National Renewable Energy Laboratory 

Innovation for Our Energy Future 

A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

A Cold-Climate Case Study for 

Affordable Zero Energy Homes 

Preprint 

P. Norton and C. Christensen 

To be presented at Solar 2006 Conference 

Denver, Colorado 

July 9–13, 2006 

Conference Paper 

NREL/CP-550-39678 

July 2006 
NOTICE

The submitted manuscript has been offered by an employee of the Midwest Research Institute (MRI), a 

contractor of the US Government under Contract No. DE-AC36-99GO10337. Accordingly, the US 

Government and MRI retain a nonexclusive royalty-free license to publish or reproduce the published form of 

this contribution, or allow others to do so, for US Government purposes. 

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any 

warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or 

usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not 

infringe privately owned rights. Reference herein to any specific commercial product, process, or service by 

trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, 

recommendation, or favoring by the United States government or any agency thereof. The views and 

opinions of authors expressed herein do not necessarily state or reflect those of the United States 

government or any agency thereof. 

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy 

and its contractors, in paper, from: 

U.S. Department of Energy 

Office of Scientific and Technical Information 

P.O. Box 62 

Oak Ridge, TN 37831-0062 

phone: 865.576.8401 

fax: 865.576.5728 

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from: 

U.S. Department of Commerce 

National Technical Information Service 

5285 Port Royal Road 

Springfield, VA 22161 

phone: 800.553.6847 

fax: 703.605.6900 

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
A COLD-CLIMATE CASE STUDY FOR AFFORDABLE ZERO ENERGY HOMES 

Paul Norton 

Craig Christensen 

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401 

Paul_Norton@NREL.gov 

Craig_Christensen@NREL.gov 

ABSTRACT

This project, supported by the U.S. Department of Energy’s 

Building America Program, is a case study in reaching zero 

energy within the affordable housing sector in cold climates. 

The design of the 1200 square foot, 3-bedroom Denver zero 

energy home carefully combines envelope efficiency, 

efficient equipment, appliances and lighting, and passive 

and active solar features to reach the zero energy goal. The 

home was designed using an early version of the BEOpt 

building optimization software with additional analysis 

using DOE2. This engineering approach was tempered by 

regular discussions with Habitat construction staff and 

volunteers. These discussions weighed the applicability of 

the optimized solutions to the special needs and economics 

of a Habitat house -- moving the design towards simple, 

easily maintained mechanical systems and volunteerfriendly construction techniques. 

1. INTRODUCTION

In October 2005, Amy Whalen and her two young boys 

moved into their new Habitat for Humanity home near 

Denver, Colorado. In doing so they became partners with 

the U.S. Department of Energy in a case study aimed at 

understanding how to create affordable zero energy homes 

in cold climates. The home was a result of collaboration 

between the National Renewable Energy Laboratory 

(NREL) and Habitat for Humanity of Metro Denver. This 

paper will detail the construction of the home including 

passive and active solar features, superinsulated walls, 

ceiling, and floor, and efficient equipment. 

A zero energy home is designed to produce as much energy 

as it consumes over the course of a full year. The home uses 

the utility power grid for storage– delivering energy to the 

grid when the photovoltaic (PV) system is producing more 

energy than is being used in the home and drawing from the 

grid when the PV system is producing less energy than 

needed in the home. This approach eliminates the need for 

battery storage in the home thereby reducing the cost, 

complexity, and maintenance of the solar electric system. 

Homes account for 37% of all U.S. electricity consumption 

and 22% of all U.S. primary energy consumption (EIA 

2005). This makes home energy reduction an important part 

of any plan to reduce U.S. contribution to global climate 

change. The goal of the U.S. Department of Energy’s 

Building America program is to create commercially viable 

zero energy homes by 2020. This project is a case study in 

reaching that goal within the affordable housing sector in 

cold climates. Zero energy is especially important in this 

sector where increasing energy cost can take a high toll on 

homeowners with limited economic resources. A zero 

energy home guarantees long term energy cost stability for 

the homeowner. 

2. DESIGN CRITERIA AND PROCESS

From its inception, the NREL/Habitat ZEH project focused 

on finding the balance between engineering ideals and realworld practicality. The team that designed the home 

included two NREL building energy researchers, two 

Habitat staff members (the Construction Manager and the 

Real Estate Development Manager), and two Habitat energy 

subcommittee volunteers. The NREL engineers made 

suggestions based on modeling results and analysis and 

presented them to the design team who then grounded the 

discussion with practical concerns and insights. This mix of 

perspectives led to a design that balances energy 

1
performance, ease of construction and low cost while 

maintaining the zero energy goal. 

