Autodesk Sustainable Design Curriculum
Lesson Five: Optimizing the Design of the Building Envelope for
Sustainability
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Carbon Neutrality and Buildings
The Design Process Overview: Steps to Carbon Neutral Design
Optimizing the Building Envelope with Respect to Daylighting Design Goals
Management of Solar Heat Gain
Natural Ventilation
Minimizing Internal Loads with Natural Ventilation
Onsite Renewable Energy
© 2009 Autodesk
2030 Voluntary Carbon Neutral Initiative
Architecture 2030, a nonprofit, nonpartisan, and independent organization founded by architect
Edward Mazria, has issued The 2030 Challenge asking the global architecture and building
community to adopt the following targets:
All new buildings, developments, and major renovations shall be designed to meet a fossil
fuel, GHG-emitting, energy-consumption performance standard of 50% of the regional (or
country) average for that building type.
 At a minimum, an equal amount of existing building area shall be renovated annually to meet
a fossil fuel, GHG-emitting, energy-consumption performance standard of 50% of the regional
(or country) average for that building type.
 Increase the fossil fuel reduction standard for all new buildings to:
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60% in 2010
70% in 2015
80% in 2020
90% in 2025
Carbon-neutral in 2030
 Achieve these targets by implementing innovative sustainable design strategies, generating
onsite renewable power, and/or purchasing (20% maximum) renewable energy and/or
certified renewable energy credits.
© 2009 Autodesk
2030 Voluntary Carbon Neutral Initiative
The following organizations have already adopted these targets:
U.S. Conference of Mayors (Resolution #50; unanimously passed) (USCM)
The American Institute of Architects (AIA)
U.S. Green Building Council (USGBC)
Royal Architecture Institute of Canada (RAIC)
County of Sarasota, FL
State of New Mexico (Governor Bill Richardson)
City of Santa Fe, NM
County of Santa Fe, NM
City of Albuquerque, NM
City of Plano, TX
Rocky Mountain Institute (RMI)
International Council for Local Environmental Initiatives (ICLEI)
World Business Council for Sustainable Development (WBCSD)
Environmental Protection Agency (EPA/Target Finder)
American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE)
National Wildlife Federation (NWF)
Society of Building Science Educators (SBSE)
AIA Committee on the Environment (AIA/COTE)
Association of Collegiate Schools of Architecture (ACSA)
Union Internationale des Architectes (UIA)
American Solar Energy Society (ASES)
© 2009 Autodesk
Carbon Neutrality and Buildings
The definition of what constitutes a carbon neutral building is in flux, with no uniform definition
existing among the regulatory or design community.
A carbon neutral building, (as defined by the Autodesk Green Building Studio web service)
accounts for regional differences in the carbon footprint of the regional electric grid, and
mandates that any fossil fuel-based electricity and fuel use be eliminated through efficiency
gains or offset by onsite nonfossil fuel-based energy sources such as PV or wind energy.
Carbon neutrality is often used interchangeably with net-zero energy consumption.
A net-zero energy project is defined as a project/building that consumes only what it produces
with respect to energy, on an annual basis.
Renewable noncarbon sources are required to meet the typical definition of a net-zero energy
building.
Noncarbon renewable sources include wind energy, solar energy, and biofuels.
In some cases, partial renewable energy “credits” are assigned for certain technologies such as
geothermal heat pumps, which use the earth as a heat source/sink.
© 2009 Autodesk
Why Is a Process Change Essential for Design of Carbon Neutral
Buildings?
BIM tools, such as Autodesk Revit Architecture, enable architects to analyze their schematic
designs using various software tools that can carry out robust energy analyses in minutes.
The process of submitting schematic designs for energy analysis can and must be repeated as a
design progresses, in order to compare results and minimize resource use for various
configurations of building form, product selection, and environmental response.
This iterative approach and the need to begin analyses early in the design process is a
fundamental departure from traditional approaches that involve specifying efficient
components late in the design process.
It must be emphasized that the design process is iterative, repeating design process steps as the
team moves ever-closer to an optimal carbon neutral design.
