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Foam-Core Panels
Foam-core panels are slabs of foam insulation
sandwiched between two facings, or "skins". These skins include metal,
drywall, and/or structural wood composition sheathing, such as plywood,
waferboard, and oriented strand board (OSB). Foam-core panels, also called
stress-skin panels, sandwich panels, structural foam panels, and structural
insulated panels are replacing stud framing in some residential construction.
The greatest advantage of these panels is that they provide ample insulation for
a home's outer structure, often with an increased R-value per inch compared to
other forms of insulation. When constructed and installed properly, they also
are more airtight than stud framing. This inhibits energy loss in the winter and
summer, making the home very energy-efficient. Another advantage is that a
builder can erect the shell of a foam-core panel home much more quickly than the
shell of a stud-framed home, saving time and money.
Applications
The two basic types of foam-core panels are
structural and non-structural. When used in house construction, structural
insulated panels may make up the primary structural support. These panels are
strong: a wall with two, half-inch (12.7 millimeters [mm]) thick OSB skins is
nearly three times stronger than a conventional 2x4 inch (51x102 mm) stud
wall. Although non-structural foam-core panels are not designed to provide
primary structural support, they may be used to enclose curtain-wall
structures like timber frames. Sometimes drywall is the only interior
sheathing option for these wall panels.
Foam-core panels are a key component of
"panelized housing," where specific fabrication occurs in a factory.
The builder needs only to assemble the pieces on the site. It takes as little
as one to three days to fully erect and weatherproof the shell of a foam-core,
panelized house. Openings in the panels, such as for doors and windows, may be
precut at the factory or cut with standard tools by the builder at the
construction site.
In 1993, a structural insulated panel
home was built adjacent to an identical, conventional wood-framed house.
Scientists at the National Renewable Energy Laboratory (NREL) found that both
types of construction performed as expected from their nominal R-values. This
contradicts previous testing by the Florida Solar Energy Center (FSEC) that
found a 12% to 17% energy savings from using foam-core panel construction. The
FSEC also monitored side-by-side foam-core and conventional wood-framed
structures in Kentucky for about two winter months. The airtightness of the
foam-core house (measured at 0.21 air changes per hour [ach]) was marginally
better than the conventional wood-framed house (measured at 0.27ach).
Types of Foam-Core Panels
Foam-core panels use a rigid-insulation
core made of one of three plastics: 1) expanded polystyrene (EPS); 2)
polyurethane; or 3) polyisocyanurate, a polyurethane derivative. Manufacturers
are examining ways of using cementitious or fibrous core insulating materials
in place of the plastic insulations.
EPS Foam-Core Panels
The majority of foam-core panel
manufacturers produce EPS panels. EPS, commonly known as beadboard, adheres to
the wood sheathing. EPS foam-core panels have a nominal R-value of about 4 per
inch (25.4 mm). Unlike the other types of foam insulations, beadboard is made
using a pentane blowing agent. The R-value of EPS remains relatively constant
as time passes. Wall panels made of EPS foam are typically 3 1/2 inches to 7
1/2 inches (89-190 mm) thick. Ceiling panels are 5 1/2 inches to 11 1/2 inches
(140-292 mm) thick. A common wall panel is 41x81 inches (1.04x1.06 meters [m])
and weighs 110 pounds (50 kilograms [kg]). Manufacturers can make some panels
as large as 81x281 inches (1.06x7.14m); these require a crane to erect.
Polyurethane/Isocyanurate Foam-Core
Panels
Wood composition sheathing may be glued to
polyurethane and isocyanurate slabs of insulation in the same way as EPS
panels. Since it is difficult to achieve a strong bond between polyurethane or
isocyanurate and wood, most manufacturers inject the foam between the two wood
skins with specialized equipment. This is a technically complex and chemically
volatile process, but it produces a strong bond between the foam and the
skins.
Polyurethane and isocyanurate foam-core
panels have a nominal R-value of around R-6 to R-7 per inch (25.4 mm) of
thickness. Both contain a blowing agent that escapes over time, reducing the
R-value of these foams. In well-sealed panels, however, the release of this
blowing agent is slower. Blowing agents such as chlorofluorocarbons (CFCs),
and to a lesser extent hydrochlorofluorocarbons (HCFCs), have been identified
with the destruction of the earth's protective ozone layer. Regulatory
measures now restrict the level of CFC production. This may adversely effect
the cost and availability of polyurethane and isocyanurate panels.
