Because of its versatility and convenience, rigid foam sheathing
has become more and more common throughout the U.S. Contributing
anywhere from R-3 to R-7.5 per inch, sheet foam is a handy way to
boost the wall's total R-value without adding too much thickness.
But foam sheathing does more than simply improve the thermal
performance of a building: Located just inboard of the wall
cladding, sheet foam can also function well as an augmented
drainage plane to help keep wind-blown water out of walls, and it
can act as both an air barrier and as a vapor barrier to defend
against the intrusion of air and water vapor. These properties make
it a good choice in most coastal climates — but only if you
get the details right. A foam-sheathed wall system has to be
designed and detailed with all functions in mind, taking into
account the site's climate and weather exposure.
Insulating Value
The various foam products on the market have different
R-values:
Expanded polystyrene (EPS), an open-cell
"thermoplastic" foam that melts at high temperatures, is made by
expanding polystyrene beads with steam inside a mold. Its R-value
varies from about R-3.2 to R-4.4 per inch, depending on the density
of the plastic and the size of the cracks between the expanded
beads (a typical value would be R-3.9).
Extruded polystyrene (XPS) is made with the same
thermoplastic material, but the molten foam is squeezed through an
extruder to harden into sheets. With closed cells and with no gaps
or cracks, an inch of XPS reaches an R-value of R-4.6 to R-5
(1-inch-thick R-5 sheets are a common product).
Polyisocyanurate (PIR) is a "thermoset" plastic that
cures by chemical reaction and won't melt (although at very high
temperatures it will char and burn). Typical polyiso sheets with
foil facings stabilize at R-6.5 per inch.
When you're designing a wall for thermal performance, foam
sheathing gives you lots of flexibility. Homes in the Houston,
Texas, market, for instance, are often built with 3/8-inch XPS
sheathing over an R-11 or R-13 fiberglass-insulated stud wall, for
an R-13 to R-15 assembly. But a superinsulated solar house in
coastal Maine might use 2x6 framing with R-19 or R-21 cavity
insulation and 2-inch R-13 sheets of foil-faced PIR, for a wall
system rated at R-32 or R-34. Between these extremes lie a whole
range of choices, with more than one way to meet or exceed
energy-code R-value minimums.
Exterior foam serves many functions, providing thermal
insulation, enhanced drainage, and protection from air infiltration
and water vapor. It's not structural, however, and should be
applied over OSB- or plywood-sheathed framing.
Vapor Permeability
In addition to being a good insulator, foam sheathing resists vapor
diffusion. Permeability varies — EPS is the most
vapor-permeable and foil-faced PIR the least — but any foam
you apply over the wall studs or wall sheathing amounts to an
exterior vapor retarder.
Vapor retarders can be problematic. They
work well when kept on the warm side of the wall, so that any vapor
they stop will stay warm and won't condense into liquid water. But
predicting which side of the wall is the warm side can be tricky
when the climate changes. Water vapor wants to move from warm,
high-humidity areas to cool, low-humidity areas, so the direction
of the vapor drive can reverse when the temperature and humidity
change.
Foam, which acts as a vapor retarder, can work on the exterior in
any climate, says building scientist Joe Lstiburek, as long as the
foam's R-value is matched to outdoor conditions — and as long
as the wall's interior face is vapor-permeable, so it can dry to
the inside.
Match R-value to climate. In the Deep South, explains
Lstiburek, an exterior vapor barrier works even if it's not also an
insulator. When you're air conditioning (so the inside is cold and
dry and the outside is hot and humid), the vapor barrier on the
outside makes a lot of sense. As you move north, conditions change:
Homes interiors are heated, and the outdoor design temperatures
grow progressively colder. "At some point, the back side of the
exterior foam [facing the interior] is going to accumulate or
condense water in the wintertime," observes Lstiburek. "So we want
to increase the thermal resistance of that layer, in order to
prevent condensation." The foam must be thick enough to insulate
the back side, keeping it above the dew point. "The farther north
you go, the colder the outdoor temperature, the greater the R-value
required, and the thicker the foam has to be. It's simple," says
Lstiburek.
