NIST Special Publication 1166

Energy Performance of the NIST 

Net Zero Energy Residential Test 

Facility relative to a Maryland 

Code-Compliant Design

Joshua Kneifel

NIST Special Publication 1166

Energy Performance of the NIST 

Net Zero Energy Residential Test 

Facility relative to a Maryland 

Code-Compliant Design

Joshua Kneifel

Applied Economics Office

Engineering Laboratory

September 2013

U.S. Department of Commerce 

Penny Pritzker, Secretary

National Institute of Standards and Technology 

Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director 
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 1166

Natl. Inst. Stand. Technol. Spec. Pub. 1166, 30 pages (September 2013) 

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 demonstration phase of the project intends to demonstrate that the 

operation of the house does perform at “net zero,” or produces as much electricity as it 

consumes over an entire year. The NZERTF began the demonstration phase in July 2013.

The purpose of this report is to compare 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+) simulations, starting 

with the Maryland code-compliant design 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 for each energy efficiency measure on energy consumption.

Keywords

Net zero energy construction; energy efficiency; residential building; whole building 

energy simulation
iv
v
vi

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 document the assumptions used to create the whole building energy 

simulation and the resulting estimated energy performance for the demonstration phase of 

the Net Zero Energy Residential Test Facility (NZERTF) project. The intended audience 

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

residential performance.

Disclaimer

Certain trade names and company products are mentioned in the text in order to 

adequately specify the technical procedures and equipment used. In no case does such 

identification imply recommendation or endorsement by the National Institute of 

Standards and Technology, nor does it imply that the products are necessarily the best 

available for the purpose.

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.
vii
viii

Acknowledgements

The author wishes to thank everyone involved in the NZERTF project. Thank you to 

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
ix
x

Contents

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

Preface............................................................................................................................... vi

Acknowledgements ........................................................................................................ viii

Author Information ....................................................................................................... viii

List of Acronyms............................................................................................................ xiv

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

1.1 Background and Purpose...................................................................................... 1

1.2 Literature Review................................................................................................. 1

1.3 Approach .............................................................................................................. 2

2 Assumptions............................................................................................................... 3

2.1 Geometry.............................................................................................................. 3

2.2 Adjustments to NZERTF Simulation................................................................... 4

3 Simulation Specifications......................................................................................... 7

3.1 Building Envelope................................................................................................ 7

3.2 Lighting, HVAC, Domestic Hot Water, and Solar Systems................................ 8

3.3 Incremental Simulation Specifications................................................................. 9

4 Simulation Results .................................................................................................. 11

4.1 Total Electricity Consumption ........................................................................... 11

4.2 Lighting .............................................................................................................. 13

4.3 Interior Equipment ............................................................................................. 14

4.4 HVAC................................................................................................................. 14

4.5 Solar Photovoltaic Generation ........................................................................... 17

4.6 Domestic Hot Water........................................................................................... 18

4.7 Interior Environment.......................................................................................... 20

4.7.1 Setpoint Temperatures........................................................................................... 21

4.7.2 Humidity Levels.................................................................................................... 22

4.7.3 ASHRAE 55 “Thermal Comfort”........................................................................... 23

4.7.4 Infiltration and Ventilation.................................................................................... 24

5 Limitations............................................................................................................... 25

6 Discussion and Future Research............................................................................ 27

References........................................................................................................................ 29
xi
xii

List of Figures

Figure 2-1 BSC Architectural Massing Model .................................................................. 3

Figure 2-2 Google SketchUp 3-D Representation of the E+ Model.................................. 4

Figure 4-1 Total Electricity Use by Building Design ...................................................... 11

Figure 4-2 Electricity Use by Category for NZERTF and 2012 IECC - kWh and 

Percentage ......................................................................................................................... 12

Figure 4-3 Total Electricity Use by Month for NZERTF and 2012 IECC - kWh........... 13

Figure 4-4 Lighting Electricity Use by Room for NZERTF and 2012 IECC– kWh and 

Percentage ......................................................................................................................... 13

Figure 4-5 Equipment Electricity Use - kWh .................................................................. 14

Figure 4-6 HVAC Electricity Use by Building Design – kWh ....................................... 15

Figure 4-7 HVAC Cooling and Heating Energy (kWh) for 2012 IECC and NZERTF 

Designs – Monthly............................................................................................................ 16

Figure 4-8 Dehumidification Performance for the NZERTF Design – Weekly.............. 17

Figure 4-9 Total Electricity Consumption and Solar PV Production (kWh) - Monthly.. 18

Figure 4-10 Total Hot Water Consumption (Percentage) - Annual................................. 19

Figure 4-11 Water Heater Electricity Use (kWh) for 2012 IECC and NZERTF – Monthly

........................................................................................................................................... 20

Figure 4-12 1st Floor Time Setpoint Not Met While Occupied for 2012 IECC and 

NZERTF - Hours .............................................................................................................. 21

Figure 4-13 2nd Floor Time Setpoint Not Met While Occupied for 2012 IECC and 

NZERTF – Hours.............................................................................................................. 22

Figure 4-14 1st Floor Relative Humidity Levels for 2012 IECC and NZERTF Designs –

Hourly ............................................................................................................................... 23

Figure 4-15 Simple ASHRAE 55-2010 Not Comfortable for 2012 IECC and NZERTF 

Designs - Hours................................................................................................................. 24

List of Tables

Table 2-1 Changes in NZERTF Assumptions................................................................... 5

Table 3-1 Framing and Insulation...................................................................................... 7

Table 3-2 Window Specifications...................................................................................... 8

Table 3-3 Infiltration Rates................................................................................................ 8

Table 3-4 Electrical and Mechanical Systems................................................................... 9

Table 3-5 Simulation Specifications................................................................................ 10

Table 4-1 Time Relative Humidity Greater than 60 % - Hours....................................... 22

Table 4-2 Average Outdoor Air Flow during Occupied Hours....................................... 24
xiii
xiv

List of Acronyms

Acronym Definition

AEO Applied Economics Office

ARRA American Recovery and Reinvestment Act

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

BA Building America

BSC Building Science Corporation

BTP Building Technology Program

CFL compact fluorescent

COP Coefficient of Performance

DHW Domestic Hot Water

DOE Department of Energy

E+ EnergyPlus

EERE Energy Efficiency and Renewable Energy

EL Engineering Laboratory

ELA Effective Leakage Area

HRV Heat Recovery Ventilator

HSPF Heating Seasonal Performance Factor

HVAC Heating, Ventilating, and Air Conditioning

IECC International Energy Conservation Code

MAT Mean Average Temperature

MEL Miscellanous Electric Load

NIST National Institute of Standards and Technology

NREL National Renewable Energy Laboratory

NZERTF Net Zero Energy Residential Test Facility

OC On Center

PV Photovoltaic

SEER Seasonal Energy Efficiency Ratio

SHGC Solar Heat Gain Coefficient

TMY Typical Meteorological Year

VT Visible Transmittance
xv
1

1 Introduction

1.1 Background and Purpose

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 

demonstration phase of the project intends to demonstrate that the operation of the house does 

perform at “net zero,” or produces as much electricity as it consumes over an entire year. The 

NZERTF began the demonstration phase in July 2013.

