NIST Special Publication 1187

Sustainability Performance of the 

NIST Net-Zero Energy Residential 

Test Facility Relative to a Maryland 

Code-Compliant Design

Joshua Kneifel

Eric O’Rear

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.SP.1187

 

NIST Special Publication 1187

Sustainability Performance of the 

NIST Net-Zero Energy Residential 

Test Facility Relative to a Maryland 

Code-Compliant Design 

Joshua Kneifel

Applied Economics Office

Engineering Laboratory

Eric O’Rear

Applied Economics Office

Engineering Laboratory

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.SP.1187

March 2015

U.S. Department of Commerce 

Penny Pritzker, Secretary

National Institute of Standards and Technology

Willie May, Acting Under Secretary of Commerce for Standards and Technology and Director
ii

Certain commercial entities, equipment, or materials may be identified in this

document in order to describe an experimental procedure or concept adequately.

Such identification is not intended to imply recommendation or endorsement by the

National Institute of Standards and Technology, nor is it intended to imply that the

entities, materials, or equipment are necessarily the best available for the purpose. 

National Institute of Standards and Technology Special Publication 1187

Natl. Inst. Stand. Technol. Spec. Pub. 1187, 57 pages (March 2015) 

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.SP.1187

CODEN: NTNOEF
iii

Abstract

The National Institute of Standards and Technology (NIST) received funding through the 

American Recovery and Reinvestment Act (ARRA) to construct a Net-Zero Energy Residential 

Test Facility (NZERTF). The initial goal of the NZERTF is to demonstrate that a net-zero energy 

residential design can “look and feel” like a typical home in the Gaithersburg area. The purpose 

of this report is to compare the sustainability performance including energy, economic, and 

environmental metrics, of the NZERTF design to a comparable Maryland code-compliant 

building design using whole building energy simulations, life-cycle costing, and life-cycle 

assessment.

Keywords

BIRDS; energy efficiency; net-zero energy; residential buildings; sustainability; whole building 

energy simulation
iv
v

Preface

This study was conducted by the Applied Economics Office (AEO) in the Engineering 

Laboratory (EL) at the National Institute of Standards and Technology (NIST). The study is 

designed to compare the sustainability performance of the NZERTF design to a comparable 

Maryland code-compliant building design using the results of EnergyPlus (E+) whole building 

energy simulations, life-cycle costing, and life-cycle assessment. The intended audience includes 

researchers in the residential building sector concerned with net-zero energy residential 

performance.

Disclaimers

The policy of the National Institute of Standards and Technology is to use metric units in all of 

its published materials. Because this report is intended for the U.S. construction industry that 

uses U.S. customary units, it is more practical and less confusing to include U.S. customary units 

as well as metric units. Measurement values in this report are therefore stated in metric units 

first, followed by the corresponding values in U.S. customary units within parentheses.
vi
vii

Acknowledgements

The authors wish to thank everyone for their advice and recommendations for the writing of this 

report, including Douglas Thomas and Dr. Robert Chapman of EL’s Applied Economics Office, 

Dr. William Healy of EL’s Energy and Environment Division, and Dr. Nicos S. Martys of EL’s 

Materials and Structural Systems Division.

Author Information

Joshua D. Kneifel

Economist

National Institute of Standards and Technology

100 Bureau Drive, Mailstop 8603

Gaithersburg, MD 20899-8603

Tel.: 301-975-6857

Email: joshua.kneifel@nist.gov

Eric G. O’Rear

Economist

National Institute of Standards and Technology

100 Bureau Drive, Mailstop 8603

Gaithersburg, MD 20899-8603

Tel.: 301-975-4570

Email: eric.orear@nist.gov
viii
ix

Contents

Abstract.............................................................................................................................iii

Preface................................................................................................................................ v

Acknowledgements ......................................................................................................... vii

Author Information ........................................................................................................ vii

List of Acronyms............................................................................................................ xiii

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

2 Building Specifications for the 2012 IECC and NZERTF Homes........................ 3

3 Performance Estimation Approaches..................................................................... 7

3.1 Energy .................................................................................................................. 7

3.2 Economic.............................................................................................................. 7

3.3 Environmental ...................................................................................................... 8

4 Sustainability Performance.................................................................................... 11

4.1 Energy Performance........................................................................................... 11

4.2 Economic Performance ...................................................................................... 11

4.3 Environmental .................................................................................................... 19

4.3.1 Environmental Impact Categories..........................................................................19

4.3.2 BIRDS Normalization and Weighting ...................................................................24

5 Variances in the Relative Efficiency of Simulated Building Designs................. 31

5.1 Isolating the Optimally Efficient Building Design ............................................ 31

5.2 Sustainability Performance of the Optimal Efficient Design............................. 34

6 Discussion, Limitations, and Future Research..................................................... 37

6.1 Discussion .......................................................................................................... 37

6.1.1 Energy Performance...............................................................................................37

6.1.2 Economic Performance ..........................................................................................37

6.1.3 Environmental Performance ..................................................................................38

6.1.4 Optimal Building Design .......................................................................................38

6.2 Limitations and Future Research........................................................................ 39

References........................................................................................................................ 41
x
xi

List of Figures

Figure 4-1 Present Value of Total First Costs for the MCC and the NZERTF Housing Designs

........................................................................................................................................... 12

Figure 4-2 Present Value of Additional First Costs by Energy Efficiency Measure....... 13

Figure 4-3 Present Value of Total Electricity Costs by Study Period ............................. 14

Figure 4-4 Present Value of Total Maintenance, Repair, and Replacement Costs by Study Period

........................................................................................................................................... 15

Figure 4-5 Present Value of Maintenance, Repair, and Replacement Costs by Component by 

Study Period...................................................................................................................... 16

Figure 4-6 Present Value of Total Residual Value by Study Period for both Building Designs

........................................................................................................................................... 17

Figure 4-7 Life-cycle Cost Measures for the Maryland Code-Compliant and NZERTF Building 

Designs.............................................................................................................................. 18

Figure 4-8 Life-cycle Impact Assessment Results for (a) GWP (b) acidification (c) smog and (d) 

energy for the Maryland design and NZERTF design...................................................... 20

Figure 4-9 Life-cycle Impact Assessment Categories ((a)-(d)) Directly Influenced by Building 

Construction Inputs and Operation for the Maryland and NZERTF designs................... 22

Figure 4-10 Relative Differences in Environmental Impacts for the Net-Zero and Maryland Code 

Compliant Designs............................................................................................................ 23

Figure 4-11 Weighted Normalized Impact Scores for the MCC and NZERTF Building Designs 

using Equal Weights......................................................................................................... 26

Figure 4-12 Weighted Normalized Impact Scores for the MCC and NZERTF Building Designs 

using BEES Weights......................................................................................................... 27

Figure 4-13 Weighted Normalized Impact Scores for the MCC and NZERTF Building Designs 

using SAB Weights........................................................................................................... 28

Figure 5-1 Changes in Life-Cycle Costs and Energy Use Reductions for Simulated Building 

Designs.............................................................................................................................. 31

Figure 5-2 Cost of Additional EEM Investment for Simulated Buildings ...................... 33

Figure 5-3 Relative Differences in Environmental Impacts for the Optimally Efficient, 

NZERTF, and MCC Designs............................................................................................ 35

List of Tables

Table 2-1 Electricity Consumption and Solar PV Production by Design (kWh) .............. 4

Table 3-1 Environmental Impact Categories and Units..................................................... 9

Table 4-1 Electricity Consumption and Solar PV Production by Design (kWh) ............ 11

Table 4-2 Annual Net Electricity Consumption and Electricity Costs............................ 13

Table 4-3 Normalization References: Annual U.S. Contributions.................................. 24

Table 4-4 Normalization Impact Assessment Scores ...................................................... 24

Table 4-5 Relative Importance Weights (%) ................................................................... 25

Table 4-6 Weighted Impact Scores.................................................................................. 29

Table 5-1 Breakdown of Energy Efficiency Measures for Simulated Buildings ............ 34

Table 5-2 Economic Performance Measures................................................................... 34

Table 5-3 Weighted Impact Scores.................................................................................. 36
xii
xiii

List of Acronyms

Acronym Definition

ACH air changes per hour

AEO Applied Economics Office

AIRR Adjusted Internal Rate of Return

ARRA American Recovery and Reinvestment Act

ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers

BA Building America

BSC Building Science Corporation

BEES Building for Environmental and Economic Sustainability

BIRDS Building Industry Reporting and Design for Sustainability

CFA conditioned floor area

CFC-11 Trichlorofluoromethane

CFL compact fluorescent lamp

CFM cubic feet per minute

CO2 carbon dioxide

CO2e carbon dioxide equivalent

COP Coefficient of Performance

CTUe Equivalent Comparative Toxicity Units for ecological receptors

CTUh Equivalent Comparative Toxicity Units for human health receptors

DHW Domestic Hot Water

E+ EnergyPlus

EERE Office of Energy Efficiency & Renewable Energy

EEM Energy Efficiency Measure

EIA Energy Information Administration

EIS Environmental Impact Score

EL Engineering Laboratory

ELA effective leakage area

EPA Environmental Protection Agency

GWP Global Warming Potential

H+ Hydrogen Ion

HPWH Heat Pump Water Heater

HRV Heat Recovery Ventilator
xiv

Acronym Definition

HSPF Heating Seasonal Performance Factor

HVAC Heating, Ventilation, and Air Conditioning

IECC International Energy Conservation Code

MCC Maryland Code-Compliant

LCA Life-Cycle Assessment

LCC Life-Cycle Cost

N Nitrogen

NIST National Institute of Standards and Technology

NOx nitrogen oxide

N S Net Savings

NZERTF Net-Zero Energy Residential Test Facility

O 3 Ozone

O C on center

PM10 particulate matter less than 10 micrometers in diameter

MRR Maintenance, Repair, and Replacement

PV photovoltaic

SAB Science Advisory Board

SEER Seasonal Energy Efficiency Ratio

SIR Savings-to-Investment Ratio

SHGC Solar Heat Gain Coefficient

SO2 sulfur dioxide

VT Visual Transmittance
1

1 Introduction

The National Institute of Standards and Technology (NIST) received funding through the 

American Recovery and Reinvestment Act (ARRA) to construct a Net-Zero Energy Residential 

Test Facility (NZERTF). The initial goal of the NZERTF is to demonstrate that a net-zero energy 

residential design can “look and feel” like a typical home in the Gaithersburg area. The purpose 

of this report is to compare the sustainability performance including energy, economic, and 

environmental metrics, of the NZERTF design to a comparable Maryland code-compliant 

building design using whole building energy simulations, life-cycle costing, and life-cycle 

assessment.