Dryable to the inside. Just as important,
a wall should not have two vapor barriers, because that could trap
moisture inside the wall (Figure 1). So if insulating sheathing is
used, no poly vapor barrier should be attached to interior wall
faces. But in very cold climates, says Lstiburek, Kraft-paper-faced
batts are recommended. These facings are semipermeable, so they
will slow vapor intrusion into the wall while still allowing
moisture to escape into the heated space.
FIGURE 1. It's inevitable that walls will get
wet at some point — either during construction, from
wind-driven rain, from a leak or a flood, or from elevated humidity
levels. For this reason, all walls must be able to dry. With foam
on the exterior, the only place to dry is to the inside, so it's
critical that no poly vapor retarder or vinyl wallpaper be
installed on the interior and that the wall be painted with a
breathable latex paint.
Foam Thickness
How thick should the foam be? That depends on the climate. In the
most general terms, an inch or less of XPS will probably work
anyplace south of Long Island. From Rhode Island through Maine, you
might need to use an inch of PIR
(R-6.5) or 11/2 inches of XPS (R-7.5) on a 2x6 wall. Of course, the
thicker the foam, the more energy efficient the wall and the safer
it is against condensation.
Lstiburek's organization, Building Science Corp., has spent years
running detailed computer simulations to predict moisture
conditions within walls and experimenting with different wall
assemblies to verify the calculations. Eventually, the group
settled on a simple way to specify exterior foam thickness: "You
take the average temperature of the three coldest months of the
year in your location," says Lstiburek. "Take the average
temperature for December, the average temperature for January, and
the average temperature for February — and you average those,
and use that average as your design temperature for outside. You
set your interior design condition as 70°F and 35% relative
humidity. Then you do a simple calculation to make sure that the
condensing surface doesn't drop below the dew point. As long as you
don't see 100% relative humidity at the interface between the foam
and the cavity insulation, you won't have condensation on the back
side of the sheathing." (See "Calculating Foam Thickness")
Lstiburek admits that his simple assumptions are not perfectly
realistic. "When someone says, ‘Yeah, but that's not really
what's going on,' well, that's true. But it's a very good
approximation — it gets us 98% accuracy with one easy
calculation." And he's backed it up with lots of experimental work
and lots of very detailed measurements and calculations. Anyone
who's not comfortable with it, he says, can always run a more
detailed simulation for the particular structure — or simply
increase the foam thickness for good measure.
Fastening foam. The thickness of the
foam, of course, affects the fastening of the siding and trim. Foam
by itself won't anchor a fastener, so nails and screws have to be
long enough to go through the foam into solid wood. According to
Lstiburek, the practical limit for normal fastening through foam is
between 1 and 11/2 inches. "For foam thicker than an inch and a
half, I go to 1x4 strapping screwed through the foam into the
framing or sheathing behind it," he says. "We've done 8-inch to
10-inch layers of foam that way. The barn at my house [near Boston,
Mass.] has 8 inches of foam on the outside, battened on using
12-inch screws."
Structural Performance
In some parts of the country, you can get away with rigid foam as
the main sheathing, with OSB or plywood used only for bracing at
wall corners, plus an occasional sheet at mid-wall. But that method
won't wash in high-speed wind zones by the ocean (Figure 2). In
general, houses near the shore will need full structural panel
sheathing under the insulating foam.
FIGURE 2. To resist the wind, as well as the
possibility of a tidal surge, coastal homes must be stiff enough to
resist racking, and they must also be anchored against sliding and
overturning. The stiffness comes from plywood or OSB sheathing;
foam isn't enough.
Racking resistance. "The main function of the wood structural panel
sheathing," explains Joe Lstiburek, "is to provide racking
resistance. It also helps support the housewrap. So I don't think
you're going to be able to build in [coastal] conditions without
sheathing your entire building with plywood or OSB."
In coastal states, homes in sheltered locations far from the water
may be able to substitute foam sheathing for wood-panel sheathing.
However, they'll still need wall bracing — either a
code-recognized method, or an engineered design (see "Wall Bracing
and the IRC," July/August 2006; www.coastalcontractor.net ). The easiest way to do
this is with fully sheathed walls. In many cases, this may require
the addition of engineered shear walls as well.
