Autodesk Sustainable Design Curriculum
Lesson Five: Design of the Building Envelope
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Daylighting
Glazing
Use the Outdoor Environment
Wall and Roof Assemblies
Cool Roofs
© 2009 Autodesk
Why Spend So Much Effort on Daylighting?
Data Source: 2005 Buildings Energy Data Book
© 2009 Autodesk
What is Wrong with This Picture?
© 2009 Autodesk
Myths from the Daylighting Collaborative
Myth: Costs More
Fact: Not if done using integrated design approach. Decreased internal load from
electric lighting allows HVAC equipment downsizing.
Myth: Complicated
Fact: No need to reinvent the wheel. Many resources are available.
Myth: Too Much Heat Gain
Fact: Light-to-heat ratio is better than using efficient electric lights. Proper
design can block most of the sun’s heat and provide 50 fc of light.
*Source of myths & facts: Daylighting Collaborative. http://www.daylighting.org/why.htm
© 2009 Autodesk
More Myths
Myth: Causes glare.
Fact: Proper designs use shading and appropriate glass to avoid too much light.
Myth: It is better to upgrade lighting and HVAC efficiency.
Fact: It is better to reduce need for electric lighting and cooling in first place
with the right building form and properly designed daylighting.
Myth: Requires use of clear glass.
Fact: Clear glass may work well for the “daylighting” glass, but depending on
opening size may cause excessive glare to be used as vision glass.
Source of myths & facts: Daylighting Collaborative. http://www.daylighting.org/why.htm
© 2009 Autodesk
More Myths
Myth: Requires use of skylights.
Fact: Building form is crucial. In many situations windows and clerestories can
provide most of the light needed.
Myth: Only works on sunny days.
Fact: Even overcast skies provide 5,000 or more foot-candles—most spaces
need no more than 50 fc. The amount of daylight through a window can be
HIGHER on a day with a thin cloud layer than with clear skies.
Myth: Requires an all-glass building.
Fact: All-glass buildings have problems with heat gain and glare. Windows in
daylit buildings typically are not different from non-daylit buildings—just
better planned. Start with a band of glass, and see how much you need.
Source of Myths & Facts: Daylighting Collaborative.
http://www.daylighting.org/why.htm
© 2009 Autodesk
Daylighting
Elec. Lighting Reduction for South Facing Office Space
Percentage Reduction in Electric Lighting from
Daylighting
Assumes:
Electric light comes on when
daylight is below 50 fc.
DEC
NOV
OCT
SEP
AUG
JUL
M o nt h
JUN
M AY
APR
Glare can be controlled by
window shades. Glare is not a
major issue for south-facing
glazing with well-designed
overhangs when the sun is high.
M AR
FEB
JAN
1
2
3
4
5
6
7
8
9
10
11 12
13 14
15 16
17 18
19 20 21 22 23 24
Ho ur
0-20
© 2009 Autodesk
20-40
40-60
60-80
Even light distribution is very
important and can be more
important than the absolute
illuminance level.
Skylights: Big Box Retail Example
Least energy use: 8%
skylight to roof ratio
(SRR)
4 – 5% SRR delivers
significant savings,
with reduced capital
costs
8% SRR is not
optimal from a
capital cost/ROI
perspective
Incremental lighting
savings decrease and
cooling penalty
increases past 5%
SRR.
Electric lighting must
be controlled to
realize savings
© 2009 Autodesk
Cooling energy increases
faster than lighting savings
Natural Light – How much is enough?
For 100%
daylighting
from skylights
– approx. 5%
of roof area
should be in
skylights, (5%
SRR).
Depends on
Building Type,
Tvis., SHGC.
The skylight
specifications
depend on
climate. All
climates
should have
high visible
transmittance
(Tvis). Hot
climates
should have
low SHGC.
Rectangular with Louvers
Skylight
Tubular
skylights
require a
much lower
SRR, approx.
1-2%.
U-value is
more
important in
cold climates
and has
energy and
condensation
implications.
Dampers (to
darken space)
are available
with most
skylights.
Tubular
© 2009 Autodesk
Skylights/Windows – What Size?
SKYLIGHTS
 Size of skylight Estimate = (Floor to Ceiling Height x 1.5)2 x SRR = area of one skylight.