The purpose of this report is to compare 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+) simulations,1 starting with the Maryland 

code-compliant design, which requires building to 2012 IECC, 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.

1.2 Literature Review

The Department of Energy’s (DOE) Energy Efficiency and Renewable Energy (EERE) Building 

Technologies Program (BTP) is responsible for funding research at the national laboratories for 

the Building America (BA) program. The BA program has been at the forefront of research of 

low-energy single-family housing design through a variety of outlets, including the BA Best 

Practices Series, case studies for new construction and retrofits, and technical reports and fact 

sheets.

2 Hendron and Engebrecht (2010) defines the BA house protocols to be implemented 

when simulating house energy performance.

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).3 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, 

 1 Department of Energy (2013) 2 Building America (2013)

3 Building Science Corporation (2009)
2

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

the ASHRAE Fundamentals Handbook.

1.3 Approach

This report uses the E+ simulation defined in Kneifel (2012) to estimate the energy performance 

of the NZERTF. The energy efficiency requirements defined in 2012 International Energy 

Conservation Code (IECC) for residential buildings are used to determine the Maryland 

code-compliant design. Each of the energy efficiency measures is removed from the NZERTF 

model, one-by-one, to reach the minimum requirements for 2012 IECC in Gaithersburg, 

Maryland (Climate Zone 4).
3

2 Assumptions

The E+ software was chosen to design the whole building energy simulations. The simulations 

are run using a one-minute timestep, and the Washington-Dulles Airport Typical Meteorological 

Year (TMY) Version 3 weather file. The general assumptions required by E+ are described in 

detail in Kneifel (2012).

2.1 Geometry

The dimensions specified in BSC (2009) and shown in the architectural massing model in Figure 

2-1 are used along with Google SketchUp and National Renewable Energy Laboratory’s (NREL)

Legacy Open Studio plug-in to construct the building geometry of the NZERTF. Total 

conditioned floor area of the E+ model is 284.6 m2 (3063 ft2

). Actual conditioned floor area of 

the NZERTF is 251.7 m

2 (2709 ft2

). There are two reasons the conditioned floor area of the 

simulation model is 32.9 m2 (354 ft2) greater than the actual house design. First, the E+ model 

does not account for the open foyer/stairway, which is approximately 19.0 m

2 (204 ft2

). Second, 

the gable walls (west wall and east wall) of the 2nd floor have built in storage under the gable, 

which decreases the conditioned floor area by approximately 14.3 m2 (154 ft2

). These two 

aspects of model account for approximately 33.3 m

2 (358 ft2

), which decreases the conditioned 

floor area to 251.3 m

2 (2705 ft2

) or a difference of only 0.4 m2 (4 ft2). Even though these two 

aspects of the house are not considered finished floor area, they their volume of space will be 

conditioned.

Figure 2-1 BSC Architectural Massing Model

Figure 2-2 shows the Google SketchUp three-dimensional geometry of the E+ model for the 

NZERTF. The model includes seven separate zones with two conditioned (1st floor and 2nd floor) 

and five unconditioned zones (open web joist space between the 1st and 2nd floors, basement, 

main attic, living room attic, and patio). The front porch and detached garage with the covered 
4

walkway are all treated as shading surfaces, which block sunlight but do not impact the thermal 

performance of the building envelope.

Figure 2-2 Google SketchUp 3-D Representation of the E+ Model

2.2 Adjustments to NZERTF Simulation

Since the publication of Kneifel (2012), there have been some adjustments to the NZERTF 

simulation model to better match the planned operation in the NZERTF during the demonstration 

phase. Each of the changes, most of which will have minor to no impact on energy performance, 

is listed in Table 2-1. The most significant changes are the lighting wattage adjustments, which 

will increase the lighting-based energy consumption.
5

Table 2-1 Changes in NZERTF Assumptions

Category Subcategory Day of Week Detail of Change

Occupancy Child A in Bedroom Saturday Starts @ 19:30 instead of 20:30

Activity Levels NA Constant 65 W Sensible, 31 W Latent

Domestic Hot Water Kitchen Sink Monday Added 1 minute @ 6:05

Master Bedroom Sink Saturday Added 1 minute @ 8:50

Dishwasher Friday Changed 20:28&21:28 from 20:15&21:15

Clothes Washing Machine Wednesday Added 1 Load @ 18:30

Electrical Equipment Range Hood NA Changed Wattage to 75 W from 330 W

Iron NA Added to Master Bedroom

Lighting Kitchen NA Changed to 118 W from 107 W

Dining Room NA Changed to 65 W from 13 W

Living Room NA Changed to 118 W from 92 W

Office NA Changed to 41 W from 28 W

Master Bedroom NA Changed to 41 W from 13 W

Bedroom 2 NA Changed to 41 W from 28 W

Bedroom 3 NA Changed to 41 W from 28 W

Master Bathroom NA Changed to 81 W from 72 W

Bathroom 2 NA Changed to 63 W from 24 W

1st Floor Bathroom NA Changed to 44 W from 46 W

Roof Assembly Insulation NA Changed to 3.81 cm (1.5 in) isocyanurate

from 2.54 cm (1.0 in)

Thermostat Cooling Setpoint NA Changed to constant 75°F

Heating Setpoint NA Changed to constant 70°F

Availability NA Changed to always available
6
7

3 Simulation Specifications

There are eleven E+ model designs used in this report, each of which represents different 

combinations of energy efficiency measures between 2012 IECC and the NZERTF design. Each 

of the energy efficiency measures is described in detail in this chapter.

