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Skylights for
Residences
Skylights and roof windows can provide
interior building spaces with the warmth and brightness of natural daylight.
Their ability to enhance almost any interior has made them increasingly popular.
However, installing a trouble-free, energy-efficient skylight can be very
difficult. In order to gain the maximum benefit from a skylight, it is important
to understand designs, materials, positioning, and proper installation.
Designs
Skylights are available in a variety of
shapes and sizes. Some open for ventilation, others are fixed. Larger
skylights that can be used as doors are sometimes called "roof
windows." Roof windows are always located within a few feet of the floor.
The most common shape for a skylight is
rectangular, but they are also available in circular, oval, diamond,
triangular, and multi-sided configurations. Non-rectangular units usually use
plastic glazing, though higher quality units use glass. The glazing can be
flat, arched, domed, pyramidal, and "warped plane" (the glazing is
flat on the low side and concave in section on the high side). Of these, the
pyramidal, arched, and domed shapes offer flexibility in placement. Their
raised design allows light to enter from more extreme angles than flat or
warped plane units. This allows more positioning options. The slope or
curvature of the glazing also helps to shed moisture and leaves. These
skylight designs also do not require the additional framing needed to slope a
flat skylight for proper drainage on flat or low-slope roofs.
Skylights can provide ventilation as
well as light. Ventilating a building through a skylight opening releases the
hot air that naturally accumulates near the ceiling. Ventilating skylights
usually open outward at the bottom, some more than others. Some units vent
through a small, hinged panel. One design uses a swing-down, inner sash with a
protected vent strip above. This can reduce the potential for rain or snow
entering the room if the vents are open. Skylights may be opened manually with
a pole, chain, or crank. Automated units with electric motors or pneumatic
devices are also available, and some models incorporate moisture sensors to
automatically close the skylight when it rains.
The physical size of the skylight
greatly affects the illumination level and temperature of the space below. Use
the following "rule of thumb" for sizing a skylight: the skylight
size should never be more than 5% of the floor area in rooms with many windows
and no more than 15% of the room's total floor area for spaces with few
windows.
In very cold weather, skylights are
often prone to water vapor condensing on the glazing. The accumulation of
water may then drip into the room. Better skylights usually have an interior
channel to collect the condensate so it can evaporate later. Thermally
efficient skylights less prone to condensation.
Recent, "high tech"
developments maximize skylights for daylighting. An "element" on the
roof becomes an aperture for collecting sunlight. It may be a sun-tracking
open-sided cylinder, a large lens-like element, or merely a conventional
skylight with a mirrored reflector mounted adjacent to it. This aperture may
then connect to a mirrored pipe, or "light pipe," which has a
diffusing lens that mounts on or is recessed into the ceiling of the room
below. These designs, relative to equivalent traditional skylights,
effectively reduce daytime overheating and nighttime heat loss, but do not
provide views or ventilation.
Fabrication and Materials Quality
The materials used in a skylight can impact
energy savings, occupant safety, and unit reliability. Consider the choice of
glazing carefully. The two types of glazing commonly used in residential
skylights are plastic and glass.
While, plastic glazings are usually
inexpensive and less liable to break than most other glazing materials, their
surfaces scratch easily and they may become brittle and discolored over time.
Many plastics also allow most of the ultraviolet (UV) rays in (unless the
glazing is coated with a special film). This increases fading damage to
furnishings. Acrylics and polycarbonates are the most commonly used plastic
glazings. Acrylics are weaker than polycarbonates, but cost less. Although
polycarbonates offer high impact resistance, some yellow with age.
Glass is usually found on the more
expensive skylights. Glass is much harder and durable than plastics and does
not discolor. All glass used for skylights must be made of "safety
glazing." This is a generic term for both tempered and laminated glass.
Tempered glass is the most impact resistant. Laminated glass is fabricated
with a thin layer of plastic embedded near the center of the glass. Both types
keep the glass from breaking into large, sharp pieces. Skylights are often
made with a tempered glass on the exterior side and a laminated pane on the
interior side. This arrangement gives maximum impact resistance while
protecting occupants from falling shards of glass.
Because skylights are located on the
roof of a building, they can result in large amounts of unwanted summer time
solar heat gains and winter time heat losses. Manufacturers use various
methods to reduce these impacts. The most common methods are multiple layers
of glazing, gas fills (of inert argon or krypton gas) between multiple layers,
and/or low-e (low-emissivity) glazings or films. Some manufacturers even
install a translucent insulation material between several glazing layers to
create a more thermally efficient assembly. Limiting summer heat gain from
skylights facing west and south can be difficult. There are skylights with
solar control tints or coatings that reduce solar heat gain. They can,
however, reduce the level of light transmission and the potentially beneficial
solar gain from your skylight in the winter.
Positioning and Slope
The location of a skylight is the primary
consideration if you want to maximize natural lighting and passive solar
heating potential. Skylights on roof s that face north provide fairly constant
but cool illumination. Those on east-facing roofs provide maximum light and
solar heat gain in the morning. While west-facing skylights provide appealing
afternoon sunlight, they can increase undesirable heat gain. South-facing
skylights provide the greatest potential for desirable winter passive solar
heat gain than any other location, but often allow unwanted heat gain in the
summer. You can prevent unwanted solar heat gain by installing the skylight in
the shade of deciduous (leaf-shedding) trees, or add a movable shading device,
such as louvers, shades, or awnings on the inside or outside of the skylight,
or buy a unit with solar control film.
