Maximizing Net Positive Homes with Software

NET POSITIVE ENERGY (NPE) HOMES:
MAXIMIZING PERFORMANCE USING ADVANCED SOFTWARE TOOLS

Mr. Bion D. Howard, Principal Consultant
Building Environmental Science & Technology
P.O. Box 2145 Valley Center, CA 92082-2145 USA
Email: info@energybuilder.com Web: www.energybuilder.com

ABSTRACT
We explore the art of the possible in advanced high
performance homes, reviewing how builders and
designers can be guided towards maximum potential
using 21st century software tools to prototype net-positive
energy home designs. Also reviewed are case studies of
home designs analyzed using the advanced computer
software, by the authors’ consulting practice.

Included is discussion of features and capabilities of
popular software tools: Energy-10TM V1.8 (Author:
National Renewable Energy Laboratory); EnergyGauge
USATM (Author: Florida Solar Energy Center); and REM-
DesignTM (Author: Architectural Energy Corp.)

INTRODUCTION
As climate change becomes an increasingly important
political issue, policymakers and the construction FIGURE 1.
community alike are grappling for solutions. How can we
make buildings more efficient in reducing their detail. The NPE home is achievable with current designs
contribution of atmospheric emissions to our climate- and off-the-shelf technology but does entail increased
challenged planet? costs of construction which are being documented.

Residential buildings comprise a significant segment of Our hypothesis was: we can use applied building science
US atmospheric emissions, particularly of Carbon forms and engineering to help designers come up with homes
of pollutions damaging to our climate. According to the that actually provide more energy (in the form of on-site
Alliance for Climate Protection (derived from US EPA electric power) back to the grid, hence on average
data, 2005) the residential sector contributes 17% of US becoming net producers of energy rather than consumers.
greenhouse gas emissions. (FIGURE 1.) So, since the Therefore, at least on the annual operating costs level (and
USA contributes nearly 21% of total global emissions. Carbon emissions) the NPE redesigned building, in our
This means, energy use at our dwellings create over 3.5% case a single family home, may be termed “net-positive”
of total global emissions of CO2. for energy.

It is shocking to look at this data and find that US There is no reason this approach – tweaked for the
residential buildings alone contribute over 1.12 billion realities of commercial building design – could not be
metric tons of CO2 emissions every year. We can do applicable for offices, schools, and retail. Perhaps the
something about this, and must. only exception might be hospitals where electric power
demand is comparably high, so that on-site renewable
Progress towards Zero-Carbon home building has a installation sizing to reach the NPE level may be eclipsed
strategic way-point which we have investigated in some by demand.

WHAT ARE NPE HOMES? According to the US DOE, Energy Information Agency,
To obtain the level of performance we consider (and can 2004 one may expect the following approximate
verify through calculations) to be a net- positive energy breakdown of purchased energy costs in a US housing
home, several design avenues need to be pursued unit; see FIGURE 2.
simultaneously.

