Fort Devens: Cold Climate, 

Energy-Efficient, Market-Rate 

Townhomes

W. Zoeller, M. Slattery, and J. Grab

Consortium for Advanced Residential Buildings

August 2013
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iii

Fort Devens: Cold Climate, Energy-Efficient, Market-Rate Townhomes 

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:

William Zoeller, Matt Slattery, and Joanna Grab

Steven Winter Associates, Inc.

of the

Consortium for Advanced Residential Buildings

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 .............................................................................................................................................. vi

Definitions.................................................................................................................................................. vii

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

1 Introduction and Background ............................................................................................................. 3

2 Research Methods/Questions............................................................................................................. 6

3 Technical Specifications...................................................................................................................... 7

3.1 Building Enclosure.............................................................................................................11

3.1.1 Above-grade Walls ................................................................................................11

3.1.2 Below-grade Walls.................................................................................................12

3.1.3 Attic Insulation.......................................................................................................13

3.1.4 Air Sealing .............................................................................................................13

3.2 Lighting and Appliances....................................................................................................14

3.3 HVAC Systems..................................................................................................................14

3.3.1 Final HVAC Specifications ...................................................................................15

3.3.2 Distribution System ...............................................................................................16

3.3.3 Fresh Air Ventilation .............................................................................................16

3.4 Domestic Hot Water ..........................................................................................................16

4 Targeted and Simulated Energy Savings......................................................................................... 18

5 Cost Data and Cost Effectiveness of Energy Saving Measures.................................................... 20

6 Research Question Conclusions ...................................................................................................... 23

Reference................................................................................................................................................... 25

Appendix A: Plan Set................................................................................................................................ 26
vi

List of Figures

Figure 1. Development sign at Devens ................................................................................................... . 1

Figure 2. Photo of the prototype triplex nearing completion. ................................................................ 3

Figure 3. Triplex basement plan, Cambridge Seven Associates, Inc (C7)............................................ 8

Figure 4. Triplex 1st floor plan, Cambridge Seven Associates, Inc (C7)............................................... 9

Figure 5. Triplex 2nd floor plan, Cambridge Seven Associates, Inc (C7)............................................. 10

Figure 6. Triplex during construction ..................................................................................................... 11

Figure 7. Above-grade walls with dense pack cellulose insulation..................................................... 12

Figure 8. Below-grade walls..................................................................................................................... 13

Figure 9. Attic insulation .......................................................................................................................... 13

Figure 10. CFL and Pin-based lighting installed.................................................................................... 14

Figure 11. Duct during framing................................................................................................................ 15

Figure 12. Air handler in basement......................................................................................................... 16

Figure 13. Tankless DHW ......................................................................................................................... 17

Figure 14. Achieved energy savings of Devens end unit over the BA Benchmark Home ................ 18

Figure 15. Comparison of modeling iterations and optimization curve.............................................. 22

Unless otherwise noted, all figures were created by CARB.

List of Tables

Table 1. Prototype Building Specifications.............................................................................................. 7

Table 2. Payback of Prototype Building Specifications........................................................................ 21

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

Definitions

ACH50 Air changes per house at a pressure difference of 50 Pascal 

between the inside and outside

AFUE Annual Fuel Utilization Efficiency

BA Benchmark Building America Benchmark 2010 home

BEopt Building Energy Optimization software

C7A Cambridge Seven Architects

CARB Consortium for Advanced Residential Buildings

CFL Compact fluorescent lamp

ccSPF Closed-cell spray polyurethane foam

DHW Domestic hot water

EEM Energy efficiency measure

EF Energy factor

HERS Home Energy Rating System

o.c. On center

PV Photovoltaic

RDI Rural Development, Inc.

SEER Seasonal Energy Efficiency Ratio

SHGC Solar heat gain coefficient
1

Executive Summary

Advanced planning and design considerations can leverage market-present technologies to yield 

high performance, marketable homes. Developing appropriate strategies based on location and 

site conditions early in architectural design and building specification is critical. In the Northeast, 

the first priority is often a tight high performance building envelope to reduce the heating load up 

front. By combining energy-conscious site and building planning with cutting-edge but marketready products and techniques, the Consortium for Advanced Residential Buildings (CARB)

developed effective strategies to reduce the overall building energy use in line with Building 

America goals in this modestly priced market rate project. 

In 2009, MassDevelopment issued a request for qualifications and subsequent request for 

proposals for teams to develop moderately priced high efficiency homes on two sites in the 

Devens Regional Enterprise Zone. MassDevelopment, a Massachusetts agency that owns the 

Devens site (formerly Fort Devens Army Base, in Harvard, Massachusetts), set a goal of 

producing a replicable example of current and innovative sustainable building practices with a 

near-zero energy potential (see Figure 1). Metric Development, as primary developer and 

construction manager, formed one of the successful teams that included CARB and Cambridge

Seven Architects (C7A).