© 2009 Autodesk
Process Change is Essential for
Effective Sustainable Design
Set Savings
Target
Optimize
Building Form
& Openings
Minimize
Internal Loads
Select HVAC
System
Onsite
Renewables
Commissioning
Purchase Green
Power &
Carbon Credits
© 2009 Autodesk
Iterate to Improve Design and Reduce Energy Use
© 2009 Autodesk
Residential vs. Commercial Buildings: A Brief Primer
Comparison of Energy End Use
30%
25%
Residential
Commercial
20%
15%
10%
Source: 2005 Buildings Energy Data Book, 2003 data
Other
Computers
Ventilation
Cooking
Electronics
Refrigeration
Water Heating
Space Cooling
0%
Lighting
5%
Space Heating
% of Total Building Energy Use
35%
The graph illustrated
compares the percentage of
energy end-use for residential
buildings in the U.S.
compared to commercial
buildings in the U.S. As you
can see, in residential
buildings, the largest
percentage of energy is
devoted to space heating—
nearly 35%; however, in
commercial buildings, space
heating comprises only about
16% of the total building
energy end-use.
This tells us that measures that reduce heating loads (passive solar design, increased
levels of insulation, windows with a lower u-value) play a larger role in designing carbonneutral houses than in designing carbon neutral commercial buildings.
© 2009 Autodesk
The Design Process Overview – Steps to Carbon Neutral Design
1.
Set Savings Target
2.
Optimize Site and Building Form and Openings
3.
Minimize Internal and External Loads
4.
Select HVAC System
5.
Onsite Renewables
6.
Commissioning
7.
Purchase Green Power & Carbon Credits
© 2009 Autodesk
The Design Process Overview – Steps to Carbon Neutral Design
Set Savings Target
How will you know if your design has “arrived” if you do not first determine a destination?
Without establishing an energy savings target, the architect is “working blindly.”
There are many Fossil Fuel Reduction Goals established by prominent organizations, including the following:
• A carbon neutral building, as defined by ASHRAE Standard 189, the U.S. EPA (draft only) and others.
• A net-zero energy project annually consumes only what it produces.
• American Institute of Architects – “Promote integrated/high performance design including resource
conservation resulting in a minimum 50% or greater reduction in the consumption of fossil fuels used to
construct and operate new and renovated buildings by the year 2010 and promote further reductions of
10% or more in each of the following 5 years.” - High Performance Building Position Statements ©2005,
The American Institute of Architects, Washington, DC.
• Architecture 2030 – “All new buildings and major renovations reduce their fossil-fuel GHG-emitting
consumption by 50% by 2010, incrementally increasing the reduction for new buildings to carbon neutral
by 2030.”
• U.S. Green Building Council – All new commercial LEED projects are required to reduce CO2 emissions by
50 percent when compared to current emission levels.
• U.S. Conference of Mayors – Adopted the “2030 Challenge” for City Buildings and calls on the Conference
of Mayors to increase the fossil-fuel reduction standard for all new buildings to carbon neutral by 2030
© 2009 Autodesk
Optimizing the Building Envelope with Respect to Daylighting
Design Goals
A general rule of thumb is that the daylighting
zone may be considered to be a depth of about
1-1/2 to 2 times the window head height.
For example, if a space has windows with a
head height of six feet, it may be feasible to
daylight up to about 12 feet deep into the space
(assuming no internal partitions block the light).
A narrow building will have a greater proportion
of its interior in the usable daylight zone.
Daylight penetration can be increased to up to
2.5 times the window head height by using light
shelves or other means of reflecting the light
farther into the space.
Light shelves simultaneously shade and reflect incoming daylight
Source “Daylighting” http://www.arch.hku.hk/~kpcheung/teaching/b4link/b4-link1.htm
© 2009 Autodesk
Window Materials, Light Levels, and Thermal Tradeoffs
Cool daylighting design carefully controls light entering the building using several key
techniques:
• Exterior shading prevents too much direct sunlight from entering the building.
• Carefully placed windows admit light up high, where it illuminates the ceiling (simulating
our experience of the sky) and shines deep into a building's interior.
• High-performance, low-transmittance glass prevents glare from interfering with computerbased tasks, but the visible transmittance of the glass must be balanced with the need to
bring in sufficient, high-quality daylight.