2.1 Project Design Criteria

A Habitat for Humanity house presents an unusual 

opportunity: thanks to volunteers, much of the labor comes 

at no cost and it is possible to get some of the equipment 

donated or at reduced cost. We established the following 

criteria for the home design: 

1. Zero net energy. Zero net energy can be defined in terms 

of site energy (used at the building site) or source energy 

(sometimes called primary energy). For electricity 

purchased from a utility, the source energy used to produce 

and distribute the electricity is typically about three times as 

much as the delivered electricity. From a societal point of 

view, source energy better reflects the overall consequences 

of energy use. The home was designed to meet the 

definition of zero energy of the US Department of Energy’s 

Building America (BA) residential energy efficiency 

research program (1). It must have predicted zero net source

energy consumption over the course of a year using typical 

meteorological year (TMY2 (2)) weather data and BA 

Benchmark (3) assumptions on occupant behavior based on 

average US behavior in terms of temperature setpoints, 

miscellaneous electric loads, and hot water use. 

2. It should be replicable by Habitat for Humanity. 

Construction techniques and energy efficiency technologies 

were vetted for their probability to be repeated in future 

homes. 

3. It should take advantage of Habitat volunteer labor. When 

considering construction alternatives, we took into account 

that Habitat’s approach to building with volunteer labor 

presents a unique opportunity to reduce building cost. 

Construction techniques that were “volunteer friendly” and 

tended towards low material cost were favored. 

Fig. 1: The NREL/Habitat Zero Energy Home 

4. Tradeoffs for zero energy were done at full material cost. 

Although some of the equipment in the house was donated 

or bought with grants at no cost to Habitat, we considered 

the full value of these items to find the balance between 

efficiency and PV production. 

5. No special operation of the building needed. After 

construction this house was sold to a Habitat family. It was 

important to the design team to have the home’s energy 

efficient attributes as invisible to the family as possible. 

From the family’s perspective it should be a normal home 

with no extra owner operating needs. 

6. No prototypes! We designed the homes with off-the-shelf 

proven technologies available in the marketplace today. 

Although optimal research systems were discussed as part 

of the design process, the final design aimed to use 

commercially available products to come as close as 

possible to the ideal. Because the home is expected to 

outlive all of the mechanical systems in the home, we 

wanted these systems to be easily replaceable by technicians 

the owners could find in the local yellow pages. 

7. Keep it simple! Many of the recently designed zero 

energy homes include complicated interconnected 

mechanical systems designed to maximize renewable 

energy use and distribution. We too were often tempted in 

this direction. We tempered this temptation by a continual 

striving to keep it simple. We believe a simpler system will 

have fewer problems and a greater chance at longevity. 

2.2 The Use of Computer Simulation in the Design

The home design process used a combination of computer 

simulations and heuristic judgment. Three simulation tools 

were used both sequentially and iteratively during the 

design process: TRNSYS Transient System Simulation 

software (4), DOE2 Building Energy Model (5), and BEOpt 

Building Energy Optimization Program (6,7). 

3. FINAL HOME DESIGN

The envelope design began by looking at Habitat of Metro 

Denver’s standard home plans. We sorted these plans for 

their applicability to the site and adaptability for a passive 

solar design. A standard 3-bedroom 26’ by 46’ design with 

a crawlspace was chosen. The floorplan was mirrored from 

its original design to accommodate the site. 

Motivated by the BEOpt simulation recommendation for a 

superinsulated envelope, the design team considered a wide 

variety of approaches including structural insulated panels, 

insulated concrete forms, straw bale, and double stud wall. 

Structural insulated panels (SIPs) and Insulated Concrete 

Forms (ICFs) were eliminated because they tend to have 

high material costs and low labor costs – the opposite of 

what is needed to take advantage of Habitat volunteer labor. 

Straw bale construction was carefully considered because it 

has low material cost and high labor. However after 

reviewing the literature on straw bale construction and 

speaking with other Habitat affiliates who have built with 

straw we eliminated this option because of the lack of 

standard techniques and details and the low probability of 

2
replication by Habitat Metro Denver. We chose a double 

stud wall with fiberglass batt construction because it has 

low material costs, uses familiar volunteer-friendly 

construction techniques, and proven construction techniques 

and details are available from the National Affordable 

Housing Network (NAHN: http://NAHN.com). 

