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Low-E and Solar
Control Glazings, Films, and Coatings for Windows
Before recent innovations in glass, films,
and coatings, a typical residential window with one or two layers of glazing
allowed roughly 75-85% of the solar energy to enter a building. Internal shading
devices such as curtains or blinds could reflect some of that energy back
outside of the building. Most of the energy-primarily heat-remained inside,
which effected the interior's thermal comfort. The weak thermal characteristics
of windows became a prime target for research and development in the attempt to
control the indoor temperatures of buildings.
Low emissivity, or "low-e,"
glass and films control heat gain and loss, reduce glare, minimize fabric
fading, provide privacy, and occasionally provide added security in wind,
seismic, and other high-hazard zones. New construction and window replacement
applications commonly use glazings with these coatings.
Solar control glazings and coatings reduce
the impact of the sun's heat without sacrificing view. These include tinted
glass and spectrally selective (transmits visible light while reflecting the
long-wave infrared portion of sunlight) glazings or coatings. Spectrally
selective materials may also exhibit low-emissivity (low-e). Improved glazing
products typically fall into one of three categories: chemically or physically
altered glazings, coated glazings or films, and multiple-layered assemblies.
Chemically or Physically Altered Glazings
Tinted glass is made by alteration of
the chemical properties of the glass. Although tinted glazing is the best
example, laminated glass also falls into this category. Both glass and plastic
may be tinted. The tints absorb a portion of the sunlight and solar heat. The
primary use of tinted glazing is to reduce glare and the amount of solar gain
passing through the glass. Tinted glazings reduce the latter by 25-55%.
"Heat absorbing" tinted glass maximizes its absorption across some
or all of the solar spectrum. Unfortunately, the absorbed energy often
transfers through radiation and convection to the inside.
Tinted glazing reduces both heat gain
and visible (light) transmittance (VT). Spectrally selective tints reduce heat
gain while allowing more light to pass through. For most residential or
commercial structures dependent on daylight for lighting, a spectrally
selective material offers the better choice.
Spectrally selective glazings transmit
the desired visible portion of the sunlight while absorbing the near-infrared
long wave radiation that commonly passes through clear glass. The ultraviolet
portion of the solar spectrum is also blocked. As a glazing, they are best
used only as the outside sheet of glass of a multi-glazed assembly; when
combined in a low-e coating, their performance is further improved. These
glazings have a light blue or green tint.
When compared to conventional bronze- or
gray-tinted glass, spectrally selective glass exhibits lower solar heat gain
coefficients (SHGC) and higher visible transmittance (VT) values. The ratio
between the two factors, called the light-to-solar gain ratio (LSG), provides
a gauge of the relative efficiency of different glass types in transmitting
daylight while blocking solar gain. The greater the LSG, the better the unit
performs, as the following equation indicates:
LSG = VT/SHGC
Coatings and Films
A reflective surface lowers solar heat
gain. Reflective coatings usually consist of thin layers of metal applied to
clear or tinted glazing. The thickness, reflectivity, and its location on the
glass directly affects the solar heat gain. As with tinted glazing, these
coatings reduce the amount of visible light passing through them. The
reflective glazings, while reducing solar heat gain, nonetheless allow a large
portion of solar heat gain to pass through (consistent with uncoated,
non-low-e glazings). Most window manufacturers now use one or more layers of
low-e coatings in their product lines.
Coating a glass surface with a low-emittance
material, then facing that coating into the air gap between glass layers,
blocks a significant amount of radiant heat transfer. Thermally, this is
roughly equivalent to adding an additional pane of glass to a multi-pane
assembly. While virtually invisible, low-e coatings reduce long-wave radiation
heat transfer by a factor of 5 to 10. The lower the emissivity value, the
better the material reduces the heat transfer. Most low-e coatings also
slightly reduce the amount of visible light transmitted through the glazing
relative to uncoated glass. Here are emmissivity values for:
- Glass, uncoated: 0.84
- Glass with single hardcoat low-e: 0.15
- Glass with single softcoat low-e: 0.15
Apyrolytic coating baked on at a high
temperature constitutes a "hardcoat" low-e coating. At least one
layer of a multilayered coating is made of metallic oxide deposits. One layer
is about 1/10,000 the diameter of a human hair. These are not yet available
for application on plastic glazing materials.