• High performance glazing (low solar heat gain) also generally reduces the required size of
the HVAC system, peak electrical demand, lifetime building energy use, and ongoing
maintenance costs for a facility. In addition, through first cost reallocation, it is also less
expensive than that portion of the HVAC system it replaces.
• Window blinds offer extra shading when the sun is low in the sky or if total darkness is
needed.
• Paint and fabric colors, when carefully chosen, can maximize reflected light and minimize
potential glare.
(Source: The Daylighting Collaborative, http://www.daylighting.org)
© 2009 Autodesk
Window Materials, Light Levels, and Thermal Tradeoffs
Once daylight enters a room, the surrounding wall, ceiling, and floor surfaces become important
light reflectors.
The most important interior light-reflecting surface is the ceiling.
Using high reflectance surfaces will better bounce daylight around the room and also reduce
extreme brightness contrast.
The daylight that arrives at a work surface comes from three sources:
1.
2.
The exterior reflected component, which includes:
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Ground surfaces
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Pavement
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Adjacent buildings
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Wide windowsills
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Other objects
The direct sun/sky component
3.
The internal reflected component
© 2009 Autodesk
Modeling Daylight
Modeling of sunlight on the exterior facade of a building, and how it casts shade and
creates shadows on various surfaces, is relatively easy.
Most 3D CAD applications can model the movement of the sun in the sky
at any time of the day and year for any location globally.
These tools are now increasingly able to model the contributions of diffuse
light from the sky and interreflection.
Modeling daylit interiors is a far more challenging matter.
Most sunlight entering inside the building needs to be controlled by
numerous fenestration features that help to diffuse, redirect, and
distribute it.
Three tools that capture all these aspects of daylight design are Radiance
(http://radsite.lbl.gov/radiance/) and
Autodesk® Ecotect® and Autodesk® 3ds Max® software.
The Autodesk Green Building Studio tool automatically analyzes qualification for the LEED
daylighting credit.
© 2009 Autodesk
Windows
Properly placed windows and other openings are indispensable for both daylighting and natural
ventilation strategies.
Natural ventilation cooling strategies must be complemented by effective control of solar gains.
Typically, light from a window is able to penetrate a depth of approximately 15 feet into a space
for daylighting purposes.
Some important definitions relating to glass and windows are:
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U-value or U-factor indicates how well a window resists conduction. The rate of heat conductivity is
indicated by the U-value (inverse of the R-value) of a window assembly. The lower the U-value, the
greater a window’s resistance to conductive heat flow and the better its insulating value.
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SHGC measures a window's ability to block radiant heat transfer, typically from sunlight. The SHGC is
the fraction of incident solar radiation admitted through a window. SHGC is expressed as a number
between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits.
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Visible light transmittance (Tvis or VLT) measures the transmission of visible light. The visible
transmittance is an optical property that indicates the amount of visible light transmitted. Most VLT
values are between 0.3 and 0.8. The higher the VLT, the more light is transmitted. A high VLT is typically
desirable to maximize daylight; however, too much light transmission can cause glare.
© 2009 Autodesk
Skylights and Roof Glazing
How much roof glazing is enough?
For 100% daylighting from traditional skylights, approximately 5% of roof area should be in
skylights, (5% Skylight-to-Roof Ratio or SRR).
In general, a high VLT/SHGC ratio is desirable, but the ideal skylight specification depends on
climate. All climates should have high visible transmittance (Tvis or VLT).
Hot climates should have low SHGC. It is almost always cheaper to use skylights to light a space
than photovoltaic panels to generate renewable electricity that would be used in turn to power
electric lighting.
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Tubular skylights require a much lower SRR than traditional skylights, approximately 1-2%.
U-value is more important in cold climates and has energy and condensation implications:
the lower the U-value, the greater the insulation value.
Dampers (to darken space) are available with most skylights.
© 2009 Autodesk
Management of Solar Heat Gain
Once you have explored a variety of daylighting strategies, it is essential to consider the
implications each of these strategies has on solar heat gain.
BIM tools can enable designers to assign measures of density, solar absorption, and solar
transmittance relevant to the analysis of solar heat gain to each specific building component,
including:
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External exposed thermal mass
Windows and doors
Curtain walls, masonry, and envelopes
Internal exposed thermal mass (walls, floors ceilings and casework, and so on)
Insulation
BIM tools can be leveraged to enable a designer to experiment with and consider the following
in their design:
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Experiment with a variety of building massing models and layouts in a variety of
orientations to discover how they affect penetration of daylight.
