compared to the 

popular strategy of constructing an unvented cathedralized attic with spray polyurethane foam, 

which for these plans could cost in the range of $10,000–$15,000, according to DWH.

2. What is a cost-effective and practical method of achieving 50% source energy savings versus

the 2006 IECC for a hot-humid production builder? 

BSC speculates that the 2015 IECC will mandate an energy efficiency threshold equal to 50% 

energy savings versus the 2006 IECC. Analysis was performed with REM/Rate to identify the 

most economical upgrades to achieve 50% savings versus the IECC 2006. Moving the HVAC 

system to conditioned space was a key improvement for this analysis and in fact constituted the 

largest increase in efficiency compared to any other individual upgrade. In addition to moving 

the HVAC system to conditioned space, the builder identified upgrades that are economical to 

implement in its production environment. This includes upgrades to the wall and ceiling 

insulation, as well as increases in air conditioner and water heater efficiencies.

3. How accurate are the pre-construction whole-house cost estimates compared to confirmed 

post-construction actual costs?

The initial cost estimates for moving the HVAC system to inside space were close to the postconstruction confirmed costs. DWH does estimate that it can save around 40%–50% in installed 

costs for both the mechanical and enclosure components of the duct design strategy. This 

translates to around $2,000–$4,000 in savings per house once the builder and HVAC contractor 

optimize this strategy.

The predicted cost estimates for the other mechanical and enclosure upgrades unrelated to the 

duct design strategy are very close to the confirmed costs, as this production builder has 

extensive experience in costing various options for its product.
43 

References

Andrews, J. 2003. “Residential Duct Diagnostics and Repair.” Air Conditioning Contractors of 

America. 

ASTM Standard E779. (2003a), Standard Test Method for Determining Air Leakage Rate by Fan 

Pressurization. West Conshohocken, PA, ASTM International.

ASTM Standard E1554. (2003b), Standard Test Methods for Determining External Air Leakage 

of Air Distribution Systems by Fan Pressurization. West Conshohocken, PA, ASTM 

International. 

Bailes III, A. (2012). “Efficiency Programs Struggle to Stay Ahead of Energy Codes.” Green 

Building Advisor, www.greenbuildingadvisor.com/blogs/dept/building-science/efficiencyprograms-struggle-stay-ahead-energy-codes. Accessed June 6, 2012. 

Baker, P. (2006). “RR-0604: Incorporating Insulating Sheathing into the Design of the Thermal 

and Moisture Management System of the Building Enclosure.” Building Science 

Corporation, www.buildingscience.com/documents/reports/rr-0604-incorporating-insulatingsheathing-into-design-of-thermal-and-moisture-management-system-of-building-enclosure/. 

Accessed June 6, 2012. 

Burdick, A. (2011). “Advanced Strategy Guideline: Air Distribution Basics and Duct Design” 

DOE/GO-102011-3461. Pittsburgh, PA: IBACOS. 

BSC (2007). “Guide to Insulating Sheathing.” Building Science 

Corporation, www.buildingscience.com/documents/guides-and-manuals/gm-guide-insulatingsheathing/. Accessed June 6, 2012. 

BSC (2009). “Info-201: Common Advanced Framing Details.” Building Science 

Corporation, www.buildingscience.com/documents/information-sheets/2-framing/informationsheet-common-advanced-framing-details/. Accessed June 6, 2012. 

BSC (2010). “Designs that Work - Cold Climate: Habitat for Humanity High R-Value 

Prototype.” Building Science Corporation, www.buildingscience.com/documents/casestudies/cs-ma-westford-hfh/. Accessed June 6, 2012. 

Chasar, D.; Withers, C. (2013). “Measured Cooling Performance and Potential for Buried Duct 

Condensation in a 1991 Central Florida Retrofit Home” FSEC-CR-1946-13. Cocoa, FL: FSEC.

Hendron, R.; Anderson, R.; Barley, D.; Rudd, A.; Townsend, A.; Hancock, E. (2008). “Field 

Test of Room-to-Room Distribution of Outside Air with Two Residential Ventilation Systems.” 

Prepared for ASHRAE IAQ 2007 Baltimore, Maryland October 15–17, 2007. NREL/CP-550-

41210. Golden, CO: National Renewable Energy Laboratory, 19 pp. 
44

Griffiths, D.; Zuluaga, M.; Springer, D.; Aldrich, R. (2004). “Insulation Buried Attic Ducts: 

Analysis and Field Evaluation Findings.” In Proceedings of the 2004 ACEEE Summer Study on 

Energy Efficiency in Buildings. Washington, DC: American Council for an Energy-Efficient 

Economy.

ICC (2012). “International Residential Code for One and Two –Family Dwellings”. International 

Code Council.

Lstiburek, J.W (2005). Builder’s Guide to Hot-Humid Climates. Westford, MA: Building 

Science Press.

