Unvented Attics with Cathedralized Half-Pound Foam Insulation
The problem with conventional attic insulation is that attic vents
used in conjunction with recessed lighting fixtures and other
penetrations through the sheetrock ceiling create an air leakage
path from the inside of the house to the outside, and visa versa.
At Advanced Insulation we believe a state-of-the-art home built in an
extremely hot climate should have an unvented attic with half-pound foam
insulation installed on the underside of the roof sheathing. This
technique is known as an unvented attic with cathedralized insulation.
In this section, we will discuss half-pound foam insulation and unvented
attics with cathedralized insulation in depth.
We believe that the underside of
roof sheathing is where half-pound foam provides the best value. As a
company, we also believe that foam is of questionable value when installed in
conventional 2x4 or 2x6 exterior walls for a variety of reasons, which
we will explore later in this article.
problem with conventional attic insulation is that attic vents used in
conjunction with recessed lighting fixtures and other penetrations
through the sheetrock ceiling create an air leakage path from the inside
of the house to the outside, and visa versa. These air leakage paths
mean that dust, noise, hot attic air, insects … have a ready access path
into the indoor environment. The other major shortcoming of insulating
at the bottom chord of the truss is that the air distribution system,
i.e. ductwork, is left in an extremely hot attic. We believe that in
extreme climates insulating at the bottom chord of the truss with a
vented attic is an outdated way of designing and building homes.
By insulating the underside of the roof sheathing, the air
distribution system is brought inside the thermal pressure envelope,
which means that this system is no longer exposed to extreme
temperatures. Many people in the building industry make the mistake of
over emphasizing attic R-value, while ignoring the fact that ductwork
typically has an R-value of only 4. In the metropolitan Phoenix area,
builders run 58ºF air in duct work with an R-value of 4 in an
environment of summer time temperatures as high 135ºF. This is poor
By eliminating the need for attic vents and insulating on the
underside of the roof sheathing we bring the ductwork into a conditioned
space and almost eliminate heat gain through the air distribution
system. A further advantage is that duct leakage becomes less critical
since these leaks are now within the conditioned space of the building.
The unvented attic with cathedralized insulation is not a new
technique. Pulte Homes of Phoenix and Las Vegas began incorporating this
strategy in many of their subdivisions in the late 90’s. Pulte used
wire-up batts and netted cellulose. They eventually gave up on the wired
up batts due to performance issues. This strategy, coupled with the
commitment to get the other basics of building science correct, provided
Pulte Homes the confidence to guarantee the energy performance in their
Advanced Insulation insulates many of metropolitan Phoenix’s finest
homes. The builders of these homes know an unvented attic with
cathedralized half-pound foam insulation is a superior building
strategy. They have come to this conclusion through their first hand
experience from both wiring-up batts and spraying half-pound foam
as-well as feedback from satisfied customers.
In further discussion, we will attempt to answer key questions about
half-pound foam insulation. We do not assume the view that half-pound
foam is a panacea. We also take pride in our fiberglass and wall spray
cellulose installations. We are more committed to total home performance
than any single product and we have done our best to provide you with
the most objective and accurate information.
Sprayed Foam Insulation
Spray foam insulation is usually defined by its density. There are
three main types of spray foam that are installed in homes. Three-pound
foam is installed on roofs. Occasionally, an inch of two-pound foam is
installed in exterior walls, which then gets covered with other
insulation products. Half-pound foam is the insulation you'll typically
find installed at the underside of roof sheathing and in exterior walls.
The denser the foam the more expensive it is. Think of it this way,
one cubic foot of three pound foam has six times more raw material than
one cubic foot of half-pound foam. This is one of the principal
advantages of half-pound foam; it simply costs less than denser
sprayed-in foams. Foam is expensive to begin with, so using lighter
density products makes sense.