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Experiment with a variety of reflective devices, including light shelves.
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Experiment with a variety of wall and ceiling treatments (exposed materials, colors,
shades and tints of paint, tiles, and other surface treatments), and also consider the
psychological impacts of color.
Consider different glazings with varying visible light transmittances and tints.
© 2009 Autodesk
Management of Solar Heat Gain
Solar Heat Gain Rules of Thumb
In cold climates, for optimal performance:
• Heat-gaining spaces should face within 15 degrees of true solar south. In most parts of the
U.S., optimizing for solar orientation can cut total energy usage by 30 to 40 percent at no
additional cost.
• The building's south face should receive sunlight between the hours of 9:00 a.m. and 3:00
p.m. (sun time) during the heating season.
• Expose thermal mass and its properties during the heating season; in a direct gain system,
the thermal mass floors and walls are functional parts of the building.
• For every square foot of south-facing glass, use 150 pounds of masonry or 4 gallons of water
for thermal mass.
The surface area of mass should be six times the area of the accompanying glazing; less may be
necessary in overcast climates. Consider different glazings with varying visible light
transmittances and tints.
© 2009 Autodesk
Material Choices for Management of Solar Heat Gain
Metal Framing: While valued for its structural benefits and recycled content, metal framing has the
drawback of a lower assembly insulation value than a wood-framed component has because of the high
conductivity of the metal. It is critical to ensure that metal-framed buildings use thermal breaks, both for
energy and moisture reasons.
Wood Framing: Wood construction offers a better overall insulation quality than steel, but it is not suitable
for high-rise construction. Also, wood is vulnerable to insect damage and rot.
High Mass Construction Materials: The use of concrete/brick/stone and other high mass materials for
building facades is typically limited to low-rise buildings for cost and structural reasons. In climates with
large diurnal temperature swings, high mass buildings may be able to store energy if the building’s interior
temperature is allowed to float and if the mass is “charged” during the warm or cool hours.
Insulation (R-Value): Does your project need a low conduction (High R-value) roof and/or walls for high
performance? It depends on the building type, climate, and occupancy schedule. You need to perform whole
building energy analysis on your project to answer the question.
Continuous Insulation: Insulation that covers the framing members and thereby provides a greater insulation
value. This is especially important for metal framing to control condensation.
Cool Roof: The use of reflective roofing materials or cool roof systems is one means of keeping solar heat out
of buildings and increasing energy savings. Cool roofs (generally white) stay as much as 70 degrees F cooler
at peak times than traditional asphalt roofs, offering important benefits to building owners and also
protecting the environment from the negative effects of urban heat islands.
glazings with varying visible light transmittances and tints.
© 2009 Autodesk
Material Choices for Management of Solar Heat Gain
Green or Living Roof: Green roofs, garden roofs, or planted roofs are “specialized
roofing systems that support vegetation growth on rooftops.”
Researchers have analyzed the thermal performance of green roofs using three
different approaches: field experimentation, numerical studies, and a combination
of laboratory or field experiments with numerical models.
All three types of analysis have confirmed that green roofs can significantly reduce
heat transfer between building and outside environment.
This reduction in heat transfer can lead directly to energy savings and cost reduction
for the building owner.
One study measured a 75% reduction in average daily demand for space
conditioning. glazings with varying visible light transmittances and tints.
© 2009 Autodesk
Material Choices for Management of Solar Heat Gain
Non-Traditional Construction Materials and Methods
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•
Structural Insulated Panels (SIPs) are rigid panels of foam insulation joined to oriented strand board
(OSB). They are typically used as floors, walls, and roofs on smaller buildings. Benefits of using SIPS in
construction generally include lower labor cost, less job-site waste, shorter construction time, and
better-insulated buildings. SIPS are prefabricated in the factory with window openings and
electrical/plumbing chases. Especially in the case of larger projects, the use of SIPS should be
considered based on economies of scale. Outside finishes can be customized to meet whatever “look”
is desired. More information about this technology, including links to suppliers, can be found at
www.sips.org.