Kneifel (2012) documents the assumptions made to create a whole building energy simulation 

model in the E+ simulation software estimating the energy performance of the NZERTF. The 

geometry, building envelope, and hard-wired lighting design as well as some energy 

performance requirements are based on the specifications defined by the NZERTF project’s 

architectural firm, Building Science Corporation (BSC).

1 Based on the BSC specifications, the 

contractor selected interior equipment and lighting to meet those specifications. Occupant 

behavior assumptions for the NZERTF are defined based on Phase I operation. For some 

operating conditions, the model uses assumptions defined in Hendron and Engebrecht (2010). 

Additional documents that assist the model design are American Society of Heating, 

Refrigerating and Air-Conditioning Engineers (ASHRAE) 90.2-2007, ASHRAE 62.2-2010, and 

the ASHRAE Fundamentals Handbook.

Kneifel (2013) compares the energy performance of the NZERTF design to a comparable 

Maryland code-compliant building design using whole building energy simulations. The analysis 

includes a total of eleven EnergyPlus (E+)

2

simulations, starting with the Maryland codecompliant design, which requires building to 2012 International Energy Conservation Code

(IECC),

3

and then adding energy efficiency measures incrementally until all measures are 

included to reach the NZERTF design. This approach allows for a comparison across energy 

efficiency measures to determine the incremental impact of each energy efficiency measure on 

energy consumption.

Kneifel (2014) compares the life-cycle cost performance of the NZERTF design to a comparable 

Maryland code-compliant building design using the results of E+ whole building energy 

simulations and a contractor report estimating the associated construction costs. The use of 

life-cycle cost analysis is important because the cost flows associated with the NZERTF design 

and a Maryland code-compliant house design are different, with the NZERTF design realizing 

greater initial costs, but lower (negative) annual energy costs. By accounting for all costs 

 

1 Building Science Corporation (2009)

2 Department of Energy (2013)

3

International Code Council (2011)
2

associated with both building designs for the home owner’s investment time horizon, it is 

possible to allow a direct comparison of the economic performance across designs.

This study expands on the analysis in Kneifel (2013) and Kneifel (2014) by analyzing the 

sustainability performance of the NZERTF design and comparing it to the performance of the 

alternative Maryland code-compliant design based on 2012 IECC. The analysis uses the recently 

developed low-energy residential database that will be incorporated into NIST’s BIRDS 

(Building Industry Reporting and Design for Sustainability) web interface, and includes detailed 

comparisons in energy, economic, and environmental performance of the two designs to 

determine the benefits and costs of going beyond current residential energy code requirements to 

reach net-zero energy consumption in a single-family dwelling. The analysis includes some 

preliminary analysis of the incremental energy efficiency measure adoption that is available in 

the BIRDS low-energy residential database. 
3

2 Building Specifications for the 2012 IECC and NZERTF Homes

The current residential code adopted by the state of Maryland is based on the 2012 International 

Energy Conservation Code (IECC) for residential buildings. The IECC defines minimum 

construction requirements for various aspects of a building that impact its energy demand. Our 

analysis compares the simulated performance of a house that is compliant with 2012 IECC to the 

same house built to the NZERTF specifications. The inclusion of ten energy efficiency measures 

(EEMs) distinguishes the NZERTF design from the Maryland Code-Compliant (MCC) home.

They come in the form of improvements to the building envelope, interior equipment and 

lighting systems, as well as the installation of an on-site solar photovoltaic (PV) system. Table 

2-1 provides a simple breakdown of the building specifications for each home design.

The building envelope is different across the two building designs. The wall assembly in the 

MCC design is typical framing with R-13 batt insulation in the wall cavity and R-5 rigid exterior 

insulation while the NZERTF design implements advanced framing (Lstiburek 2010) with R-20 

blown-in cellulose insulation in the wall cavity and R-32 rigid exterior insulation. The MCC 

design includes R-38 of blown-in insulation on the attic floor while the NZERTF has the 

insulation installed in the roof assembly with R-45 blown-in insulation in the rafters and R-30 of 

rigid exterior insulation on the outside of the roof. The windows installed in the NZERTF design 

have a lower U-factor and solar heat gain coefficient (SHGC) than those installed in the MCC 

design. The building envelope in the NZERTF design is much “tighter” than the MCC design, 

leading to less air flow through the building envelope.
4

Table 2-1 Electricity Consumption and Solar PV Production by Design (kWh)

Building 

Category

MCC NZERTF

Windows U-Factor 1.99 W/m2-K (0.35 Btu/h*ft2-F) 1.14 W/m2-K (0.20 Btu/h*ft2-F)

SHGC 0.35 0.25

VT 0.40 0.40

Exterior Wall 

Assembly

Framing 5.1 cm x 10.2 cm - 40.6 cm OC

(2 in x 4 in - 16 in OC)

23 % Framing

5.1 cm x 15.2 cm - 61.0 cm OC

(2 in x 6 in - 24 in OC)

15 % Framing

Insulation† RSI-3.5 or RSI-2.3+0.9 (R-20 or R-13+5)* RSI-3.5+4.2 (R-20+24)*

Basement Wall Insulation† RSI-1.8

(R-10)

RSI-3.9

(R-22)

Roof/Ceiling

Assembly

Framing 11 % Framing 11 % Framing

Insulation† Ceiling: RSI-8.6 (R-49) Roof: RSI-7.9 + 5.3 (R-45 + 30)*

Infiltration Air change rate 3.00 ACH50 0.61 ACH50

Effective 

Leakage Area

1

st Floor = 403.6

2

nd Floor = 368.1

1

st Floor = 98.8

2

nd Floor = 90.2

Lighting 75 % efficient built-in fixtures 100 % efficient built-in fixtures

HVAC Heat/Cool Air-to-air heat pump

(SEER 13/HSPF 7.7)

Air-to-air heat pump

(SEER 15.8/HSPF 9.05)

Outdoor Air** Min. Outdoor Air (0.04 m3/s) Separate HRV system (0.04 m3/s)

Domestic Hot 

Water

Water Heater 189 L (50 gal) electric water heater

(0.98 efficiency)

189 L (50 gal) heat pump water 

heater (COP 2.6)

Solar Thermal None 2 panel, L(80 gal) solar thermal 

storage tank

Solar PV system None 10.2 kW

* Interior Wall Cavity + Exterior Continuous Insulation

**Minimum outdoor air requirements are based on ASHRAE 62.2-2010.

†Units: m2K/W (ft2F/(Btu/h))

The 2012 IECC requires that at least three-quarters of built-in lighting be efficient (energy 

conservative bulbs). This analysis assumes that the MCC and NZERTF designs use 75 % and 

100 % efficient built-in lighting, respectively. All heating and cooling demands for the MCC 

design are met using a standard efficiency (Seasonal Energy Efficiency Ratio (SEER) of 13 and 
5

Heating Seasonal Performance Factor (HSPF) 7.7) air-to-air heat pump system. The NZERTF 

design uses a higher efficiency (SEER 16.5/HSPF 9.05) air-to-air heat pump system and 

incorporates a separate Heat Recovery Ventilator (HRV) system. The HRV system is necessary 

for the NZERTF design in order to maintain the required outdoor air exchange as defined in 

ASHRAE 62.2-2010. The domestic hot water system installed in the MCC is a 189 L (50 gal)

electric water heater (efficiency of 0.98 with uniform skin loss coefficient of 0.41 W/m2-K). A

189 L (50 gal) heat pump water heater (HPWH) with a coefficient of performance (COP) of 2.6 

is installed in the NZERTF. An electric back-up element (thermal efficiency of 0.98) internal to 

the water heater tank is installed in the case that the HPWH fails to bring the water temperature 

to setpoint levels. An additional EEM installed in the NZERTF includes two solar thermal panels 

and a 302.8 L (80 gallon) storage tank that preheats water entering the HPWH. A solar PV 

system sized according to the surface area of the roof (10.2 kW) is installed on the north side of 

the NZERTF. The solar PV system installation and operation offsets the household electricity 

demands.
6
7

3 Performance Estimation Approaches

The approaches implemented to calculate performance for each category of sustainability 

(energy consumption, economic competitiveness, and environmental impacts) are discussed 

below.