Wall panels made of polyurethane or
isocyanurate are typically 3 1/2 inches (89 mm) thick. Ceiling panels are up
to 7 1/2 inches (190 mm) thick. The length of available composition
sheathings, currently 28 feet (8.5m), limits panel length. Polyurethane/isocyanurate
panels, although more expensive, are more fire and vapor-diffusion resistant
than EPS panels.
Costs
Inch per inch, polyurethane and
isocyanurate panels cost about twice as much as their EPS equivalent. However,
polyurethane and isocyanurate panels have R-values nearly double that of EPS.
EPS panels usually cost slightly less than equivalent R-value polyurethane or
isocyanurate panels. Foam-core panels normally cost more than competing wall
and roofing products, but they require considerably less labor to install.
Consequently, a frameless foam-core house may cost nearly the same as a
well-insulated stud-frame house with standard 2x6 inch (38x138 mm) stud walls.
Advantages of Foam-Core Panels
Foam-core panel walls are superior to
conventional walls in a number of ways. Foam-core panels combine a high level
of insulation with speed and ease of construction. Unlike certain types of
insulation, which lose some of their insulation value when exposed to
moisture, normal home moisture levels do not significantly affect the R-value
of foam-core panels. The solid foam core virtually eliminates air convection
within the walls and thermal bridges through wood studs and insulation voids.
The panels also reduce air infiltration that, with proper installation, make a
tightly sealed house.
Disadvantages of Foam-Core
Panels
Fire safety and insect problems are the two
greatest problems associated with foam-core panels. The concern over fire
safety mostly concerns EPS panels. Polyurethane and isocyanurate burn like
wood, remaining intact until it burns through. EPS, however, can begin to
deform at temperatures as low as 167°F (75°C), melt at around 200°F
(93°C), and flow at around 250°F (121°C). In addition to causing structural
failure, melting polystyrene can actually fuel a fire. Although all plastic
insulations emit toxic gases while burning, experts believe that toxic gases
released by carpets, furnishings, and foam-filled furniture appear more
dangerous than the foam insulation hidden behind drywall and composition
board.
When installed according to
manufacturers' recommendations, EPS panels pass the American Society for
Testing and Materials (ASTM) tests and meet national building codes.
Polyurethane/isocyanurate supporters nonetheless claim that the ASTM test
(ASTM E84) for flame spread and smoke is not appropriate for testing EPS.
There appears to be a consensus among the testing community to change the test
so that it more realistically duplicates EPS burning patterns. Such a change
is likely to raise the EPS smoke and flame levels.
EPS panel manufacturers, however,
maintain that fire testing of EPS panels should deal with assemblies (for
example, the walls) made up of panels. Several full panel simulation tests
have been conducted. In one instance, the Underwriters Laboratory (UL)
performed a test on EPS panels with waferboard skins (drywall-EPS-waferboard).
UL exposed the EPS wall and ceiling panel room to a wood fire set in the
corner of the room. The EPS foam-core melted back about two inches from the
skin in the vicinity of the fire, but the panel skins and the EPS foam-core
did not sustain notable damage elsewhere. Winter Panel Corporation, a
polyurethane panel manufacturer, commissioned three tests on EPS panels with
totally different results. In these tests, each panel failed in just 16 to 19
minutes. The major difference between these tests and the UL test is that
Winter's panels lacked the waferboard skins on their EPS panels; the EPS foam
adhered directly to the drywall.
Associated Foam Manufacturers, a trade
association of EPS foam manufacturers, conducted two heat endurance tests.
Various-sized panels, loaded with heavy weights to simulate a 3-story
building, were heated in an oven to very high temperatures, then doused with
water to simulate fire fighter intervention. In both tests, the core failed to
melt and showed no sign of panel bowing, bending, or deflection. Moreover, the
bond between the foam-core and the OSB skins remained so strong that they had
to use a crowbar to pry the skins off the EPS core.
In actual cases of fires in buildings
constructed of EPS panels, the panels have fared well. In one case, a
panelized restaurant in Kentucky caught fire in September 1987 Although
temperatures are believed to have exceeded 1,000°F (538°C) in the ceiling
areas and 200°F (93°C) near the floors, the kitchen wall panels and much of
the ceiling remained intact. A limited examination of several wall panels
revealed that the foam-core had neither melted nor delaminated from the skins.
In similar cases, a lack of oxygen reportedly caused fires in foam-core
buildings to extinguish themselves. Fire uses up available oxygen rather
quickly. The air supply in a structural insulated panel home may quickly be
consumed in a fire. This might suggest an additional hazard for human survival
compared to conventionally built houses.
The results of these tests and case
studies of actual fires indicate the importance of having both drywall and
wooden sheathings to protect the EPS foam core. When installed properly, the
fire safety of EPS panels appears dramatically improved.