Insulated Concrete Forms (ICFs) are forms/molds or hollow blocks with built-in insulation. These are
stacked and filled with reinforcing bar and concrete.
Both SIPS and ICFs have a typical insulation value of approximately R5 per inch. ICFs also have thermal mass
benefits. Both also reduce infiltration/noise.
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Straw Bales: Many factors can affect the insulating quality of a straw bale wall, including moisture
content, bale size, and bale orientation. Autodesk Green Building Studio has adopted the California
Energy Commission’s default assumption insulation value of R-30 for Straw Bale construction.
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Radiant Barriers: Radiant barriers consist of shiny “foil” like materials that lessen the ability of a hot
surface to transfer heat by radiation to adjacent surfaces. Consider specifying a layer of radiant barrier
film to the underside of the roof decking for those areas with plenums, and foil-faced insulation for
those areas where the insulation is at the roof deck. This measure effectively cuts down the radiant
heat entering the space directly below the roof, which means mechanical systems face lower cooling
loads.
with varying visible light transmittances and tints.
© 2009 Autodesk
Natural Ventilation
Natural ventilation occurs when outdoor air is introduced to a space without the use of
mechanical systems, such as a fan.
Most often, natural ventilation is assured though operable windows but it can also be achieved
through temperature and pressure differences between spaces.
There are two types of natural ventilation for buildings: wind driven ventilation and stack
ventilation.
The pressures generated by buoyancy, also known as "the stack effect," are quite low (typical
values: 0.3 Pa to 3 Pa), while wind pressures are usually far greater (~1 Pa to 35 Pa).
Although the majority of buildings rely primarily on wind-driven ventilation, stack ventilation also
has numerous benefits. The most efficient design for a natural ventilation building implements
both types of ventilation.
To adequately ventilate a building using the stack effect, the inside and outside temperatures
must be different so that warmer indoor air rises and escapes the building through higher
apertures, while colder, denser air from the exterior enters the building through lower level
openings.
The stack effect increases with greater temperature differences and increased height between
the higher and lower apertures.
© 2009 Autodesk
Natural Ventilation Rules of Thumb
In order to adequately ventilate a building via the stack effect, the inside and outside temperatures must be
different so that warmer indoor air can rise and escape the building at higher apertures, while colder, denser
air from the exterior can enter the building through lower level openings
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Do not obstruct inlet and outlet openings with nearby objects.
Locate windows in opposing pressure zones since this usually will increase ventilation rate.
Keep enough vertical distance between apertures to produce stack effect.
Avoid multiple openings at the same level and near the ceiling or else much of the air flow may bypass
the occupied zone.
Use architectural elements like wing walls, parapets, and overhangs to promote air flow into the building.
Use topography, landscaping, and surrounding buildings to redirect airflow and give maximum exposure
to breezes.
Maximize air velocities in occupied zones to promote bodily cooling if you live in a hot, humid climate.
Orient the long façade of the building and the door and window openings with respect to the prevailing
wind direction to admit wind airflow.
Make window openings accessible to and operable by occupants if possible.
Use vertical shafts and open staircases to increase and generate stack effect.
Reduce the number of openings near the neutral pressure level because they are less effective for
thermally induced ventilation.
Ensure that all inlet and outlet openings are of nearly equal areas to obtain a balanced and greater
ventilation.
(“Air Movement and Natural Ventilation,” Dr. Sam C M Hui Department of Mechanical Engineering, The University of Hong Kong,
http://www.arch.hku.hk/teaching/lectures/airvent/sect03.htm).
© 2009 Autodesk
Compressorless Cooling
In Persia, during the Medieval era, builders created a natural cooling technique that is still in use today in
modern Iran. Buildings were built over cisterns and capped with wind towers to cool subterranean rooms
during the hot season. The cisterns are large, open, below-ground pools that are fed by subterranean water
canals called qanats.
The air from the qanat is drawn into the tunnel at some distance away and is cooled both by contact with the
cool tunnel walls and water, and by giving up latent heat as water evaporates into the air stream. In dry desert
climates, this can result in a greater than 27°F/15°C reduction in the air temperature coming from the qanat.
This technology has been in use for over 1,000 years.