Choose a size that is appropriate. Start with 5% SRR and modify depending on climate
and building use. More windows => fewer skylights. In a high rise…skylights may not be
possible/useful.
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For example with a 12' ceiling and 5% skylight-to-roof ratio the right size skylight would
be approximately:
(12 x 1.5)^2 x 5% = 16.2sf
=> should use a 4x4 or 8x2' skylight for good light distribution.
WINDOWS
 Great deal of variation in window area depending on climate and building type.
 In a sunny climate, a 2.5 – 3 ft. perfectly clear opening (does not exist) will provide 40 –
50 fc in perimeter offices. If the glass has a visible transmittance of 50%, you need twice
as much glass to achieve the same amount of light.
 Glazing should be divided into “daylighting glass” and vision glass. A light shelf of
approximately 2 – 2.5 ft. shades the lower vision glass and distributes light from the
daylighting glass above.
© 2009 Autodesk
Common Question –
Should I use PVs or skylights on the roof?
Skylights and PVs in a School
Turning Off vs. Powering a Lighting System
Cover a 5x6' roof area with PV or Skylight
1,200
1,000
$1,115 w rebates
$900 w/o rebates
800
kWh/yr Energy
OR
600
$ Installed Cost
400
The Small Print
•1500 hrs/yr daylight/sunlight that
can be used.
•10% PV System Efficiency
•Automatic controls turn off electric
when daylight is available.
•PV cost - $8/Watt without incentives,
$4/Watt with incentives
•No rebates assumed for skylight
•Northern California climate and
rebate assumptions
200
0
Skylight/Lighting Control
Avoided/Produced kWh per YEAR
PV System
First Cost including Incentives
Cost per kWh over 10 yrs
Photovoltaic - $0.267/kWh
Daylighting - $0.12/kWh
© 2009 Autodesk
Site, Shading, and Daylighting – Autodesk Ecotect
Environmental Analysis
Desktop Application
Schematic
•Lighting
•Daylighting/Shading
•Overhangs
•Also used as a visualization
interface for analyses that
take place outside of Ecotect
•Very visual, presentation
graphics
© 2009 Autodesk
Daylighting
© 2009 Autodesk
Site, Shading and Daylighting - A Few Graphics
© 2009 Autodesk
© 2009 Autodesk
The Building Envelope – Modes of Heat Transfer
Conduction
Convection
Radiation
Which is most
important?
Modes of Heat Transfer in Residential Buildings
© 2009 Autodesk
Heat Transfer
Conduction
Through Solids
Convection
Through Fluids
(gases and
liquids)
Dependent on
Velocity
Radiant
Heat
Transfer
Proportional to
T4 Difference
Does not
Require Matter
for Transmission
© 2009 Autodesk
Alternative Envelopes
Structural Insulated Panels (SIPS) - rigid
panels of foam insulation joined to OSB. Typically
used as floors, walls, and roofs on smaller buildings.

Insulated Concrete Forms (ICFs) are forms/molds
or hollow blocks with built-in insulation. These are
stacked and filled with reinforcing bar and concrete.
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Both SIPS and ICFs approximately R5 per inch, ICFs
have thermal mass benefit. Both also reduce infiltration/
noise, mostly residential/light commercial.
Many variants – Benefit is dependent on climate.
Do not obsess on a single component (i.e., opaque envelope) – The right question is:
Do I need low conduction (High Rval) for high performance? Depends on building
type, climate, and occupancy pattern.
© 2009 Autodesk
Alternative Envelopes – Metal Framing
R-19 in metal framing => Utotal = .183 = R 5.5 with metal framing
R-19 in wood framing => Utotal = .074 = R 13.5 with wood framing
Do I need low conduction (High Rval) for high performance?
Depends on building type, climate, and occupancy schedule.
House in Maine – High R-value – Good, winter dominated climate.
Retail store in Los Angeles – High R-value may increase energy use.
Develop a theory and test your assumptions by analyzing the impact of improving the
thermal envelope
© 2009 Autodesk
Case Study - Community Center
Question: Will this project benefit from high R-value, but
expensive wall/roof systems, or should the resources be spent
on other efficiency measures?