"Softcoat" low-e coatings are
sputtered onto glazing at lower temperatures than hardcoatings. They often use
a layer of silver that is then protected from humidity and other contact. They
are even thinner than pyrolytic coatings. Softcoat low-e applications within
insulated glazing assemblies should last the life of the window, and they
offer lower emittances than hardcoat low-e coatings.
The only spectrally-selective coatings
now available are modified softcoat low-e coatings. The near infrared and
long-wave elements of the solar spectrum are reflected by modifying the
coating's thickness and number of layers. A spectrally selective tinted
glazing with a pyrolytic coating serves a similar purpose. These spectrally
selective hardcoats are currently under development.
Window films are easy to apply on
glazing up 36 square inches (91.5 square centimeters). They are often applied
to the glass with a water soluble adhesive. To reduce the possibility of
bubbles and wrinkles on large windows, have the film installed professionally.
Most films should be applied to the inside surface of the glass, since they
are not designed to withstand the elements. If you plan to install the film
yourself, be careful to select the appropriate film for your needs, and read
all directions before beginning.
Performance Selection Factors
When selecting windows, compare the
U-value (thermal conductivity value), solar heat gain coefficient (measure of
solar spectrum transmitted or absorbed inward), visible transmittance
(percentage of visible light passing through the glass), air leakage rating
(measure of the rate of air loss around a window under a specific pressure
differential), and light-to-solar-gain ratio (measure of glazing to provide
light without excessive solar heat gain). Since air leakage ratings do not
expressly correlate with glazing characteristics, further explanation about it
will be omitted.
The U-value measures how easily heat
travels through a material. As the U-value increases, heat transfer through
the window becomes greater. Some manufacturers rate thermal performance using
R-values. R-values are the inverse of U-values, i.e., 1/U = R, 1/R = U. The
R-value of a material of U-0.25 is R-4.0. The total U-value of any window
depends on the glazing types, frame type and size, coatings, and inert gas (if
any) used between the panes. Here are typical ranges of U-Values for windows
that are:
- Single glazed: 0.91 - 1.11
- Double glazed: 0.43 - 0.57
- Triple glazed: 0.15 - 0.33
The solar heat gain coefficient (SHGC)
has superseded the shading coefficient (SC) as the standard indicator of a
window's shading ability. The SHGC is the fraction of solar radiation admitted
through the window or skylight and subsequently released inward. This includes
both directly transmitted and absorbed radiation. The lower the SHGC, the less
the window transmits heat and the greater its shading ability. The SHGC may be
expressed in terms of the glass alone or can refer to the entire window
assembly. Passive solar heating projects in cold climates are served better
with high SHGC assemblies, while hot climates are better designed using the
lower SHGC assemblies.
The visible transmittance (VT) refers to
the percentage of the visible spectrum (380-720 nanometers) weighted by the
sensitivity of the eye, that is transmitted through the glazing. When daylight
is a supplementary light source (as in showrooms and studios), glazing that
admits much of the visible light is a logical choice. Bronze, gray, or
reflective-film windows are a better choice when outward view is a low
priority (as in office buildings or for privacy).
To reduce interior glare, select a glass
or film with a low visible light transmittance. Solar-control glass with a low
solar heat gain coefficient does not necessarily reduce interior glare. A
typical clear, single-pane window has a VT of 0.90, meaning it admits 90% of
the visible light.
Here are values for typical window
characteristics for different glazing systems. The first value is for a Whole
window and the second, in ( ), is for the Center of Glass.