3.1 Building Envelope

The NZERTF design adds energy efficiency measures to each aspect of the building envelope

listed in Table 3-1, Table 3-2, and Table 3-3: framing, wall, roof, fenestration, and infiltration. 

The NZERTF is constructed using “advanced framing,” which uses 2”x6” 24” on center (OC)

framing instead of the common practice of 2”x4” 16” OC framing. The thicker framing allows 

for greater levels of insulation within the wall cavity while decreasing the amount of wood 

required for framing the house, making it easier to increase the thermal performance of the 

building envelope.

Table 3-1 Framing and Insulation

Insulation NZERTF 2012 IECC – Zone 4

Framing 2”x6” 24” OC 2”x4” 16” OC

Exterior Wall R-20+24 R-20 or R-13+5

Basement Wall R-22 R-10

Roof R-45+30 R-49 or R-45+4

Note 1: Interior + Exterior R-Value

Note 2: Basement Floor Insulation is the same for both 

The 2012 IECC wall insulation requirement for Climate Zone 4 is R-20 in the wall cavity or 

R-13 in the wall cavity and R-5 of rigid insulation. The NZERTF design uses advanced framing 

and adds an additional R-24 of rigid insulation to the R-20 in the wall cavity. The basement wall 

requirement for 2012 IECC is R-10 of rigid insulation while the NZERTF adds R-12 to the 

interior of the basement wall. The 2012 IECC design with typical framing uses blown-in 

insulation on the attic floor to reach R-49 of continuous insulation. The 2012 IECC design with 

advanced framing uses R-45 blown-in insulation in the rafters with R-4 rigid insulation on the 

exterior of the roof. The NZERTF roof construction uses the R-45 insulation in the rafters and 

adds rigid insulation to the exterior roof to reach an additional R-30.

The fenestration surface construction materials for windows are defined based on three simple 

parameters: U-factor, Solar Heat Gain Coefficient (SHGC), and Visible Transmittance (VT). 

This approach allows the rated window performance to be modeled while simplifying window 

“materials” and “constructions” in the simulation. The window parameters can be seen in Table 
8

3-2, and are based on the minimum requirements specified in 2012 IECC and the BSC window 

specifications.

4

Table 3-2 Window Specifications

Field Units NZERTF 2012 IECC – Zone 4

U-Factor W/m2-K 1.1356 1.9875

Solar Heat Gain Coefficient 0.25 0.35

Visible Transmittance 0.40 0.40

The maximum envelope air leakage rate in the 2012 IECC allowed for residential structures in 

Climate Zone 4 is 3 air changes per hour at 50 Pa. The air tightness of the NZERTF was 

measured at 0.61 air changes per hour at 50 Pa using a blower door test.5 These results, shown in 

Table 3-3, are converted into effective leakage area (ELA) for the simulations and then split 

between the 1st floor and 2nd floor based on fraction of volume.

6

Table 3-3 Infiltration Rates

Air Leakage NZERTF 2012 IECC – Zone 4

Air Changes at 50 Pa 0.61 3.00

ELA – 1st Floor (cm2) 98.8 403.6

ELA – 2nd Floor (cm2) 90.2 368.1

3.2 Lighting, HVAC, Domestic Hot Water, and Solar Systems

Table 3-4 shows that the NZERTF design implements energy efficiency measures in the lighting, 

heating, ventilation, and air conditioning (HVAC), and domestic hot water (DHW) systems, and 

installs a solar thermal hot water system and solar photovoltaic system. The 2012 IECC requires 

75 % of all light fixtures to be high efficiency. The NZERTF replaces the remaining 25 % of 

non-high efficiency light bulbs (incandescent) with high efficiency compact fluorescent (CFL) 

bulbs.

 4 These parameters assume no difference in performance of the windows regardless of the window type (awning or 

double hung).

5 Everyday Green (2012) 6 The ELA should have been split based on the fraction of surface area for each floor, not volume. However, this 

should not make a significant difference in the results.
9

Table 3-4 Electrical and Mechanical Systems

Building System Component NZERTF 2012 IECC-based System

Lighting Light Bulbs 100 % Efficient Lighting 75 % Efficient Lighting

HVAC Air Conditioning Heat Pump (SEER 15.8) Heat Pump (SEER 13.0)

Heating Heat Pump (HSPF 9.05) Heat Pump (HSPF 7.7)

Ventilation/Outdoor Air Heat Recovery Ventilator Min. Outdoor Air (0.04 m3

/s)

DHW Water Heater Tank Heat Pump Water Heater Electric Water Heater

Solar Solar Thermal System 2 Panel with 80 gallon tank None

Solar PV System 10.2 kW None

 * SEER = Seasonal Energy Efficiency Ratio

** HSPF = Heating Seasonal Performance Factor

The 2012 IECC design assumes a federal minimum efficiency heat pump with continuous

outdoor air of 0.04 m3

/s.

7 The NZERTF replaces the minimum efficiency heat pump with a 

high-efficiency heat pump, and adds a dedicated outdoor air system with a heat recovery 

ventilator (HRV). The NZERTF replaces the electric water heater with a thermal efficiency of 

0.98 in the 2012 IECC design with a heat pump water heater with a coefficient of performance 

(COP) of 2.6 and electric back with thermal efficiency of 0.98. Additionally, the NZERTF 

installs two solar thermal panels and 80 gallon storage tank to preheat water entering the heat 

pump water heater. The NZERTF installs the largest possible solar photovoltaic (PV) system 

(10.2 kW) based on the surface areas of the roof.