One of the advantages of the denser foam is that the R-value per inch
is greater. Instead of R-values ranging from R-3.6 to R-4.0 per inch for
half-pound foam, the denser foams trap a denser gas than air and thus
have aged R-values of approximately 6. We install both half-pound and
two-pound foam, and in most cases we feel half-pound foam is a much
better value because of the economics.
Another consideration is that half-pound foam has characteristics
that permit the foam and roof assembly to dry in the event of a roof
leak. A denser foam will trap moisture between the foam and the roof
sheathing, which can create building durability problems. We believe
that the greater the drying potential of a building assembly, the
greater the building durability and building durability is a key
component of our criterion for a high performance home.
There are a variety of manufactures of half-pound foam in the market.
The most widely known is Icynene. Icynene is good foam, just like
Demilec Foam, Biobased Foam, and North Carolina Foam. These foams are
known as semi-open celled and semi-rigid when installed. They are
water-blown foams, contain no formaldehyde, and emit no harmful VOC’s or
CFCs. As mentioned, half-pound foams have R-values ranging from R-3.6 to
R-4.0 per inch.
At Advanced Insulation we only use half-pound foams that are
qualified by having an International Code Council (ICC) number or an ESR
or NER report. We will not install foam products that do not have proper
Installation of Foam
We strongly believe that our foam crew’s installations are the best
in the Arizona. Simply ask any of our foam customers. For those who
would like references, we will gladly provide a current list of builders
and superintendents we've recently worked for. It is our company’s
philosophy and commitment to getting the job done right that sets our
installations apart from most other companies. We are proud of our
culture and the quality of the insulation work we perform on a daily
At Advanced Insulation we also believe that the installation of the
foam is more important than the brand of foam, assuming the product
meets the proper qualifications. Any product that is installed on a job
site can be undermined by sloppy installation. This is true of framing
materials, roof systems, sheetrock…foam is no different.
Foam insulation is not a panacea, as some foam manufactures and foam
insulation contractors would like you to believe. We have inspected foam
projects, where the foam was installed poorly. We found areas where the
depth was too shallow and worse, areas where the pressure envelope was
not sealed with foam to the outside thus undermining one of the big
benefits of the foam.
Our philosophy is that workmanship and company culture are essential
to achieving a quality job. Michael Uniacke, owner of Advanced
Insulation, was featured as a consumer advocate and expert on a NBC
Dateline expose’ on cheating in the insulation industry. Insulation
doesn’t receive the same scrutiny as other building systems since the
consequences of poor insulation are usually not safety concerns, but
rather compromised comfort and high utility bills.
When we apply spray foam on an attic lid, we don’t tell our crews how
much foam to spray. They simply know to spray enough foam until the job
is done correctly. There is no incentive to skimp on material. We also
go to great lengths to protect the building – covering windows,
decorative beams, heating & cooling units, recessed cans … with plastic
keeping them free of foam over spray. Pre-spraying preparation, or
“prepping” as we commonly call this, is as important as the spraying
How much R-value is Enough?
In unvented attics with cathedralized insulation, we recommend 6" of
foam installed to the underside of roof sheathing having an R-value of
24. This is based on the excellent research done by Department of Energy
(DOE) “Build America” program. Pulte Homes has insulated entire
subdivisions having unvented attics with cathedralized insulation with
an R-value of 21. As a result of the incorporation of this technique,
Pulte was able to guarantee the energy costs in these homes.
To appreciate why 6” of foam is more than sufficient, consider that an unvented attic with cathedralized insulation places the
air distribution system inside the thermal pressure envelope. These duct
systems typically have an R-value of 4. By incorporating an unvented
attic with cathedralized insulation the air distribution systems no
longer sees extreme attic temperatures in the hot season and the heat
gain through the duct work is eliminated.