3.1 Energy

Energy performance is evaluated using results from EnergyPlus (E+) 8.0.0 (DOE 2013) whole 

building energy simulations designed to the specifications of the NZERTF as operated during the 

demonstration phase and a comparable home built to meet 2012 IECC energy efficiency 

requirements. Energy consumption is estimated by E+ using the 3rd iteration of the Typical 

Meterological Year (TMY3) data for the weather station located at the Dulles International 

Airport near Sterling, Virginia. See Kneifel (2012) and Kneifel (2013) for additional details on 

the whole building energy simulations.

3.2 Economic

Economic performance is evaluated using the life-cycle cost methodology defined in Handbook 

135 (Fuller and Petersen 1996). Life-cycle costing accounts for all costs (in present value terms) 

associated with constructing and operating a building over the selected study period. Four 

different LCC metrics are used in the analysis: LCCs, net savings (NS), savings-to-investment 

ratio (SIR), and adjusted internal rate of return (AIRR). Life-cycle costs (LCC) are the sum of the 

costs (Ct) realized in each year (t) of the study period (N) discounted using discount rate d into 

present value terms as shown in Formula 1. The alternative that has the lowest LCC is the best 

option.

(1) LCC = ∑

Ct

(1+d)

t

N

t=0

For a building design, LCC can also be analyzed as shown in Formula 2, where LCC is the 

present value initial investment costs (I) plus present value capital replacement costs (RC) minus 

present value building residual value (RV) plus present value energy-related operating costs (E) 

plus present value non-energy related operating, maintenance, and repair costs (OM&R).

Residual value is the value of the building at the end of the study period (i.e., resale value).

(2) LCC = I + RC − RV + E + OM&R

Net savings (NS) is the difference between the LCC for the baseline (LCCbase) and the LCC for 

alternative i (LCCi) as shown in Formula 3. Positive NS means that alternative i is preferable to 

the baseline case.

(3) NSi = LCCbase − LCCi
8

Formula 4 shows that SIR is the difference in present value costs of future non-investment 

related costs over the study period relative to the baseline case (ΔE + ΔOM&R) divided by the 

difference in present value investment costs relative to the baseline case (ΔI+ΔRC-ΔRV) where Δ

is the alternative case investment costs minus the baseline case investment costs. An SIR of 

greater than 1.0 occurs when an alternative leads to greater present value cost savings than the 

additional present value investment costs required to obtain those savings. 

(4) SIR =

ΔE+ΔOM&R

ΔI+ΔRC−ΔRV

AIRR uses the SIR to calculate an annualized return on investment using Formula 5, where r is 

the reinvestment rate at which realized savings is reinvested, which is the minimum acceptable 

rate of return (MARR). An AIRR greater than the MARR, which is often the assumed discount 

rate (d), means that the alternative leads to greater returns than the baseline case.

(5) AIRR = (1 + r) ∗ (SIR)

1

N

⁄

Each of these four metrics will be used in the economic analysis within this report assuming a 

3 % discount rate (Rushing, Kneifel et al. 2014) and a study period ranging from one year to 40 

years. The economic analysis excludes any incentives offered to the homeowner for energy 

efficiency gains or renewable energy production, including the 30 % tax credit for the installed 

cost of the solar PV system. See Handbook 135 (Fuller and Petersen 1995) for additional 

information on LCC methodology.

3.3 Environmental

Environmental performance is evaluated using a hybrid life-cycle assessment (LCA) approach, 

which combines “top-down” LCA input-output environmental flow data with “bottom-up” 

process-based environmental flow data to calculate the total environmental flows associated with 

constructing and operating each design over the study period of interest (Suh and Lippiatt 2012).

The flows for a “baseline” single-family dwelling (i.e., MCC design) are estimated using the 

input-output environmental flows. Since input-output environmental flows are based on a per 

dollar basis, the baseline construction costs (Constbase) and the baseline maintenance and repair 

costs (M&Rbase) are multiplied by the associated flows (Flow Per $) as shown in Formula 6 

where i is an array of 12 environmental impact categories.

(6) LCAbase,i = Constbase ∗ Flow Per $Const,i + M&Rbase ∗ Flow Per $M&R,i

The flows for the building design changes associated with the energy efficiency measures 

implemented in the NZERTF design not included in the MCC design are derived from 

“bottom-up” process-based LCA data. The flows for building envelope products are based on a 

per square foot basis while the flows for building equipment are estimated on a per installed unit 

basis. See Suh and Lippiatt (2012) for additional details on the hybrid LCA approach.
9

The consideration of only a building design’s carbon footprint could miss other important 

environmental impacts. For this reason, building designs will be analyzed across the twelve 

environmental impact categories in Table 3-1. Global Warming Potential (GWP) serves as a 

measure of the degree to which human activities (e.g., the building and use of a small family 

dwelling) contribute to the greenhouse gas effect phenomenon through the releasing of heattrapping gases or greenhouse gases (GHG) like carbon dioxide and methane. Acidification 

involves the mixing of acidifying compounds, such as nitrogen oxides (e.g., NOx) and sulfur 

dioxide (SO2), in water to form acid rain which can be detrimental to both local and global 

ecosystems. A common source of acidification of water is the pollution released into the 

atmosphere (SO2 and NOx) from the burning of coal to produce electricity. Smog formation is 

the result of sunlight reacting with nitrogen oxides and volatile organic compounds released by 

local industries and vehicles including fossil fuel-based electricity production. This by-product 

has proven to be harmful to individuals suffering from chronic respiratory issues and to local 

vegetation. Primary energy consumption includes energy consumption from building operation 

as well as the energy embodied within the building products. The thinning of earth’s ozone layer 

– referred to as “ozone depletion” – results in more radiant energy reaching the planet’s surface.

The implications of this can be detrimental, with agriculture and human health being at risk.

Eutrophication refers to the depositing of additional mineral nutrients to soil or water. The 

harming of surrounding ecosystems from industrial pollutants is accounted for in the “ecological 

toxicity” or “ecotoxicity” impact category. The potential impacts on human health can occur 

from the exposure to both industrial and natural substances.

Table 3-1 Environmental Impact Categories and Units

Category Unit

Global Warming Potential kg CO2e

Acidification mol H+ eq

Ozone Depletion kg CFC-11 eq

Ecotoxicity CTUe

Water kg

Land Acre

Smog kg O3 eq

Eutrophication kg N eq

Energy 1000 Btu

Carcinogens CTUh

Non-Carcinogens CTUh

Respiratory Effects kg PM10 eq

The additional components incorporated into the NZERTF design have embodied environmental 

flows. There are two approaches to allocating these flows over a study period. First, the flows 

can be treated as “sunk flows” associated with the initial year of construction. This approach 

tends to overweight the embodied environmental flows for short study period lengths. Second, 
10

the flows can be split equally across all years of the product’s or component’s lifetime, which is 

more consistent to the life-cycle costing residual value calculation approach. This study treats

environmental flows on an annualized basis to maintain consistency across both the life-cycle 

costing and life-cycle assessment methodologies.
11

4 Sustainability Performance

The sustainability performance results for the MCC design and NZERTF design are compared 

and contrasted in this chapter across the three metrics (energy consumption, LCC, and LCA) 

defined in Chapter 3.

4.1 Energy Performance

Both the NZERTF and MCC designs are assumed to operate solely on electricity. The annual 

electricity consumption, production, and net consumption of the two designs are shown in Table 

4-1. Given the assumed occupant activity and setpoint conditions, the MCC design consumes 

23 604 kWh of electricity annually while the NZERTF design consumes 11 231 kWh of 

electricity annually. The solar PV system installed on the roof of the NZERTF design produces

15 474 kWh of electricity, which leads to net consumption of -4243 kWh, or 38 % more 

production than its consumption. In other terms, 8 of the 32 solar PV panels could be removed 

and the NZERTF design would still reach net zero energy consumption.

Table 4-1 Electricity Consumption and Solar PV Production by Design (kWh)

MCC NZERTF

Electricity Consumption 23 604 11 231

Electricity Production 0 15 474

Net Consumption 23 604 -4243

4.2 Economic Performance

Total first costs (initial investment costs) for the MCC and NZERTF designs are shown in Figure 

4-1. Total first costs are the sum of all costs of construction, which can be broken down into two 

categories: incremental costs associated with energy efficiency measures (EEMs) and costs 

independent of EEMs. Cost of purchasing and preparing the land for construction is excluded 

from the analysis because those costs are a constant across the two designs. Constructing to meet 

the NZERTF design costs an additional $108 049 (26 %) relative to the MCC design ($521 722 

versus $413 673) due to the additional expenses of incorporating more energy conservation

technologies and on-site electricity production.
12

Figure 4-1 Present Value of Total First Costs for the MCC and the NZERTF Housing 

Designs

Figure 4-2 provides a breakdown of the additional first costs related to the EEMs included in the 

NZERTF design relative to the MCC design. The on-site solar PV system costs an additional 

$58 091 – the most expensive of all seven adopted energy efficiency measures. The advanced 

framing and additional insulation in the walls ($14 807) and roof ($11 597) combine to cost 

$27 440 while the installation of a more efficient HVAC system (combination of the air-to-air 

heat pump, electric resistance heating element, and separate HRV) costs an additional $12 945.