3.3 Incremental Simulation Specifications

Table 3-5 shows the Maryland code-compliant design simulation (baseline) and the ten 

incremental simulations considered in this study, with each changing one component in the 

baseline simulation to meet the NZERTF component design. Simulation 1 replaces typical 2”x4” 

16” OC framing with “advanced framing,” which uses 2”x6” 24” OC framing to decrease 

fraction of the wall and roof accounted for by wood framing. Simulation 2 through Simulation 4 

replace the wall construction, roof construction, and windows with the NZERTF designs, 

respectively. Simulation 5 decreases the infiltration rate from 3 air changes per hour at 50 Pa in 

the Maryland code-compliant design to 0.61 air changes per hour at 50 Pa, which is the rate 

measured by the NZERTF blower door test. Simulation 6 increases the percentage of light 

fixtures with high efficiency light bulbs from 75 % to 100 %. Simulation 7 replaces the typical

heat pump with a high efficiency heat pump and separate HRV system. Simulation 8 replaces the 

 7 Low air leakage rates without including mechanical ventilation of outdoor air into the house could lead to concerns 

over indoor air quality. Therefore, 2012 IECC requires that any house with an air leakage rate of less than 3 air 

changes per hour must include mechanical ventilation that meets either the International Residential Code or 

International Mechanical Code. The Maryland state energy code for residential buildings requires a minimum 

ventilation rate that is equivalent to those defined in the ASHRAE 62.2-2010. Since the HRV system is designed to 

meet ASHRAE 62.2 requirements, the mechanical ventilation rate for the simulations without the HRV system is 

assumed to be equivalent to those rates. The heat pump fan is used to supply a constant outdoor air flow, and the 

associated electricity consumption is captured in the “HVAC Fan” category.
10

electric water heater with a heat pump water heater. Simulation 9 adds a solar thermal system to 

the DHW system. Simulation NZERTF adds the 10.2 kW solar PV system to the roof of the 

house to complete the NZERTF design.

Table 3-5 Simulation Specifications 

Simulation Design Detail

Sim. 2012 IECC 2012 IECC – Climate Zone 4 2x4 16 OC; R-49 Ceiling - Floor; R-13+5 Wall

Sim. 1 2012 IECC – Climate Zone 4 2x6 24 OC; R-45+4 Ceiling - Roof; R-20 Wall

Sim. 2 Sim. 1 + NZERTF Wall R-20+24 Ext. Wall; R-10+12 Basement Wall

Sim. 3 Sim. 2 + NZERTF Roof R-45+30 Roof

Sim. 4 Sim. 3 + NZERTF Windows U = 0.25, SHGC = 0.25

Sim. 5 Sim. 4 + NZERTF Infiltration Air changes per hour at 50 Pa = 0.6

Sim. 6 Sim. 5 + NZERTF Lighting 100 % high-efficiency; 206W LED, 387W CFL, 60W LF

Sim. 7 Sim. 6 + NZERTF HVAC Air-to-Air Heat Pump with separate HRV System

Sim. 8 Sim. 7 + NZERTF Water Heater Heat Pump Water Heater

Sim. 9 Sim. 8 + Solar Thermal 2 Solar Panels, liters 303 liter (80 gallon) storage tank

Sim. NZERTF Sim. 9 + Solar PV 10.2 kW system
11

4 Simulation Results

The energy efficiency measures implemented in the NZERTF design require an incremental 

analysis approach to determine the amount of total energy consumption reductions that result 

from the adoption of each measure. This chapter will analyze the results across building 

components and building performance metrics.

4.1 Total Electricity Consumption

Figure 4-1 shows that constructing to the NZERTF specification leads to a 16 242 kWh (60 %) 

reduction in annual electricity use relative to constructing to meet residential 2012 IECC 

requirements for Climate Zone 4 (10 742 kWh versus 26 983 kWh), with net electricity 

consumption (solar photovoltaic electricity production minus electricity consumption) 

of -4731 kWh.

Figure 4-1 Total Electricity Use by Building Design

The most significant reductions are a result of decreasing infiltration in Simulation 5 (4292 kWh 

or 15.9 %), implementing advanced framing in Simulation 1 (2533 kWh or 9.4 %), installing a 

high efficiency heat pump with dehumidification and a separate HRV system in Simulation 7 

(2214 kWh or 8.2 %), increasing the R-value of the wall construction in Simulation 2 (1953 kWh

or 7.2 %), and replacing an electric water heater with a heat pump water heater in Simulation 8 

(1633 kWh or 6.1 %). However, the order in which the energy efficiency measures are adopted 

in this study impact the incremental reductions realized in each simulation. For example, 

installing the more efficient HVAC system in Simulation 1 instead of Simulation 7 would lead to

-5000

0

5000

10000

15000

20000

25000

kWh

Net Elect.

Consumption

Inverters

Heat Recovery

Fans

Water Systems

Interior Lighting

Interior

Equipment

Cooling

Heating
12

a larger reduction in electricity use associated with the more efficient HVAC system while 

decreasing the reduction associated with all the energy efficiency measures adopted in 

Simulation 1 through Simulation 6.

Figure 4-2 shows the breakdown of electricity use by category for the 2012 IECC and NZERTF 

designs, both in total kWh and percentage. By far, heating realizes the greatest reduction in 

electricity use (12 169 kWh to 1675 kWh or 86 %) while the amount of cooling electricity 

decreases less significantly (2083 kWh to 1747 kWh or 16 %) because it includes the 

consumption for the dehumidifiers. Water systems require 82 % less electricity consumption 

(3456 kWh versus 533 kWh) to meet the same domestic hot water loads due to the combination 

of the more efficient water heater and solar thermal system. Electrical equipment is unchanged at 

5025 kWh because the 2012 IECC design includes the same appliances as the NZERTF design.

Figure 4-2 Electricity Use by Category for NZERTF and 2012 IECC - kWh and 

Percentage

The fraction of electricity use profile differs significantly between the 2012 IECC and NZERTF 

designs. The efficiency of the NZERTF results in HVAC operation (the combination of heating, 

cooling, fans, and heat recovery) accounting for 41 % of total electricity use, which is 23

percentage points lower than the 2012 IECC design (64 %). Electrical equipment accounts for a 

significantly greater amount of total electricity use for the NZERTF than the 2012 IECC design 

because the total electricity use decreases while the electricity use for equipment remains 

constant at 5025 kWh.

Figure 4-3 shows the monthly electricity use profile for the NZERTF and 2012 IECC designs.

The reductions in electricity use are most prevalent in the winter months due to the reduction in 

heating loads.

12169

45%

2083

8%

1219

4%

5025

19%

3028

11%

3456

13%

2012 IECC

(26 983 kWh)

1675

16%

1747

16%

653

6%

5025

47%

556

5%

78

1%

472

4% 533

5%

3

0%

NZERTF

(10 742 kWh)

Heating

Cooling

Interior

Lighting

Interior

Equipment

Fans

Pumps

Heat

Recovery

Water

Systems

Generators
13

Figure 4-3 Total Electricity Use by Month for NZERTF and 2012 IECC - kWh

4.2 Lighting

Figure 4-4 shows how lighting varies between the NZERTF and 2012 IECC designs. Lighting is 

reduced for all rooms except for the kitchen because the 25 % of non-high efficiency fixtures 

(incandescent light bulbs) in the 2012 IECC design are assumed to be replaced with CFL light 

bulbs. Since the kitchen uses 100 % LED lighting, the room does not have any light bulbs that 

can be replaced. Replacing 25 % of light fixtures decreases total lighting electricity consumption 

by 567 kWh, or 46 %.

