As a general contractor, I was taught that attic and
cathedral ceiling assemblies should always be vented. Since
then, however, studies have shown that properly designed and
installed unvented attic assemblies outperform vented
assemblies. They reduce energy loss and protect against rot and
mold by preventing moisture from passing through the insulation
and condensing on cold surfaces. Although many builders —
and even some building inspectors — are unfamiliar with
them, unvented assemblies are already part of the 2006 IRC and
will soon be allowed by most building codes (see sidebar).
Code Provisions for Unvented
Every state except California and Hawaii has adopted
some version of the IRC. And California is expected to
adopt it in 2008.
Until recently, the IRC required all attics and
enclosed rafter spaces to be vented. But the latest
version allows unvented attic assemblies if certain
conditions are met.
According to Section R806.4 of the 2006 IRC, unvented
assemblies are allowed if "no interior vapor retarders
are installed on the ceiling side (attic floor) of the
unvented attic assembly" and if "air-impermeable
insulation is applied in direct contact with the
underside/interior of the structural roof deck."
There is an exception that allows air-permeable
insulation (fiberglass and cellulose) to be used in
unvented assemblies in certain parts of the South
(climate zones 2B and 3B).
It has long been possible to get an unvented assembly
approved by the inspector as an "alternate construction
method." But once states update their codes to the 2006
IRC, it will no longer be necessary to get special
approval for unvented assemblies.
In the meantime, the fact that the 2006 IRC allows
unvented assemblies should make it easier to get
special approval in states that have adopted earlier
versions of the code.
Do not build an unvented attic assembly without first
talking to the local building inspector. Unvented
assemblies are new in the IRC, and your state might be
using an older version of the code. Also, the committee
that wrote this section is still working on it, so more
changes may be on the way.
I work for a company in Northern California that installs
spray polyurethane foam (SPF) insulation, and we are frequently
asked to insulate unvented assemblies. Sometimes the building
has a flat roof or a cathedral ceiling that would be difficult
or impossible to ventilate (Figure 1). In other cases, the
existing framing cavities are too shallow to accommodate a
sufficient amount of insulation plus a vent space. And
occasionally customers request unvented attics because they
make the building more comfortable and energy-efficient.
Figure 1. Spray foam is a good choice for
roofs that are difficult to vent, like a turret with converging
rafters (top) or a flat roof with its rafters hung between
flush beams (bottom).
Why Install Roof Venting?
Traditionally, venting has been used to deal with problems that
occur when heat or moisture escapes into the attic (Figure
Figure 2. While attic ventilation can
mitigate problems caused by ineffective insulation or leaky air
or vapor retarders, a better approach is to build the attic as
an unvented assembly. The foam insulation used for unvented
attics stops air movement and with it the transport of
moisture. Any hvac equipment located in the attic is within the
conditioned shell of the house, which also cuts energy
In cold climates, the escaping heat can cause ice dams by
melting the snow on the roof. Venting the space above the
insulation helps keep the roof cool by carrying this heat away.
If moisture enters the attic through the ceiling (usually as an
air leak), the vents are supposed to allow it to exit before it
condenses on something cold.
However, ventilating above fiber insulation comes with an
energy penalty. Fiber insulation is designed to be enclosed in
an airtight cavity. When air flows over and through fiber
insulation, there is a substantial loss of thermal
Also, most hvac ducts and air handlers leak to some degree, so
when these are installed in vented attics, conditioned air is
lost to the exterior. And because vented attics are subject to
extreme high and low temperatures, additional energy is lost
through the thin insulation on the hvac equipment.
In cooling climates, venting the attic can bring humid outdoor
air into contact with attic ductwork. If the ducts are not
properly insulated, they can be cold enough to cause
Venting and shingle temperature. It's
a common misconception that code-required venting significantly
lowers the summer temperature of the roof surface. In fact,
tests have shown that it lowers the surface temperature of
asphalt shingles by at most about 5°F.
For many years, roofing manufacturers required that shingles be
installed over vented substrates, but today, several companies
— including Elk and CertainTeed — will guarantee
shingles installed over properly constructed unvented
How Unvented Assemblies Work
A properly constructed unvented attic is immune to the moisture
problems that occur in vented assemblies and is much more
likely to be energy-efficient.
In an unvented assembly, anything below the insulation —
including an attic — is considered conditioned space.
Turning the attic into conditioned space saves energy; if heat
or air escapes from the hvac equipment, it remains within the
conditioned space (Figure 3).
Figure 3. The ducts visible in this
unvented attic will be concealed after drywall is installed.