The slope, or tilt, of the skylight also
has a great effect on solar heat gain. A low-slope will admit relatively more
solar heat in the summer and less in the winter, exactly the opposite of what
is desirable. A general rule of thumb is to achieve a slope equal to your
geographical latitude plus 5 to 15 degrees. For example, the optimum slope for
a south-facing skylight in Columbus, OH, at 40o North lattitude, is 45o to
55o. At least one skylight manufacturer makes a prefabricated, tilted base
that increases the angle of a skylight above the roof.
Preventing Leaks
Leaks are a common problem with skylights.
To avoid leaks, you should mount the skylight above the roof surface, properly
install a curb (a raised, watertight lip that helps to deflect water away from
the skylight) and flashing, thoroughly seal joints, and follow the
manufacturer's guidelines. It is also prudent to apply a layer of sheet
waterproofing over the flanges/flashing of the skylight. This is generally
installed under the finish roofing material as an aid in protecting against
ice dams. Avoid water diversion devices such as roof crickets or diverter
strips, as they often create more problems than they solve.
Buying Your Skylight
Skylight prices vary widely. A small
skylight can cost $300 or more. Expect to pay more for special glazing
materials, venting, shading devices, or other options. Installation costs
depend on the complexity of the job. If you choose to have a professional
install your skylight, be sure to obtain several estimates and a list of
references. Contractor bids and fees can vary tremendously. Be wary of any bid
that seems too high or too low.
The United States Department of Energy
(DOE) and the Environmental Protection Agency (EPA) have established the
Energy Star® labeling program for many consumer appliances and residential
building products. Energy Star®-labeled skylights exceed a defined minimum
energy performance requirement for three broad climate regions of the country
- sometimes by as much as 40 percent! Additionally, all Energy Star®
qualifying windows and skylights will carry a label of the National
Fenestration Rating Council (NFRC). The NFRC label indicates specific energy
performance characteristics, including the U-Value (thermal performance) and
Solar Heat Gain Coefficient (SHGC) of the skylight. These independent ratings
provide the basis for Energy Star® performance requirements. For more
information, contact:
Energy Star Program
Phone, toll-free: (888) STAR-YES [ (888)-782-7937) ]
Email: info@energystar.gov
World Wide Web: http://www.energystar.gov
National Fenestration Rating Council
1300 Spring Street, Suite 120, Silver Spring, MD 20910
Phone (301) 589-NFRC; Fax: (301) 588-0854
Email: nfrcusa@aol.com
World Wide Web: http://www.nfrc.org
Bibliography
The following publications and articles
provide additional information about skylights. This bibliography was reviewed
in May 1999.
Magazine and Journal Articles
"Australian Skylight Design Allows
Lights to be Left Off in Single-Story Buildings," T. Rees, The Solar
Letter, (5:29) p. 444, December 22, 1995.
"Buy Better: Dish in Dome," Home
Mechanix, (92:805) p. 14, May 1996.
"Framing for Skylights," D.
Hopper, Fine Homebuilding, (No. 90) pp. 70-74, August/September 1994.
"Headaches from Sunpipes," J.
Nisson, Energy Design Update, (16:8) pp. 7-8, August 1996.
"Ice Dam Solution," Letter from
M. Rosenbaum, Journal of Light Construction, (13:11) p. 5, August 1995.
"Light Pipes for Homes," J.
Nisson, Energy Design Update, (15:9) pp. 8-12, September 1995.
"On the House: Flat Roof
Skylights," H. Spies, Journal of Light Construction, (9:3) p. 33,
December 1990.
"Remodeler's Guide to Building
Skylight Wells," Jim Hart, Journal of Light Construction, (12:11)
pp. 36-39, August 1994.
"Selected Detail: Glass Roof
Structure," M. Crosbie, Progressive Architecture, (No. 90) p. 131,
June 1995.
"Shallow Skylights Cause
Overheating," Letter from S. Larson, Fine Homebuilding, (No. 95) pp.
6, 8, December 1995/January 1996.
"Shedding Light on Skylights,"
R. Osborn, Fine Homebuilding, (No. 102) pp. 48-53, April/May 1996.
"Skylight Glass Spacing," Solplan
Review, (No. 41) p. 14, October/November 1991.
"Skylights in the Eaves," A.
Simmonds, Fine Homebuilding, (No. 70) pp. 40-44, November 1991.
"Skylight Nightlight," Home
Mechanix, (92:806) p. 10, June 1996.
"Skylight Options and
Accessories," J. Wagner, Journal of Light Construction, (14:7) pp.
47-50, April 1996.
"SunPipe-A Practical Energy-Saving
Alternative to Skylights," J. Nisson, Energy Design Update, (12:9)
pp. 8-10, September 1992.
"Tough Skylight Earns Dade Hurricane
Approval," Journal of Light Construction, (14:7) p. 10, April 1996.
"When Architecture Meets Energy
Efficiency," M. Rosenbaum, Fine Homebuilding, (No. 65) p. 62, March
1991.
Report
Evaluation of Solar Gain through
Skylights for Inclusion in the SP53 Residential Building Loads Data Base, J.
Hanford and Y. Huang, Lawrence Berkeley Laboratory, 1993. Available from the
National Technical Information Service (NTIS) (see Source List below). 23 pp.,
$19.50, NTIS order number DE94004940.
Source List
American Solar Energy Society (ASES)
2400 Central Avenue, Suite G-1, Boulder, CO 80301
Phone: (303) 443-3130; Fax: (303) 443-3212
Email: ases@ases.org; World Wide Web: http://www.ases.org
National Technical Information Service (NTIS)
5285 Port Royal Road, Springfield, VA 22161
Phone: (800) 553-6847
Email: orders@ntis.fedworld.gov;
World Wide Web: http://www.fedworld.gov
NTIS adds charges for shipping and handling. Check the price and availability
before placing an order.
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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