Figure 4-4 Lighting Electricity Use by Room for NZERTF and 2012 IECC– kWh and 

Percentage

0

1000

2000

3000

4000

Electricity [kWh]

NZERTF

2012 IECC

231

19%

256

21%

6

44 0%

4% 53

4%

61

5%

119

10%

64

5%

386

32%

2012 IECC

(1293 kWh)

50

8%

256

39%

3

0%

25

4%

33

5%

28

4%

56

9%

31

5%

172

26%

NZERTF

(653 kWh)

Dining Room

Kitchen

1st Floor Bath

2nd Floor Bath

Master Bath

Master

Bedroom

2nd Bedroom

3rd Bedroom

Living Room
14

4.3 Interior Equipment

Electricity consumption for interior equipment is constant across all eleven simulations because 

the 2012 IECC design is assumed to use the same equipment and maintain the same occupant 

activity as installed in the NZERTF design. The clothes dryer consumes the most amount of 

electricity (1361 kWh) followed by the miscellaneous electrical loads (MELs) for the office 

(703 kWh), living room MELs (567 kWh), and master bedroom MELs (561 kWh).

Figure 4-5 Equipment Electricity Use - kWh

4.4 HVAC

The electricity consumption required to meet the HVAC load for each building design is shown 

in Figure 4-6, and is split into 5 subcategories: heating, cooling, HVAC fans, HRV operation, 

and HRV fans. The heating electricity use consistently decreases from the 2012 IECC design 

through Simulation 5, which is a result of increases in the thermal performance of the building 

envelope. There is a slight increase from Simulation 5 to Simulation 6 because Simulation 6 

replaces the remaining low-efficiency light fixtures with high-efficiency light fixtures, which 

decreases internal heat gains and increases the required heating load. There is an additional 

decrease in heating loads from Simulation 6 to Simulation 7 when the high-efficiency HVAC 

system (air-to-air heat pump with dehumidification and HRV system) replaces the minimum 

efficiency heat pump. The decrease in electricity consumption in heating is partially offset 

(10 %) by an increase in electricity consumption for cooling due to the increase in outdoor air 

through mechanical ventilation from constant operation of the HRV system. The HVAC 

1361

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44

561

703

61

567

192

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15

electricity consumption is relatively constant for Simulation 7 through the NZERTF design 

because the remaining energy efficiency measures target the DHW system (water heater 

efficiency and solar thermal system) and electricity production (solar PV system). There is a 

slight increase from Simulation 7 to Simulation 8 because the more efficient heat pump water 

heater increases the heating load in the house.

Figure 4-6 HVAC Electricity Use by Building Design – kWh

The monthly cooling and heating loads for the NZERTF and 2012 IECC simulations can be seen 

in Figure 4-7. Similar to the results in electricity consumption, the heating load is significantly 

larger than the cooling load for the 2012 IECC design while the cooling load is greater than the 

heating load for the NZERTF design. However, the difference is not as significant for the heating 

and cooling loads as for heating and cooling electricity use. Cooling loads account for 30 % and 

64 % of total thermal loads for the 2012 IECC and NZERTF designs, respectively. Meanwhile 

cooling electricity consumption accounts for 15 % and 51 % of combined heating and cooling 

electricity consumption, respectively. 

The reason that cooling thermal loads account for such a greater percentage than cooling 

electricity consumption is that the equipment is more efficient when operating in cooling mode 

than in heating mode. The 2012 IECC design heat pump has a nominal COP of 3.28 for cooling 

and COP of 2.26 for heating. Similarly, the heat pump in the NZERTF design has a COP of 3.72

for cooling and COP of 2.65 for heating.

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HRV Fans

HVAC Fans

Heat Recovery

Cooling

Heating
16

Figure 4-7 HVAC Cooling and Heating Energy (kWh) for 2012 IECC and NZERTF 

Designs – Monthly

Figure 4-8 shows the weekly dehumidification performance of the two zone dehumidifiers. As 

would be expected, most dehumidification occurs during the summer months, but there is some 

dehumidification that occurs in the spring and fall as well. The efficiency of the dehumidifiers is 

relatively constant between 0.90 L and 1.00 L of water removed from the air per kilowatt of 

electricity consumed, which is slightly lower than the assumed rated performance of the zone 

dehumidifiers.

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[kWh]

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Cooling [kWh]

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[kWh]

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17

Figure 4-8 Dehumidification Performance for the NZERTF Design – Weekly

4.5 Solar Photovoltaic Generation

The E+ model estimates annual solar PV production of 15 471 kWh, which is 144 % of the 

house’s total electricity use (10 740 kWh). Given the assumptions used in the current simulation

the NZERTF not only reaches but surpasses well beyond its net zero energy goal, as shown in 

Figure 4-1.

8 Alternative solar PV estimates, both through different parameter values and an 

alternative PV software model, are used to test the sensitivity of the electricity production of the 

NZERTF. Two alternative E+ solar PV modeling approaches are used to estimate electricity 

production. Assuming the “simple” solar PV modeling approach with a 19.6 % PV efficiency 

results in an estimate of 14 650 kWh (136 %). A more conservative 18.0 % PV efficiency further 

reduces electricity production to 13 500 kWh (126 %). Additionally, a simplified PV model 

(NREL’s PVWATTS) was used by a NIST solar PV expert, and based on the 18.4 degree tilt of 

the NZERTF solar PV panels, the predicted production is 12 177 kWh (113 %). The actual 

efficiency performance of the solar PV panels will make a significant difference in the energy 

performance of the NZERTF. However, under all four approaches the NZERTF reaches its 

net-zero energy goal.

Figure 4-9 shows the solar PV production for the minimum and maximum potential outcomes 

and consumption by the NZERTF by month. As would be expected, the summer months are 

when the most energy is produced while the winter months are when production lags. However, 

even with the varying monthly production, eight of the twelve months realize significantly 

greater production of electricity than is consumed by the NZERTF for the minimum performance 

estimate.

