Understanding U-Factors
Integrated Façade Design (IFD) recommendations
and training draw on the technical expertise of all
Apogee architectural business units, to bring
balanced and competitive energy solutions to the
design community.
©2010
Viracon
Apogee Enterprises
Tubelite Linetec Harmon
Wausau
Apogee Enterprises is a Registered Provider with The American Institute of Architects Continuing
Education Systems. Credit earned on completion of this program will be reported to CES Records for AIA
members. Certificates of Completion for non-AIA members available on request.
This program is registered with the AIA/CES for continuing
professional education. As such, it does not include content that
may be deemed or construed to be an approval or endorsement
by the AIA of any material of construction or any method or
manner of handling, using, distributing, or dealing in any
material or product.
Questions related to specific materials, methods, and services
will be addressed at the conclusion of this presentation.
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© Apogee Enterprises, Inc. 2010
Learning Objectives
Upon completion of this program you will be able to:
• Identify five attributes affecting window U-Factor.
• Employ at least three design options to improve window UFactor.
• Differentiate between NFRC and AAMA testing and certification
processes.
• Draft non-defective specification requirements for U-Factor.
• Recognize other important energy related window design factors
and the importance of a balanced design.
Introduction to
U-Factors
Model Energy Codes are requiring
better performance for many reasons
Building operating costs
HVAC capacities $$
Useable perimeter space
Occupant comfort and productivity
Sanitary conditions
View
Maintenance damage
Need for secondary glare control
Environmental responsibility LEED™
If every existing window in the U.S. was upgraded ONLY to the level of double insulating glass, we
would save 2% of the country’s gross energy consumption. (Energy Act of 1991)
Product Selection Concerns
Window selection based upon only one performance number.
Calculating projected energy savings. Level of confidence.
Accountability for the actual results.
Uncertainty.
Calculations based upon clearly defined numbers rather than
perception.
We will help you understand the
fine print.
Definition of U-Factor
U-Factor (a.k.a. U-Value) is a measure of thermal transmittance.
(conduction, convection, radiation)
Heat flow per unit area, time, and °F temperature difference
(units are BTUs/ ft2-hr-°F)
U-Factor allows the HVAC engineer to calculate peak loads and
energy consumption for any size window in any climate. A key
component is determining a buildings total annual energy costs
U-Factor is the reciprocal of R-Value
U = 1/R
(R-3 is the same as U = 0.33)
Featured Project:
Boston University Life Sciences
Location:
Boston, Massachusetts
Climate Zone 5
Design Team:
Cannon Design
Products:
Triple insulating, argon-fill, low-e glass,
warm edge spacer
Multi-level thermal barrier framing
Performance:
NFRC U-Factor 0.18 BTU/hr. ft2.°F
SHGC 0.25 VT 60%
AAMA CRF 83
STC 40, OITC 32
Design Features
Affecting U-Factor
Components and U-Factor
Frame
0.90+/-
• Center of glass (COG)
• Edge of glass (EOG)
• Frame
Center of
Glass
(COG)
0.29+/-
Assembly U-Factor = The “area weighted”
average thermal transmittance of all
components
( UFRAME . AreaFRAME ) + ( UEOG . AreaEOG ) + ( UCOG . AreaCOG
)
____________________________________________
Total Area
2-1/2”
• Window Area
• Window Configuration
Edge of Glass (EOG)
0.34 +/-
Area and U-Factor
Area = 24 ft2
U-Factor = 0.4
Area = 8 ft2
U-Factor = 0.5
(25% worse)
COG = 3.3 ft2
(41%)
4’
6’
COG = 15.7 ft2
(65%)
EOG = 3.6 ft2
(15%)
FRAME = 4.7 ft2
(20%)
EOG = 2 ft2
(25%)
FRAME = 2.7 ft2
(34%)
2’
4’
Configuration and U-Factor
Vent
U-Factor = 0.4
Fixed over Vent
U-Factor = 0.5
Area = 24 ft2
Area = 24 ft2
(25% worse)
COG = 12.3 ft2
(51%)
6’
COG = 15.7 ft2
(65%)
EOG = 3.6 ft2
(15%)
EOG = 4.6 ft2
(19%)
FRAME = 4.7 ft2
(20%)
FRAME = 7 ft2
(30%)
4’
4’
Frame Type and U-Factor
Thermal barriers in frames also
improve EOG performance
Non thermal
Thermally broken
15mm polyamide nylon
thermal break
Frame 1.32
EOG 0.40
COG
0.29
Frame 0.85 (36% better)
EOG 0.37 (8% better)
COG
0.29
Glass Options
Glass Makeup and U-Factor
1/4” Monolithic
9/16” Laminated
1” Insulating
