Most residential structures are insulated with either
fiberglass batts or cellulose because both are cheap sources of
added R-value. But there's more to insulation than R-value. For
best results, the insulation must be accurately cut to fit the
joist or stud cavities, and an effective air barrier is needed
to keep unconditioned outdoor air from penetrating the
insulation like wind blowing through a sweater. In most
climates, a poly or kraft-paper vapor retarder is also needed
to limit the flow of moisture-laden air and prevent
condensation from forming within the insulation.
None of that is exactly rocket science, but doing the job
right does take some care and attention to detail.
Unfortunately, because both fiberglass and cellulose
installations are typically subbed out to the lowest bidder,
vapor retarders, air barriers, and the insulation itself are
often thrown into place with little regard to quality.
When quality is a more important consideration than price,
spray-applied polyurethane foam is emerging as the first choice
of a growing number of builders. Although it costs up to
several times as much as its competitors — an R-11
application of low-density foam goes for at least $1.00 per
square foot of wall, compared to about 65¢ for spray
cellulose, and 25¢ to 55¢ for fiberglass batts
— foam eliminates many of the installation headaches
associated with fibrous insulating materials.
First, foam has exceptional air-sealing ability. When sprayed
or injected into a framing cavity, it sticks tight to the
sheathing and framing and rapidly expands to fill every crack
and opening in the exterior shell. This is especially valuable
around rim joists and other difficult-to-seal areas. Some types
of foam are also effective vapor retarders, so it's often
possible to omit the separate poly or kraft-paper vapor
Finally, going with foam can provide added flexibility in
designing a framing package: Because dense varieties of foam
offer a lot of insulating value per inch of thickness, it's
often possible to size studs and rafters based on structural
loads rather than the amount of space needed for
Foamed-in-place polyurethane was developed in Europe. It was
first used in North America during the 1960s, first as an
insulator for commercial cold-storage buildings and later as a
commercial roofing material.
Polyurethane foam has had a harder time penetrating the
residential market. During the 1970s and early '80s, a
foamed-in-place product known as UFFI — an abbreviation
for urea formaldehyde foam insulation — was widely used
for retrofitting uninsulated houses but was later found to
offgas potentially harmful amounts of formaldehyde into living
spaces. The resulting uproar left all foamed-in-place
insulating materials with an image problem that they have only
Today's foamed-in-place insulation does not contain urea
formaldehyde. Current products are made from isocyanate —
a material derived from petroleum — and urethane resins,
which are often made from sugar cane or soybeans. Potentially
toxic vapors may be present while the foam is actually being
applied, but the cured material is nontoxic and will not offgas
Equipment and materials.
Application methods vary somewhat depending on the proprietary
product used, but most residential foam contractors arrive on
the site in a small box truck that contains the necessary drums
of chemicals, a pumping machine, and several feet of hose. The
pumping machine precisely meters out the two components of the
foam and heats them to accelerate the chemical reaction that
causes them to foam when combined.
The chemicals pass through separate lines that are combined in
a single hose until they mix at the nozzle. The liquid that
emerges expands almost instantly from a paint-like consistency
to a thick foam that sets up into a durable solid.
Density and R-value. There
are many brands of proprietary foams on the market, and they
vary widely in density and insulating power. Commercial flat
roofs, for example, are often insulated with a high-density
material that weighs about 3 pounds per cubic foot, which makes
it hard and strong enough to walk on without damage. But most
residential foam insulation weighs between .5 and 2.0 pounds
per cubic foot.
With most common building materials, lower density translates
into higher insulating value. That's why fiberglass batts
insulate better than wood and wood insulates better than
concrete. But the opposite is true of foam. A 1/2-pound foam
such as Icynene, for example, has an R-value of about 3.5 per
inch — roughly the same as fiberglass batts or loose-fill
A denser, 1.8-pound foam, on the other hand, has an R-value of
about 7. But because the 1.8-pound foam contains nearly four
times the amount of chemicals per unit of volume as the
1/2-pound material, the square-foot cost is substantially
Trimming the foam.