The additional cost of reducing infiltration is $2766 while the HPWH and solar thermal system 

cost an additional $1504 and $5963, respectively. Shifting the 25 % of built-in light fixtures from 

incandescent bulbs to high efficiency lighting (compact fluorescent bulbs) is inexpensive at an 

additional cost of $30.

$413,673

$521,722

$ 0

$100 000

$200 000

$300 000

$400 000

$500 000

$600 000

US Dollars

Present Value Total First Costs

MCC

NZERTF
13

Figure 4-2 Present Value of Additional First Costs by Energy Efficiency Measure

Annual net electricity consumption and electricity costs for both designs are shown in Table 4-2. 

Net annual consumption for the MCC design is 23 604 kWh at a cost $3153.4 The operation of 

the solar PV system by the NZERTF design provides enough on-site electricity to more than 

offset annual energy consumption, leading to a production surplus of 4243 kWh of electricity. 

The NZERTF design realizes annual energy costs of -$567, or annual energy cost savings of 

$3720 relative to the MCC design.5

Table 4-2 Annual Net Electricity Consumption and Electricity Costs

MCC NZERTF

Net Electricity Consumption 23 604 kWh -4243 kWh

Electricity Costs $3153 -$567

Figure 4-3 displays total present value electricity costs across study periods ranging from 1 year 

to 40 years. Household electricity expenditures are steadily increasing in the case of the MCC 

design given the projected annual increases in electricity prices. The opposite occurs for the 

NZERTF design with energy costs growing increasingly negative (cost savings) as the study 

 

4 Based on the state average residential cost per kWh of electricity for Maryland (EIA 2012)

5 The value of the excess electricity sold back to the utility is also based on EIA (2012).

$30 $347 $1,504 $2,766 

$5,963 

$11,597 $12,945 

$14,807 

$58,091 

$ 0

$10 000

$20 000

$30 000

$40 000

$50 000

$60 000

$70 000

US Dollars

Present Value Incremental EEM First Costs

Lighting EEM

Windows EEM

HPWH EEM

Infiltration

EEM

Solar Thermal

EEM

Roofs EEM

HVAC EEM

Walls EEM

Solar PV EEM
14

period length increases. The divergence in costs between the two building designs grows larger 

over time. By year 40, there is a difference of more than $95 000 – once again showcasing the 

present value of potential energy cost savings that can be recouped by constructing the NZERTF 

design.

Figure 4-3 Present Value of Total Electricity Costs by Study Period

All future maintenance, repair, and replacement costs are necessary to complete life-cycle cost 

analysis. Figure 4-4 shows the total maintenance, repair, and replacement (MRR) costs for the 

two building designs. The NZERTF design realizes notably larger MRR costs than the Maryland 

compliant home in all study periods. The most significant differences occur in study periods 

longer than 25 years where NZERTF MRR costs are between 2 to 3 times higher than the MCC

design.

-$20 000

-$10 000

$ 0

$10 000

$20 000

$30 000

$40 000

$50 000

$60 000

$70 000

$80 000

$90 000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

US Dollars

Years

Present Value Electricity Costs

MCC

NZERTF
15

Figure 4-4 Present Value of Total Maintenance, Repair, and Replacement Costs by Study 

Period

Figure 4-5 reveals that the significantly higher MRR costs for the NZERTF building design are 

associated with the replacement of the home’s solar PV system, which has an assumed lifetime 

of 25 years. Higher MRR costs seen in earlier periods (before year 25) are primarily related to 

the costs of replacing other technologies adopted under the NZERTF design, such as the high 

efficiency HPWH, solar thermal system, and the high efficiency HVAC. The annual cost of 

maintaining the buildings’ heat pump comprise the majority of the MRR costs for the MCC 

design from year 0 to year 16 at which time the air-to-air heat pump is replaced. Non-EEM 

related costs (baseline costs) have not been included Figure 4-5. Baseline cost is the money spent 

maintaining, repairing, and replacing house components not associated with the adopted EEMs 

(windows, HVAC, DHW, and lighting systems) and is the same for both designs. Lighting 

system MMR costs proved to be negligible for the two designs and are not included in Figure 

4-5.

$ 0

$10 000

$20 000

$30 000

$40 000

$50 000

$60 000

$70 000

$80 000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

US Dollars

Years

Present Value Total MRR Costs

MCC

NZERTF
16

Figure 4-5 Present Value of Maintenance, Repair, and Replacement Costs by Component 

by Study Period

A house has a long lifetime (assumed to be 100 years), which is much longer than the maximum 

study period considered in this analysis (40 years). As a result, both designs have some value at 

the end of the study period (residual value or resale value). Figure 4-6 illustrates the residual 

value for both building designs across all study periods. Residual values for the NZERTF design 

are consistently higher than the MCC design across all study periods given the additional EEMs 

incorporated into the NZERTF design.

6 Residual values for both building designs are steadily 

declining as the study period increases except in year 25 where the residual value for the newly 

replaced solar PV system drives up the residual value for the NZERTF design.

 

6 Assumes the homeowner can realize the additional residual value at resale of the house

$ 0

$5 000

$10 000

$15 000

$20 000

$25 000

$30 000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

US Dollars

Years

Maryland Code Compliant Present Value MRR Costs 

Windows

Heat Pump

HRV

Water

Heater

(a)

$ 0

$5 000

$10 000

$15 000

$20 000

$25 000

$30 000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

US Dollars

Years

NZERTF MRR Present Value Costs

Windows

Heat Pump

HRV

Water

Heater

Solar

Thermal

Solar PV

(b)
17

Figure 4-6 Present Value of Total Residual Value by Study Period for both Building 

Designs

Total LCCs are the sum of first costs, replacement costs, maintenance and repair costs, and 

energy costs minus the residual value. It is used in this analysis to gauge the economic feasibility 

of the NZERTF design. Figure 4-7 shows (a) total LCC and (b) the changes in LCC between the 

two building designs. According to Figure 4-7(a) the NZERTF consistently has a higher LCC 

than the MCC design. Differences in costs grow wider as study period length increases. Future 

energy cost savings realized by the NZERTF are not large enough to offset the higher initial and 

future investment costs associated with the energy efficiency measures incorporated in its design.

$ 0

$100 000

$200 000

$300 000

$400 000

$500 000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

US Dollars

Years

Present Value Total Residual Value

MCC

NZERTF
18

Figure 4-7 Life-cycle Cost Measures for the Maryland Code-Compliant and NZERTF 

Building Designs

Changes in LCC between the designs captured in Figure 4-7(b) further supports the above 

conclusion that the MCC design is more economical than the NZERTF design. The difference in 

LCCs begins to plateau near year 25, at which point the solar PV system is replaced. However, a 

homeowner could build and live in the NZERTF design, consuming no net energy over a year at 

additional present value costs of $20 000 (6 years) to $35 000 (40 years), which excludes any 

financial incentives offered to homeowners for building a more energy efficient house or 

installing the solar PV system.

$ 0

$100 000

$200 000

$300 000

$400 000

$500 000

$600 000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

US Dollars

Years

Total LCC

MCC

NZERTF

(a)

$ 0

$5 000

$10 000

$15 000

$20 000

$25 000

$30 000

$35 000

$40 000

$45 000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

US Dollars

Years

(b) Difference in LCC
19

4.3 Environmental

The environmental performance of the NZERTF design and MCC design are analyzed by 

individual environmental impact category as well as a combined environmental impact score 

(EIS) based on a weighted average of all impact categories. The EIS takes into account the 

environmental benefits of lower energy consumption and the additional environmental impacts 

embodied within the EEM components.

4.3.1 Environmental Impact Categories

The environmental performance of the NZERTF design relative to the MCC design varies 

depending on the environmental impact category. Because of this result, it is imperative that all 

environmental impacts are considered as opposed to a single impact such as GWP. Twelve 

impact categories are assessed for both building designs: GWP, Primary Energy Consumption, 

Human Health (Respiratory Effects, Cancer Effects, and Non-cancer Effects), Water 

Consumption, Ecological Toxicity, Eutrophication Potential, Land Use, Smog Formation, 

Acidification Potential, and Ozone Depletion.

