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Foam Board Insulation
Foam boards—rigid panels of insulation—can be used to insulate almost any
part of your home, from the roof down to the foundation. They provide good
thermal resistance and often add structural strength to your home. Foam
board insulation sheathing reduces heat conduction through structural
elements, like wood and steel studs.
Types of Foam Board
The most common types of materials used in making foam board include
polystyrene,polyisocyanurate or polyiso, and polyurethane.
Molded Expanded Polystyrene Foam Board
Molded expanded polystyrene (MEPS) is a closed-cell material that can be
molded into many everyday items, such as coffee cups and shipping
materials, or into large sheets of foam board insulation. MEPS foam board
insulation is commonly known asbeadboard.
To make beadboard, loose, unexpanded polystyrene beads containing liquid
pentane are mixed with a blowing agent and poured into an enclosed
container. The mixture is heated to expand the beads many times their
original size. The beads are then injected into a mold. Under more heat and
pressure, they expand to become foam blocks, which are shaped as needed.
The physical properties of MEPS foam board vary with the type of bead
used. It's manufactured at various densities, depending on the application.
Beadboard for roofing materials has to be dense enough to walk on without
damage; wall insulation foam boards are several times less dense than roof
boards. R-values range from 3.8 to 4.4 per inch (2.54 cm) of thickness.
MEPS foam board is available with a variety of facings. Since spaces
between the foam beads can absorb water, a vapor diffusion retarder is
necessary if water transmission through the insulation might become a
MEPS foam board also is often used as the insulation for structural insulated
panels (SIPs) and insulating concrete forms (ICFs).
Extruded Expanded Polystyrene Foam Board
Extruded expanded polystyrene (XEPS) is a closed-cell foam insulation
similar to MEPS. To make it, the polystyrene pellets are mixed with various
chemicals to liquefy them. A blowing agent is then injected into the mixture,
forming gas bubbles. The foaming, thick liquid is then forced through a
shaping die. When cooled, the panel is cut as required. Foam densities are
typically 1.5 pounds per cubic foot (24 kilograms per cubic meter).
XEPS is more expensive than MEPS. Like MEPS, the R-value depends upon
the density of the material and is generally about R-5 per inch. It's also much
more consistent in density and has a higher compressive strength than
MEPS, making it better suited for use on roofs or for wall panels. Extruded
polystyrene also has excellent resistance to moisture absorption.
Like MEPS, XEPS is available with a variety of facings and is also often used
as the insulation for SIPs and ICFs.
Polyisocyanurate and Polyurethane Foam Board
Polyisocyanurate or polyiso and polyurethane are very similar, closed-cell
foam insulation materials. Because both materials offer high R-values (R 5.6
to R 8) per inch of thickness, you can use a thinner foam board to achieve
the required thermal resistance. This can be an advantage if you have space
Polyiso foam board insulation is available in a variety of compressive
strengths.Compressive strength refers to the ability of a rigid foam board to
resist deformation and maintain its shape when subjected to a force or load.
Also, polyiso remains stable over a wide temperature range (-100ºF to
+250ºF). This makes it good as roofing insulation. And when used with a
laminated aluminum foil facing, polyiso foam board provides an effective
moisture or vapor barrier.
These foam boards can also be used to make SIPs.
The maximum performance of foam board insulation depends heavily on
proper installation. Therefore, it's best to have a certified insulation installer
do it. If you'd like to have it installed professionally, you should do the
• Obtain written cost estimates from several contractors for the R-value
you need. Don't be surprised to find quoted prices for a given R-value
installation to vary by more than a factor of two.
• Ask contractors about their air-sealing services and costs as well, if
To evaluate batt installation, you can measure batt thickness and check for
gaps between batts.
If you want to install it yourself, you should try to obtain instructions and
safety precautions from the manufacturer. Carefully follow these instructions.
You should also check your local building and fire codes.
Avoiding Direct Sunlight Damage
Protect all types of foam insulation from direct sunlight. Over time, the sun's
ultraviolet rays can damage the insulation. For roofs, this is generally done
by applying a coating such as tar, acrylic, silicone, or rubberized paint. You
can also cover the foam with a rubber or plastic membrane, or a layer of
asphalt and roofing felt. Make certain you are using compatible products.
The solvents in some coatings dissolve certain plastics.
Avoiding Potential Moisture Problems
In cold weather, warm inside air containing water vapor can get past the wall
finish and insulation, condensing inside the colder wall cavity. In hot, humid
climates the same thing can happen, just in the reverse direction. Humid
outdoor air in the summer can condense inside cool, air conditioned wall
cavities. If enough of this happens and the water cannot escape, wood rot,
mold, and other moisture-related problems can occur. For this reason,
building codes often require installing a vapor diffusion retarder on the
warmest side of the wall cavity.
Foam board insulation is commonly placed between the exterior finish (i.e.,
siding, brick) and the studs of exterior walls. To prevent air infiltration, you
should place rigid insulation boards tightly together and seal the seams with
tape or caulk. However, this practice may worry some builders in cold
climates since the foam board may act as a second vapor diffusion retarder.
Studies have shown, however, that condensation rarely occurs in these
areas unless something else is seriously wrong with the wall assembly (i.e.,
massive uncontrolled air leakage into the walls from the house). If the
assembly is constructed correctly, the inside surface of the foam board stays
warm enough to keep water vapor in its gaseous state long enough for it to
Avoiding Potential Insect Problems
When insulating a foundation you need to consider that, although insects
don't eat foam board, they can easily tunnel through it. Insect burrows reduce
the R-value and structural integrity of the insulation. For these reasons, some
manufacturers treat their foam products with an insecticide, usually a borate
compound. Many building jurisdictions also mandate treating the earth
around the building with insecticides. These jurisdictions may also want an
inspection area several inches wide and all around the foundation of a house
kept bare of insulation board.
A better solution for below-grade walls in need of insulation is to install the
foam board over the interior of the basement walls rather than on the
exterior, which is more common. Interior applications prevent ground-dwelling
insects from finding the foam board at all, and they eliminate the need for the
bare inspection area. Insulating interior walls, however, requires careful
attention to moisture control.
Most jurisdictions also require installing a fire barrier over the interior foam
board. While this adds extra cost, the thermal performance of this method is
superior in most cases to the more common exterior foam board application.
This equates with a dollar savings in energy that can repay you many times
over for the additional cost of an interior application. If you plan to convert a
basement into a living space, there is almost no additional cost.
Ensuring Fire Protection
Foam insulation is relatively hard to ignite, but when it is ignited, it burns
readily and emits a dense smoke containing many toxic gases. The
combustion characteristics of foam insulation products vary with the
combustion temperatures, chemical formulation, and available air.
Because of these characteristics, foams used for construction require a
covering as a fire barrier. One half-inch thick (1.27 cm) gypsum wallboard is
one of the most common fire barriers. Some building codes, however, do not
require an additional fire barrier for certain metal-faced, laminated foam
products. Check with your local building code/fire officials and insurers for
specific information on what is permitted in your area.