GREEN BUILDING MANUAL
NEW RESIDENTIAL UPDATED 2-MAY-11
Windows and Skylights Glare and Heat Gain Reduction
What is Glare and Heat Gain Reduction?
Glare and heat gain reduction includes strategies for
lowering the heat and light that enters a building with
protective barriers such as heat control glazing or
physical barriers such as shades, blinds, awnings or
vegetation. Heat gain, or solar gain, is the amount of
heat that enters a building as a result of solar radiation
penetrating through windows, skylights or other
transparent materials. Choosing the right type of
fenestration (windows, doors, skylights, etc.) is
important in preventing unwanted glare and solar
Figure 1 - Disability Glare (Source: Florida
Solar Energy Center)
gains.
Glare is a natural phenomenon that occurs when the
sun’s light enters a room at a steep angle and inhibits
a person from visually focusing on his or her task
either by casting a veil of light between the task and
the viewer or by flooding the eyes with light, making
them unable to focus on the task. For example, sun
glare on a document makes the black text almost as
bright as the white background, reducing the contrast
and making the document difficult to read. Reducing
this effect allows residents to enjoy activities without
the visual discomfort of sun glare. 1
Figure 2 - Discomfort Glare (Source: Florida
Solar Energy Center)
Heat gain is the solar radiation or warmth that enters a residence through a window when the sun
shines through it. While this may be a welcome benefit during the winter months, heat gain must
often be mitigated during the summer with air conditioning.
While glare causes discomfort and inconvenience and can be hazardous, heat gain can
significantly increase a household’s air conditioning bills in the summer months. 2 UV rays that
1
Florida Solar Energy Center. How Windows Work – Glare Tutorial.
http://www.fsec.ucf.edu/en/consumer/buildings/basics/windows/how/glare.htm (accessed October 15, 2010).
2
CaregiverNJ. Caregiver Guide. http://www.state.nj.us/caregivernj/basic/guide/pg3.3-improve.shtml (accessed
October 16th, 2010).
GREEN BUILDING MANUAL
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enter through windows can also damage furnishings and fabrics. Reducing heat gain helps keep
the home comfortable. 3
How to Implement Glare and Heat Gain Reduction
One of the best ways to prevent unwanted glare and heat gain in a building is through installing a
proper selection of windows and skylights. Look for the ENERGY STAR label when selecting
windows. ENERGY STAR products must meet the criteria established by the ENERGY STAR
standards for energy efficiency. 4 Today, windows and skylights can be fabricated to reduce the
amount of ultraviolet (UV) light and heat that penetrates into the building. Windows can be made
with low-e coatings that provide greater insulation than conventional windows. This low-e
coating that is applied to the interior glass surface reflects harmful radiation from the sun, thus
reducing heat gains and maintaining heat already in the building. For warmer climates, there is
the hot climate low-e coating that greatly reduces solar heat gain while still allowing visible light
to penetrate. This coating reflects half of the solar radiation that hits the window. When selecting
a window based on performance, look for visible transmittance (VT) levels between 0.6 and 0.8
(60% and 80% transmittance). SHGC numbers for optimal performance should be 0.4 (40%) or
lower. These numbers will be labeled on all windows by the National Fenestration Rating
Council (NFRC). 5
Along with low-e coatings, there are other types of window glazing choices to help reduce glare
and heat gains. Tinted glazings and reflective glazings are effective in that they absorb and block
light, respectively, but it is at the cost of low light transmittance. Instead, spectrally selective
glazings block heat while allowing in natural light. This is achieved because the window is
designed to block infrared wavelengths and allow visible light wavelengths, giving this glazing a
great shading coefficient. Other glazing technologies include electrochromic, liquid crystal,
thermochromic, and photochromic, but professional guidance should be attained before trying
these newer technologies. 6
Removable treatments may be used seasonally when appropriate. Another option for moderate
climates like New Jersey’s may be “smart” coatings that have cooling benefits in the summer
and heating benefits in the winter.
3
National Fenestration Rating Council. The Facts about Solar Heat Gain and Windows.
http://www.nfrc.org/documents/SolarHeatGain.pdf (accessed October 15, 2010).
4
Energy Star.
http://www.energystar.gov/index.cfm?fuseaction=find_a_product.showProductGroup&pgw_code=WI (accessed
April 30, 2011).