Insects such as carpenter ants,
carpenter bees, and termites may become a problem in foam-core panels. EPS,
polyurethane, and isocyanurate foam provide the ideal environment for an
insect nest. In a short period of time, insect colonies can completely
honeycomb foam insulation. Some foam-core panel manufacturers issue guidelines
for preventing insect infestation. These steps include applying insecticides
to the panels, treating the ground with insecticides both before and after
initial construction backfilling, maintaining indoor humidity levels below
50%, locating outdoor plantings at least 18 inches (457 mm) away from the
foundation, and trimming away any tree limbs that may overhang the roof. Boric
acid-treated insulation panels are occasionally available in the market.
Insecticidal boric acid is a low toxicity insecticide and fire retarder used
in other insulation materials.
Summary
An increasing number of houses are being
built with foam-core panels. They attract many people because of their high
insulation value and the ease and speed in construction. Insect infestation
can become a serious problem without adequate prevention measures. Although
fire safety is also a concern, houses with properly installed foam-core panels
appear to be quite safe.
Bibliography
The following articles provide additional
information about foam-core panels. This bibliography was reviewed in August
1997.
Conference Proceedings Papers
"Cost Analysis for a Stressed Skin
Insulating Core Panel Demonstration House, Springfield, Oregon," K. Aires,
et al., The 20th National Passive Solar Conference Proceedings,
Minneapolis, MN, July 15-20, 1995, pp. 156-61. Available from the American Solar
Energy Society (ASES) (see Source List below). $4.00 (reprint), $100.00 (full
softcover proceedings).
"Energy Diagnostic Testing and
Monitoring of Six Panelized Housing Units," G. Brown, et al., The 20th
National Passive Solar Conference Proceedings, Minneapolis, MN, July 15-20,
1995, pp. 162-67. Available from ASES (see Source List below). $4.00 (reprint),
$100.00 (full softcover proceedings).
Reports
Alternative Framing Materials in
Residential Construction: Three Case Studies, National Association of
Homebuilders (NAHB) Research Center for the U. S. Department of Housing and
Urban Development (HUD), July 1994. Available from HUD-User (see Source List
below). 69 pp., $4.00 (softcover), Order No. ACCN-HUD6507. Also available from
NTIS (see Source List below). $27.00 (softcover), Order No. PB95 182325/LL.
Alternatives to Lumber and Plywood in
Home Construction, National Association of Homebuilders (NAHB) Research
Center for the U. S. Department of Housing and Urban Development (HUD), July
1994. Available from HUD-User (see Source List below). 74 pp., $4.00 (softcover),
Order No. HUD-PDR-1409(1).
Side-by-Side Evaluation of a
Stressed-Skin Insulated-Core Panel House and a Conventional Stud-Frame House:
Final Report, A. Rudd and S. Chandra, FSEC. Available from the FSEC (see
Source List below). Free, Order No. FSEC-CR-664-93.
Articles
"Building with Panels," S.
Winter, Progressive Architecture, (No. 11) pp. 88-90, November 1993.
"Building with Structural
Panels," A. Boyce, Custom Builder, (2:10) pp. 21-24, October 1987.
"A Compact Timber-Frame
Farmhouse," J. Sousa and L. Johnson, Fine Homebuilding, (No. 105)
pp. 98-103, April/May 1995.
"Demo Highlights Cost and Efficiency
of Panel Houses," Fine Homebuilding, (No. 95) p. 38, April/May 1995.
"EIFS: Exterior Insulating Finish
System," R. Kakulski, Solplan Review, (No. 68) pp. 17-18, May 1996.
"Fire Safety and Insect Problems in
Foam-Core Panel Systems," J. Nisson, Energy Design Update, (No. 75)
pp. 7-11, May 1988.
"Foam-Core Panels: Speedy, High-R
Alternatives to Stud Framing," J. Hufnagel, Better Homes and Gardens,
(No. 67) pp. 90-92, March 1989.
"Foam-Core Panel versus Stud-Frame
Construction-More Side-by-Side Test Results," J. Nisson, Energy Design
Update, (14:3) p. 6, March 1994.
"Foam-Core Panel versus Stud-Frame
Construction-Side-by-Side Test Results," J. Nisson, Energy Design Update,
(13:7) pp. 4-7, July 1993.
"Foam-Core Panels with Recycled PVC
Studs," J. Nisson, Energy Design Update, (13:12) p. 14, December
1993.
"Foam Panels: The Nuts, Bolts, and
Bottom Line," S. Andrews, Custom Builder, (3:3) pp. 25-32, March
1988.