 8 This is driven by the assumed optimal solar PV performance, which can vary significantly depending on its 

operating conditions.

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L/kWh

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18

Figure 4-9 Total Electricity Consumption and Solar PV Production (kWh) - Monthly

Assuming the most conservative estimate (PVWATTS), the home owners are able to sell a net 

1437 kWh back to the electric utility ($225 annually at 15.69¢/kWh)9

. Putting that in context, 3

of the 32 solar photovoltaic panels could be removed from the roof and the NZERTF would still 

consume less electricity than it produced on an annual basis. An alternative use of the excess 

electricity would be to power an electric car for 4224 miles.10 The 2012 IECC building design 

consumes 26 983 kWh, which leads to an annual electricity bill of $4234. Given the negative 

electricity costs of $225 for the NZERTF design, the electricity cost savings is $4459 annually.

The annual electricity cost savings of $4459 for the NZERTF design increases the value of a 

home the owner is able to afford. The average monthly electricity cost savings is $372. 

Assuming an average 30-year fixed mortgage rate of 4.37 % and a back-end debt-to-income ratio 

of 38 %,11 treating the $372/month of electricity cost savings as additional income increases the 

owner’s ability to pay by $141/month or $28 295 in additional initial mortgage loan value.

12 This 

result gives credence to the assertion that mortgage companies could consider energy 

performance of a house when determining the maximum mortgage value a home owner can 

afford.

4.6 Domestic Hot Water

The domestic hot water (DHW) system uses the same volume of water across all eleven 

simulations. Figure 4-10 shows total water use is 106 970 L (28 258 gal.), or 293.1 L (77.4 gal.) 

 9 The average price per kWh of electricity is based on data from the EIA (2011) for PEPCO during 2010. 10 Assumes an electric car gets 2.94 miles per kWh (EPA, 2012). 11 U.S. Weekly Average Primary Mortgage Market Survey for July 18, 2013 (Freddie Mac, 2013). 12 Bankrate’s Mortgage Calculator used to reverse engineer the additional mortgage loan value.

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19

per day: 53 % of which is used in baths and showers, 32 % in sinks, 14 % by the clothes washer, 

and 1 % for the dishwasher. Showers, baths, and sinks use a mix of hot and cold water while the 

washers use only hot water. The hot water use by the clothes washer would decrease if the 

occupants were to use cold water for a portion of their loads of laundry.

Figure 4-10 Total Hot Water Consumption (Percentage) - Annual

The change in electricity consumption to meet the DHW demand is driven by the efficiency of 

the equipment installed. The 2012 IECC design uses an electric water heater with a thermal 

efficiency of 0.98, which is the least efficient type of water heater, while the NZERTF design 

implements a heat pump water heater with a COP of 2.6, a back-up electric resistance heater with 

a thermal efficiency of 0.98, and a solar thermal system. Simulation 8 represents the incremental 

addition of the heat pump water heater with electric resistance back-up without preheating with

the solar thermal system. The 2012 IECC design consumes 3457 kWh to meet the DHW 

demand. Replacement of the electric water heater with the heat pump water heater in 

Simulation 8 reduces electricity use for heating water to 1623 kWh. Adding the solar thermal 

system further reduces electricity use to 613 kWh.

Figure 4-11 shows that electricity use to meet DHW demand varies by month for each of the 

three designs discussed above. For all three designs, electricity use trends down as the mean 

average temperature (MAT) increases and then begins to rise as the MAT decreases. The 

installation of the heat pump leads to a significant reduction in electricity consumption because 

the more efficient heat pump meets most of the heating load. However, electric resistance still 

accounts for 30 % of electricity use to meet the DHW demand. The incorporation of the solar 

thermal system in the NZERTF design to preheat water entering the heat pump reduces 

electricity use for both the heat pump and electric resistance heating, with resistance heating only 

used minimally during seven months of the year and accounting for 9 % of electricity used in 

domestic water heating. The solar thermal system requires 79 kWh of electricity to operate, but 

reduces the electricity consumption of the water heater by 1010 kWh.

4.2

1% 39.9

14%

95.3

32%

142.4

49%

11.3

4% Dishwasher - 1.57 gal/cycle (1 hr)

Clothes Washer - 12.25 gal/cycle (45 min)

Sinks - 1.5 gpm

Showers - 1.75 gpm

Baths - 1.75 gpm
20

Figure 4-11 Water Heater Electricity Use (kWh) for 2012 IECC and NZERTF – Monthly

4.7 Interior Environment

Analysis up to this point has focused on the energy performance. However, another important 

aspect of building performance is the indoor environmental conditions. Given the unique 

characteristics of the NZERTF (high insulation, low infiltration, and mechanical ventilation 

control), there are concerns that the comfort levels in the house will not meet target levels. There 

are a number of ways in which to compare indoor environment performance using the 

temperature and humidity levels inside the house.

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[kWh]

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[kWh]

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21

4.7.1 Setpoint Temperatures

The simplest approach to determine if the house performs to the desired indoor conditions is to 

determine if the house is maintained at the setpoint temperatures. Figure 4-12 shows that the 

number of hours the setpoint temperatures are not met in the control zone (1st floor) are

significantly lower for the 2012 IECC design (0.03 hours) relative to the NZERTF design 

(8.69 hours), with cooling primarily being the setpoint that cannot be maintained. However, for 

both designs the total number of hours throughout the year that the setpoint is not met is quite 

low (0.001 % of hours in a year or lower) because the 1st

floor is the control zone for the house.

Figure 4-12 1st Floor Time Setpoint Not Met While Occupied for 2012 IECC and NZERTF 

- Hours

Figure 4-13 shows the amount of time the setpoint temperatures are not met on the 2nd floor for 

the 2012 IECC and NZERTF designs, which are significantly higher than for the 1st floor

because the 2nd floor is not the control zone. Unlike the 1st floor, the 2nd floor of the 2012 IECC 

design performance is worse than the NZERTF design, primarily because it does not meet its 

setpoint temperature for heating. The heating setpoint is not met for 2570 hours and the cooling 

setpoint not met for 623 hours. The hours that the setpoint is not met for the 2nd floor of the 

NZERTF design are primarily for the cooling setpoint (947 hours) versus heating (227 hours), 

with most hours occurring from May through September. The NZERTF design realizes 37 % 

fewer hours for which the setpoint temperature is not met on the 2nd floor. Note that in the 

simulations, there is no air flow transferred between the 1st floor and 2nd floor because they are 

simulated as separate zones. As a result, the warmer air created by occupants and equipment on 

the 1st floor does not rise to the 2nd floor as would occur in an actual house.