But because they are in conditioned space, they won't be
subject to the extremes of temperature typical of
If enough energy is saved in this manner, the hvac system can
actually be downsized, reducing installation and operating
A number of insulation materials can be used in an unvented
assembly, but the one with the greatest applicabil-
ity across the country is SPF. It's an extremely effective
insulation and air barrier all in one, and since it's
spray-applied, it conforms to irregular shapes that otherwise
might be difficult to insulate and seal (Figure 4).
Figure 4. This barrel ceiling (top) would
be difficult to insulate and seal with traditional materials.
It's an ideal candidate for spray foam, which conforms to its
irregular surfaces (bottom).
Despite the multiple brands of SPF, there are only two main
kinds: open-cell foam and closed-cell foam. Chemically, all
brands are nearly identical — contrary to some
advertising claims — and contain about the same
proportion of agriculturally derived resin from corn, sugar
beets, sugarcane, or soybeans. None of the spray foams contain
formaldehyde or use toxic or ozone-depleting blowing
The important differences between products have to do with
density, R-value, and permeability.
Open-cell foam. The typical open-cell
foam weighs 0.5 pound per cubic foot and has an insulation
value of R-3.5 per inch of thickness. This type of foam is
relatively permeable; at 5 inches thick it is rated at about 10
perms. Open-cell foam is an air barrier but not a vapor
When sprayed, open-cell foam expands to about 100 times its
liquid volume, so it usually has to be trimmed flush to the
framing. Fortunately, it's soft and easy to trim.
Closed-cell foam is denser and less
permeable than open-cell material. The typical closed-cell foam
weighs 2.0 pounds per cubic foot and provides R-6.6 per inch of
When sprayed, closed-cell foam expands from 30 to 50 times its
liquid volume, making it easy to apply without completely
filling the framing bay. If the bay must be filled completely,
the applicator can overfill it and then trim off the
Trimming closed-cell foam is not as easy as trimming the
open-cell material, but it can be done.
Advantages of Closed-Cell Foam
Both types of SPF are excellent insulation materials, but our
company uses closed-cell material in unvented assemblies
because we think it provides the best overall performance. With
it, we can pack more R-value into a small space, which is
helpful when the existing rafter bays are shallow; for example,
we can get R-30 into a 41/2-inch space.
In our climate zone, it's important to avoid excessive vapor
diffusion, and we think the best way to do this is to use
closed-cell foam. One of the great benefits of closed-cell foam
is that if you install it to a thickness of at least 2 to 2 1/2
inches, it will have a permeance of 1.0 perm or less.
This means that in addition to being an air barrier,
closed-cell foam is a vapor retarder. It's actually a vapor
retarder from both sides, so it ends the debate about which
side of the insulation to put the vapor retarder on in climates
where interiors are both heated and cooled.
Some companies that make both open-cell and closed-cell foam
advise insulation contractors not to use the open-cell material
in unvented assemblies — or to do it only in certain
climates where vapor diffusion will not be a problem.
In conditions of extreme vapor drive — an indoor pool or
spa, for instance — it may be necessary to further reduce
the permeability of closed-cell foam by coating it with a
spray-applied liquid vapor barrier.
In a vented cathedral ceiling, the insulation is in contact
with the back of the drywall and there's an air gap (the
venting space) above. But in an unvented assembly, the
insulation must be against the bottom of the sheathing.
Sometimes, if the rafter bays are unusually shallow, we have to
fill them all the way up with closed-cell foam (Figure 5). But
because this type of foam has such a high R-value, in most
cases we have to fill the cavities only partway.
Figure 5. Open-cell foam, which expands to
about 100 times its liquid volume, typically has to be trimmed
flush to framing members — an easy task, since the foam
is so soft. Because of its lower expansion rate and higher
R-value per inch, closed-cell foam doesn't usually have to be
trimmed. When it does, as in this shallow rafter bay (top), the
author's crew uses a scraper — in this case a horse curry
comb — to clean the framing in preparation for drywall
Contractors often ask about the air space below the foam; most
were taught that it's bad to leave an air space below
insulation. This is true of fiber insulations because
convection currents can form in gaps and degrade the
insulation's thermal performance. But it is not true of foam,
which can't be infiltrated and is relatively unaffected by
surrounding air currents.