1-3/4” Triple
Insulating
COG
1.02
COG
0.95
COG
0.47
COG 0.30
Edge
0.93
Edge
0.85
Edge
0.49
Edge
0.39
Glass Coatings and U-Factor
Low-e coatings improve both
COG and EOG U-Factors
1-3/4” Triple
Insulating
Uncoated
1-3/4” Triple
Insulating Double
Low-e Coated
1” Insulating
Uncoated
1” Insulating
Low-e Coated
COG 0.47
COG
0.29
COG
0.30
COG
0.16
EOG
Frame
EOG
Frame
0.37
0.85
EOG
Frame
0.39
0.71
EOG
Frame
0.27
0.69
0.49
0.86
38% better
47% better
Glass Coatings and U-Factor
Coatings added to laminated
glass improve SHGC, but do not
improve COG or EOG U-Factors
9/16” Laminated
Uncoated
9/16” Laminated
Low-e Coated
COG
0.95
COG
0.95
EOG
Frame
0.85
1.01
EOG
Frame
0.85
1.01
NO CHANGE
Glass Spacer and U-Factor
IG Spacers can affect the frame more
than the edge of glass
Stainless steel
“warm edge” spacer
Aluminum spacer
stainless steel
aluminum
Frame 0.85
EOG 0.37
Frame 0.79 (7% better)
EOG 0.35 (5% better)
COG
COG
0.29
0.29 no change
Gas Fill and U-Factor
Gas fills affect the COG more than the EOG
Argon filled
Air filled
1” Insulating low-E coated
COG
EOG
0.29
0.37
Frame
0.85
COG
EOG
0.24 (17% better)
0.34 (8% better)
Frame
0.84
Argon Retention
Over time, argon gas will dissipate from an insulating glass unit.
The rate at which this occurs depends upon the type of edge
seal, the quality of materials, and manufacturing assembly
processes.
In the absence of a formal U.S. standard, industry has accepted
a dissipation rate of 1% per year.
Select an insulating glass manufacturer that is IGCC certified
argon, and uses PIB primary and silicone secondary seals.
PRIMARY SEAL
POLISOBUTYLENE (PIB)
SECONDARY SEAL
SILICONE
Combined Effects on U-Factor
1-3/4” Triple
1-3/4” Triple
1” Insulating
Insulating Double
1” Insulating
Low-e Coated Insulating Double
Low-e Coated,
Low-e Coated, Argon, Stainless Low-e Coated,
Argon, Stainless
Air, Alum Spacer
Air, Alum Spacer Steel Spacer
Steel Spacer
COG 0.29
EOG 0.37
Frame 0.85
GOOD
COG 0.24
EOG 0.32
Frame 0.80
COG 0.16
EOG 0.27
Frame 0.69
COG 0.12
EOG 0.22
Frame 0.64
BEST
Manufacturer’s
U-Factor Claims
Watch out for…
Sizes larger than NFRC test size -May not be acceptable per
local code
1/8” (3 mm) glass thickness
Pillowing, flatness, size limits, coating options, breakage
Residential spacer systems
Sightline, color, radius corners, SSG, size, availability
Flip the window “inside-out”
Outside-glazed vision, lower CRF, appearance
Krypton as gas fill
Cost versus benefit
Suspended films
Wrinkles, corrosion, warranty, replacement cost
For a given glass type, frame sightline, and thermal barrier,
there is little that can be done to lower U-Factor
Featured Project:
Syracuse University Newhouse Center
Location:
Syracuse, New York
Climate Zone 5
Design Team:
Polshek Partnership Architects
Products:
Unitized four-side silicone
curtainwall
Performance:
NFRC U-Factor 0.59 BTU/hr.sqft.°F
SHGC 0.38 VT 70%
AAMA CRF 63
STC 35, OITC 30
Rating and Testing Programs
AAMA
American Architectural Manufacturers Association
NFRC
National Fenestration Rating Council
AAMA
AAMA established in 1936 to develop
standards providing third-party validation of
product performance and quality
Uses physical testing (AAMA 1503.1) to
measure window assembly U-Factor and
CRF (no SHGC).
In 2009, changed single-lite fixed and
operable sizes in AAMA 1503 to be the
same as NFRC.
Offers combination (fixed and
operable) non-residential configurations.
AAMA 507-03
Applicable for storefront, curtainwall, window
wall and other products for commercial
buildings.
Performance is building-specific.
Accounts for vision area and spandrel area.
Accounts for size variation.
Accounts for glass performance.
AAMA 507-03
Building-specific U-Factor
determination from charts
Determine the glass COG U-factor.
(yellow)
Determine the glass % vision area (pink)
Follow the intersection point of those two
lines. (circle point)
Read across to the right for the system U
Factor. (red)
The U factor for this example is
approximately 0.52 BTU/hr.sqft.°F
NFRC
Established in 1989 - A joint effort of
government and residential manufacturers
to create a window labeling system in
response to the need for energy efficiency
Uses physical testing (NFRC 102) and
thermal modeling to determine
Window Assembly U-Factor (NFRC 100) and
SHGC (NFRC 200)
Uses standard residential sizes and
configurations in lieu of those typically
found on commercial projects.