Figure 1. Development sign at Devens

The performance goals were to reach a target Home Energy Rating System (HERS) Index of 40 

(to meet the MassDevelopment requirements) and 30% source energy savings over the Building 

America (BA) Benchmark 2010. Combining these aggressive goals with affordability and 

replicability dictated that the process begin in the basic planning stages and incorporate site 
2

design for solar access and building planning for simplicity of design and ease of construction. 

The resulting design called for four triplex two-story townhomes, each containing two bedrooms 

and 1900 ft2 of space, including basements. Performance optimization studies were performed 

using BEopt software, which determined the insulation levels, overall airtightness, fenestration 

specifications, and other thermal enclosure attributes. The developer’s marketing requirements 

meant that conventional, although highly efficient, mechanical systems were specified. Design 

integration kept all systems, including heating, ventilation, and air conditioning (HVAC) ducts 

within conditioned space. 

A conventional vented attic with loose fill cellulose insulation formed the ceiling plane thermal 

boundary. Arriving at the final wall assembly (R-38, double-wall with dense pack cellulose) 

required more effort, as the initial design with 3 in. of closed-cell foam was deemed too 

expensive. 

With the elimination of the foam insulation, the air sealing protocol for the vertical walls became

critical. Specifically advocated by CARB and adopted by Metric Development is the use of the 

exterior sheathing plane as the primary vertical air barrier. All the panel butt joints were treated 

with proprietary butyl/polyurethane tape; and the panel edges, terminations, and penetrations 

with low-expansion foam to meet the aggressive air sealing target of 2.0 ACH50.

Conclusions and Recommendations

The energy efficiency measures (EEMs) implemented at the Devens triplex surpass the 

anticipated energy performance target. Examining the costs of the energy-efficient upgrades 

reveals the most- and least-effective measures. In the Devens end unit, the enclosure

improvements cost the most, but also saved the most energy in the biggest end use category. 

(The middle unit enclosure upgrades were less costly by virtue of the reduced surface area 

relative to the end units). Lighting and appliances were the most cost effective in terms of 

payback period and provided the simplest changes, but also yield limited energy savings. 

Mechanical improvements struck a balance between the moderate price and substantial energy 

savings. The overall improvements cost $7,804 and saved 46.06 MBtu/yr for a payback period of 

13.44 years. This package of improvements proved to be cost effective with a short payback 

period and relatively low first cost for significant energy savings. 

Final testing of the initial prototype resulted in a HERS Index of 39, exceeding the targeted 40. 

Pricing the incremental cost increases for each upgraded system from standard practice 

determined cost effectiveness of the final prototype as compared to the specifications and costs 

of the BA Benchmark. Based on these outcomes, Metric Development is moving forward with 

the next buildings with no significant changes to plans and specifications. 
3

1 Introduction and Background

MassDevelopment, a Massachusetts agency that owns the Devens Regional Enterprise Zone, 

(formerly Fort Devens Army Base in Harvard, Massachusetts) issued a request for qualifications

in 2009 and a subsequent request for proposals for teams to develop moderately priced high

efficiency homes on two sites in the enterprise zone (see Figure 2). Metric Development, as 

primary developer and construction manager, formed one of the successful teams that included 

CARB and C7A. MassDevelopment’s goal was to produce a replicable example of current and 

innovative sustainable building practices with a near-zero energy potential. The homes, to be 

built and marketed in a mostly working class area north of Boston, are intended to showcase how 

they could be built and sold profitably, and that the results can be scaled to much larger 

developments and to infill opportunities. 

Figure 2. Photo of the prototype triplex nearing completion

CARB’s objective was to demonstrate and document how advanced planning and design 

considerations, as well as optimized whole-house technology, can leverage market-present 

technologies and yield high performance, marketable homes. CARB’s performance target was to 

improve efficiency levels to 30% better than Building America House Simulation Protocols for 

new homes (Hendron and Engebrecht 2010). MassDevelopment’s performance requirements are 

actually even higher, and therefore formed the project’s overall performance goals. Initially 
4

intended to be capable of net zero-energy performance (after photovoltaic [PV] arrays were 

installed), the team looked at efficiency levels balanced against projected costs and available roof 

area for PV and arrived at a target HERS of 40. The final agreement between the developer and 

MassDevelopment included HERS 41 as a minimum performance level to trigger the land 

purchase incentives. This contractual requirement ensured that the developer remained 

committed and helped to focus the design and planning activities. 

The planning process started with a nearly clean sheet of paper, requiring 12 homes to be 

constructed on the flat 1-acre infill site. The homes were to be priced at $225,000–$350,000 and 

provide a reasonable return to the developer. To achieve these goals, basic planning decisions 

were paramount. A design charrette with the architect, developer, and CARB research team was 

conducted to develop and evaluate planning options and refine the most promising ones. From 

this exercise the basic two-story triplex building configuration was selected for economy of 

construction, marketability, enclosure-to-floor-area ratio, and solar site planning. 