Figure 4-8 provides an illustration of the four impact categories primarily affected by operating 

energy consumption: global warming potential, ozone depletion, smog, and primary energy 

consumption. The GWP impacts (in kg CO2 eq.) in Figure 4-8(a) for both the MCC and 

NZERTF designs show continuous growth in emissions as the study period length increases –

with the MCC design having significantly greater CO2e emissions because of the greater 

electricity consumption. In the case of a 40-year study period, the impacts of the MCC design on 

GHG emissions are more than five times that of the NZERTF design. As shown in Figure 4-8(b), 

both designs contribute to acidification levels (mol H+ eq.). The contributions of the NZERTF

design are considerably lower than the MCC design for all study periods. Figure 4-8(c) shows 

that the NZERTF design contributes less to smog formation than the MCC design across all 

assumed study period lengths. MCC design impacts are as much as five times greater (40-year 

study period) because of its higher electricity consumption. Since electricity production relies on 

fossil fuels, at least partially as a fuel source, increasing the energy efficiency of a house will 

impact fossil fuel resource supplies. The “Primary Energy Consumption” metric in Figure 4-8(d) 

shows that the contributions of the NZERTF design are consistently lower than the less 

energy-efficient MCC design. The difference is considerable for a number of study periods. For 

example, the difference is 1725 kWh for a 40-year period. Since the metric includes both energy 

embodied within the building materials and operating energy consumption, the NZERTF GWP 

value is still positive even though the NZERTF is a net producer of energy.
20

Figure 4-8 Life-cycle Impact Assessment Results for (a) GWP (b) acidification (c) smog 

and (d) energy for the Maryland design and NZERTF design

 0

200 000

400 000

600 000

800 000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

kg CO2 eq

Years

Global Warming

MCC

NZERTF

(a)

 0

50 000

100 000

150 000

200 000

250 000

300 000

350 000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

mol H+ eq

Years

Acidification

MCC

NZERTF

(b)

 0

10 000

20 000

30 000

40 000

50 000

60 000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

kg 03 eq

Years

Smog Formation

MCC

NZERTF

(c)

0

500

1000

1500

2000

2500

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

kWh

Years

Primary Energy Consumption

MCC

NZERTF

(d)
21

Given the above results, the NZERTF design will fare better than the MCC design in impact 

categories driven by energy consumption (GWP, Ozone Depletion, Smog Formation, and 

Primary Energy Consumption). Its impact on the remaining impact categories must also be 

considered in order to evaluate the overall “environmentally-friendliness” of the NZERTF design 

relative to the MCC design.

Household energy demands somewhat influence the impact categories associated with Figure 

4-9. However, the embodied environmental flows in the building materials and components 

themselves are primarily responsible for these impacts. Figure 4-9(a) shows that the NZERTF 

design has a much larger impact on ozone depletion than the MCC design, which is driven by the 

embodied emissions in the additional rigid insulation in the roof and wall assemblies. The 

divergence in impacts grows larger over time to 0.40 kg CFC-11-eq given a 40-year study 

period. Figure 4-9(b) shows that the NZERTF design has a consistently greater impact on 

eutrophication levels than the MCC design. The building and operation of both the MCC and 

NZERTF designs are likely to contribute to ecotoxicity. Figure 4-9(c) shows that the potential 

contributions of the NZERTF design will be more than twice as high as that of the MCC design 

in most study periods due to the emissions embodied in the solar PV system. Figure 4-9(d) 

highlights differences in impacts for the NZERTF and MCC designs on human health – more 

specifically, the non-carcinogenic effects. Construction and use of the NZERTF design lead to

greater (non-cancerous) human health impacts than the MCC design. Impact differences grow as 

large as 0.011 CTUh (assuming a 40-year study period length).
22

Figure 4-9 Life-cycle Impact Assessment Categories ((a)-(d)) Directly Influenced by 

Building Construction Inputs and Operation for the Maryland and NZERTF designs

0.00

0.10

0.20

0.30

0.40

0.50

0.60

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

kg CFC-11-Eq

Years

Ozone Depletion

MCC

NZERTF

(a)

0

100

200

300

400

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

kg N eq

Years

Eutrophication

MCC

NZERTF

(b)

 0

50 000

100 000

150 000

200 000

250 000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

CTUe

Years

Ecotoxicity

MCC

NZERTF

(c)

0.00

0.02

0.04

0.06

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39

CTUh

Years

Human Health (Non-Carcinogenics)

MCC

NZERTF

(d)
23

Only 8 of the 12 impact categories have been accounted for in Figure 4-8 and Figure 4-9. The 

effects of the two designs on the remaining 4 categories (land, water, human health –

carcinogenics, and respiratory effects) are relatively small. Both the NZERTF and MCC designs

will increase the potential for the occurrence of these impacts; however, there is no real benefit 

of building and operating one home over another in regards to minimizing its impact on one of 

the five categories.

Figure 4-10 summarizes the above results by measuring the impacts of the NZERTF on all 12 

environmental impact categories relative to the MCC design (in percentage terms) given a

40-year study period. The most noticeable difference in impacts occurs for ozone depletion. The 

contributions of the NZERTF are three times greater (a 200 percentage point difference) than

that of the MCC design. Similar impacts are seen with ecotoxicity, where the NZERTF impacts 

are 172 percentage points greater than the MCC design. The effects of the NZERTF on GWP, 

acidification, and smog formation are 20, 6, and 19 percentage points less than the MCC 

alternative.

Figure 4-10 Relative Differences in Environmental Impacts for the Net-Zero and Maryland 

Code Compliant Designs

Large differences between the impacts of MCC and NZERTF designs on individual 

environmental flows make it difficult to gauge which home is more environmentally-friendly.

The following section discusses a reliable approach to reaching a more holistic conclusion about 

each design’s overall environmental impact. 

0%

50%

100%

150%

200%

250%

300%

Ozone depletion

Global warming

Acidification

Respiratory effects

Non carcinogenics

Carcinogenics

Eutrophication

Smog

Ecotoxicity

Energy

Land

Water

MCC NZERTF
24

4.3.2 BIRDS Normalization and Weighting

As mentioned in the previous section, impacts are expressed and measured in different units, 

which prevents adequate comparisons across environmental flows. This analysis implements a 

technique that allows such comparisons to be made through normalized impact scores, which can 

then be used to compare impacts. Fixed scale normalization references based on the relative 

annual contributions of total U.S. economic activity to each of the 12 environmental flows shown 

in Table 4-3 are used in calculating the scores (Lippiatt, Kneifel et al. 2013).

Table 4-3 Normalization References: Annual U.S. Contributions

Impact Category Normalization reference 

(Annual)

Units

Global Warming 7.16E+12 kg CO2 eq.

Primary Energy Consumption 3.52E+13/1.20E+14 kWh/kBTU

HH – Criteria Air 2.24E+10 kg PM10 eq.

HH – Cancer (Carcinogenic) 1.05E+04 CTUh

Water Consumption 1.69E+14 L

Ecological Toxicity 3.82E+13 CTUe

Eutrophication 1.01E+10 kg N eq.

Land Use 7.32E+08/1.81E+09 hectare/acre

HH – Non-cancer (Non-Carcinogenic) 5.03E+05 CTUh

Smog Formation 4.64E+11 kg O3 eq.

Acidification 1.66E+12 mol H+ eq.

Ozone Depletion 5.10E+07 kg CFC-11-eq.

Dividing each initial impact assessment by the total US contribution to that particular flow 

normalizes the results to permit direct comparisons. Table 4-4 lists the first series of normalized 

impact assessment scores for the MCC design and NZERTF design assuming a 10-year study 

period length, which is the same study period used for the life-cycle cost analysis.

Table 4-4 Normalization Impact Assessment Scores

Impact Category MCC NZERTF

Global Warming 2.73E-08 5.47E-09

Primary Energy Consumption 1.60E-08 3.73E-09

HH – Criteria Air 3.05E-09 3.31E-09

HH – Cancer (Carcinogenic) 3.93E-08 4.04E-08

Water Consumption 7.14E-09 7.47E-09

Ecological Toxicity 5.10E-10 1.39E-09

Eutrophication 5.22E-09 8.74E-09

Land Use 6.89E-09 6.97E-09

HH – Non-cancer (Non-Carcinogenic) 1.38E-08 1.93E-08

Smog Formation 3.04E-08 5.91E-09

Acidification 4.86E-08 2.81E-09

Ozone Depletion 9.73E-10 2.95E-09
25

An overall environmental impact score (EIS) can then be calculated by taking a weighted 

average of the normalized scores. The higher the EIS, the more impact the building will have on 

the environment. The selected weights for the impact categories will affect the relative 

environmental performance. Weights developed by the EPA Science Advisory Board (SAB) and 

a stakeholder panel hosted by NIST for the Building for Environmental and Economic 

Sustainability (BEES) software7(referred to as the BEES Stakeholder Panel through the 

remainder of this document), along with a set of equal weights (8.3 % for each category), are 

three examples of weighting approaches that are used to assess environmental flows. The “Equal 

Weighting” approach assumes that each impact category is valued the same. GWP is just as 

important as Ecotoxicity, Smog, and so forth. However, it is likely that some impacts are 

considered more important than others. The Science Advisory Board weights consider land use 

and global warming as the “highest-risk problems” and therefore are weighted more heavily than 

the others. Weights based on BEES Stakeholder Panel judgments attribute higher weights to 

GWP and energy consumption. For further discussion on the BIRDS normalization and 

weighting approaches, please refer to Lippiatt et al. (2013). Weights based on the SAB and 

BEES panel are shown in Table 4-5.

Table 4-5 Relative Importance Weights (%)

Impact Category Science Advisory Board BEES Stakeholder Panel

Global Warming 18 29.9

Primary Energy Consumption 7 10.3

HH – Criteria Air 7 9.3

HH – Cancer (Carcinogenic) 8 8.2

Water Consumption 3 8.2

Ecological Toxicity 12 7.2

Eutrophication 5 6.2

Land Use 18 6.2

HH – Non-cancer (Non-Carcinogenic) 5 5.2

Smog Formation 7 4.1

Acidification 5 3.1

Ozone Depletion 5 2.1

Figure 4-11, Figure 4-12, and Figure 4-13 are graphical illustrations of how the different 

weighting approaches generate noticeably different EISs. The equal weighting of normalized 

environmental flows in Figure 4-11 produces an overall higher EIS for the MCC design. Because 

all environmental flows are assumed to be valued the same, impact categories more negatively 

affected by the NZERTF design than the MCC design, like ecotoxicity and ozone depletion, end 

up contributing less to the overall EIS.