1. Sustainable site considerations must be taken into
account, including orientation, local vegetation,
proximity to adjacent structures, distances to water
bodies and roadways.
2. The thermal envelope must be exceptionally energy
efficient, including the insulation system, window
and door selections, and extensive measures to
control air-leakage. Selection of appropriate
foundation design and thermal protection is also vital.
3. The architectural design must be responsive to the
local climate in order to maximize the benefits of
passive solar heating, and natural gains-avoidance
and cooling strategies. FIGURE 2. (Source: DOE/EIA 2004)
4. The exterior colors of the buildings surfaces need to
be “tuned” to the climate in order to maximize the Our specific targets have evolved around the heating, air-
potential for reducing solar driven loads where conditioning, water heating, the refrigerator and
necessary. appliances loads – such as dishwasher, laundry units, and
5. Water consumption needs to be reduced to the televisions. In the typical project we see these usages
absolute minimum while still maintaining serviceable often account for in excess of 70% of overall purchased
levels of consumer satisfaction; particularly the hot energy. Therefore, they become a sizable target of
water service system. opportunity to engineer-out waste and excess atmospheric
6. Electric demand needs to be reduced to micro-load pollution emissions.
levels by incorporating super-efficient and right sized
appliances, and mechanical systems. Source-energy Fortunately there are super efficient products on the
analysis is used. market today, thanks in large part to the market
7. On-site consumption of fossil fuel resources must transformation efforts of EAP/DOE EnergyStar
similarly be reduced, since in the overall energy programs. Many new appliances provide exceptional
“budget” of typical North American homes shows performance while providing consumers with reasonable
heating is a big usage. life-cycle investment returns from energy savings. It is
important for designers and builders to actively consider
In an NPE home, the logical approach is to directly attack installing the most energy efficient appliances in their
the biggest pie segments through super-efficiency and products.
then apply renewable energy solutions to further reduce
the demand; then finally implement cost-effective PV and THE ASSESSMENT PROCESS
small-wind energy solutions to overcome the remaining Early intervention in a specific project is absolutely vital
billable energy usage. In our assessment, we shoot for a to success. The father along a project becomes the more
10% annual energy “surplus” that the home provides back difficult and costly the design changes appear to be.
to grid, usually via net-metering (in jurisdictions where Since elevated investments are considered, and hence
it’s permitted). higher first costs will derive from the approach, these
methods may not be accepted readily by merchant
To achieve NPE “status” the home needs to be net (production) home builders. For now our studies and tech
positive on an annual basis. While some months may be support remain, largely the province of architects and
above or below the NPE line, the consumer and the design/build firms.
environment likely care more about what happens
annually and over the long term. This is because when Our approach is fairly simple, and is relatively similar to
the consumer thinks about investing in a home their time that used in certified Home Energy Ratings today.
horizons are usually 7 to 10 years (typical occupancies) Currently we conduct our assessments in the pre-design
and they also look at 30 year, and 15 year fixed phase of the new home design, specification and
mortgages.

2

construction process. Future work will explore NPE counterpart “reference” homes and would exceed
existing homes. EnergyStar Homes criteria by almost 45% (depending on
climate and reference code requirements). This has been
In general the assessment protocol follows these bullet encouraging, but getting to a Carbon-Neutral home design
points: is more complex and more broadly bounded life-cycle
assessment problem.
• Obtain program information, schematic drawings and
building site information. The end result of deploying super-efficient home thermal
• Compile necessary climate data, utility and fuel designs using passive solar energy strategies has been the
schedules, local cost correction factors. reduction in size and cost of the photovoltaic electric
• Develop a “reference home” that complies with local power system needed to tip over the line to annual NPE
energy codes, or statewide model codes if no local performance capability. This is not a trivial point. It
code is in force. This reference home is similar to validates a key building energy engineering and passive
that used in HERS assessments by RESNET solar design axiom around since the early 1980’s, that
accredited professional rating providers. “efficiency first” should be a principal rule of thumb.
(www.natresnet.org)
• Compile alternate super-efficient building shell COMPUTER ASSISTED DESIGN BENEFITS
(envelope) thermal protection package. “Follow the money” – is a statement true in many forms
• Compile via iteration, the most effective passive solar of human endeavor. There are no more Darwinian
heated variant of the home. economics than those imposed by real-world construction
• Re-run the analysis to minimize collateral excess air- projects. Using computer based design tools definitely
conditioning demand from the selected passive solar helps foster better overall cost-effectiveness of building
heating package; continue to optimize natural heating projects. Imagine trying to make hundreds of different
and cooling. calculations of energy savings value, installation cost
• Using the “optimal” thermal envelope, passive solar differences, thermal loads and weather variations over and
design, and cooling avoidance solutions determined over, all at the same time. Nope, can’t do it without
to this point, make preliminary attempt to configure computers!
and size a prototype space conditioning solution that
is right-sized and reasonable in first cost. Part of the cost-effectiveness equation is improving
• Implement a mechanical ventilation solution, performance of the green design strategies selected by the
providing adequate fresh air (re-run). designer, by looking at competing options for achieving
• Recheck the overall performance of emerging high- desired results. There are often competing options with
performance prototype; tweak. different performance claims, some measured and some
• Model the home again with a super-efficient estimated by vendors, which need to be considered.
appliance and lighting package; tweak. Sustainable design strategies also need to be “optimized”
in combination with energy efficiency strategies, that also
• Size an appropriate photovoltaic electric system.
have cost and savings (value) propositions that exist
Consider small-scale wind as needed.
without solar devices or techniques being applied.
• Prepare report and meet with client team
• Respond to comments, and re-run if necessary
CASE STUDIES (Actual pre-construction designs
By the conclusion of the assessment process generally analyzed for NPE capabilities)
described the energy engineer should have a good handle
on not only how the home will perform overall, but will Home Design A. (Software: Energy-10™)
have addressed interactions – such as between air-leakage In this project we used the recently updated NREL
control strategies and sufficiency of fresh air ventilation. Energy-10 model. Energy-10 Version 1.8 (with PV) is an
So far in every case we have needed to include an advanced capability tool to help integrate whole building
occupant controlled mechanical ventilation system in energy efficiency studies, including photovoltaic (PV)
these NPE home prototypes. Otherwise, the air-tightness solar electric power simulations, and solar domestic hot
levels required to sufficiently reduce heating and cooling water. These solar features were added to ENERGY-10
costs can not deliver sufficient fresh air via leakage, to be during 2004 – 2006 development cycle lead by software
considered healthy. experts at NREL. Since its inception, Energy-10 has
provided design support information for small
In several cases, the NPE assessment protocol has commercial and residential projects of 10,000 square feet,
produced prototype home designs that utilize as little as or less, conditioned floor area.
20% of the projected energy consumption of their