Glazing System: Characteristic SHGC / SC / VT / LSG
Single-glazed, clear: 0.79(0.86) / -(1.00) / 0.69(0.90)/ 0.87 (1.04)
Double-glazed, clear: 0.58(0.76) / -(0.89) / 0.57(0.81) / 0.98 (1.07)
Dble-glzd, bronze: 0.48 (0.62) / -(0.72) / 0.43 (0.61) / 0.89(0.98)
Sngl-glzd, spectrally selective: 0.31(0.41) / -(0.47) / 0.51 (0.72) /
1.65(1.75)
Dble-glzd, spectrally selective: 0.26(0.32) / -(0.38) / 0.31(0.44) /
1.19(1.38)
Triple-glzd, new low-e: 0.37(0.49) / -(0.57) / 0.48(0.68) / 1.29 (1.39)
Other factors to consider when choosing
glazing for a structure are climate, building design, building orientation,
and external shading. Check with manufacturers for specific product testing
results and specifications.
Calculating Savings
The energy savings from using solar
control glazing are difficult to accurately predict. Since solar heat gain,
the primary cause of high cooling costs, is a function of solar heat gain
factor (SHGC), shading coefficient (SC), and cooling load factor (CLF) (the
ratio of actual total cooling compared with total steady-state cooling during
the same period at constant ambient conditions), predictions of savings are
based on these factors. Some manufacturers and data books combine these three
variables into one figure, frequently called the heat transfer multiplier (HTM).
The HTM will vary with location, seasonal changes, time of day, shading,
orientation, temperature, and building color.
Computer programs for determining
savings are available from some window manufacturers. These programs contain
the necessary weather and window data to help determine savings. Compare the
estimated savings with the purchase price of the glazing to determine the
simple payback period.
Some solar control films are very costly
and may have a long payback period. There are other shading devices such as
overhangs, solar screens, shutters, roller shades, blinds, and draperies that
also help to reduce solar gain. These types of products provide privacy with
varied aesthetic appeal. The effectiveness of any internal shading device
depends on its ability to reflect incoming solar radiation before it can heat
the building interior. The following are the percentages of solar radiation
that are transmitted, absorbed, or reflected, by different mechanical shading
devices.
Device: % Transmitted / Absorbed / Reflected
Roller Shades: up to 25% / 15-80% / 20-65%
Vertical Blinds: 0% / 23% / 77%
Venetian Blinds: 5% / 40-60% / 35-55%
Bibliography
The following publications contain
additional information about reflective films and glazings. This bibliography
was reviewed in March 1998.
Books and Reports
1997 ASHRAE Handbook of Fundamentals,
American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE).
Available from ASHRAE, 1791 Tullie Circle, NE, Atlanta, GA 30329-2305; Phone:
(404) 636-8400; Fax: (404) 321-5478; Email: ashrae@ashrae.org ; World Wide Web: www.ashrae.org
. $124.00 (hardcover), ASHRAE Code 81970 (I-P: English) or 81971 (SI: Metric).
See Chapter 27.
Energy Management Handbook (2nd
Ed.), W. Turner, Association of Energy Engineers (AEE), 1993. Available from AEE,
4025 Pleasantdale Road, Suite 420, Atlanta, GA 30340: Phone: (770) 447-5083;
Fax: (770) 446-3969; World Wide Web: www.aeecenter.org
. $119.00 (hardcover). See pages 200-201.
Energy and Cost Evaluation of Solar
Window Film Use in an Office Building, S. Treado, National Bureau of
Standards, 1983. Available from the National Technical Information Service (NTIS),
see Source List below. 127 pp., $35.00 (softcover), NTIS Order Number
PB83214692.
Glazing Materials for Solar and
Architectural Applications: Final Report, C. Lampert, Lawrence Berkeley
Lab., Berkeley, CA, 9/94. Available from NTIS (see Source List below). 58 pp.,
$21.50 (softcover), NTIS Order Number DE95008350.
Handbook of Energy Audits (4th
Ed.), A. Thumann, Fairmont Press, April 1995. Available from the Fairmont Press,
700 Indian Trail, Lilburn, GA 30247, (770) 925-9388. 412 pp., $75.00, Publisher
Code 356, ISBN 0-88173-211-7.
Manual J Load Calculation for
Residential Buildings, Air Conditioning Contractors of America (ACCA),
1995. Available from ACCA (see Source List below). 126 pp., $35.00, Catalog Code
J.
Manual N Load Calculation for
Commercial Buildings, Air Conditioning Contractors of America (ACCA), 1997.