 

7 Lippiatt, B., et al. (2010). BEES Online: Life cycle analysis for building products, National 

Institute of Standards and Technology, US Department of Commerce.
26

Figure 4-11 Weighted Normalized Impact Scores for the MCC and NZERTF Building 

Designs using Equal Weights

Figure 4-12 and Figure 4-13 show how different weighting approaches can change the EIS. The 

MCC design generates a higher EIS under both approaches. In the case of the BEES Stakeholder 

panel, impact categories in which the NZERTF design performs worse than the MCC design 

(i.e., ecotoxicity and ozone depletion) are not heavily weighted, leading to small EIS 

contributions. The contributions of GWP and primary energy consumption to the EIS are 40 %. 

Better performance by the NZERTF design in these impact categories drives down its EIS 

relative to the MCC design.

0.00

0.50

1.00

1.50

2.00

2.50

MCC NZERTF

EIS

Equal Weighting

Carcinogenics(CTUh) Global warming(kg CO2 eq) Acidification(mol H+ eq)

Respiratory effects(kg PM10 eq) Non carcinogenics(CTUh) Ozone depletion(kg CFC-11-Eq)

Eutrophication(kg N eq) Smog(kg O3 eq) Ecotoxicity(CTUe)

Energy(thousand BTU) Land(acre) Water(kg)
27

Figure 4-12 Weighted Normalized Impact Scores for the MCC and NZERTF Building 

Designs using BEES Weights

A similar outcome occurs with the SAB weights. Environmental flows more negatively impacted 

by the NZERTF design compared to the MCC design are not weighted heavily (with the 

exception of ecotoxicity), leading to rather small contributions to the EIS than other impact 

categories where the NZERTF design performs better. The EIS for the MCC design is more than 

two times higher than the NZERTF design.

0.00

0.50

1.00

1.50

2.00

2.50

MCC NZERTF

EIS

BEES Stakeholder Panel

Carcinogenics(CTUh) Global warming(kg CO2 eq) Acidification(mol H+ eq)

Respiratory effects(kg PM10 eq) Non carcinogenics(CTUh) Ozone depletion(kg CFC-11-Eq)

Eutrophication(kg N eq) Smog(kg O3 eq) Ecotoxicity(CTUe)

Energy(thousand BTU) Land(acre) Water(kg)
28

Figure 4-13 Weighted Normalized Impact Scores for the MCC and NZERTF Building 

Designs using SAB Weights

Computed impact scores using the three aforementioned weighting systems for both the MCC

design and NZERTF design are shown in Table 4-6. All 3 EIS estimates suggest that the 

NZERTF design will have less of an overall environmental impact than the MCC design. The 

MCC design fairs just as well or better than the NZERTF design in 8 of the 12 impact categories.

However, large divergences in GWP, acidification, and smog formation impacts make the 

NZERTF design more environmentally-friendly than the MCC design. Under the equal 

weighting approach, the NZERTF design realizes an EIS that is 41 % smaller than the MCC 

design. This divergence is further exacerbated by the SAB and BEES weights since both place 

great emphasis on GWP. All results must be considered with caution. Our results are based only 

on three weighting systems. Alternative weighting approaches could produce significantly 

different outcomes. Since environmental flows for building components are annualized, the 

study period length would impact the magnitude of the results. Additionally, the relative 

performance of the building designs would change as the study period increases in length 

because the fraction of total flows associated with operating energy consumption increases.

0.00

0.50

1.00

1.50

2.00

2.50

MCC NZERTF

EIS

Science Advisory Board

Carcinogenics(CTUh) Global warming(kg CO2 eq) Acidification(mol H+ eq)

Respiratory effects(kg PM10 eq) Non carcinogenics(CTUh) Ozone depletion(kg CFC-11-Eq)

Eutrophication(kg N eq) Smog(kg O3 eq) Ecotoxicity(CTUe)

Energy(thousand BTU) Land(acre) Water(kg)
29

Table 4-6 Weighted Impact Scores

Equal Weighting Science Advisory Board BEES Stakeholder Panel

Impact Category MCC NZERTF MCC NZERTF MCC NZERTF

HH – Cancer 0.33 0.34 0.71 0.73 1.17 1.21

Global warming 0.23 0.05 0.19 0.04 0.28 0.06

Acidification 0.40 0.02 0.34 0.02 0.45 0.03

HH – Criteria Air 0.03 0.03 0.02 0.03 0.03 0.03

HH – Non-cancer 0.11 0.16 0.04 0.06 0.11 0.16

Ozone depletion 0.01 0.02 0.01 0.04 0.01 0.02

Eutrophication 0.04 0.07 0.03 0.04 0.03 0.05

Smog Formation 0.25 0.05 0.55 0.11 0.19 0.04

Ecological Toxicity 0.00 0.01 0.00 0.01 0.00 0.01

Primary Energy 

Consumption 0.13 0.03 0.11 0.03 0.07 0.02

Land Use 0.06 0.06 0.03 0.03 0.02 0.02

Water Consumption 0.06 0.06 0.04 0.04 0.02 0.02

Total 1.66 0.90 2.07 1.16 2.38 1.64

Pct. Difference 59 % 57 % 36 %

Note: All values listed in the table have been scaled by 10

8
30
31

5 Variances in the Relative Efficiency of Simulated Building Designs

Design of the NZERTF requires the implementation of seven additional EEMs. Chapter 4

revealed that these additional measures lower household electricity demands relative to the MCC

design, but at a significantly higher cost to the homeowner. Ideally, homeowners wanting to 

improve the energy efficiency (relative to the MCC) of their new home are seeking cost-effective 

solutions. Alternative designs that are less costly to construct and still lead to considerable 

reductions in energy use could be considered. This chapter includes preliminary analysis of the 

incremental adoption of the EEMs in the NZERTF design as are found in the BIRDS low-energy 

residential database. 

5.1 Isolating the Optimally Efficient Building Design

Figure 5-1 is a scatterplot of 864 incremental designs assuming a ten-year study period. Each 

point represents the observed level of energy efficiency and change in life-cycle costs associated 

with a specific combination of EEMs specified in Section 2 relative to the baseline case of the 

MCC design (Point A). All designs or “points” captured on the lower envelope of the solution set 

(points along the black line) are the most inexpensive combination of EEMs for a given level of 

energy reduction. We characterize a design as being closer to optimal the lower its change in 

LCCs. Based on this characteristic, Point B proved to be the LCC optimal building design. The 

design lowers annual energy use by 46 % while decreasing life-cycle costs by $6304 relative to 

the MCC design and nearly $32 000 less than the NZERTF design (Point F). Four of the seven 

EEMs described in Section 2 (i.e., high efficiency windows, high efficiency HVAC system, 

100 % efficient lighting, and improved infiltration) are implemented in the optimal design.

Figure 5-1 Changes in Life-Cycle Costs and Energy Use Reductions for Simulated 

Building Designs

$25 579 (F)

$0 (A)

-$6304 (B)

-$606 (C)

Net Zero (D)

$7633 (E)

-$10 000

-$5 000

$ 0

$5 000

$10 000

$15 000

$20 000

$25 000

$30 000

$35 000

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 110% 120%

US Dollars

Energy Reduction [Percent of Baseline ]

Simulated Building Cost and Energy Performance
32

Homeowners may be willing to select a design alternative that is not LCC optimal in order to 

gain additional energy efficiency as long as the LCC will not increase relative to the MCC 

design. Point C is an example of a combination of EEMs (6 in total) that produce noticeable 

energy savings (51 %) at roughly the same LCC as the MCC design. In fact, homeowners will 

save $606 over the course of ten years.

Homeowners driven by reducing their energy use and/or environmental impacts are likely to 

consider house designs that are more expensive to build and operate, but are more energy 

efficient and sustainable in nature. Figure 5-1 shows reductions in energy consumption beyond 

51 % are achievable but come at higher LCCs to the homeowner. Points further to right of the 

graph incorporate the 10.2 kW solar PV system in its design. Additional electricity produced 

on-site is enough to offset most or all of the household energy demands. However, the additional 

cost of the system makes these designs much less economical. Point D and Point E are examples 

of building designs that reach net-zero energy performance using different EEM combinations. A 

home built to the specifications of Point D will just reach net-zero. However, a different 

combination of EEMs, used in the design at Point E, generates an additional 10 % of net energy

consumption reductions for approximately $4000 less in LCCs. 

Figure 5-2 captures, for a given level of energy efficiency, the additional investment costs 

associated with the adoption of various combinations of EEMs. Only designs located on the 

lower envelope of the solution set displayed in Figure 5-1 are shown. According to the figure, 

homeowners incorporating higher efficiency windows, 100 % efficient lighting, and/or 

improvements in infiltration in their home, assume only slightly higher investment costs.