The simple compact form results in four rectilinear triplexes and allows all homes to be placed 

with direct southern exposures for passive solar gain and for future placement of roof-mounted 

PV arrays. With the townhouse triplex form, the high floor area to thermal enclosure area results 

in economy of construction and allows high-R wall strategies to be incorporated with lower 

incremental cost increases. The compact townhome form is also a traditional staple of New 

England that was thought to enhance marketability. Lastly, the design enhances energy 

performance by optimizing usable floor area relative to thermal enclosure area.

The energy strategy was to reduce the thermal loads as much as practical through enclosure

efficiency, and then apply efficient, low capacity systems to meet those reduced loads. CARB 

used lessons learned from previous Building America cold-climate research to incorporate 

several of the enclosure strategies developed with Rural Development Inc. (RDI), including the 

double-frame walls and dense-pack cellulose fill (Aldrich 2012). These homes are meant to be 

“market rate” not definitionally “affordable,” so two major departures from the successful RDI 

homes were incorporated: fully insulated and semi-conditioned basements and traditional HVAC 

systems. (For cost savings the RDI homes were insulated at the first floor framing, leaving the 

basement unconditioned, and conventional HVAC systems were omitted, instead relying on a 

single unit heater in each dwelling for space heating). The insulated basements used strategies 

that were researched and developed by CARB during side-by-side testing analysis completed in 

Chicago in 2004 (Aldrich and Zuluaga 2006) and included in a recent CARB implementation 

guide. (Aldrich et al. 2012) In this instance R-13 aluminum faced polyisocyanurate boards were 

adhered directly to the interior concrete surfaces. 

Ensuring the enclosure assemblies would perform at their intended levels meant that extreme 

care was given to air sealing. CARB provided on-site contractor training, observation, quality

control, technical installation support, and verification. Following project completion, CARB 

conducted the final test-out and building commissioning. The target HERS Index of 40 was 

surpassed at each unit. MassDevelopment intends to build three more triplexes based on the 

lessons learned from the completed prototype.

Through the prototype, the additional costs beyond the baseline home have been gathered and 

evaluated, and research gaps have been identified. This research provides valuable data on best-
5

in-class whole-house solutions with the potential to meet the source energy savings goals. 

Working on a prototype test building also allowed training opportunities for the developer and 

contractor for better installation when the techniques used are replicated in the remaining 

triplexes and in future projects. 
6

2 Research Methods and Questions

The following research questions will be answered by this project and represent a continuation of 

CARB’s work with RDI.

• What solution package can be readily implemented in cold climate homes to achieve at 

least 30% energy savings compared to the BA Benchmark?

• Is that solution package commercially viable? Where are opportunities to reduce costs in 

this solution package?

• What are the specific gaps to achieving the solution package at a production scale (cost, 

risk adversity, implementation complexity, etc.)? 

• Based on the results of this test home, what other alternative energy efficiency solution 

packages should be considered? 

• What are the market interest and consumer reactions, developer and builder reactions and 

feedback loops, and stakeholder enthusiasm for replicating the package?

• How effectively does each EEM meet its specific cost and performance targets? How 

effective is each when integrated into a whole-house package? 

• How effectively and cost effectively does the double wall solution provide R-35 plus 

performance? What is the medium-term moisture performance of this assembly in a 

northern climate? (This last question will not be answered in this report but will be the 

subject of a study in a follow-up report.) 

The team approached these questions by identifying the largest energy end uses, and therefore 

the most potential savings, weighing the costs and benefits of each EEM, and determining the

point at which further improvements to a particular building component were warranted or if 

diminishing returns came into play. 
7

3 Technical Specifications

Building components were chosen for their local market availability, improvement over the 

BA Benchmark, and cost effectiveness. Table 1 summarizes the EEMs implemented at the 

end unit of the Devens triplex prototype, compared to the BA Benchmark. The 1,975-ft2

attached townhome has two levels plus an unfinished insulated basement, two bedrooms, and 

1.5 bathrooms. Figure 3 through Figure 5 show floor plans of the prototype triplex. Figure 6 

shows the triplex under construction.

Table 1. Prototype Building Specifications

Component BA Benchmark 2010 Prototype Specification

Foundation Assembly

Uninsulated basement slab; R-10 

(2-in. extruded polystyrene at 

R-5/in.) at exterior OR R-19 

(5.5-in. fiberglass batt insulation) 

on interior of basement cavity wall

Uninsulated basement slab; 

R-13 (2-in. polyisocyanurate

at R-6.5/in.) at interior side of 

foundation wall

Above-Grade Exterior 

Walls

Wood 2 × 4 stud frame, 16 in.

o.c.

a

; R-13 (3.5 in. fiberglass batt)

+ R-5 continuous insulation 

(1 in. extruded polystyrene 

at R-5/in.)

Wood 2 × 4 double stud 

frame walls, 16 in. o.c., R-38 

(10 in. dense pack cellulose at 

R-3.8/in.)

Ceiling Assembly R-38 fiberglass batt 

insulation

R-59 loose fill cellulose

(16 in. at R-3.6/in.)