Lstiburek, J.W.; Grin, A. (2010). “RR-1004: Building America Special Research Project—

Advanced Framing Deployment.” Building Science 

Corporation, www.buildingscience.com/documents/reports/rr-1004-ba-special-researchadvanced-framing-deployment/. Accessed June 6, 2012.

Lubliner, M.; Kerr, R.; Gordon, A.; Murray, C. (2008). “Moving Ducts Inside: Big Builders, 

Scientists Find Common Ground.” In Proceedings of the 2008 ACEEE Summer Study on Energy 

Efficiency in Buildings. Washington, DC: American Council for an Energy-Efficient Economy.

Rudd, A. (2008). “RR-0304: Central Fan Integrated Supply Ventilation—The Basics.” Building 

Science Corporation, www.buildingscience.com/documents/reports/rr-0304-central-fanintegrated-supply-ventilation-the-basics/. Accessed June 6, 2012.

Rudd, A. (2006). “Design Process for Sizing: Cooling and Heating System Capacity, Room Air 

Flows, Trunk and Runout Ducts, and Transfer Air Ducts.” Building Science 

Corporation, www.buildingscience.com/documents/guides-and-manuals/gm-system-sizingpro/view. Accessed April 4, 2013.
45 

Appendix

Whole-House Energy Analysis

BEopt software was utilized to calculate the annual source energy savings versus the BA

Benchmark. The BEopt software includes an optimization capability that uses user-supplied cost 

data and energy use information for a specified set of energy-saving measures to determine 

combinations of measures that are optimal or near optimal in terms of cost effectiveness. BEopt 

uses a sequential searching technique so that not every possible combination of options is 

simulated. BSC compiled cost figures from DWH and updated the BEopt Cost Library.

Table 23 lists the whole unit, new construction, cost data provided by DWH for the 

characteristics implemented in these three research homes. 

Table 23. DWH Cost Data for BEopt Analysis

Component Cost

R-50 Blown Fiberglass Ceiling Insulation $0.78/ft2

2 × 6 R19 Fiberglass Batts + 3⁄4-in. XPS $3.76/ft2

R-30 Batts Cantilever Floor $0.72/ft2

Windows (Double Glazed, Argon Filled, U = 0.29, SHGC = 0.22) $25.36/ft2

96% AFUE Furnace $2,365

16 SEER Two-Stage Air Conditioner $3,420

Fan Cycling Controller With Mechanical Damper $345

The following sections list the specific BEopt output for each of the three floor plans. The BEopt 

parametric function was utilized for each plan in establishing the most cost effective building 

characteristics out of the options that were discussed with DWH. 

Plan 4127 BEopt 

Figure 36 compares the predicted source energy use to the BA Benchmark, as calculated by 

BEopt, and is broken down into various end uses. Plan 4127 is estimated to save 36% in source 

energy use versus the BA Benchmark. This is equivalent to around 56.3 MBtu/yr. 
46

Figure 36. Plan 4127 BEopt source energy savings versus the BA Benchmark—end use 

breakdown

Figure 37 is the BEopt optimization graph that plots the various combinations of characteristic 

that were calculated. The circled point for the technology package in the swoosh curve is the 

proposed technology package. This point has the lowest annualized energy related costs and is 

represented as the most cost-optimized configuration for the options studied.

Figure 37. Plan 4127 BEopt parametric graph
47

Plan 4128 BEopt 

Figure 38 compares the predicted source energy use to the BA Benchmark, as calculated by 

BEopt, and is broken down into various end uses. Plan 4128 is estimated to save 32% in source 

energy use versus the BA Benchmark. This is equivalent to around 52.3 MBtu/yr.

Figure 38. Plan 4128 BEopt source energy savings versus the BA Benchmark—end use 

breakdown

Figure 39 is the BEopt optimization graph that plots the various combinations of characteristic 

that were calculated. The circled point for the technology package in the swoosh curve is the 

proposed technology package. This point has the lowest annualized energy-related costs and is 

represented as the most cost-optimized configuration for the options studied.

Figure 39. Plan 4128 BEopt parametric graph
48

Plan 4069 BEopt 

Figure 40 compares the predicted source energy use to the BA Benchmark, as calculated by 

BEopt, and is broken down into various end uses. Plan 4128 is estimated to save 27% in source 

energy use versus the BA Benchmark. This is equivalent to around 59.3 MBtu/yr.

Figure 40. Plan 4069 BEopt source energy savings versus the BA Benchmark—end use 

breakdown

Figure 41 is the BEopt optimization graph that plots the various combinations of characteristics

that were calculated. The circled point for the technology package in the swoosh curve is the 

proposed technology package. This point has the lowest annualized energy-related costs and is 

represented as the most cost-optimized configuration for the options studied.

Figure 41. Plan 4069 BEopt parametric graph duct layouts
49

Plan 4127
50

Plan 4128
51
52

Plan 4069
53
54
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