Integrating a high efficiency HVAC system in the home has a considerable impact on investment 

costs. The first noticeable jump in initial investment costs (and resulting jump in energy 

consumption reduction) is a result of the high efficiency HVAC system being implemented and 

requires an initial investment of approximately $13 000 (Figure 5-2). The additional investment 

costs incurred ($16 280) by moving from the Point B design to Point C is directly related to the 

inclusion of both advanced framing with additional insulation for the walls and a HPWH in the 

building design. The greatest increase in first costs is related to the installation and use of the

solar PV system, which drives up investment costs by approximately $58 000 while increasing 

energy reduction by about 60 percentage points. Our findings suggest that additional investment 

costs are not necessarily a function of the number of measures adopted, but more so the types of 

EEMs adopted.
33

Figure 5-2 Cost of Additional EEM Investment for Simulated Buildings

A breakdown of the different combinations of EEMs employed by four of the simulated building 

designs is shown in Table 5-1. The four are: (1) MCC design; (2) a design that is more energy 

efficient than the MCC design with comparable LCCs (MCC*); (3) LCC optimal design; and (4) 

NZERTF design. Relative to the MCC design, a 51 % improvement in energy efficiency can be 

attained by implementing 6 of the 9 EEMs at no additional LCC to the homeowner (MCC*). The 

inclusion of the high efficiency HVAC system is largely responsible for the reductions in energy 

use. Only four measures are adopted by the LCC optimal design (high efficiency HVAC system, 

high efficiency windows, air infiltration improvements, and 100 % efficient lighting). The 

NZERTF adopts all EEMs and improves energy efficiency by 118 %, but is one of the least costeffective of all the building designs considered in the BIRDs low-rise residential database.

$0 (A) $3,173 $16,119 (B)

$32,399 (C)

$75,991 (E)

$108,049 (F)

$ 0

$10 000

$20 000

$30 000

$40 000

$50 000

$60 000

$70 000

$80 000

$90 000

$100 000

$110 000

$120 000

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 110% 120%

US Dollars

Energy Reduction [Percentage of Baseline]

Additional Investment Costs

Lighting, 

Windows,

Infiltration

Solar PV

HVAC

Adv. Framing w/ Additional

Insulation and HPWH
34

Table 5-1 Breakdown of Energy Efficiency Measures for Simulated Buildings

Building 

Design

Energy 

Reduction

Change 

in LCC Additional Energy Efficiency Measures

MCC - - -

MCC* 51 % -$606

(1) High efficiency windows; (2) High efficiency HVAC system; 

(3) improved air infiltration; (4) 100 % energy efficient lighting; 

(5) HPWH; and (6) advanced framing with additional insulation.

Optimally 

Efficient 46 % -$6304

(1) High efficiency windows; (2) High efficiency HVAC system; 

(3) improved air infiltration; and (4) 100 % energy efficient 

lighting.

NZERTF 118 % $25 579

1) High efficiency windows; (2) High efficiency HVAC system; 

(3) improved air infiltration; (4) 100 % energy efficient lighting; 

(5) solar PV system; (6) HPWH with (7) additional solar thermal 

storage; (8) shift from ceiling to roof insulation with additional

exterior rigid insulation; and (9) advanced framing with additional 

insulation in wall.

5.2 Sustainability Performance of the Optimal Efficient Design

Isolating the optimal building design was done by comparing its relative cost-effectiveness with 

those of the other 863 designs. In this section, we provide an in-depth analysis of the LCC 

optimal design by considering both its economic and environmental performance relative to the 

MCC and NZERTF designs. Along with LCC, the 3 metrics described in Section 3.2 (NS, SIR, 

and AIRR) can be used to compare design alternatives to the established baseline case (MCC 

design).

Table 5-2 lists the NS, the SIR, and AIRR for the optimal and NZERTF designs under the 

assumption of a ten-year study period. The optimal design generates a positive net savings 

($6304), a SIR greater than one (1.9), and an AIRR greater than 3 % (10.0 %). All three 

measures suggest that our LCC optimal building design is a cost-effective alternative to the MCC 

home. The NZERTF produces a NS of -$25 570, a SIR of 0.54, and an AIRR of -3.1 %, 

implying that it is in fact not a cost-effective project alternative.

Table 5-2 Economic Performance Measures

Economic Performance Measures LCC Optimal NZERTF

Net Savings $6304 -$25 579

Savings-to-Investment Ratio 1.9 0.5

Adjusted Internal Rate of Return 10.0 % -3.1 %
35

Energy efficiency improvements can be cost-effective, as the results have shown, given the 

correct combination of EEMs. However, homes built close to or in complete accordance with the 

NZERTF specifications, will not be economical after 10 years.

Figure 5-3 shows the impacts of both the LCC optimal and NZERTF designs on all twelve 

environmental impact categories relative to the MCC design. The LCC optimal design performs 

just as well or better than the MCC design in all but four impact categories (Human health - noncarcinogenics, eutrophication, ecotoxicity, and water consumption). The additional impacts on 

human health, eutrophication, ozone depletion, and water consumption are marginal, i.e. 1 % to 

2 %. Only in the case of ecotoxicity are the impacts considerably larger (a difference of 17 %), 

although much lower than the NZERTF design because the optimal design does not include the 

solar PV system. The NZERTF performs better than the optimal design in the global warming, 

acidification, smog formation, and energy use impact categories.

Figure 5-3 Relative Differences in Environmental Impacts for the Optimally Efficient, 

NZERTF, and MCC Designs

The results of Figure 5-3 do not provide an adequate measure by which to determine if the 

optimal building design is more environmentally-friendly overall compared to the MCC design.

0%

50%

100%

150%

200%

250%

300%

Ozone depletion

Global warming

Acidification

Respiratory

effects

Non

carcinogenics

Carcinogenics

Eutrophication

Smog

Ecotoxicity

Energy

Land

Water

MCC NZERTF Optimal
36

Similar to Section 4.3.2, environmental flows must be normalized and weighted to develop 

meaningful impact scores. Table 5-3 shows the weighted impact scores for all three designs. The 

LCC optimal design has lower EISs than the MCC design across all three weighting systems, 

while the NZERTF design continues to be the most environmentally-friendly alternative.

Table 5-3 Weighted Impact Scores

Equal Weighting Science Advisory Board BEES Stakeholder Panel

MCC NZERTF Optimal MCC NZERTF Optimal MCC NZERTF Optimal

Score 1.66 0.90 1.29 2.07 1.16 1.64 2.38 1.64 2.00
37

6 Discussion, Limitations, and Future Research

6.1 Discussion

This study uses whole-building energy simulations, life-cycle costing, and life-cycle assessment 

to compare the sustainability performance of the NIST NZERTF design relative to that of a 

Maryland code-compliant design constructed according to the 2012 IECC. Comparisons are 

made based on the energy, economic, and environmental performance of the two designs. This 

section discusses key findings, research limitations, and future research interests.

All key findings are categorized and described according to the three aforementioned metrics: 

energy, economic, and environmental performance. Additionally, there is a preliminary 

discussion of the variations in the relative performance for the 864 different building designs 

included in the BIRDs low-energy residential database.

6.1.1 Energy Performance

The MCC design and NZERTF design are assumed to rely solely on electricity to satisfy energy 

demands. Annual electricity consumption by the MCC design and NZERTF designs are 

23 604 kWh and 11 231 kWh, respectively. The installed solar PV system incorporated into the 

NZERTF design produces 15 474 kWh, resulting in an annual net consumption of -4243 kWh.

The NZERTF design proves to not only be net-zero, but actually a net producer of electricity 

(38 % excess production).

6.1.2 Economic Performance

Incorporating all nine EEMs into the NZERTF design increases initial investment costs by 

$108 049, an increase of 26 % relative to the MCC design. More than half of these additional 

first costs are due to the installation of the on-site solar PV system ($58 091). Use of advanced 

framing, additional roof and wall insulation, and a high efficiency HVAC system collectively 

raises first costs by another $40 385.

The added costs of the NZERTF, however, are partially offset by the 4243 kWh production 

surplus and the consequent energy cost savings each year. Annual savings for the NZERTF 

design is $3720 relative to the MCC design. The divergence in cumulative energy cost savings 

between the two designs increases as the assumed study period length increases, with the 

NZERTF design saving a homeowner a total of $95 000 (in present value terms) in energy costs 

over the course of 40 years.

Total LCCs consider initial investment costs, replacement costs, maintenance and repair costs, 

energy costs (savings in the case of the NZERTF), and the residual value of the house at the end

of the study period. Using this metric to compare the economic feasibility of the designs, the

NZERTF design will have a higher LCC than the MCC design across all assumed study period 

lengths. In other words, the MCC design will consistently be the more economical alternative of 
38

the two designs. The additional $108 049 investment required by the NZERTF design coupled 

with consistently higher maintenance, repair, and replacement costs more than offset the 

potential energy cost savings and residual value recouped by the homeowner. Differences in

LCC range from $8746 after 1 year to $42 224 after 40 years. Even though the NZERTF design 

is not more cost-effective, the LCC premium is not as significant as some may expect even 

without considering the financial incentives available for building a more energy efficient house 

or installing renewable energy systems.

6.1.3 Environmental Performance

A life-cycle assessment of the environmental performance for the MCC design and NZERTF 

design is conducted based on twelve different impact categories described in Section 4.3.1. 

Under the assumption of a 40-year study period, the NZERTF design performs better than the 

MCC design in the four impact categories primarily affected by household energy consumption 

(i.e., GWP, acidification, smog formation, and primary energy consumption). The effects of the 

MCC design on acidification levels are roughly seventeen times greater than the NZERTF 

design. Additionally, its impacts on GWP, smog formation, and the availability of primary 

energy sources are four to five times greater.