3

TABLE 1. NPE PROJECT HOUSE A.
Features Location: Baltimore/Washington USA Criteria
Size: 1825 SF (reduced from 2380 SF)
Energy Efficiency Walls, Roof/ceiling, Foundation R-34 wall (frame)
R-49 attic
R-19 foundation
Windows and Doors Low-E Ar., Fiberglass
frames, super-spacer;
R-4.2 doors
Heating/Cooling/Ventilation Approach Heat Pump SEER17,
HSPF >8.6
Lighting and appliances “best in class”
ACEEE Guide ‘06
EnergyStar / HERS rating summary HERS (“0”)
(prior to implementing PV system) HERS (23)
Renewable Energy
Passive Solar: increased glazing, thermal Calculated 62% solar
mass, and attached sunspace. Included heating fraction
significant overhangs omitted in first cut. (ASHRAE)
Solar hot water 75% solar fraction
Electrical System (PV included) 2.4 kW peak required
Water Efficiency
Assumed low-flow devices; all hot water Max available effic.
taps; relocated wet areas to “stack”
Home run PEX piping assumed Estimated 40% water
reduction annually
Marginal Cost Per Percentage basis: 16.5% Reference home
Square-foot (Extra hard $178/SF
construction cost, less (MEANS 2006 costs and Green Guide) NPE Prototype
land) $207/SF
Comments: (Not yet built)

Included are many straightforward and user friendly large niche in building construction; the myriad small-
capabilities to analyze: highly insulated building commercial structures that historically tended to “fall
envelopes; passive solar heating and cooling; natural through the cracks” when it comes to energy and solar
ventilation; daylighting applications, high-performance design.
windows, efficient electric lighting systems, many types
of mechanical equipment options and most recently, solar Energy-10 is cited in the US Green Building Council’s
photovoltaic and hot water energy systems. The best LEED®-NC 2.2 Reference Guide section on effective
feature of Energy-10 is its ability to create automatically a sustainable low-energy building design. An extensive
base-line (reference) building, and then through a write-up on this software is available on line, at:
consistent naming process, the user may create a large set www.sbicouncil.org.
of prototype variations to test various design strategies. If
carefully done, a useful set of iterative comparisons of Home Design B. (Software: EnergyGauge)
different potential strategies on the same building can be EnergyGauge USA is a sophisticated home energy
created for analysis. simulation software tool designed specifically for the easy
and accurate evaluation of home energy-efficiency.
Indeed, with the new capabilities to analyze building-
integrated PV systems, Energy-10 is one step closer to We found that upon completing use of the built-in help
full realization of its potential. The author hopes that screens, and exploration of the FSEC Support Page for the
development of this tool will continue since it serves a software that the tool could be put into full use in about