Project Description
 Community Center – 21,000 square feet
 Location – Northern Ca
 Building Form – As shown below
© 2009 Autodesk
Wall Insulation Sensitivity
Wall Insulation Sensitivity--R19 Roof
Total Building Energy Use (MMBtu/year)
1,600
1,400
1,200
1,000
San Jose, CA
800
W. Palm FL
Anchorage, AK
600
400
200
0
R7 Wall
© 2009 Autodesk
R11 Wall
R15 Wall
R19 Wall
Roof Insulation Sensitivity
Roof Insulation Sensitivity--R11 Walls
For this building form, adding
insulation beyond code
minimum has only minimal
benefit in San Jose, CA and
no benefit in Miami. There is
considerable benefit in
Fairbanks, AK.
Total Building Energy Use (MMBtu/year)
1,600
1,400
1,200
1,000
San Jose, CA
800
W. Palm FL
Anchorage, AK
600
400
200
0
R11 Roof
© 2009 Autodesk
R19 Roof
R30 Roof
R38 Roof
© 2009 Autodesk
National Fenestration Rating Council
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Windows require NFRC label for factory built windows
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Label must contain:
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U-value
Solar Heat Gain Coefficient (SHGC)
Visible Light Transmittance
Air Leakage Value
© 2009 Autodesk
Windows: U-Factor (Conduction)
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The U-factor defines how well a window prevents conductive heat flow.
 The rate of heat conductivity is defined by the U-value (inverse of the Rvalue) of the window assembly. The lower the U-value, the greater a
window’s resistance to heat flow and the better its insulating value. (source:
Efficient Windows website)
The definition of a “good” U-factor depends on the building type and location.
 In Los Angeles – U-factor does not matter as much.
 In a cold climate, a low U-factor is important – 0.25 - 0.40 Btu/hr-ft2-degF.
 Low U-factors are important for skylights in cold climates for both energy
and water condensation reasons.
© 2009 Autodesk
Windows: Solar Heat Gain Coefficient (SHGC) Radiation
The SHGC measures the portion of incident solar energy that passes through a
window.
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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.
The definition of “good” depends on building location and the amount of
glass.
 In Tucson, Miami, and Dallas a low is beneficial – 0.20 - 0.35 would be good.
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In Fairbanks, AK a high SHGCis Good – 0.5-0.7 would be good.
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SHGC is very important for windows/skylights in hot climates and for
buildings with a large amount of unshaded glass.
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Some building designs vary the SHGC of the glass by orientation.
© 2009 Autodesk
Windows: Visible Light Transmittance (VLT or VT)
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Visible light transmittance measures the portion of visible light that passes
through a window.
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The NFRC's tabulation of VT is a whole window rating and includes the impact
of the frame, which does not transmit any visible light. Most VT values are
between 0.3 and 0.8. The higher the VT, the more light is transmitted. A high
VT is typically desirable to maximize daylight.
Definition of Good Depends on Building Type/Amount of Glass.
 Too high and => glare if large glass area.
 Too low and => spent money on glass and did not capture enough daylight.
 Modern selective coatings combine high VLT and low SHGC – Do not need
mirrored glass to get low heat gain.
 High performance glass can have 65% VLT, 30% solar heat gain and 0.30 Uvalue.
© 2009 Autodesk
I Specify Low-E Windows…Good Enough?
All of the glass listed above is Low-E.
-Some of the glass is suitable for Fairbanks, Alaska
-Some is suitable for Fresno, California
You would not need such a low “U-value” in Fresno, and could use an even lower
one in Fairbanks, AK.
© 2009 Autodesk
Radiant Barriers
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Reduce heat transfer from the underside of the roof deck—primarily used
in residential and small commercial.
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Radiant barriers/foil faced insulation are either metallic foils or plasticized
foils that reduce the amount of radiant heat transfer between the roof and
the plenum or conditioned space. Radiant barriers should be installed
with the shiny side facing the interior of the building.
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Radiant barriers require air spaces to be effective. If a radiant barrier is in
contact with sheathing or facing the wrong direction, they are not
effective.
© 2009 Autodesk
Shading and Solar Studies
Use BIM tools for shading studies. The combination of
tools enable you to visually control direct solar (such as
Autodesk® Ecotect ®) and quantify Energy/cooling load
impacts (such as Autodesk® Green Building Studio®).
© 2009 Autodesk
Cool Roofs
•Rolled, BUR, composition roofs
absorb between 70-90% of incident radiation.
•Cool roofs absorb 25-40% of
incident radiation (aged performance.)