Dissimilar findings are seen in regard to the performance of the NZERTF design in categories 

mainly influenced by the actual construction of the building and the incorporated technologies 

(i.e., ozone depletion, eutrophication levels, ecotoxicity, and human health (non-carcinogenic)).

The NZERTF design performs worse than the MCC design in all four categories with ozone 

depletion and ecotoxicity being the most negatively affected at close to three times those of the 

MCC design. The impacts of the two building designs on the remaining four categories are 

minimal and fail to exploit the preference of one design over the other. 

Environmental flow impacts cannot be adequately compared across flows of different types. An 

EIS can be generated for each design by normalizing and weighting flows based on different 

weighting assumptions. These scores can then be compared to determine which design will be 

the least harmful to the local ecosystem and environment. The Equal Weighting, BEES 

Stakeholder Panel, and EPA Science Advisory Board series of weights all lead to a similar 

conclusion – construction and operation of a home built according to NZERTF specifications 

will be more environmentally-friendly than the less energy-efficient MCC design. EISs between 

the two designs differ by as much as 59 % (Equal Weighting). Although the results are robust 

across the 3 selected weighting approaches, other weight sets could lead to a different conclusion 

depending on which environmental impact categories are determined to be of most importance.

6.1.4 Optimal Building Design

All 864 different designs included in the BIRDS low-rise residential database were analyzed, 

each of which is representative of a different combination of adopted EEMs, leading to different 

levels of energy consumption reduction and LCCs. Assuming a 10-year study period length, the 
39

LCC optimal design, or the one that provides the most “bang-for-your-buck”, lowers annual 

energy use by 46 % at $6304 and $32 000 less in LCCs than the MCC design and NZERTF 

design, respectively. The LCC optimal design incorporates only four of the nine EEMs (i.e., high 

efficiency windows, high efficiency HVAC system, 100 % efficient lighting, and improved 

infiltration).

A number of other design alternatives improve energy efficiency, each with a different impact on 

total life-cycle costs. At roughly the same LCC as building and operating the MCC design for ten 

years, a more energy efficient design incorporating six of the nine EEMs can generate 51 % in 

energy savings each year. Alternative designs that achieve net-zero include the 10.2 kW on-site 

solar PV system as one of its adopted EEMs and raise overall LCC significantly despite the 

additional energy cost savings recouped through excess electricity generation. The alternative 

design that reaches net-zero energy performance at the lowest cost leads to an additional $7633 

in LCCs.

Net Savings, the Savings-to-Investment Ratio, and the Adjusted Internal Rate of Return 

measures shed some light on the relative cost-effectiveness of the LCC optimal design.

Assuming that the MCC design is the baseline case, the LCC optimal design generates a net 

savings of $6304, an SIR of 1.92, and an AIRR of 10.0 % for a 10-year study period, suggesting 

that the LCC optimal design is a cost-effective alternative. In contrast, the NZERTF design is not 

cost-effective with a negative net savings (-$25 570), a SIR less than one (0.54), and an AIRR 

less than 3 % (-3.1 %).

Based on the EIS, the optimal building design performs better than the MCC design. Differences

between the two differ by as much as 22 % when relying on the Equal Weighting system. The

environmental impact scores for the NZERTF are lower than both the MCC design and LCC 

optimal design, showcasing its ability to be the more environmentally-friendly design despite 

being more costly.

6.2 Limitations and Future Research

The analysis in this study is limited in scope and scale, and future research should consider a 

number of factors: updated simulation models, alternative assumption values, additional cost 

data and approaches, and more increments for EEM alternatives.

Since the creation of the database used in this analysis, an updated NZERTF simulation model 

has been developed and documented in Kneifel (2015). The validated model is significantly 

different than the model developed before the demonstration phase used in Kneifel (2012), 

Kneifel (2013), and Kneifel (2014). The new NZERTF model should be used as the basis to 

update the existing low-energy residential database to ensure the results are consistent with the 

measured performance of the NZERTF.
40

Although all costs associated with the building designs are included in the current analysis, there 

are several assumptions that should be analyzed in greater detail. The assumed discount rate may 

not be representative of the typical homeowner. Other discount rates should be considered to 

determine the sensitivity of the results. This study assumes an all cash purchase of the house 

even though financing of a new house is more typical of the market. Financing options should be 

considered in future analysis. The state average electricity price may vary significantly from the 

local electricity price, both in terms of magnitude as well as complexity because actual electricity 

rates often vary by time of year and/or day. Additionally, the compensation for net production 

sold back to the utility is not always equivalent to the electricity price. Local electricity pricing 

schedules, including net metering rates, should be included in future research. Financial 

incentives (i.e., federal tax credits) have been excluded from the current analysis, but could have 

significant impacts on the LCC analysis. LCC methodology clearly defines how to calculate the 

residual value of a building at the end of a study period, which assumes a linear depreciation of 

the house value. It is unclear what the market value is for reductions in energy consumption. 

Alternative approaches to valuing the resale value of a house should be considered in future 

research. Each of these factors is accounted for in Kneifel (2014), and should be used as a basis 

for testing the sensitivity of the results to these assumptions.

The underlying cost data for the current analysis is based on a publically available database that 

is becoming outdated. Alternative data sources should be identified to see how sensitive the 

results are to up-to-date data. This study uses an engineering cost approach to estimating the 

incremental costs of EEMs while contractors may deal with different costs due to unforeseen 

factors (e.g., uncertainty or non-energy related “green” products). The engineering approach 

defined in this study should be compared to the contractor estimate in Kneifel (2014). The nonenergy related features in the NZERTF design that lead to a LEED Platinum Certification are 

ignored even though these amenities may add value at the time of resale. Future research should 

consider both the additional first cost as well as the future value of these “green” amenities to 

incorporate them into the NZERTF design LCC analysis.

Although this study does consider incremental combinations of EEMs, each EEM only includes 

two or three levels. For example, the wall insulation R-values either meet the 2012 IECC 

requirements or the specifications of the NZERTF design even though the optimal R-value for 

wall insulation may be somewhere in between the two R-values. Future research should include 

incremental options between the MCC design and NZERTF design.
41

References

ASHRAE (2009). "American Society of Heating, Refrigerating and Air-Conditioning Engineers 

(ASHRAE) Fundamentals." Retrieved 2012, from www.ashrae.org.

ASHRAE (2010). "American Society of Heating, Refrigerating and Air-Conditioning Engineers 

Standard 62.2-2010." 2012, from www.ashrae.org.

ASHRAE (2012). "American Society of Heating, Refrigerating and Air-Conditioning Engineers 

90.2-2007." 2012, from www.ashrae.org.

Building Science Corporation (2009). "Net Zero Energy Residential Test Facility Construction 

Documents Set." 2012, from http://www.nist.gov/el/nzertf/.

Department of Energy (DOE) (2013). EnergyPlus Energy Simulation Software Version 8.0.0, 

Building Technologies Program.

Fuller, S. and S. Petersen (1996). "Life-Cycle Costing Manual for the Federal Energy 

Management program, 1995 Edition." NIST handbook 135.

Hendron, R. and C. Engebrecht (2010). Building America House Simulation Protocols. Energy 

Efficiency and Renewable Energy, National Renewable Energy Laboratory.

International Code Council (ICC) (2011). Residential Provisions 2012 International Energy 

Conservation Code, International Code Council: R29-R41.

Kneifel, J. (2012). Annual Whole Building Energy Simulation of the NIST Net Zero Energy 

Residential Test Facility Design. NIST TN 1767. National Institute of Standards and 

Technology. Gaithersburg, MD.

Kneifel, J. (2013). Energy Performance of the NIST Net Zero Energy Residential Test Facility 

relative to a Maryland Code-Compliant Design. NIST SP 1166. National Institute of Standards 

and Technology. Gaithersburg, MD.

Kneifel, J. (2014). Life-Cycle Cost Comparison of the NIST Net Zero Energy Residential Test 

Facility to a Maryland Code-Compliant Design. NIST SP 1172. National Institute of Standards 

and Technology. Gaithersburg, MD.
42

Lippiatt, B., et al. (2010). BEES Online: Life cycle analysis for building products, National 

Institute of Standards and Technology, US Department of Commerce.

Lippiatt, B., et al. (2013). Building Industry Reporting and Design for Sustainability (BIRDS) 

Technical Manual and User Guide, NIST TN 1814. National Institute of Standards and 

Technology. Gaithersburg, MD.

Lstiburek, J. (2010). "Building Science Insights - BSI-030: Advanced Framing." from 

http://www.buildingscience.com/documents/insights/bsi-030-advancedframing/#F04.

Rushing, A., et al. (2014). Energy Price Indices and Discount Factors for Life-Cycle Cost 

Analysis - 2014, Annual Supplement to NIST Handbook 135. National Institute of Standards and 

Technology.

Suh, S. and B. C. Lippiatt (2012). "Framework for hybrid life cycle inventory databases: a case 

study on the Building for Environmental and Economic Sustainability (BEES) database." The 

International Journal of Life Cycle Assessment 17(5): 604-612.

US Energy Information Administration (EIA) (2012). "Detailed Electricity Survey Data Files -

Annual State Data Tables." 2013, from http://www.eia.gov/electricity/data.cfm#sales.