4

TABLE 2. NPE PROJECT HOUSE B.
Features Location: Atlanta Metro area, USA Criteria
Size: 2650 SF (reduced from 3280 SF)
Energy Efficiency Walls, Roof/ceiling, Foundation R-28 wall (frame)
Note: SPIS alternate
R-42 roof/clg. (SIPS)
R-10 slab perimeter, R-5
under slab
Windows and Doors Low-E Ar., Fiberglass
frames, super-spacer;
R-3.2 doors
Heating/Cooling/Ventilation Approach Heat Pump SEER16, HSPF
>8.3
ACEEE Guide ‘06
Renewable Energy
Passive Solar: increased glazing, thermal Calculated 78% solar
mass, and attached sunspace. Included heating fraction (ASHRAE)
significant overhangs omitted in first cut.
Solar hot water 86% solar fraction
Water Efficiency
Assumed low-flow devices; all hot water Max available effic.
taps; relocated wet areas to “stack”
Home run PEX piping assumed Estimated 35% water
reduction annually
Marginal Cost Per Percentage basis: 18.3% Reference home
Square-foot (Extra hard $162/SF
construction cost, less (MEANS 2006 costs and Green Guide) NPE Prototype
land) $192/SF
Comments: Out for bids 11/2007

three days, by an experienced energy engineer or HERS making. The EnergyGauge USA complies with all
rater. We learned quickly to frequently save when requirements of the International Energy Conservation
changing data inputs, because on our PC platform there Code (IECC) for energy code compliance calculations
were occasional lock-ups and crashes (program seems to and reporting. It has also been subject to and approved in
consume a lot of “resources” on IBM-PC Windows XP national accreditation procedures, and meets all technical
Service Pack-2). guidelines, for Home Energy Rating Systems (HERS),
including the HERS BESTEST (Building Energy
This software provides an hourly simulation design tool Simulation Tool Test, National Renewable Energy Lab)
for the design of high-performance homes and the procedures.
evaluation of energy use and peak demand impacts of
home energy-efficiency improvements. It is capable of EnergyGauge USA capabilities (Edited from source:
evaluating both existing and proposed (new) homes. The FSEC Web site) include:
software uses the powerful and widely-respected DOE
2.1-E hourly building energy simulation software to Detailed, hourly simulation modeling and reporting using
simulate energy use and provides for the combined an FSEC-enhanced version of the widely-respected DOE-
evaluation of both the energy use and the economic and 2.1E building energy simulation program.
financial impacts of home energy-efficiency decision-