Window Glazing U-0.35, SHGCb

-0.35, double pane,

non-ENERGY STAR®

U-0.24, SHGC-0.22, 

double pane, vinyl

Cooling System 1.5-ton air conditioner, SEERc 13

1.5-ton air conditioner, 

SEER 16, programmable 

thermostat

Heating System 78% AFUEd gas furnace 96.5% AFUE condensing gas 

furnace

Water Heating Non-ENERGY STAR gas storage 

water heater, 40-gal tank, EFe 0.59

96% condensing gas tankless 

water heater, EF 0.96

Lighting 66% incandescent, 21%

CFLf, 13% T-8 fluorescent

0% incandescent, 95% CFL, 

5% T-8 fluorescent

Appliances Non-ENERGY STAR appliances ENERGY STAR

Air Sealing 7.4 ACH50 1.6 ACH50

Ducts In conditioned space, R-0 In conditioned space, R-8, 

0 LTOg, 85 TLh

a On center

b Solar heat gain coefficient c Seasonal Energy Efficiency Ratio

d Annual Fuel Utilization Efficiency

e Energy factor

f Compact fluorescent lamp

g Leakage to outside

h Total leakage
8

Figure 3. Triplex basement plan, C7A

(courtesy of Cambridge Seven Associates, Inc.)
9

Figure 4. Triplex first floor plan, C7A

(courtesy of Cambridge Seven Associates, Inc.)
10

Figure 5. Triplex second floor plan, C7A

(courtesy of Cambridge Seven Associates, Inc.)
11

Figure 6. Triplex during construction

3.1 Building Enclosure

In the Northeast with its typically cold winters, a high performance building enclosure greatly 

reduces the building thermal loads, and thus the overall energy consumption. Enclosure

improvements were therefore the top priority in achieving whole-house improvements at the

prototype Devens triplex. Insulation R-values were significantly increased above the BA

Benchmark levels; modeling optimization was used to determine the levels needed to meet the 

performance target. The R-value of attic insulation was 35% higher than the BA Benchmark, and 

the wall insulation more than doubled the R-value of the BA Benchmark. 

3.1.1 Above-Grade Walls

Based on early parametric studies, the design team understood that to meet the HERS 40 target, 

the above-grade walls’ R-values would need to approach R-40. Multiple approaches, including 

structurally insulated panels (SIPs) and thick exterior foam sheathing, were investigated; the 

developer selected the double-wall method for cost and constructability reasons. Once the basic 

assembly was finalized, the team investigated the insulation materials and methods. The 

significant air sealing advantages of closed-cell spray polyurethane foam (ccSPF) led to its 

inclusion in the initial assembly used in the design development drawings. Originally specified at 

2-in. continuous against the inside of the sheathing, the remainder of the wall was to be filled 

with dense-pack cellulose, for a nominal wall R-value of 42. 

Cost estimating by the developer forced this assembly to be reconsidered because the cost for a 

moderate market-rate product was higher than desired. CARB re-evaluated the available options 

and determined that by limiting the SPF to rim-joist applications and general air sealing, the 

target HERS Index could be met, but with more aggressive air infiltration targets (see Sections 
12

3.1.2 and 3.1.3). The final specification for the above-grade walls thus evolved to 10 in. of dense 

pack cellulose (R-3.8/in.) for a nominal value of R-38, with ccSPF used only at mudsills, rim 

joists, and for other air sealing purposes. 

Because no exterior continuous insulation is used for this assembly, an interior side vapor

retarder becomes code required. To simplify the assembly, and to keep potential water vapor out 

of the assembly altogether, CARB specified a vapor retarder latex paint primer on the gypsum 

wall board, which CARB has used successfully in the past. 

Figure 7. Above-grade walls with dense pack cellulose insulation 

3.1.2 Below-Grade Walls

As noted in Section 3.1.1, the developer desired that the full basement foundations for these 

homes be usable and potentially occupiable. This dictated that the basement be treated as 

conditioned space and that the vertical walls define the thermal enclosure, and not the first floor 

framing. CARB’s experience is that polyisocyanurate adhered directly to the foundation walls is 

the most cost-effective and practical method, and allows the insulation to remain exposed (if 

rated for exposure as is the specific polyiso used here), or to be covered in drywall should the 

future owner decide to finish the space. 

The optimal R-values also needed to be considered, and again with HERS 40 as the target, the 

design team arrived at R-13, or 2-in. of polyisocyanurate. The material was installed full height 

and was adhered directly to the interior concrete wall surfaces with construction adhesive. All 

seams were covered with foil Underwriters Laboratories-rated duct tape.
13

Figure 8. Below-grade walls 

3.1.3 Attic Insulation 

The Devens triplexes use standard raised-heel roof trusses and vented attics for cost, 

constructability, durability, and energy performance reasons. The optimal ceiling insulation 

levels can thus be economically achieved by various types of loose fill insulation: either mineral 

fiber (fiberglass or mineral wool) or cellulose. As the wall assemblies were to use dense-pack 

cellulose, the decision was made to use loose fill cellulose at the ceiling plane as well. Energy 

modeling optimization suggested R-59, or 16 in. at R-3.6/in. 