Any space left below the foam is considered conditioned space
Figure 6. Fiberglass and cellulose
insulation are usually installed in contact with the back of
the drywall; the concern is that leaving a space there allows
convective air currents to degrade the insulation's thermal
performance. Because closed-cell foam is unaffected by air
movement, the space between it and the drywall is not a
Dealing With Can Lights
It's easier and more energy-efficient to build a cathedral
ceiling as an unvented assembly, but dealing with recessed
light fixtures can be a real challenge.
There are two issues: how to insulate and seal the area above
the fixture, and how to provide enough space around it so it
doesn't overheat. Even if the fixture is an IC unit, you can't
embed it in foam.
Insulating above. If we're lucky,
there will be room to spray a full thickness of foam above the
fixture and still maintain the desired 2 to 3 inches of
clearance between foam and fixture.
If there isn't enough space or access to spray above a fixture,
we sometimes install a piece of nonperforated foil-faced rigid
foam above it instead. Before spraying, we mask the fixture to
keep it clean, then create an airtight seal by lapping the SPF
onto the rigid foam (Figure 7). If the rigid foam butts to
framing, we caulk that joint with polyurethane sealant.
Code requires that a space be left between
can lights — even IC-rated cans — and spray foam
insulation. In shallow bays, the author's crew installs
foil-faced rigid foam above fixtures and creates a seal by
lapping the spray foam onto it (top). An alternate method,
which may soon be required in California, is to isolate
fixtures from the foam by installing them in metal boxes
Clearances. Few building codes contain specific
requirements about clearances between foam and can lights, so
it's a good idea to talk to the building inspector about the
issue. SPF is such a good insulator it can cause a fixture to
overheat, tripping the temperature-limit switch and cutting
power to the light. Excess heat could also damage the wire
sheathing or even the foam itself.
In California, new code provisions are being developed that
will require builders to take one of three measures with
recessed lights: leave 3 inches of clearance around a fixture,
box around it, or wrap it with 2 inches of mineral fiber. A
3-inch clearance is already required around hot appliance
SPF is compatible with PVC and CPVC, so it's okay to spray it
on Romex, PVC pipe, and CPVC sprinkler pipe.
Any surface we spray will be sealed against the movement of
air, but there are always some surfaces we can't spray.
For example, the gaps between doubled-up framing members are
too small to spray with foam, yet a significant amount of air
can leak through at these spots. It's best to seal these joints
during framing by installing compressible foam gaskets between
the members. If that isn't done, you can caulk the joints after
the foam is installed.
When the gaps are too wide for caulk, we fill them with foam
from a can. The canned foam should be the low-expansion type;
it contains more closed cells than the high-expansion material.
We stay away from the latex foams because they're very
When the unvented assembly is a cathedral ceiling, the foam
will be covered with drywall, which is a code-approved thermal
barrier. In an attic, though, the rafter bays are not normally
covered by drywall, so the issue of fire-resistance comes into
play (Figure 8).
Figure 8. When insulating an unvented roof
assembly, the author prefers closed-cell to open-cell foam
because it's both an air barrier and a vapor retarder. To
finish an unvented cathedral ceiling insulated with closed-cell
foam, most codes require a layer of 1/2-inch drywall or an
equivalent thermal barrier (top). Depending on local code, the
spray foam in an unvented, or "cathedralized," attic (bottom)
may not require drywall covering unless the area is accessible
for servicing equipment. In some cases, the foam may have to be
sprayed with an intumescent coating.
This can be a gray area in the code, so be sure to check with
your building department before building an unvented attic
space. Most codes state that if the attic is accessible for the
service of utilities, the foam must be covered with an ignition
barrier. Certain water-based intumescent coatings qualify as
If the attic area is not accessible or is not "accessed for the
service of utilities," it may be possible to leave the SPF
exposed. Many contractors are confused about how to treat this
enclosed attic space. Providing access through a ceiling hatch
is okay but not necessary; venting to the room below is
prohibited by the fire code.
Unlike fiber insulation, which can be blown through a hose or
stuffed into hard-to-reach areas, SPF can't be installed
without sufficient access. The applicator must be able to get
close enough to the sheathing to spray from 16 to 24 inches
away — and do it from pretty much straight on.
Cost. In our area, the installed cost
of an average-size closed-cell foam insulation project is
between $1.10 and $1.40 per board foot of material.
For R-30, that comes to about $5 per square foot of roof area.
That's more than other insulation materials would cost, but not
much more if you factor in all of SPF's advantages —
future energy savings, increased comfort and moisture control,
the greater design flexibility that comes with being able to
fit the necessary R-value into small framing cavities, and the
possibility that the mechanical system can be downsized.
James Morshead is senior project manager and
technical director for American Services Co. in Dublin,