Analysis modules include: information becomes available. This is particularly useful
to designers that want to incorporate energy efficiency
• computation of heating and cooling system part into the project early in the schematic phase, and then
load performance improve the details as the project evolves.
• prediction of indoor air relative humidity
• Coil airflow and system sizing impact on heat REM/Design tool also provides automatic (and editable)
pump and air conditioner performance sizing of heating and air-conditioning equipment, and can
help determine whether a project complies with numerous
• accurate ground contact modeling
standards and model codes for both the prescriptive and
• attic modeling (including attic ventilation/
performance compliance paths. Solar energy systems can
reflective roofing and radiant barriers)
be modeled in conjunction with efficiency improvements,
• Detailed hourly modeling of duct heat transfer including: sunspaces, photovoltaic solar electric systems,
and air leakage and solar water heating.
• Whole building heating and cooling system
sizing Passive solar design analysis is somewhat limited, and
• Energy code compliance in accordance with the users will need to be creative to include high-thermal
International Energy Conservation Code mass applications. All the utility rates, component costs,
• (IECC) 1998 and IECC 2000 whole-building and default building characteristics inputs may be
performance method. customized by expert users, while the novice or
• Ideal for evaluation of low energy and Zero intermediate user may remain assured the default data is
Energy Building Designs. very representative. We like REM/Design because it is
• Hourly prediction of solar electric PV system flexible, powerful, and easy to use for iterative
performance. comparison of different combinations.
• Hourly prediction of solar hot water heating
systems. REM/Design Web site:
• Hourly prediction of end-use electrical and gas [http://www.archenergy.com/products/rem/ ]
energy consumption for evaluation of peak
impacts.
ADDITIONAL SOFTWARE PACKAGES
More information on EnergyGuage USA
http://www.energygauge.com/usares/default.htm. HEED (Formerly Solar-5 )
SOLAR-5 was incorporated into Home Energy Efficient
Home Design C. (Software: REM/Design V12.2 Design (HEED) a more user-friendly version in mid-
and REM Rate) 2000. HEED is quite user friendly at all levels of energy-
We have been using the Architectural Energy Corporation modeling and solar design expertise. It is so simple to
(AEC) REM series programs for efficiency policy start off, that some homeowners have employed the tool
research, building design assessment, and HERS ratings to make retrofit decisions on their dwellings. Other users
since initial release. Now in its 12th generation, the EEBA include architects, builders, students, energy program
Excellence 2000 award-winning REM software provides a managers, weatherization pros, and green building
really useful tool for residential designers, remodelers, consultants.
project managers, weatherization program managers, and
energy professionals. REM/Design calculates heating, It employs a graphic interface to draw a floor plan, and
cooling, hot water, lights and appliance loads, then users click and drag various elements like windows
consumption and costs. Climate files are included for to selected locations. Default lists of standard wall and
about 250 North American cities. It is compatible with roof constructions are available and may be edited. A
MS Windows™ PC platforms. simple base-case building can be generated, then subject
to further analysis. Advanced users may employ detailed
The tool uses a specially configured seasonal adaptation design input options and display output graphics similar
of full-annual simulation methods, based on work done by to the original SOLAR-5 tool. HEED expands on
the National Renewable Energy Laboratory (NREL). SOLAR-5 features including: editable utility rates, new
This reduces run-times on PC platforms, while fuel rates for oil and propane, editable air pollution data,
maintaining good levels of accuracy. Users can provide attic radiant barriers, operable shading, enhanced thermal
specific inputs or use a “Simplified” initial approach to mass algorithms and dealing with previous minor
start developing a building model. The user may then software bugs.
move all the previous work into a detailed model, as more

6

TABLE 3. NPE PROJECT HOUSE C.
Features Location: Grand Junction CO, USA Criteria
Size: 2125 SF (spec plan book house)
Energy Efficiency Walls, Roof/ceiling, Foundation R-36 wall (panels)
R-60 attic
Note: SIPS alt. bid
R-30 shallow cellar (was crawl
space)
Windows and Doors Low-E Ar., Fiberglass frames,
super-spacer;
R-4.2 doors
Heating/Cooling/Ventilation Approach LP Gas Tankless Boiler N=91
No-AC, possible use of Mini-
splits later
ACEEE Guide ‘06
Renewable Energy
Passive Solar: increased glazing, high Calculated 86% solar heating
thermal mass, and integrated sunroom. fraction (ASHRAE)
Significant use shading and wing-walls.
Night flush ventilation (high elevation)
Solar hot water (Aux = combo system) 100% solar fraction
Water Efficiency
Assumed low-flow devices; Max available effic.
all hot water taps micro-flow restrictors;
relocated wet areas to “stack”
Rainwater catchment system bid
Home run PEX piping Estimated 50% water reduction
annually
Marginal Cost Per Square- Percentage basis: 16.5% Reference home
foot (Extra hard $178/SF
construction cost, less land) (MEANS 2006 costs and Green Guide) NPE Prototype
$207/SF
Comments: Going out for bid

HEED’s primary analysis component SOLAR-5 is a full- This handy software is available at no cost from the
annual hourly simulation program, and was mostly UCLA Department of Architecture and Urban Design,
recently updated in June 2000. It has been validated either on line by free download, or by contacting
compared to DOE-2 and BLAST using the NREL Research Professor Murray Milne [Email:
BESTEST procedure. HEED self-installs on stand alone Milne@ucla.edu ]. Note: HEED is represented as a
non-networked MS Windows™ 95/98 and XP operating BETA release since it is undergoing continuing
systems. An Apple Macintosh OS X version is also development.
available. Climate data is available for over 500
locations. HEED has also been translated into the Home Energy Efficient Design Web site:
Spanish language. http://www2.aud.ucla.edu/heed/download.html