Stabilizing the shell. Foam can also have a positive effect on the
structural performance of the wall. By placing an insulating,
air-blocking, and vapor-blocking skin between the house's framing
and sheathing and the exterior weather, foam sheathing lets the
builder bring the home's wood structure into a relatively protected
zone that is closer to the conditioned indoor environment. Notes
Lstiburek: "Wall frames move because of moisture-change
differentials between the inside faces of the studs and the outside
face of the studs. That moisture-content difference increases if
the temperature difference is greater. When you put insulating
sheathing outside, the wall frame sees more constant and uniform
conditions, and you actually reduce drywall cracking and
building-frame movement."
Drainage Planes
Just as important in any wet climate, foam serves as a building's
drainage plane for rainwater management. A "Guide to Insulating
Sheathing" posted among the technical resources on the
buildingscience.com Web site offers several ways to detail the foam
skin under siding. But for the severe weather of coastal
conditions, Lstiburek calls for a more robust system.
Fully sheathe the building, he says, and then apply a layer of
drainable housewrap (Figure 3). "Attach the windows and doors
directly to the sheathing, and flash everything just as if you
weren't applying the foam." After that, the foam gets installed
over the housewrap and flashings (Figure 4). Most rainwater will be
deflected by the cladding or by the foam sheathing beneath it, says
Lstiburek, but any water that penetrates further will be shed by
the flashings and housewrap; and any minor, incidental leaks should
be able to dry into the conditioned space.
FIGURE 3. Contractor Craig Caulkins of
Caulkins Building & Design in Niantic, Conn., routinely applies
1/2-inch or 1-inch Dow Styrofoam to home exteriors under vinyl
siding (top). While foam is water resistant and provides a good
barrier against the bulk of the weather, wind-driven rain can find
its way around panels and through joints, so Caulkins relies on a
housewrap and careful window flashing installed over a fully
sheathed structure before installing the foam (bottom).
FIGURE 4. In combination with Andersen flanged
vinyl windows, says Caulkins, no special furring or attaching
method is required: window flanges extend far enough from the wall
to cover all foam edges. Crews flash the window openings with
self-adhering membrane, fasten the window directly to OSB-sheathed
wall, and then apply a two-piece window surround from CertainTeed.
The base piece of the window surround is nailed through the foam
into the window rough frame (top), and the matching top piece snaps
into place over it (bottom).
Robust performance. The field experience
of others supports Lstiburek's recommendations. Dennis McCoy of Ram
Builders, Inc. (www.rambuilders.com), a specialist in the repair of
failed stucco-clad walls, says he's observed that foam sheathing
can improve the weather perfor-mance of wall systems and protect
against moisture damage. McCoy's company has torn apart and
repaired or rebuilt thousands of moisture-damaged stucco walls in
the hot, humid, coastal climate of Houston, Texas. "Walls with foam
sheathing, in our experience, show less moisture damage, especially
when the interior plastic vapor barrier is omitted from the walls,"
notes McCoy. Even if there is only one layer of building paper on
the wall (good stucco practice requires two layers), walls with
foam sheathing generally do better than walls without, he
reports.
Ideally, McCoy would like to see stucco-clad wall frames protected
by two layers of building paper and flashing, then a layer of foam,
before the lath and stucco are applied. "We call the building paper
a secondary weather barrier," he says. "The stucco cladding is the
primary weather barrier. But if you include the foam, now you
actually have a third weather barrier to help handle
rainwater."
In cases he has investigated, McCoy reports, the foam indeed seems
to protect against both exterior rain and interior moisture. "The
walls don't get condensation on the back side of the OSB
sheathing," he says. "And where there are leaks, they seem to be
able to dry to the inside as long as there's no plastic vapor
barrier in the way. I can't explain the science, but it works. It's
a hard sell to customers, especially after all the trouble that has
happened with EIFS [exterior insulated finish systems]. But if
someone's willing to pay for it, I'd like to put exterior foam
insulation on every wall we fix." ~
Ted Cushman reports on the building industry
from his home base in Great Barrington, Mass.