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22

Figure 4-13 2nd Floor Time Setpoint Not Met While Occupied for 2012 IECC and 

NZERTF – Hours

4.7.2 Humidity Levels

Another aspect of thermal comfort is the humidity level. Table 4-1 shows that the NZERTF 

design leads to excellent humidity level control, with the 1st floor and 2nd floor realizing relative 

humidity levels greater than 60 % for a total of three hours and 1 hour for the year, respectively. 

The relative humidity levels fluctuate more significantly for the 2012 IECC design, with the 1st 

floor and 2nd floor realizing humidity levels greater than 60 % for 15 % (1318 hours) and 16 % 

(1415 hours) of all hours during the year, respectively.

Table 4-1 Time Relative Humidity Greater than 60 % - Hours

Hours 1st Floor 2nd Floor

2012 IECC 1318 1415

NZERTF 3 1

Figure 4-14 shows that for some hours during the year, the humidity levels exceed 80 % on the 1st

floor for the 2012 IECC design. Given these results, it may be appropriate to assume that the heat 

pump will be operated in cooling mode when the humidity levels are significantly over 60 %, 

which would significantly increase the electricity use for cooling. The NZERTF simulation 

maintains the humidity level because it includes a dehumidifier on each floor, which is used to 

represent dehumidification mode capability of the heat pump because it cannot currently be 

simulated in E+. Note that in the actual NZERTF, it is unlikely that the heat pump could perform 

as precisely as the dehumidifiers and would lead to additional hours for which the humidity level 

exceeds the 60 % target level.

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23

Figure 4-14 1st Floor Relative Humidity Levels for 2012 IECC and NZERTF Designs –

Hourly

4.7.3 ASHRAE 55 “Thermal Comfort”

The goal of temperature and humidity control is to maintain the indoor environment at 

acceptable thermal conditions for an inhabitant. ASHRAE Standard 55-2010 defines an approach 

to estimate a range of conditions (temperature and relative humidity) under which an occupant is 

“comfortable.” Figure 4-15 shows the number of hours for which the conditions are considered 

“not comfortable” according to ASHRAE 55 by month for the 2012 IECC and NZERTF design.13

For both designs, the winter months lead to more “uncomfortable” conditions, and the 2nd floor 

realizes a greater number of hours in “uncomfortable” conditions. The NZERTF design has 

significantly fewer hours for which the thermal comfort is not maintained relative to the 2012 

IECC design.

 13 These results are outputs from the E+ model for the simple approach of calculating acceptable indoor 

environment levels, which are based on combinations of operative temperature and humidity ratio. The calculations 

allow maximum flexibility in the insulation value of clothes worn by the occupant, which estimates the insulation 

value of summer clothes and winter clothes to be 0.078 m2

K/W (0.5 Clo) and 0.155 m2K/W (1.0 Clo), respectively.

For additional details, see the E+ documentation and ASHRAE 55-2010.

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 01/01 01/15 01/30 02/13 02/28 03/15 03/29 04/13 04/28 05/12 05/27 06/10 06/25 07/10 07/24 08/08 08/23 09/06 09/21 10/05 10/20 11/04 11/18 12/03 12/18

Relative Humidity (%)

Date

2012 IECC

NZERTF
24

Figure 4-15 Simple ASHRAE 55-2010 Not Comfortable for 2012 IECC and NZERTF 

Designs - Hours

4.7.4 Infiltration and Ventilation

Table 4-2 shows the average air changes per hour (h-1) from infiltration14 and mechanical 

ventilation calculated by E+ throughout the year for the 2012 IECC design, Simulation 5, and 

NZERTF design. The average outdoor air change rate is higher for the 2012 IECC design 

(around 0.4 h-1) relative to Simulation 5 and the NZERTF design (around 0.2 h-1) because the 

mechanical ventilation rate target is the same for all three designs, but the infiltration rate is 

reduced in Simulation 5 to meet the NZERTF air leakage rate.

Table 4-2 Average Outdoor Air Flow during Occupied Hours

Air Change Rate (h-1) Ventilation Infiltration Total

2012 IECC 1st Floor 0.13 0.22 0.35

2nd Floor 0.15 0.23 0.37

Simulation 5 1st Floor 0.13 0.05 0.19

2nd Floor 0.14 0.06 0.20

NZERTF 1st Floor 0.13 0.05 0.18

2nd Floor 0.14 0.06 0.20

 14 Air change rates for infiltration are calculated based on the effective leakage area infiltration formula based on 

Sherman and Grimsrud (1980), which is a function of outdoor air temperature, indoor air temperature, ELA at 4 Pa, 

stack coefficient, and wind coefficient.

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25

5 Limitations

Whole building energy simulation software is limited in its abilities to estimate real world energy 

performance because it is difficult to control for all potential variables that can impact the 

thermal conditions in a building. There are a number of reasons that the estimated performance 

from the E+ model may vary from actual performance of a building design. A few are discussed 

below.

Discrepancies between the E+ model and the actual NZERTF design are the most prominent 

reasons for variations in energy performance. First, the E+ model does not include the 

stairway/foyer opening, which restricts thermal transfer between the floors and increases the 

conditioned floor area relative to the actual NZERTF design. The greater conditioned floor area 

will impact the thermal load needed to meet the thermostat set points as well as the minimum 

ventilation rates required for the HVAC equipment.

Second, electrical equipment consumption may be overestimated or underestimated dependent 

on whether equipment operates as expected. For example, it is unclear at what wattage level the 

clothes dryer operates throughout its cycle, or how long the cycle may last due to the “smart” 

technology allowing the dryer to stop once it senses the clothes are dry.

Third, air infiltration may not be accurately estimated in the E+ model. The infiltration rate due 

to building envelope leakage in the E+ model is simplified to an ELA based on the most recent 

air leakage test. The model does not account for any additional infiltration induced by occupant 

activities, which will underestimate the total infiltration that will occur during the demonstration 

phase of the NZERTF project.