The walls consist of an outer 2x4 structural stud wall on 16” 

centers with R13 fiberglass batts in the cavities. Spaced 

3 1⁄2” inside this wall a second 2x4 studwall on 24” centers 

was built. Additional R13 fiberglass batts were placed 

horizontally in the space between the studwalls and 

vertically in the interior wall cavities. An outer vapor 

permeable housewrap and fibercement siding and an inner 

poly vapor barrier and drywall complete the nominal R40 

assembly. The actual whole-wall R-value of this wall will 

be much closer to its nominal value than a single stud wall 

because the thermal shorting of the studs is broken by the 

insulation in the space between the double stud walls. 

Fig. 2: Details of the double stud wall design 

Fig. 4: Raised heel trusses 

Fig. 3: Double stud wall framing 

Raised heel trusses were designed to accommodate 2 feet of 

blown-in fiberglass in the attic giving the top of the thermal 

envelope an R-60 rating. The floors are insulated to a 

nominal R-30. 

The superinsulated shell dramatically reduces heating 

energy needs; “sun tempering” was used to reduce these 

needs further. We designed the home with increased glazing 

area on the long south side and reduced glazing area on 

other orientations. As a policy, Habitat Metro Denver does 

not equip their homes with air conditioning, so we were 

sensitive to overheating potential. Using DOE2 we 

evaluated different southern glazing areas and types and 

overhangs. We compared the simulated heating energy and 

simulated cooling energy (as if there were an air 

conditioning system) in the ZEH and the identical standard 

3
construction home for each window combination. We chose 

the design that maximized heating reduction without 

increasing cooling energy over the standard construction. 

Double glazed, low emissivity (U-value = 0.30 BTU/hr-Fft2

), high solar heat gain coefficient (SHGC=0.58) glass was 

chosen for the southern windows. Double glazed clear 

windows would have provided more solar heating, but also 

would have increased the overheating potential and would 

have lacked the thermal comfort and ultraviolet screening 

benefits. Double glazed low emissivity (U-value = 0.22 

BTU/hr-F-ft2

) low SHGC (0.27) were used for the east, 

west, and north windows. 

3.1 Ventilation System

Because we intended to build the home with very low air 

leakage, a mechanical ventilation system was required. To 

provide fresh air to the home while minimizing energy 

losses we chose to use a balanced energy recovery 

ventilation (ERV) system. The ERV exhausts air from the 

kitchen and bathroom and supplies fresh air to the living 

room and the bedrooms. The warmth of the exhaust air is 

used to heat the incoming fresh air. This significantly 

reduces the heat loss due to ventilation. We chose an ERV 

with efficient electronically commutated motors. 

3.2 Space Heating 

Having a very low design heating load is a blessing and a 

challenge. The blessing is obvious – it takes very little 

energy to keep this home warm! The challenge is that most 

commonly available heating systems are oversized for this 

home and the low heating energy needs cannot justify a 

complicated or expensive system. We considered a wide 

variety of heating systems for the home including: 

 Active solar thermal with radiant floor, baseboard 

heaters, or fan air coil in the ERV supply 

 Ground-coupled heat pump 

 Point-source natural gas furnace (no duct system) 

 Electric resistance baseboard heating 

The design team considered a solar “combisystem” that 

combines active solar thermal space heating and water 

heating. However, this approach requires a relatively large 

equipment investment. During the summer and during 

periods when the house is passively solar heated during the 

fall, winter and spring there is no need for active heating. 

Therefore the additional equipment investment for space 

heating is delivering no energy benefit for most of the year. 

Few integrated solar combisystems are commercially 

available in the U.S. so most of these systems are custom 

designed and built and can be quite complicated. We 

decided that the high first cost, low utilization, complexity 

and custom design of this approach was not consistent with 

our design criteria. 

For an all electric ZEH, using a ground coupled heat pump 

(GCHP) for heating has attractive benefits. The GCHP can 

deliver three to four units of heat for each unit of electricity 

used. In contrast, electric resistant heat delivers one unit of 

heat for every unit of electricity consumed. In addition the 

GCHP can deliver cooling in the summer. However, the 

ground loop for the GCHP and the heat pump itself are quite 

expensive and would require an air handler and duct system 

Fig. 5: Windows have been moved to the south wall to provide solar heating. 

North Elevation 

East Elevation South Elevation 

West Elevation 

4
or a hydronic system to distribute the heating. The compact 

size and superinsulated shell of the Habitat ZEH reduced 

heating needs to so low a level that the cost of the GCHP 

was not justified. 

