Bubble Wraps
Source: BUILDER
Magazine
Publication date: 2005-09-01
By Charles Wardell
Q: What are the pros and cons of open-cell and closed-cell
foam insulations? And where are the best places to use each?
A: BLOWN-IN-PLACE
POLYURETHANE foams are a blend of isocyanates and
urethane resins that are mixed in a spray nozzle, then
blown into a wall or ceiling cavity. A blowing agent makes the foam expand and
fill the space.
Open-cell and closed-cell foams use different blowing
agents. They also differ in hardness, insulating value, vapor permeability, and
cost.
AIR AND GAS
The most familiar brands of open-cell foams are Icynene and Demilec. These use
water as a blowing agent and insulate by trapping air in microscopic cavities
that are connected to one another like the voids in a sponge.
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Closed-cell foams are blown with gas, which also acts as
the insulator. As the foam expands, it traps the gas in a matrix of tiny
spheres, each about the diameter of a human hair—a kind of microscopic,
three-dimensional bubble wrap. Unlike the air in an open-cell product, very
little of the gas trapped in these bubbles leaks out. Insulation manufacturers
buy the blowing agent, package it with the urethane
mixture, then sell the system to insulation contractors.
Until recently, the blowing agents used in closed-cell
foams included chlorine-based chemicals, which deplete the earth's ozone layer,
but manufacturers are switching to more environmentally benign products. For
instance, Honeywell's new Enovate gas (HFC-245 fa), which is used by some of the biggest insulation
manufacturers, is a hydro-fluorocarbon gas that includes non-ozone-depleting
fluorine.
COMPOSITION AND DENSITY
Open- and closed-cell foams have different compositions and
densities. Open-cell foams are soft and pliable, with densities of ½ to ¾
pounds per cubic foot. “They're like a cushion, or the packaging material
molded inside a plastic bag to fit a fragile object being shipped,” says Steve
Riddle, vice president of sales with North Carolina Foam Industries
(www.ncfi.com), a nationwide installer of open- and closed-cell foams.
Closed-cell foam, by contrast, has a density of around 2
pounds per cubic foot; in fact, it's stiff enough to walk on without crushing
it. Says Riddle: “The bubbles in a closed-cell foam
are strong enough to take a lot of pressure, like the inflated tires that hold
up an automobile.”
Riddle's installers find that closed-cell foam, because
it's so rigid, will actually stiffen wall assemblies by helping lock a home's
framing members together, making it the insulation to use in high-wind areas.
This field experience has been confirmed by laboratory research.
“There has been a lot of testing done on the racking
strength provided by different types of spray foams,” says Tom Kenny, of the NAHB's
However, Kenny points out that this strength isn't
acknowledged by building codes. In other words, while closed-cell foam can help
stiffen a structure in a high-wind zone, you can't reduce the structural
bracing as a trade-off.
INSULATING VALUE
The insulating value of any foam depends on the gas trapped
inside of it. Because a gas like Enovate is a better
insulator than air, closed-cell foams give you more R-value per inch. Some
open-cell manufacturers like to differentiate their product by pointing out that
it has a stable R-value, while gas leaking from the closed-cell product
degrades its effectiveness over time. Riddle says this is misleading.
Closed-cell foams do lose some gas soon after installation, but they stabilize
over time, and their long-term R-value is still impressive. In fact, his
company advertises its closed-cell foam's long-term R-value of 6.8 per inch,
which is nearly twice that of open-cell's R-value of 3.5 per inch.
AIR AND VAPOR SEALING
Both foam types resist the passage of water vapor better
than fiberglass or cellulose, but closed-cell foams do it best. In fact, some
closed-cell foams have a perm rating of less than 1.0, the standard perm rating
for a vapor retarder.
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Under normal circumstances this may not mean much:
Condensation in walls and ceilings is usually caused by leakage of moist air,
not differences in vapor pressure, and even low-density foams do a good job of
blocking air movement. However, Riddle recommends using closed-cell foam where
outside water is a concern: on the coast, in flood-prone regions, in crawl
spaces, and in attics. (His company even uses it as a flat roof covering. It
sprays it on the roof surface, then covers it with
gravel.) It's also a good choice where you need to get a lot of insulating
value into a small space.
THE BOTTOM LINE
One disadvantage of closed-cell foam is it costs more per inch
than open-cell, since it's denser and requires more material. Cost per R-value
can vary in different parts of the country, so you'll have to check with your
installer.
The choice of foam should be based on the requirements for
strength and vapor control, available space, and so on. “We select a foam
system depending on the particular requirements of our clients' projects,” says
Riddle. The table on this page includes some suggested uses.
Charles Wardell is a freelance
writer based in Vineyard Haven, Mass.
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