High-density foams are usually applied to a total thickness
that's significantly less than the depth of the framing. An
experienced applicator will take care to avoid getting much
foam on the exposed edges of the studs, since any stray drops
or spatters have to be scraped off before the drywall goes on.
Low-density foams, by contrast, expand much more and usually
bulge out beyond the framing. This excess material must be
trimmed off with a long, flexible saw blade before the wall or
ceiling finish can be applied.
Framing dimensions. With
low- density foam, as with fiberglass batts or cellulose, the
dimensions of the framing are driven more by the insulation
value required than by structural considerations. For example,
the 2x6 wall studs used on so many residential jobs are
overkill from the standpoint of supporting the weight of the
building. The real reason for using them is that they provide
stud bays deep enough to accommodate R-19 fiberglass batts.
Because the R-value of low-density foam is comparable to that
of fiberglass, the framing requirements are also similar.
But when a denser foam is used, it's possible to pack more
R-value into a shallower bay. With 1.8-pound foam, you can
frame walls with 2x4s and still achieve an R-value of 24 (see
Figure 1). Another option is to frame with 2x6s and fill the
cavities only partially, leaving an open space for running
pipes or wires.
Figure 1.Polyurethane foam expands to between 30
and 100 times its wet volume. Dense, closed-cell material such
as this has twice the R-value per inch of light, open-cell
In walls or ceilings insulated with porous insulating
materials such as fiberglass, a poly or kraft-paper vapor
retarder is usually installed on the warm side of the
insulation (that is, on the inside in heating climates and on
the outside in cooling climates) to prevent condensed moisture
from wetting the insulation. But because foam itself is
resistant to water vapor, it may be possible to omit this added
step. The question of whether to install a separate vapor
retarder will depend partly on the specific foam you choose and
partly on your local building inspector.
Tiny bubbles. Dense foams
have what's known as a closed-cell structure, which means that
the gas bubbles that form during the application process remain
permanently locked into the cured foam. The result is something
like a three-dimensional bubble wrap with extremely tiny
bubbles. Because there are no interconnections between
individual bubbles, the foam absorbs little water and also
resists the passage of water vapor. According to most building
codes, a vapor retarder must have a perm rating of less than
1.0, and some dense foams meet this standard.
Low-density open-cell foams, on the other hand, have a
structure more like a very fine-grained sponge. The cured
material consists of a series of tiny interconnected
passageways. These open cells are too small to permit the
passage of much air, but they are more permeable to water vapor
than closed-cell foams. Unless there's an exceptional amount of
vapor drive, though, that isn't usually a problem. Most
condensation in framing cavities is caused by leakage of moist
air, not differences in vapor pressure, and even low-density
foams block air movement so effectively that problems are
unlikely. Some building inspectors will allow you to omit the
vapor retarder even if the perm rating of the foam is above the
required minimum value (Figure 2).
Figure 2.Open-cell foam is more permeable to vapor
than closed-cell material. All foam is designed to be used
without a vapor barrier, but some inspectors will make you use
it, as in this kitchen that was insulated with 1/2-pound
Trading places. Proponents
of foam claim that it's an ideal insulating material for mixed
climates, where the warm and cold sides of the building
envelope reverse during the year. During the heating season,
the vapor retarder belongs on the inside of the wall, but when
the air conditioning kicks on during the summer, it belongs on
the outside. This is a practical impossibility with permeable
insulating materials. But because foam is uniformly solid, it
resists the passage of vapor equally well in either
Roofs and Attics
Cathedral ceilings are notoriously difficult to insulate
effectively. Unlike walls, ceilings don't have air barriers
like Tyvek and are usually vented to maintain a cool roof
surface and prevent ice dams. But venting makes it easier for
cold air to infiltrate batt insulation, which reduces its
effective R-value. Ceiling penetrations like recessed lights
are also common sources of air leakage.
Cold roofs and foam. One way
to deal with these sorts of troublesome leaks is to fill the
ceiling with spray foam. According to Matt Momper — whose
Indiana-based company is one of the region's largest installers
of foam, fiberglass batts, and other materials — foamed
cathedral ceilings should be vented if possible.