Left: Traditional cisterns with double domes and windcatchers in the central desert city of Naeen, near Yazd, Iran. Photo by Zereshk , 2005,
Wikimedia Commons. Creative Commons Attribution ShareAlike 3.0 License, http://creativecommons.org/licenses/by-sa/3.0/
Right: Windcatcher used for cooling in conjunction with qanat. Illustration based on concepts as discussed in article by Bahadori, M. N.,
"Passive Cooling Systems in Iranian Architecture" Scientific American, February 1978, pp. 144-154,
© 2009 Autodesk
Compressorless Cooling
Other non-mechanical approaches to air-cooling include:
Night sky cooling, which cools water by spraying it over a building roof at night. Once cooled, the water is
collected from the roof, filtered, and stored. It can be used the following day to cool the building either in
conventional water-cooled HVAC systems or by passive cooling of the roof or floor.
Evaporative coolers use water to cool air. Air blowing through a wet medium (a bathing suit, cellulose
membranes, and so on) evaporates some of the water and its dry bulb temperature is lowered. The cooling
effect of an evaporative cooler depends on two factors. The first is level of humidity (the difference between
the air’s dry bulb and wet bulb temperatures); the second is the cooling device’s efficiency.
Evaporative coolers typically come in three general designs:
•
Direct evaporative cooling. With direct evaporative cooling, outside air is blown through a watersaturated medium (usually cellulose) and cooled by evaporation. A blower circulates the cooled air. Direct
evaporative cooling adds moisture to the air stream until the air stream is close to saturation. The dry
bulb temperature is reduced, while the wet bulb temperature stays the same.
•
Indirect evaporative cooling. With indirect evaporative cooling, a secondary (scavenger) air stream is
cooled by water. The cooled secondary air stream goes through a heat exchanger, where it cools the
primary air stream. A blower circulates the cooled primary air stream. Indirect evaporative cooling does
not add moisture to the primary air stream. Both the dry bulb and wet bulb temperatures are reduced.
During the heating season, an indirect system’s heat exchanger can preheat outside air if exhaust air is
used as the secondary air stream.
•
Indirect/direct evaporative (IDEC) cooling. With indirect/direct evaporative cooling, the primary air
stream is cooled first with indirect evaporative cooling, and then cooled further with direct evaporative
cooling.
© 2009 Autodesk
Onsite Renewable Energy
Electrical energy can be generated from a variety of renewable sources that are available on the
building site.
These sources can include:
•
•
•
•
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Solar photovoltaic power
Wind power
Micro-hydroelectric power
Tidal/wave power
Renewable biofuels
Regents Quarter development at Kings Cross, London, UK, with wind
turbines on the roof, photo by Judith, London, UK, 2008, Wikimedia
Commons, Creative Commons Attribution 2.0,
http://creativecommons.org/licenses/by/2.0/
© 2009 Autodesk
Photovoltaic solar panels on the roof of a house
near Boston, Massachusetts, photo by Gray
Watson, 2006, http://256.com/solar/, Wikimedia
Commons, Creative Commons Attribution
ShareAlike 3.0 License,
http://creativecommons.org/licenses/by-sa/3.0/
Voltage profile of an on site wind power turbine generator, Marco Ortiz, Juan
Ríos, Manuel Acosta, “Wind Generation and Power System Interaction
Analysis Using Probabilistic Techniques”, Electrical Systems Planning Division
CVG EDELCA Caracas, Venezuela, "2008 International Conference on
Renewable Energy and Power Quality (ICREPQ´08)," Santander, Spain, 2008,
http://www.icrepq.com/icrepq-08/277-ortiz.pdf.
Summary
The design of the building envelope for carbon neutrality can be optimized through a
variety of daylighting, passive solar heating and cooling, and natural ventilation
techniques. The renewable energy producing potential of a building site can also be
effectively modeled.
© 2009 Autodesk
Autodesk, AutoCAD, Civil 3D, Ecotect, Green Building Studio and Revit are registered trademarks or trademarks of
Autodesk, Inc. and/or its subsidiaries and/or affiliates, in the USA and/or other countries. All other brand names, product
names, or trademarks belong to their respective holders. Autodesk reserves the right to alter product offerings and
specifications at any time without notice, and is not responsible for typographical or graphical errors that may appear in
this document.
© 2009 Autodesk, Inc. All rights reserved
© 2009 Autodesk
.