Fourth, building components may not perform at the manufacturer specifications. Solar PV 

production may vary drastically by weather and operating conditions. The simulation assumes 

optimal electricity production based on the TMY3 weather file. A single year may vary enough 

from the historical average to either increase or decrease solar PV performance. The solar PV 

system may be less productive than the specifications claim for some or all weather conditions, 

which would result in less electricity production than is estimated by the E+ model. There is 

minimal information available for the energy performance of the HVAC and DHW systems, 

which could perform significantly better or worse than the simulation and drastically impact the 

NZERTF’s energy performance. Interior equipment and lighting may also differ from the 

expected performance.

Fifth, the monitoring and control of the NZERTF systems may change due to unforeseen 

circumstances. Monitoring and controlling every aspect of the NZERTF will be a complex and

difficult task. Related problems could result in significant variations in electricity use depending 

on if systems are used more or less often than intended.
26

These limitations will be addressed whenever possible to improve the accuracy of the E+

simulation results. Updates based on the changes to the simulations and the related impacts on 

the results will be made as they occur.
27

6 Discussion and Future Research

The estimated performance of the NZERTF design relative to the 2012 IECC design leads to 

several interesting results. First, the NZERTF leads to a reduction of 60 % in energy 

consumption relative to the 2012 IECC design while doing a better job at controlling the indoor 

environment in terms of temperature and relative humidity. If the NZERTF were to be compared 

to an older IECC edition, the percentage reduction would be even more significant, especially 

since the goal for 2012 IECC was to be 30 % more efficient than 2006 IECC.

Second, the greatest reduction in electricity consumption is realized from reducing unwanted 

infiltration (16 %), implementing advanced framing (9 %), installing a high efficiency HVAC 

system (8 %), increasing the R-value of wall construction (7 %), and installing a heat pump 

water heater (6 %). However, the order in which the energy efficiency measures are adopted in 

this study may impact the incremental reductions realized in each simulation.

Third, the electrical equipment (appliances and miscellaneous plug loads) become more 

important as a house becomes more efficient because the electrical equipment has the same 

performance across all simulations while electricity use in the other categories decreases. In 

order to further decrease electricity use in the NZERTF, it will require more efficient appliances 

and lower miscellaneous plug loads through more efficient electronics (e.g., TVs) and changes to 

occupant behavior in plug load use (e.g. unplugging laptop chargers when not in use) to further 

reduce electricity consumption.

This study does not consider the resulting energy efficiency performance of each combination of 

the ten energy efficiency measures. A more in depth study should be done to determine how the 

order in which energy efficiency measures are adopted impact the incremental increase in energy 

efficiency for each measure. This comprehensive set of E+ models covering all potential 

combinations of energy efficiency measure adoption can be used in collaboration with life-cycle 

costing and life-cycle assessment approaches to analyze the incremental benefits and costs of 

building to reach net-zero annual energy consumption. The resulting sustainability performance 

analysis will assist in determining the optimal combination of energy efficiency measures for a 

low-energy house design given current economic conditions.

Given all the assumptions required to generate an E+ model, the use of sensitivity analysis to test 

the importance of these assumptions will assist in better understanding the aspects of the 

NZERTF. Sensitivity analysis can be used to look at a number of building characteristics (e.g. air 

tightness and orientation), occupant behavior (e.g. setpoint temperatures and DHW demand), and 

mechanical equipment performance (e.g. HVAC and water heater efficiency). The sensitivity of 

the energy performance for factors outside of the building can also be tested, such as shading 

from surrounding trees or buildings or extreme weather. The E+ model can also be altered to 

determine how additional energy efficiency measures would impact the energy performance of 

the NZERTF, including equipment that is already installed in the NZERTF. For example, each of 
28

the three geothermal heat pump systems could be included in the analysis to determine the 

impacts of systems on the NZERTF’s energy performance. Natural ventilation could be 

introduced by opening and closing windows based on the outdoor temperature to account for 

common human behavior.
29

References

ANSI/ASHRAE Standard 55-2010. “Thermal Environmental Conditions for Human 

Occupancy.” American Society of Heating, Refrigerating and Air-Conditioning 

Engineers, Inc. www.ashrae.org.

ASHRAE Fundamentals. 2009. American Society of Heating, Refrigerating and AirConditioning Engineers, Inc. www.ashrae.org.

ASHRAE Standard 90.2. 2007. American Society of Heating, Refrigerating and AirConditioning Engineers, Inc. www.ashrae.org.

ASHRAE Standard 62.2. 2010. American Society of Heating, Refrigerating and AirConditioning Engineers, Inc. www.ashrae.org.

Bankrate Mortgage Calculator. http://www.bankrate.com/calculators/mortgages/mortgagecalculator.aspx

Building America. 2013. http://www1.eere.energy.gov/buildings/residential/ba_index.html

Building Science Corporation. 2009. 65 % Net Zero Energy Residential Test Facility 

Construction Documents Set.

Department of Energy, Building Technologies Program, EnergyPlus energy simulation software 

Version 8.0.0, 2013, http://apps1.eere.energy.gov/buildings/energyplus/.

Energy Information Administration. 2011. Average retail price of electricity to ultimate 

customers: By end-use sector and utility - residential sector by state and utility - PEPCO. 

http://www.eia.gov/electricity/data.cfm#sales

Environmental Protection Agency. 2012 Nissan Leaf EPA Fuel Economy Rating. 

http://www.fueleconomy.gov/feg/noframes/32154.shtml

Everyday Green, 2012, Building Leakage Test of the Net Zero Energy Residential Facility.

Freddie Mac. U.S. Primary Mortgage Market Survey. July 18, 2013.

http://www.freddiemac.com/pmms/.

Google SketchUp 8 software. http://www.sketchup.com/.

Hendron, R., Engebrecht, C., October 2010, Building America house simulation protocols, 

National Renewable Energy Laboratory, Building Technologies Program.

International Code Council, Inc. 2011. 2012 International Energy Conservation Code – Chapter 

4: Residential Energy Efficiency.
30

Kneifel, Joshua. September 2012. Annual Whole Building Energy Simulation of the NIST Net 

Zero Energy Residential Test Facility Design. NIST TN 1767.

National Renewable Energy Laboratory. PVWATTS software. 

http://www.nrel.gov/rredc/pvwatts/.

Sherman, M.H. and Grimsrud, D.T. 1980. Infiltration- Pressurization Correlation: Simplified 

Physical Modeling. ASHRAE Transactions, 86(II), 778-807.