PV DesignPro (Solar Design Studio) effectively, they thrive on good information with prompt
Previously available as a stand-alone application, PV turn-around times when questions arise.
DesignPro by Maui Solar Energy Software Corporation is
now part of a design tool suite called Solar Design Studio Computer tools also provide data sources for program
delivered via CD-ROM. Compatible with recent versions compliance verification, such as in LEED® for Homes,
of MS Windows® including XP, this tool requires NAHB Green, and EnergyStar™. Without such
considerable expertise in solar energy systems design and information, provided in a manner consistent with sound
analysis to wring out maximum results, but can be used engineering judgment and environmental principles, it
by a novice since it has embedded help functions. would be virtually impossible to tell a “green building”
from one slathered with “green-wash” instead.
Inputs include default values and database entries that can The next logical step is to include this type of analysis in
be edited and customized. Numerous configurations of broader design assessment research on how to create truly
panels, connections and wiring strings may be entered, Carbon-Neutral home designs, and to look at the
along with performance parameters for AC inverters, associated economics.
power conditioning, battery charging, and grid interfaces.
Numerous climate files are included along with a weather BIBLIOGRAPHY
file generator to customize for site micro-climates.
"Solar-Design Tools for Green Building" Special Solar
Energy & Efficiency Issue, Spring 2007. © Home Energy,
Results available include for example, solar-fraction
Berkeley, CA
charts; annual and monthly performance details; battery
charge status; and economics estimates based on energy
“Software Secrets” Column - Quarterly, 2000 - 2005. ©
sold; life-cycle system costs; and rates of return over the
Home Energy magazine, Berkeley, CA
life of the project based on user editable prices per kWh.
"Whole Building Design Approaches and Useful Tools
Additional features of the Solar Design Studio include: a for Implementing Energy Efficiency." (invited)
solar hot water design tool, a climate generator tool, Energy2001, June 2001. Kansas City, MO. US Dept. of
global insolation database, video tutorial on CD-rom, and Defense, US Dept. of Energy.
graphing tools for 3-D and I-V-P plotting of results.
Special order variants include larger scale PV systems, "Your Computer as a Green Building Resource" (Invited
water pumping, and time-of-use rates, along with a seminar presentation) AIA '99 National Convention,
Spanish version. Dallas, TX. May 1999.

Maui Solar Energy Software Web site: "Advances in Passive Solar Design Tools." 1992. In
http://www.mauisolarsoftware.com Proc. of Thermal Performance of the Exterior Envelopes
of Buildings V. Proc. DOE/ASHRAE Conference.
SUMMARY AND CONCLUSIONS ASHRAE, Atlanta, GA.
Our research shows that NPE Designed homes are
technically possible, but prior to there being more interest BIOGRAPHY
by builders and consumers it is apparent that more cost
information is needed Bion Howard via his consulting practice, (dba) Building
Environmental Science & Tech. (B.E.S.T.) provides technical
Project “green teams” thrive on good input for decision- assistance and training on green building including energy
efficiency, indoor environmental quality, and renewable energy
making. It is remarkable that engineers and architects system implementation in buildings.
produced so many buildings before today’s vast
computation power came onto the scene. Design tool Bion Howard’s prior service includes the U.S. Environmental
software help designers pick through potential mine-fields Protection Agency (helped design EnergyStar Homes program);
of completing claims, and the results provide improved Alliance to Save Energy (Senior Program Manager - Buildings),
information for project teams. and the National Association of Home Builders Research Center
(Senior Energy Analyst).
One of the most important breakthroughs in the evolution
of sustainable design, whether it is for buildings, sites, or Member: ASHRAE, ASES (life member), National RESNET
(serves on Standing Technical Committee for HERS ratings). In
other infrastructure, has been the improved
the USGBC he serves on LEED Tech. Advisory Group - Energy
interdisciplinary communication and evolution of and Atmosphere. Education: BA and Masters (Wisconsin –
effective project teams. For these teams to function Green Bay), environmental studies.

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