Another major shortcoming in using R-value as a gage of insulation
performance is that it does not factor in the foam’s ability to also act
as an air barrier. Spray foam out performs fiberglass and cellulose
since it creates its own air barrier. There is no air movement around
and through the insulation. So, the foam only gets credit for its
conductive properties in the R-value rating and not it’s air barrier
properties, which is why the R-value is not a complete indicator of
There are foam contractors in the marketplace using terms such as
“effective R-value,” in order to get credit for the air sealing
properties of the foam. We understand these contractors' frustration
with traditional R-value measures, but their efforts often mislead
consumers. Sometimes they intentionally mislead consumers, which is bad
for the industry. For this reason, Advanced Insulation does not use
terms like “effective R-value.”
The very same contractors who advertise “effective R-value” will also
install less than 6” of foam on the underside of roof sheathing. Without
proof, (from third party independent laboratories), supporting
applications of less than 6”, we do not recommend R-values less than a
The anecdotal evidence we have gotten from our past half-pound foam
customers has been truly remarkable. We have customers who now will only
use foam in their attics. This evidence coupled with the infrared
scanning we have done, supports our belief in the half-pound foam.
Foam versus Fiberglass Batts and Dense Pack Cellulose
It is possible to wire-up batts to the underside of the roof
sheathing or to install netting between the top chords of the trusses
and then blow in cellulose insulation. Through first-hand experience
from spraying half-pound foams, wiring up fiberglass batts, and doing
dense pack insulation, we have concluded spraying foam is, by far, a
superior method. We routinely see houses that have
attics in which the framing is so complex that the attic can not be
properly sealed without the benefit of foam.
When specified our company does an excellent job of wiring-up batts
to the underside of roof sheathing. Having said this, careful
examination of our work, as well as that of any of our competitors, shows a
multitude of gaps and voids that occur because of the webs of trusses,
cuts due to hip roofs, decorative framing details, and more. If the top
chord of the truss is a 2x6 framing member then the fiberglass batts
often will not touch, creating gaps that undermine the R-value.
During the summer of 2005 we performed infrared scanning analysis in
several attics with both wired up batts and foam. We also took numerous
temperature readings in order to observe where the insulation was
failing and to determine where the thermal defects occurred.
Scanning attics with wired up batts that are failing within
three years of installation.
Heat created by these gaps seen through the infrared camera.
Thermal defects and wired up fiberglass batts
What was most disturbing was that we observed wires starting to fail
in attics with wired-up batts. We have surmised that heat transmitted
through the truss web is transferred into the wire, which begins a
series of expansions and contractions, thus weakening the wire.
We also know that some custom homes with complicated ceilings
have many areas that are impossible to seal and insulate
correctly unless foam is used. We have customers who will have wired-up batts
in the field and foam the more complex areas in order to maintain the
integrity of the thermal / pressure envelope.
If you combine the air sealing qualities of foam and the reduction of
thermal defects due to the inherent quality of the sprayed-in process,
you have a superior system. We genuinely believe that 6” of half-pound
foam will outperform wired-up R-38 fiberglass batts.
Half-Pound Foam in 2x4 or 2x6 Exterior Walls
At advanced Insulation we are energy pragmatists and not energy
purists. We realize no one product makes the home and that budgets often
drive the decision making process. This is why we advocate the use of
foam on the underside of roof sheathing and not necessarily on 2x4 or
2x6 exterior walls.
We have learned by blower door testing of homes that it is the roof
and not the exterior walls where we need to concentrate on air sealing.
Between the attic vents and penetrations in the ceiling, i.e. recessed
cans, sprinkler heads, duct leaks - the ceiling is where most air leaks
We also have conducted thermal analysis (Manual J Heat Gain
Calculations) of homes in the desert and know that an exterior wall
with an inch of rigid foam on the outside is not a major contributor to
heat gain. This is especially true if the walls have porches – they are
shaded and thus not in direct sunlight. The majority of stucco homes
have one inch of rigid insulation on the exterior, which makes a huge
difference in reducing thermal bridging and sealing the pressure
One could argue that one-inch of foam on a stucco house is a better
value than sprayed half- pound foam between the framing cavities.