The use of natural gas for heating, cooking, and clothes 

drying within a ZEH is somewhat controversial. There are 

those who believe that since a ZEH exports only electricity, 

it must consume only electricity. However, in most of the 

U.S., the electricity consumed comes primarily from fossil 

fuels. So the home is consuming fossil fuels when it is using 

electricity and offsetting that consumption when it is 

producing excess photovoltaic electricity. This is similar for 

a ZEH that consumes natural gas. The photovoltaic system 

is sized to produce an excess of electricity to offset the 

natural gas used. The source energy use is net zero. 

The economics of the use of natural gas or all-electric differ. 

For an all-electric home too small and efficient to use a 

GCHP, the all-electric approach requires a larger PV system 

and is substantially more expensive. The all-electric 

approach has the advantage of eliminating the monthly fixed 

cost of having a natural gas hookup which is about 

$9.00/month in the Denver, CO area. However, the Habitat 

ZEH design team decided to use natural gas in the home, 

reducing the required PV array size by 1.1 kW and to take a 

hybrid approach to space heating. 

The space heating system combines a point-source direct 

vent natural gas furnace in the living/dining area of the 

home and small baseboard electric resistive heaters in the 

three bedrooms. This approach is relatively low cost, 

elegantly simple and provides zone heating because each 

appliance has its own independent thermostat. 

3.3 Water Heating System

Although we ruled-out a solar combisystem, the results of 

the early BEOpt runs convinced us to incorporate a high 

solar saving fraction solar water heating system into the 

home design. TRNSYS was used to do parametric studies to 

design the system. We found that mounting the collectors at 

the roof pitch rather than raising them to their optimal angle 

incurred only a small annual energy penalty (8). We found 

that a 96 sq. ft. collector area with 200 gallons of water 

storage would result in an 88% annual solar savings fraction 

using TMY2 weather data and BA Benchmark hot water 

use. This solar savings fraction includes pump energy use. 

We specified a natural gas tankless water heater as a backup 

to the solar system. Unlike tank water heaters, the tankless 

system uses no heating energy when the solar water tank is 

at or above the 115 degree F hot water delivery temperature, 

although it may use electricity in standby. The disadvantage 

of using the tankless system is the added cost compared to a 

tank system. We considered using the tankless water heater 

for space heating also, but ultimately decided to use separate 

systems to avoid the complexity of the combined system. 

3.4 Photovoltaic System Sizing

Once all possible energy loads in the house were 

significantly reduced, the photovoltaic system was sized to 

meet the remaining electricity needs and offset the expected 

Fig. 6: Energy end uses for a typical design and the zero energy home. 

5
natural gas use. In a similar home built to BA Benchmark 

standards, about one quarter of the energy in the home is 

consumed by lighting, appliances, and miscellaneous 

electric loads (“LAME” loads). We reduced the lighting 

load by using compact fluorescent lights throughout the 

home. We reduced the appliance load through the use of 

energy star appliances. This leaves the miscellaneous 

electric loads (MELs) that include everything plugged in by 

the occupants.... TV, hair dryer, toaster oven, computer, 

aquarium, etc. Because all other loads have been 

dramatically reduced, the MELs in the Habitat ZEH are 

expected to consume 57% of all energy used annually (see 

Fig. 6). Although the BA Program is pursuing research into 

ways to reduce these loads, they are currently out of the 

control of the home designer. Furthermore, these loads are 

highly unpredictable and vary substantially from household 

to household. So the ZEH designer is faced with sizing a PV 

system for a home where the largest load is really not 

known with any accuracy. 

The BA Benchmark includes assumptions that we used to 

estimate the MELs and size the 4kW PV system. These 

assumptions are based on the best available nationwide 

studies of energy use. So the home’s PV system is sized 

with the assumption that it will be occupied by a “typical” 

American household. If the actual household and weather 

are typical, the home will achieve zero energy. If the 

household or weather is atypical, the home may not achieve 

zero energy or may be a net producer. 

4. FOR MORE INFORMATION ON THE PROJECT

We installed a data acquisition system in the home in 

January 2006 and will monitor the performance of the home 

for at least one year. Periodic email updates on the project 

are available from NREL. If you would like to receive these 

updates, send your email address to 

Paul_Norton@NREL.gov. A conference paper on the 

home’s performance will be available early in 2008 and a 

full technical report on the project will be available in mid2008. These reports will be posted on the BA website at 

http://BuildingAmerica.gov. 