"Some roofing manufacturers won't warrant their shingles if
the roof isn't vented," he says. Before spraying the
closed-cell foam, Momper installs polystyrene baffles below the
sheathing to create a channel connected to soffit vents and a
continuous ridge vent.
But if the rafters aren't deep enough to leave room for a vent
channel, or if the design of the roof makes it impractical to
install a ridge vent, Momper has found that unvented ceilings
also work well.
Unvented attics. Foam is
also effective in areas where codes permit unvented attics.
This technique is especially popular in parts of the South,
where the humidity is high and it's common to put air handlers
in the attic. Spraying the underside of the sheathing and the
gable-end walls turns the attic into a conditioned space and
prevents humid air from entering and condensing on cold
ductwork (Figure 3).
Figure 3.Spray-applied foam insulation allows you
to build cathedral ceilings without venting or vapor barriers.
It also allows you to build unvented attics.
Placing the air handler in the relatively cool environment of
a sealed attic also decreases the load on the hvac system and
may allow you to install smaller, less expensive equipment.
Finally, any air that leaks from ductwork located in the attic
will help cool the conditioned space rather than escaping
uselessly to the outdoors.
Spray foam works well under floors because it won't sag or
fall down the way batts sometimes do. This makes it a good
choice for rooms over exterior porches or small additions built
on elevated piers. Foam is especially useful for insulating
truss-framed assemblies and other areas that are difficult or
impossible to insulate with batts (Figure 4).
Figure 4.Rim joists are difficult to insulate and
nearly impossible to fully seal with traditional insulation and
vapor barriers. Foam allows you to do a much better job
insulating areas that are often poorly done.
Unvented crawlspaces. Spray
foam adheres well to masonry of all kinds, including the
irregular stone foundations sometimes encountered in old
houses. As a result, it's becoming a popular choice for sealing
and insulating the perimeter walls of crawlspaces, especially
in areas where unvented crawlspaces are permitted by
Like unvented attics, unvented crawlspaces aim to prevent
condensation and moisture problems by keeping humid air outside
the conditioned envelope. The air-sealing properties prevent
the entry of airborne moisture, but it's also important to seal
out moisture in the soil. The usual way of doing this is to
cover the dirt floor of the crawlspace with a continuous poly
Foam and batts: hybrid or
bastard? Some insulation contractors install foam and
batts in the same framing cavity in order to combine the air-
sealing and vapor-resistant properties of foam with the economy
of fiberglass. Momper uses this technique regularly. The
framing cavities are first sprayed with a 1/2-inch layer of
closed-cell foam before the rest of the cavity is filled with
batt insulation to beef up the overall R-value.
Momper reports no problems with this approach, but the
technique is a controversial one within the spray-foam
industry. Opponents of this method refer to it as "flash and
dash," the implication being that it's a shoddy way to do the
job. They claim that putting foam outside the fiber insulation
may result in a wrong-side vapor retarder in heating climates.
Proponents say that it's an effective system because the foam
will prevent air from infiltrating the wall, and vapor usually
gets into walls because of air infiltration, not because of
Foam and structural
strength. There's both anecdotal and scientific evidence
to suggest that spray-in-place foam also adds strength and
stiffness to wood-framed buildings. Builder Joseph Jackson, of
Faust Contracting in Little Silver, N.J., recalls framing a
house that moved slightly every time the wind blew. Once the
walls were sprayed with 2-pound foam, Jackson reports, the
structure felt absolutely rigid.
According to Craig DeWitt of RLC Engineering in Clemson, S.C.,
Clemson University has performed extensive testing to evaluate
the structural value of foam. Racking tests showed that walls
filled with sprayed-in-place foam were stiffer than walls
filled with fiberglass batts. Tests also showed that spray foam
significantly strengthened the bond between rafters and
sheathing, which is a plus in high-wind areas. DeWitt cautions
that building codes do not recognize sprayed foam as a
structural component. But he says that engineers can include
the strength of this bond in the structural calculations for