Recently implemented the new component
modeling approach (CMA) program enabling
whole product energy performance ratings for
non-residential projects.
Currently referenced in Model Energy
Codes as a compliance option
NFRC Code Compliance
NFRC labels mandatory only in California and Seattle – Other
jurisdictions have easier alternate compliance paths
The NFRC Certified Products Directory lists all products that can
be labeled
Lower U-Factors based on “project-specific sizes” are often
acceptable
Aluminum windows can be made in larger sizes than PVC,
wood, or fiberglass, and exhibit superior durability, finish,
longevity and strength – Attributes not recognized by NFRC
Model Energy Codes like
ASHRAE 90.1 and IECC cite
NFRC U-Factors, but stop
short of requiring NFRC labels
NFRC Test Sizes Matter …a lot
NFRC is residentially-oriented, so NFRC operable window test sizes are small
Small NFRC test sizes make a huge difference in U-Factor for
Aluminum windows (…but not for PVC, wood, or fiberglass)
Fixed window and curtainwall test sizes are more representative
(so U-Factors are much lower, even though frame performance is worse!)
The NFRC Certified Products database includes dozens of glass types
for each product
Users must sort through a lot of numbers to make valid comparisons
NFRC Size
casement
U 0.44 BTU/hr.sqft.°F Double IG
U 0.36 BTU/hr.sqft.°F Triple IG
Up to 40% higher!
Typical Project Size
fixed / casement
U 0.36 BTU/hr.sqft.°F Double IG
U 0.26 BTU/hr.sqft.°F Triple IG
NFRC versus AAMA
NFRC 100 U-Factors are about
10% LOWER than AAMA 1503 U-Factors
Spandrel areas don’t “count”, only vision
glass and adjacent framing
Can’t use thermal models for labeling of
products with between-glass blinds
Thermal models are fairly accurate for UFactor, but NOT necessarily for surface
temperatures (CRF)
Featured Project:
Minneapolis Central Library
Location:
Minneapolis, Minnesota
Climate Zone 6
Design Team:
Pelli Clarke Pelli Associates
Products:
Unitized four-side silicone
curtainwall
Performance:
NFRC U-Factor 0.45 BTU/hr.sqft.°F
SHGC 0.25 VT 40%
AAMA CRF 65
STC 35, OITC 30
Specification
Requirements
Energy Star®
Misapplication of a great idea
Energy Star® for Windows is for residential products only
Using Energy Star® window requirements for commercial
buildings will result in a bad design
Energy Star® for Buildings uses utility data versus nationwide
benchmarks
For commercial window design, Model Energy Codes like
ASHRAE 90.1 and IECC reflect best practice
Specification Checklist
 Don’t use any single performance criteria to select a
system.
 Don’t confuse COG U-Factor with “whole window” U-Factor
 U-Factor and CRF are very different
 For windows, cite U-Factor rather than R-Value
 If using published product NFRC U-Factors, be sure to
review the project sizes and configurations
 Be consistent with SHGC and SC
 Know your Climate Zone and local energy codes
 Make sure Division 8 Metal Windows specifications for UFactor and SHGC match the Glass and Glazing
specifications, and both match code requirements and
building permit values.
Integrated Façade
Design Tips
Design Criteria
“Balanced design” is key. Selection should be
based on all applicable criteria, not on any single
number rating system.
Code Compliance
Structural Integrity
Weather-ability
Energy Efficiency
Condensation Resistance
Ventilation and Cleaning Access
Sustainable Design
Durability
Cost
Aesthetics
Emergency Egress
Hurricane Impact
Psychiatric Detention
Blast Hazard Mitigation
Noise Control
Seismic Movements
Smoke Evacuation
etc.
Featured Project:
1800 Larimer
Location:
Denver, Colorado
Climate Zone 6
Design Team:
RNL Design
Products:
Unitized four-side silicone
curtainwall and captured
window wall
Performance:
NFRC U-Factor 0.50 BTU/hr.sqft.°F
SHGC 0.19 VT 20%
AAMA CRF 65
STC 35, OITC 30
Summary
U-Factor is only a number
Thermal testing and modeling is not that precise. When
comparing manufacturer’s products, the second decimal
place is usually immaterial.
Statistically, U 0.39 can be equal to U 0.36.
For energy efficient designs, also consider daylighting,
natural ventilation, LSG, shading devices, and air
infiltration.
Conductive heat loss can actually reduce demand in
swing seasons.
Most commercial buildings are cooling mode-dominated,
even in cold climates. For these buildings, solar heat
gain control is of primary importance.
Learning Objectives Recap
Now you can:
• Identify five attributes affecting window U-Factor.
• Employ at least three design options to improve window U-Factor.
• Differentiate between NFRC and AAMA testing and certification
processes.
• Draft non-defective specification requirements for U-Factor.
• Recognize other important energy related window design factors
and the importance of a balanced design.
Thank You
This concludes The American Institute of Architects
Continuing Education Systems Program
Kevin Haynes
[email protected]