"Foam Sweet Home," C. Wardell, Popular
Science, (249:1) p. 48, July 1996.
"The Frameless House Comes of
Age," S. Bliss, Progressive Builder, (1:1) pp. 10-12, Spring 1985.
"Jury Still Out on Borate Treated
Foam Insulation," Journal of Light Construction, (12:9) p. 8, June
1994. (Answer in Journal of Light Construction, (12:9) p. 6, July 1994.)
"Nailbase Foam Sheathing," J.
Nisson, Energy Design Update, (16:8) pp. 8-9, August 1996.
"New EIFS Standard Ignores Water
Drainage Issue," Energy Design Update, (16:11) p. 2, November 1996.
"NREL's Tests Confirm Good SIP
Thermal Performance," R. Judkoff, J. Balcomb, et al., Spotlight on SIPA,
(4:1) pp. 2-3, Spring 1995.
"Side-by-Side Tests Show Favorable
Results," S. Bush, Spotlight on SIPA, (3:1) p. 2, Spring 1994.
"Skinless Foam Wall Panels," J.
Nisson, Energy Design Update, (15:7) pp. 9-13, July 1995.
"Slab-Edge Insulation Requirement
Partially Eliminated from Energy Code," Energy Design Update,
(16:11) pp. 1-2, November 1996.
"Some Perspective on 'Stressed-Skin'
Panels," B. Rose, New England Builder, (3:7) pp. 15-16, April 1985.
"Stramit Partition Walls," Environmental
Building News, (4:6) p. 11, November/December 1995.
"Stramit USA Closes Down," Environmental
Building News, (5:3) p. 3, May/June 1996.
"Stresskin Panels: Where is the
Technology Taking Us?" S. Chappell, Joiner's Quarterly, (No. 15) pp.
8-14, Summer 1990.
"Study Aims to Reduce Homeowner's
Energy Costs," R. Roeding, Spotlight on SIPA, (4:1) p. 5, Spring
1995.
"Styrofoam-Core Structural
Panels," J. Nisson, Energy Design Update, (13:5) p. 11, May 1993.
"Synthetic Stucco," S.
Culpepper, Fine Homebuilding, (No. 105) pp. 66-71, October/November 1996.
"Synthetic-Stucco Makers See the
Light," S. Culpepper, Fine Homebuilding, (No. 108) p. 38, April/May
1997.
"Taking on Board," S. Winter, Building,
(No. 11) pp. 54-55, January 1991.
"Two Types of Exterior Insulation
Systems Emerge from North Carolina Moisture Disaster," Energy Design
Update, (16:10) pp. 2-3, October 1996.
"University of Oregon Studying SSIC
Panel Technology," American Plywood Association, Spotlight on SIPA,
(4:1) p. 6, Spring 1995.
"Ventilate Roof Decks, Advises
Industry Group," Journal of Light Construction, (15:2) p. 10,
November 1996.
"Wall-Sheathing Choices," B.
Greenlaw, Fine Homebuilding, (No. 106) pp. 50-55, December 1996/January
1997.
Source List
American Solar Energy Society (ASES)
2400 Central Avenue, Suite G-1Boulder, CO 80301
Phone: (303) 443-3130; Fax: (303) 443-3212
E-mail: ases@ases.org
World Wide Web: www.ases.org/solar/
Florida Solar Energy Center (FSEC)
1679 Clearlake Road, Cocoa, FL 32922
Phone: (407) 638-1000; Fax: (407) 638-1010
E-mail: info@fsec.ucf.edu
World Wide Web: www.fsec.ucf.edu/
HUD-User
P. O. Box 6091, Rockville, MD 20849
Phone: (800) 245-2691or (301) 251-5154
E-mail: huduser@aspensys.com
Gopher: huduser.aspensys.com:73
National Technical Information Service
5285 Port Royal Road, Springfield, VA 22161
Phone: (800) 553-6847or (703) 487-4650; Fax: (703) 321-8547
E-mail: orders@ntis.fedworld.gov
World Wide Web: www.fedworld.gov/
Foam-core panels have become considerably
easier to find in the last decade. Builders and do-it-yourselfers are likely to
find at least one manufacturer of foam-core panels in their nearest major
metropolitan area. To locate a manufacturer, check your local yellow pages under
"Building Materials,""Insulation Materials," or contact the
national advocacy group below:
Structural Insulated Panel Association
3413 'A' 56th Street NW, Gig Harbor, WA 98335
Phone: (253) 858-7472; Fax: (253) 858-0272
E-mail: staff@sips.org
World Wide Web: www.sips.org
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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