•Trade-off with roof insulation.
•In-house tests of coating on pitched
new composition roof—disappointing.
•Better luck on flat roofs.
•San Francisco vs. Tucson—where is a cool roof more important?
California Requirement as an example
Applies to Low Slope Roofs (2:12 slope or less)
Minimum solar reflectance of 0.70
Minimum Thermal Emittance of 0.75
Certified by Cool Roof Rating Council
© 2009 Autodesk
More information
www.coolroofs.org
www.coolroofs.info
www.energystar.gov
http://eetd.lbl.gov/coolroofs/
Time Erodes the Cool Factor
17°F
Fresh application
of cool roof coating
© 2009 Autodesk
8°F
10-month old
cool roof coating
Uncoated “light”
asphalt shingles
A Cool Roof Coating on a Composition Roof - Results
151.9F
160
143.6F
150
140
130
120
Temperature F
110
100
90
80
70
60
50
40
roof standard
aged roof cool coating
outdoor ambient
30
© 2009 Autodesk
Cool Roof Conclusions
Asphalt shingles are cheap and prevalent. All roofers know how to install
them. BUT, they are a very poor choice for reflecting incoming solar
radiation.
The solar reflectance of all commercial asphalt shingles is low (premium
white shingles are only about 30% reflective, and other colors reflect
less.) The low solar reflectance can be attributed to several factors.
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Limited amount of pigment in the granule coating.
Roughness of the shingle contributes to multiple scattering of light, and thus
to increased absorption.
The black asphalt substrate is not 100% covered, and reflects only about 5%
of the light that strikes it.
© 2009 Autodesk
Why do I need AC when it is 60 degrees outside?
20
80.0
18
16
75.0
65.0
14
12
60.0
10
55.0
8
6
70.0
50.0
40.0
4
2
35.0
0
Tim e
Supply air typically at approximately
50 - 55 degrees
© 2009 Autodesk
8/6 19:12
8/6 16:48
8/6 14:24
8/6 12:00
8/6 9:36
8/6 7:12
45.0
Unit Power (kW)
Outside air – 60 – 70
deg F this day.
85.0
8/6 4:48
Air Tem perature (°F)
Oakland AC-4 Air Temperatures and Unit Power
Supply
Return
Outside
Mixed
Unit Pow er
Are we addicted to air-conditioning?
AC is assumed for new buildings.
 Buildings are uninhabitable without AC.
 Thermal comfort is #1 occupant complaint.
 AC systems can have comfort and IAQ problems.

Need to think in terms of comfort and not in terms of tons of cooling.
Source: Erik Ring, PE, Glumac, April 2007 Presentation at the PEC
© 2009 Autodesk
Most of the World’s Roofs
Packaged airconditioning
units.
This is the
fresh
air intake
© 2009 Autodesk
The More Difficult But More Efficient Cooling
Strategy: Passive/Low-Energy Cooling
Natural Ventilation
 Thermal mass
 Evaporative Cooling
 Backup with compressors where needed
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This type of scheme is
HARDER than traditional
HVAC, even though it may
require fewer tons of cooling and will save energy.
© 2009 Autodesk
Bioclimatic Charts
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Examines four passive cooling strategies:
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Natural Ventilation
High Thermal Mass
High Thermal Mass with Night Ventilation
Evaporative Cooling
Limitations
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Charts typically applicable for residences and projects with small internal heat
gain.
For more information: G.Z. Brown. Sun, Wind, and Light. John Wiley &
Sons, 1985. Pg 50
© 2009 Autodesk
Natural Ventilation
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Design issues:
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Shade
Must have inlet and outlet for air
Building should have thin cross-section
Challenges
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Not effective above 90ºF
More difficult at higher humidity
© 2009 Autodesk
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High Thermal Mass
Design issues:
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Shade
Need wide daily temperature variation straddling comfort zone
Isolate from exterior temperatures
Mass should be distributed
Challenges
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Not effective above 95ºF
More difficult at higher humidity
© 2009 Autodesk
Evaporative Cooling
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Design issues:
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Shade
Requires air movement
Provides greatest savings at highest temperatures
Challenges
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Not effective at high humidity
Evaporative media and pump requires maintenance
Requires water
© 2009 Autodesk
Autodesk, Ecotect, and Green Building Studio 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
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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
.