Figure 9. Attic insulation

3.1.4 Air Sealing

With the financial decision to eliminate the 2-in. ccSPF insulation from the above-grade walls, 

the design team understood the need for a more aggressive air infiltration target. Even though 

this was developers’ first attempt at such high performance targets, CARB lowered the 
14

infiltration specification to 2.0 ACH50 from 3.0 ACH50 to make up for the slightly lower Rvalue in the walls. With this, the execution of the air sealing became critical, so CARB provided

the strategy and on-site training to the contractor. 

The vertical framed wall air sealing was handled mostly at the exterior sheathing surface, which 

used a proprietary integrated water-resistive barrier oriented strand board and urethane-butyl 

joint tape. This product works exceptionally well on the vertical planes, leaving only the edges, 

terminations, and penetrations to otherwise address. The basement mudsill and the rim joist were 

covered with 3 in. ccSPF, and low expansion foam was used at all penetrations. 

All penetrations at the attic ceiling plane were also sealed with low expansion foam. This

included covering electrical boxes for surface-mounted devices such as smoke detectors with 

fire-stop ccSPF. Exposed frame wall top plates were similarly covered and sealed before the 

cellulose loose fill insulation was installed.

Final blower door testing on April 6, 2012 revealed that the home achieved 1.6 ACH50, 

exceeding the target of 2.0 ACH50.

The final aspect of the high performance building enclosure is the windows. Rather than the BA 

Benchmark-specified double-pane non-ENERGY STAR windows with U-0.35 and SHGC-0.35, 

the Devens triplexes have vinyl double-paned windows with U-0.24 and SHGC-0.22. Vinyl lowe windows with these impressive performance characteristics have become far more common

and available at only minimal cost premiums. 

3.2 Lighting and Appliances

The BA Benchmark specifies 66% incandescent bulbs, 21% CFLs, and 13% T-8 fluorescent 

bulbs. At the end unit in the Devens prototype, there are no incandescent bulbs, 95% CFLs, and 

5% T-8 fluorescent bulbs. An ENERGY STAR refrigerator and dishwasher were installed, which 

was an improvement over the non-ENERGY STAR appliances in the BA Benchmark. 

Figure 10. CFL and pin-based lighting installed

3.3 Heating, Ventilation, and Air-Conditioning Systems

Given the very high performance thermal enclosure intended for the Devens homes, the potential 

options for appropriate HVAC systems are numerous. At RDI’s Wisdom Way development,

similar buildings made do with 11,000-Btu unit heaters and no real distribution system (Aldrich 

2012). Because these homes are market rate, the developer thought such an unconventional 
15

approach would limit sales and insisted on a more conventional approach. The two systems 

shortlisted for further scrutiny were ducted air-source heat pumps and condensing gas furnaces 

with conventional air-conditioning outdoor units. The advantage of the air-source heat pump was 

that natural gas would not need to be brought to the site, which saved considerable costs. The 

lack of natural gas would, however, limit options for domestic hot water (DHW). The gas 

furnace/conventional air conditioner combination would be a less expensive first-cost option, but 

would require that natural gas infrastructure be brought to the site. 

Figure 11. Duct during framing

3.3.1 Final Heating, Ventilation, and Air-Conditioning Specifications

Weighing and comparing the various options eventually led to the selection of a conventional but 

very high performing 95.6 AFUE condensing gas furnace, and a 1.5-ton 16 SEER airconditioning system. This single-zone setup includes a programmable thermostat, and was 

chosen because it provides the highest energy efficiency of all the options investigated. 
16

Figure 12. Air handler in basement

3.3.2 Distribution System

The next challenge was to integrate the ducted distribution system into the compact plan 

completely within conditioned space. Early design decisions had considered this and resulted in 

the use of open web floor trusses on both levels to accommodate the ducts. The basic premise 

included running the first- and second-floor ducts separately in each floor system and using floor 

registers for distribution in this heating-dominated climate. 

Several iterations with C7A resulted in a final variation of this basic concept; the first-floor 

kitchen is served through a ceiling register fed from the second floor ducts. There are two return 

registers, one at the base of the stair near the first-floor ceiling, the other at the top of the stair 

near the second-floor ceiling. 

Final testing of the ducted HVAC distribution system found total leakage at 25 pascals to be 85 

CFM; leakage to the outside was zero.

3.3.3 Fresh Air Ventilation

CARB has had excellent results using exhaust-only ventilation strategies to meet ASHRAE 

Standard 62.2 ventilation requirements in similar cold-climate projects, and incorporated that 

strategy again here. The low-sone ENERGY STAR bath fan located in the second-floor 

bathroom incorporates a continuous operation mode that can be set to continuous flow, when the 

occupant turns off the boost feature. The bedroom count (two) and size of these dwellings dictate 

a continuous exhaust flow rate of approximately 43 CFM. Final testing confirmed that the design 

ventilation rate was met. 