5
Florida Solar Energy Center - University of Central Florida. "How Windows Work - About Solar Gain."
http://www.fsec.ucf.edu/en/consumer/buildings/basics/windows/how/solar_gain.htm (accessed August 10, 2010).
6
Pacific Gas & Electric Company . "Energy-Efficient Window Glazing Systems for Commercial Facilities."
http://www.pge.com/includes/docs/pdfs/about/edusafety/training/pec/inforesource/glazsys.pdf (accessed August 10,
2010).
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A number of mounted physical barriers such as blinds, awning, shutters and screens can be
employed to block UV rays from reaching the windows or skylights. An alternative, if feasible,
is to plant a shade tree near the window or skylight in order to shade it during the summer
months and let as much light and heat in as possible during the winter months.
Allowing the right amount of heat in is also important, especially in northern climates during the
winter. Taking advantage of sunlight during the winter through various transparent access points
on the building's envelope can help heat the building naturally rather than solely by the
mechanical heating system. Net energy savings can be amounted if solar heat gains during the
winter more than offset day and night time conduction heat losses.7
Figure 3 - Exterior Window Shading Strategies (Source: Florida Solar Energy Center)
Providing natural light, also known as daylighting, is an important component in achieving
energy savings, increasing occupancy comfort, and is part of the holistic approach in establishing
a more efficient lighting system. Daylighting uses direct, diffuse, or reflected daylight to provide
partial or all light in a building's interior. Daylighting is effective in minimizing heat gains
because it produces less heat per unit of illumination than artificial lighting systems, thus
7
Florida Solar Energy Center -University of Central Florida. "Windows."
http://www.floridaenergycenter.org/en/consumer/buildings/commercial/windows.htm (accessed August 10, 2010).
GREEN BUILDING MANUAL
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reducing the amount of electricity spent on air conditioning. In collaboration with sensor
controls, daylighting can provide greater occupant comfort and a more productive work
environment. To prevent unwanted glare, exterior and interior shading devices can be
implemented to reduce the amount of direct sunlight. 8
Benefits
The benefits of heat gain and glare reduction are
numerous. Residents who take action to reduce
these effects can anticipate greater visual comfort in
their homes while enhancing safety for family
members with mild to serious vision problems.
Residents can also expect more consistent indoor
temperatures as well as considerably reduced
energy costs during the warm months. Preventing
excess solar radiation from entering the home can
help reduce air conditioning costs. Excess heat
coming from sunlight into the home means more
electricity is used trying to cool the building. Using
less electricity on air conditioning means cost
savings can be achieved.
Figure 4 - Window Performance Rating Label
(Source: National Fenestration Rating Council)
Costs
Costs will vary depending on the types of strategies implemented.
Windows and skylights with low-e coatings are widely available. Check with the local home
improvement store to explore the full range of price and product choices available in the area.
The cost of planting a tree or shrub for shade may range from as little as $10 to hundreds of
dollars depending on materials and labor. 9
External window coverings range in price from $150 for an aluminum awning 10 to tens of
thousands of dollars for a porch addition. 11 There are also additional costs associated with
maintenance of outdoor structures, particularly year-round or permanently affixed shades.
8
Center for Building Performance and Diagnostics at Carnegie Mellon University. "Maximize the Use of Daylight
without Glare and Provide Daylight-responsive Lighting Controls."
http://cbpd.arc.cmu.edu/ebids/images/group/cases/Daylighting.pdf (accessed August 10, 2010).
9
Cost averaged from several suppliers
Cost averaged from several manufacturers
10
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Incentives may be available for New Jersey residents who invest in low-e windows. For up-todate information on available incentives, please visit the ENERGY STAR and the NJ Clean
Energy Program websites:
http://www.energystar.gov/index.cfm?c=windows_doors.pr_benefits
http://www.njcleanenergy.com/residential/programs/programs
Resources
New Jersey’s Clean Energy Program
http://www.njcleanenergy.com
NJ Green
http://www.nj.gov/nj/green/
US DOE – Energy Savers
http://www.energysavers.gov
ENERGY STAR
http://www.energystar.gov
Florida Solar Energy Center
http://www.fsec.ucf.edu/
National Fenestration Rating Council
http://www.nfrc.org/documents/SolarHeatGain.pdf
CaregiverNJ
http://www.state.nj.us/caregivernj/
CostHelper.com.
http://www.costhelper.com/
11
CostHelper.com. Cost of a Porch. http://www.costhelper.com/cost/home-garden/porch.html (accessed October 16,
2010).