5. ACKNOWLEDGMENTS

The authors would like to thank the many individuals and 

organizations that supported this project including: 

homeowner Amy Whalen; The U.S. Department of 

Energy’s Building Technologies Program; Midwest 

Research Institute; Battelle; Energy Outreach Colorado; 

Xcel Energy; The Governors Office of Energy Management 

and Conservation; Secretary of Energy Samuel Bodman; 

Congressman Bob Beauprez; Lori Vaclavic, Bruce 

Carpenter, Andy Blackmun, Brady Nelson, Pandora 

Reagan, and Lynne Brown of Habitat of Metro Denver; Paul 

Kreischer of Lightly Treading; Pete Beverly; Paul Reeves 

of the Partnership for Resource Conservation; Ed Hancock 

and Greg Barker of Mountain Energy Partnership; Ren 

Anderson, Admiral Truly, Dan Arvizu, Jessie Harris, Byron 

Stafford, Kerry Masson, Lee Boughey, Sarah Barba, Jay 

Burch and Scott Horowitz of NREL; Betsy Petit of 

Building Science Corporation; Barbara Miller of the 

National Affordable Housing Network; and Rob Dumont of 

the Saskatchewan Research Council. 

6. REFERENCES

(1) For more information on the Building Energy Program 

of the Department of Energy see 

http://www.buildingamerica.gov 

(2) Marion, W. and Urban, K. User's Manual for TMY2s

Typical Meteorological Years, Derived from the 

1961-1990 National Solar Radiation Data Base, National 

Renewable Energy Laboratory, 1995 

(3) Hendron, B., et. al., Building America Benchmark 

Definition, Updated December 29, 2004, National 

Renewable Energy Laboratory, NREL/TP-550-37529, 2005 

(4) Klein, S., et al., TRNSYS: A Transient System 

Simulation Program – Reference Manual, Solar Energy 

Laboratory, University of Wisconsin, 1996 

(5) DOE2.2 Building Simulation Software, developed by 

Lawrence Berkeley National Laboratory, Berkeley, CA 

94720, available at http://www.osti.gov/estsc/ or 

http://www.doe2.com 

(6) Christensen, C.; Barker, G.; Horowitz, S. A Sequential 

Search Technique for Identifying Optimal Building Designs 

on the Path to Zero Net Energy, Proceedings of the Solar 

2004 Conference Including Proceedings of 33rd ASES 

Annual Conference and Proceedings of 29th National 

Passive Solar Conference, American Solar Energy Society, 

NREL/CP-550-36099, 2004 

(7) Christensen, C.; Barker, G.; Tupper, K., Optimal 

Building Designs on the Path to Zero Net Energy, 

Proceedings of the Solar 2004 Conference Including 

Proceedings of 33rd ASES Annual Conference and 

Proceedings of 29th National Passive Solar Conference, 

American Solar Energy Society , NREL/CP-550-36100, 

2004 

(8) Christensen, C.; Barker, G., Effects of Tilt and Azimuth 

on Annual Incident Solar Radiation for United States 

Locations, Proceedings of Solar Forum 2001: Solar Energy: 

The Power to Choose, 2001 

6
REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents 

should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a 

currently valid OMB control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

July 2006 

2. REPORT TYPE

Conference Paper 

3. DATES COVERED (From - To)

 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

A Cold-Climate Case Study for Affordable Zero Energy Homes: 

Preprint 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/CP-550-39678 

5e. TASK NUMBER

BET6.8004 

6. AUTHOR(S)

P. Norton and C. Christensen 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

8. PERFORMING ORGANIZATION 

REPORT NUMBER

NREL/CP-550-39678 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

This project, supported by the U.S. Department of Energy's Building America Program, is a case study in reaching 

zero energy within the affordable housing sector in cold climates. The design of the 1200 square foot, 3-bedroom 

Denver zero energy home carefully combines envelope efficiency, efficient equipment, appliances and lighting, and 

passive and active solar features to reach the zero energy goal. The home was designed using an early version of 

the BEOpt building optimization software with additional analysis using DOE2. This engineering approach was 

tempered by regular discussions with Habitat construction staff and volunteers. These discussions weighed the 

applicability of the optimized solutions to the special needs and economics of a Habitat house — moving the design 

towards simple, easily maintained mechanical systems and volunteer-friendly construction techniques. 

15. SUBJECT TERMS

Building America; U.S. Department of Energy; zero energy homes; affordable houses; Habitat for Humanity; BEOpt 

16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

c. THIS PAGE 

Unclassified

17. LIMITATION 

OF ABSTRACT

UL 

18. NUMBER 

OF PAGES 

19b. TELEPONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18