3.4 Domestic Hot Water

The BA Benchmark has a non-ENERGY STAR gas storage water heater with an EF of 0.59 and 

a 40-gal tank. The condensing gas tankless water heater installed at Devens has an EF of 0.96
17

(see Figure 13). This piece of equipment was chosen because of the decision to bring natural gas 

onto the site and to achieve a HERS Index of less than 41. After resolving to provide natural gas 

to the townhomes, the team focused on tankless water heaters as a relatively high efficiency 

solution to one of the higher end use energy loads. More standard, noncondensing tankless water

heaters were investigated, but their lower efficiency values (EF .84 to EF .87) forced 

consideration of the condensing units, which have become more market available. The 

approximately 10% performance improvement over the noncondensing units helped lower the 

final HERS Index below 41. The water heaters were installed in the basements with the other 

mechanical equipment.

Figure 13. Tankless water heater 
18

4 Targeted and Simulated Energy Savings

To align with the BA goal, 30% source energy savings were targeted. Figure 14 shows that the 

Devens end unit was able to achieve 30.71% savings in source energy over the BA Benchmark. 

This equates to a HERS Index of 39 for this dwelling. The building enclosure improved by 

approximately 14% over the BA Benchmark, lighting and appliances by about 4%, and HVAC

by approximately 11%. Figure 14 shows the BA Benchmark with cumulative incremental 

changes, which combine to simulate predictive energy savings at the Devens end unit.

Figure 14. Achieved energy savings of Devens end unit over the BA Benchmark

To create this chart, energy savings were isolated by comparing the BA Benchmark and the 

measure improvement. The chart was built up by modeling the measure improvements from the 

most savings (air sealing) to the least energy savings (foundation) with the programmable 
19

thermostat at the end. The programmable thermostat is placed last to better demonstrate the 

Devens end unit’s improved capabilities. 

The single greatest contributor to overall energy savings was the improved air sealing. It shows a 

7.71% drop in energy use compared to the BA Benchmark. Applying heating system upgrades

improves the home by an additional 4.32%. When the heating system improvements are looked 

at independently, there is a 5.71% improvement. Using the cumulative approach demonstrated in 

Figure 14 the overlapping energy savings of the individual measures is eliminated.

In the Northeast, the heating load often uses the most source energy—this applied to the Devens 

end unit. The BA Benchmark consumes 44.74 MBtu annually for heating. Improvements to the 

home’s air sealing, furnace, and overall enclosure insulation, and installation of a programmable 

thermostat reduced heating energy consumption by 53.87%. Decreasing the heating energy 

consumption was a critical factor in achieving the 30% targeted energy savings. 

The next biggest end use was the large appliances, using 21.34 MBtu/yr in the BA Benchmark. 

With the refrigerator and dishwasher upgrades, large appliance improvements had only a 5.30% 

decrease in large appliance energy use. This improvement did not greatly contribute to the 

reduction in energy consumed. 

The domestic hot water (DHW) system was the third largest end user, demanding 20.59 MBtu/yr 

according to the BA Benchmark. DHW energy use decreased by 41.48% with the improvements 

from a 0.59 EF gas storage water heater to an EF 0.96 condensing gas tankless water heater. 

The lighting demands 18.97 MBtu/yr per the BA Benchmark and was the fourth largest energy 

user. The change from 66% incandescent, 21% CFL, and 13% T-8 fluorescent to 95% CFL and 

5% T-8 fluorescent decreased lighting energy use by 35.37%. 

Other improvements to HVAC fan/pump and cooling had much smaller end use loads but greater 

improvements. According the BA Benchmark, the HVAC fan/pump uses 4.89 MBtu/yr and used 

66.46% less with the improvement to the air-conditioning system. The BA Benchmark cooling 

system uses 3.85 MBtu/yr and 60.26% less energy with the improvement. Cooling improvements 

increased the system efficiency, but did not substantially improve cooling because it accounts for 

such a small percentage of the total building energy use. 

Overall, the improvements save 46.06 MBtu/yr. Three of the four greatest end uses (according to 

the BA Benchmark) also saved the most energy. Only miscellaneous loads and ventilation fan 

loads remained the same from the BA Benchmark to the final design.
20

5 Cost Data and Cost Effectiveness of Energy Efficiency 

Measures

The purpose of this prototype was to build a moderate priced, highly efficient prototype home. 

Improvements over the BA Benchmark cost an additional $7,804 and saved more than 46

MBtuh/yr. Costs for each improvement were estimated using market and regional costs and 

validated by the National Renewable Energy Laboratory Retrofit Cost Database (NREL 2010). 

Cost effectiveness of these improvements is determined by simple payback. The cost is divided 

by the annual utility savings to see payback period in years. The utility savings are $580.66/yr, 

so the combined improvements have a payback period of 13.44 years. The improvements can be

grouped into three categories: enclosure, mechanical, and electrical improvements (see Table 2). 