Available from ACCA (see Source List below). 120 pp., $34.00 (softcover) Catalog
Code N.
NFRC Certified Products Directory
(7th Ed.), National Fenestration Rating Council (NFRC), April 1997. Available
from the NFRC Incorporated, 1300 Spring Street, Suite 120, Silver Spring, MD
20910; Phone: (301) 589-6372, Fax: (301) 588-0854; Email: nfrc@drintl.com ;
World Wide Web www.nfrc.org . 291 pp., $25.00
(softcover).
Residential Windows: A Guide to New
Technologies and Energy Performance, J. Carmody, S. Selkowitz, L. Heschong,
W.W. Norton & Company, 1996. Available from W.W. Norton & Company, 500
Fifth Avenue, New York, NY 10110; Phone: (800) 223-2584; World Wide Web: www.wwnorton.com
. 214 pp., $22.00 (softcover), ISBN 0-393-73004-2.
Spectrally Selective Glazings for
Residential Retrofits in Cooling-Dominated Climates, E. Lee, et al.,
Lawrence Berkeley National Laboratory, Berkeley, CA, 4/93. Available from NTIS
(see Source List below). 18 pp., $19.50 (softcover), NTIS Order Number
DE94018048.
Articles
"Annual Window Energy Rating Labels
Coming," J. Nisson, Energy Design Update, (15:7) pp. 1-2, July
1995.
"Energy-Efficient Window Retrofits:
Install with Care," J. O'Bannon and A. Grieco, Home Energy, (14:1)
pp. 35-42, January/February 1997.
"The NFRC National Window Energy
Rating System," J. Nisson, Energy Design Update, (15:8) pp. 8-9,
August 1995; and Reader's Forum follow-up, "On Unrealistic Savings for
High-Performance Windows," J. Nisson, Energy Design Update,
(15:11) pp. 5-6, November 1995.
"Out the Window with U-Guessed-It
Values," Home Energy, (10:4) pp. 5-7, July/August 1993.
"R-5 Krypton Windows from
Weathershield," J. Nisson, Energy Design Update, (16:2) pp. 12-13,
February 1996; and Reader's Forum follow-up, "On the Benefits of 'Super
Windows,'" J. Nisson, Energy Design Update, (16:6) pp. 5-6, June
1996.
"Selecting Replacement Windows,"
S. Hymes, Journal of Light Construction, (16:5) pp. 28-34, February
1998.
"Shopping for Wood Windows," C.
Wardell, Journal of Light Construction, (12:9) pp. 27-34, June 1994.
"Southwall Gets One (Award) for New
Low-E Window Film," J. Nisson, Energy Design Update, (16:2) p. 4,
February 1996.
"Superwindow Retrofits Show
Significant Energy Savings," M. Jackson, Home Energy, (11:5) pp.
37-40, September/October 1994.
"Understanding Energy-Efficient
Windows," P. Fissette, Fine Homebuilding, (No. 111) pp. 68-73,
February/March 1998.
"Window Sales by Frame Type," J.
Nisson, Energy Design Update, (16:10) p. 4, October 1996.
Source List
Air Conditioning Contractors of America
(ACCA)
1712 New Hampshire Avenue, NW
Washington, DC 20009
Phone: (202) 483-9370, Fax: (202) 232-8545
Email: acca_web@acca.org
World Wide Web: www.acca.org
National Technical Information Service
(NTIS) 5385 Port Royal Road
Springfield, VA 22161
Phone: (800) 553-6847, Fax: (703) 321-8547
Email: orders@ntis.fedworld.gov
World Wide Web: www.ntis.gov
EREC is operated by NCI Information
Systems, Inc. for the National Renewable Energy Laboratory/U.S. Department of
Energy. The statements contained herein are based on information known to EREC
at the time of printing. No recommendations or endorsement of any product or
service is implied if mentioned by EREC.
Energy Efficiency and Renewable Energy
Clearinghouse (EREC)
P.O. Box 3048 Merrifield, VA 22116
Voice: 1-800-DOE-EREC
E-mail: doe.erec@nciinc.com
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