The shortest payback period of all is in the electrical improvement category. In a single year, 

these improvements save $103.90, and it costs only $140 to upgrade the lighting. The payback 

period for electrical improvements is less than two years. The payback period for appliances 

(refrigerator and dishwasher) is negligible because the cost difference between an inefficient 

model and the efficient is incrementally small. Installing energy-efficient appliances is a simple 

way to lower energy use in new construction residential units. 

The second shortest payback period was in the mechanical improvement category, which would 

save the resident $215.31 annually. Mechanical improvements cost $2,827 and the payback 

period is 13.13 years. The mechanical improvements had relatively short payback periods, 

including the heating system upgrade (8.62 years), DHW upgrade (13.93 years), and the air 

conditioning/programmable thermostat upgrade (18.28 years).

Building enclosure improvement-related annual utility savings are $261.45 and had an upfront 

additional cost $4,837 to upgrade, making the payback period 18.50 years. Although air sealing 

alone has a payback period of just 2.15 years, it is dependent the quality of installation, improved 

insulation type, and R-value of all enclosure insulation. There are longer payback periods for the

attic insulation upgrade (27.95 years), window upgrades (28.09 years), foundation insulation 

upgrade (32.00 years), and the double wall (44.74 years). Although these enclosure

improvements have the longest payback period, they have the most significant impact on energy 

savings in the Devens end unit. 
21

Table 2. Payback of Prototype Building Specifications

Component BA Benchmark 2010 Prototype 

Specification

Upgrade 

Costs per 

Measure

Energy 

Savings (in 

MBtu/yr)

Payback 

(years)

Foundation 

Assembly

Uninsulated basement slab; 

R-10 (2-in. extruded 

polystyrene at R-5/in.) at 

exterior OR R-19 (5.5 in.

fiberglass batt insulation) 

on interior of basement 

cavity wall

Uninsulated 

basement slab; 

R-13 (2-in.

polyisocyanurate 

at R-6.5/in.) at 

interior side of 

foundation wall

$288 0.77 32

AboveGrade

Exterior 

Walls

Wood 2 × 4 stud frame, 16 

in. o.c.; R-13 (3.5 in.

fiberglass batt) + 

R-5 continuous insulation 

(1-in. extruded polystyrene 

at R-5/in.)

Wood 2 × 4 double 

stud frame walls, 16 

in. o.c., R-38 

(10 in. dense pack 

cellulose at 

R-3.8/in.)

$2880 5.31 44.74

Ceiling 

Assembly

R-38 fiberglass batt 

insulation

R-59 loose fill 

cellulose 

(16 in. at R-3.6/in.)

$469 1.27 27.95

Window 

Glazing

U-0.35, SHGC-0.35, double 

pane, non-ENERGY STAR

U-0.24, 

SHGC-0.22, 

double pane, vinyl

$900 2.51 28.09

Air Sealing 7.4 ACH50 1.6 ACH50 $300 11.56 2.15

Total Enclosure Improvements $4,837 21.42 18.50

Cooling 

System

1.5-ton air conditioner, 

SEER 13

1.5-ton air 

conditioner, SEER 

16, programmable 

thermostat

$1,000 4.19 18.28

Heating 

System 78% AFUE gas furnace

96.5% AFUE 

condensing gas 

furnace

$666 6.48 8.62

Water 

Heating

Non-ENERGY STAR 

gas storage water heater, 

40-gal tank, EF 0.59

96% condensing gas 

tankless water heater, 

EF 0.96

$1,161 6.86 13.93

Total Mechanical Improvements $2,827 17.53 13.13

Appliances Non-ENERGY STAR 

appliances ENERGY STAR n/a 1.34 n/a

Lighting

66% incandescent, 

21% CFL, 

13% T-8 fluorescent

0% incandescent, 

95% CFL, 

5% T-8 fluorescent

$140 5.77 1.65

Total Electric Improvements $140 7.11 1.35

Total Improvements

$7,804

additional 

cost

46.06

MBtu/yr

saved

13.44 year 

payback
22

The optimization curve in Figure 15 shows the curve’s relationship with the BA Benchmark, the 

final Devens end unit, and the final Devens end unit without the programmable thermostat. The 

BA Benchmark point shows the home saves little source energy and the energy-related costs are 

very high. 

The final Devens end unit point reinforces how much it saves in source energy and in energyrelated costs compared to the BA Benchmark. Without a programmable thermostat, the home 

saves less energy and energy-related costs are higher than the actual design. 

The final design point is very close to the optimization curve, indicating the final design package 

is close to ideal in terms of annual energy related cost savings and source energy savings. 

Figure 15. Comparison of modeling iterations and optimization curve
23

6 Research Question Conclusions

With the basic goal of meeting or exceeding the BA Benchmark by 30% with cost in mind, the 

team was interested in answering this overarching research question: 

Question: What solution packages can be readily implemented in cold climate homes to achieve 

at least 30% energy savings compared to the BA Benchmark?

Conclusion: This solution package successfully surpassed the target of 30% source energy 

savings, using only market-present technologies to achieve the targeted source energy savings. 

This can be attributed to early planning and design decisions, and proper installation. The 

uniform upgrade in enclosure construction contributed to more than 14% improvement over the 

BA Benchmark. 

This prototype represents a viable solution package. Its standout feature is its replicability. The

improvements, such as a double stud wall, are simply efficient construction methods that do not 

require special equipment, skills, or training, just extra attention during installation. 

Question: Is that solution package commercially viable? Where are opportunities to reduce costs 

in this solutions package?

Conclusion: This solution package is commercially viable. For $7,804, this unit’s improvements 

had a payback period of less than 14 years. This triplex building is the first prototype in a series 

of four similar buildings. The developer has indicated remaining with the exact energy efficiency 

improvement package for future triplexes as this prototype building, based on its success.

Question: What are the specific gaps to achieving the solution package at a production scale 

(cost, risk adversity, implementation complexity, etc.)? 

Conclusion: Air sealing stands as a huge barrier to effectively deploying this solution package. 

The team’s thoroughness in installation led to better than anticipated infiltration results. This 

improvement is determined by the quality of installation and by material choice, and was

confirmed with blower door testing. This construction team has determined a good air sealing 

strategy at will be replicable in the future triplexes. Specifically advocated by CARB and 

adopted at Metric Development is the use of the exterior sheathing plane as the primary vertical 

air barrier. The structural sheathing and proprietary joint-tape system used effectively seals the 

planer elements leaving only the edges, terminations, and material transitions to be sealed. The 

CARB team provided walk-through inspections and subcontractor training to point out the 

critical remaining areas to be sealed, and methods and material options available. Most 

nonsheathing air sealing was accomplished with low expansion ccSPF. 

Question: Based on the results of this test home, what other alternative energy efficiency 

solution packages should be considered?

Conclusion: To reduce source energy use and associated costs, the developer might consider 

switching from an electric to a gas clothes dryer. The developer might also consider decreasing 

window area; however, this will impact user comfort. 
24

It would be interesting to see the impact of a solar PV panel installation. Miscellaneous electric 

loads remained unchanged from the BA Benchmark to the final design. This could substantially 

decrease each unit’s electric load, especially because this building has a south-facing roof. 

Question: What are the market interest and consumer reactions, developer and builder reactions 

and feedback loops, and stakeholder enthusiasm for replicating the package?

Conclusion: This triplex building was the first of four slated for construction on this site. The 

developer’s HERS goal keeps all parties invested in creating a robust prototype by planning, 

adjusting, and replicating. It will be interesting to see what the developer retains and discards 

based on lessons from the prototype triplex. 

Initial marketing results were very encouraging. The first (middle unit) triplex dwelling went to 

buyer contract approximately 30 days following completion. Closing on that home occurred the 

end of September. Based on early interest and foot traffic the developer has commenced

construction on the final two triplexes. 

Question: How effectively does each EEM meet its specific cost and performance targets? How 

effective is each when integrated into a whole-house package?

Conclusion: Individual EEMs provide interesting results. In the Devens end unit, the enclosure

improvements cost the most, but also saved the most energy in the biggest end use category. 

Lighting and appliances were the most cost effective in terms of payback period and provided 

the simplest changes, but also yielded limited energy savings. Mechanical improvements struck a 

fine balance between the moderate price and substantial energy savings, with a payback period 

of $7,804 and energy savings of 46.06 MBtu/yr, for a payback period of 13.44 years. This 

package of improvements proved to be cost effective with a short payback period and relatively 

overall low cost for considerable energy savings. 
25

References

Aldrich, R. (2012). Wisdom Way Solar Village: Design, Construction, and Analysis of a Low 

Energy Community. Golden, CO: National Renewable Energy Laboratory. SR-5500-53564. 

www.nrel.gov/docs/fy12osti/53564.pdf. 

Aldrich, R.; Mantha, P.; Puttagunta, S. (2012). Measure Guideline: Basement Insulation Basics. 

Golden, CO: National Renewable Energy Laboratory. SR-5500-55802. 

www.nrel.gov/docs/fy13osti/55802.pdf.

Aldrich, R.; Zuluaga, M. (2006). “The Challenges of Basement Insulation.” Home Energy

January/February 2006, pp. 30–34. Accessed May 2013. www.carb-swa.com/articles/ 

in%20the%20news/HomeEnergy_The%20Challenges%20of%20Basement%20Insulation.pdf.

Hendron, R.; Engebrecht, C. (2010). Building America House Simulation Protocols. Golden, 

CO: National Renewable Energy Laboratory. TP-5500-49246. 

www.nrel.gov/docs/fy11osti/49246.pdf.

NREL. (2010). National Residential Efficiency Measures Database v2.0.0. Accessed August 10, 

2012. www.nrel.gov/ap/retrofits/about.cfm. 
26

Appendix A: Plan Set

(courtesy of Cambridge Seven Associates, Inc.)
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DOE/GO-102013-3903 ▪ August 2013

Printed with a renewable-source ink on paper containing at 

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