Approximately 20-25% of a wall is wood framing with wood having an
R-value of only 1 per inch. When sprayed between studs, foam doesn’t
deal with this thermal bridging, whereas the foam on the exterior of the
Cutting the foam flush with the face of a 2x4 or 2x6 generates a
phenomenal amount of waste. This is due to the rapid expansion rate of
the foam. It is virtually impossible to fill a cavity flush and not
We believe that the energy savings do not warrant the investment of
foam in conventional exterior walls. For walls greater than 2x6, the
spraying of foam does not generate nearly as much waste therefore the
labor is dramatically reduced, thus making the foam much more
economically attractive. For those who can benefit from foam in exterior
walls but can't afford the cost, a great compromise is our hybrid systems.
Hybrid Wall and Roof Systems
For our customers who have major budgetary constraints, Advanced
Insulation offers a “hybrid solution” to controlling the insulation
budget. Where the project budget does not permit the use of foam
exclusively, we install foam where we get the most benefit, and wired-up
batts or wall spray cellulose where it will perform the best. We know
that the construction process is fraught with compromise, and we believe
that with our “Advanced Hybrid Systems” we get optimum performance at
the lowest possible price.
We can insulate exterior walls with foam / fiberglass batt or foam /
walls spray cellulose insulation combination. This means we insulate
approximately 3% to 6% of the walls with foam and the balance with the
other materials. We spray the foam in areas that would be hard to
insulate with conventional insulation. These are typically areas where
efficient framing techniques were not utilized.
Truss webs create thermal short circuits, whereas the foam
completely envelops the framing members.
Hybrid - Foam/Fiberglass
This combination with an inch of foam on the exterior provides
the best value. A square foot of R-19 unfaced batts costs five
times less than a square foot of foam installed. The foam on the
exterior of the building eliminates thermal bridging.
Our wall spray cellulose / foam or fiberglass / foam hybrid is a much
better value in exterior walls than an all foam installation. A good
example of a hybrid system is where we insulate the roof with foam and
spot-foam portions of the exterior walls that are difficult, if not
impossible, and complete the job with conventional insulation. This way
our customers get all the benefit of foam and use fiberglass or
cellulose wall spray where it makes the most sense.
A component of any house insulated with foam should be a ventilation
system. We would rather see the money our customers save by not using
foam in the exterior walls spent on a
Venmar Hepa 3000. This
ventilation and filtration system introduces fresh air into the tightly
sealed home in an energy efficient manner, thus addressing indoor air
Garage Separation Wall
A half-pound insulation quote from Advanced Insulation will include a
wall separating the house attic from the garage attic. This is also true
if we insulate the garage attic with foam. This wall is created to make
sure that, while engines are running in the garage, none of the exhaust
gasses get drawn into the house. Although the risk of this is not that
significant, we recognize that when homeowners run kitchen fans, dryers,
and bathroom fans they put the house under a slight negative pressure.
Make-up air for these appliances could draw air from the garage and this
is why we establish an air barrier between the house and the garage.
An unvented attic with cathedralized foam insulation is a relatively
new building technique, so we advise our customers that it may be wise
to be proactive and work with their building inspectors in advance to
gain approval for this technique. When assisting our customers in
providing inspectors with the information they need, Advanced Insulation
has experienced few objections from municipal building departments while
getting approval for unvented attics and foam insulation.
The acceptance of unvented attics is gaining momentum. The work done
by the DOE program “Build America” and Pulte Homes has helped to pave
the way towards acceptance. The International Energy Conservation Code
Council accepts unvented attics with cathedralized insulation. See
Item 11.F Section R806.4.
One concern with foam insulation is its burn characteristics. Like
wood, foam will burn when exposed to a constant ignition source. All
half-pound foams should have a Class One Certifications since they are
treated with a fire retardant package. The Demilec Foam we spray
frequently, Sealection™ 500, has a flame spread index of 21 and smoke
development of 216, which satisfies the 2003 International
Residential Code (section R316). These are the key numbers building
officials want to see.
To place the safety of foam insulation in an attic in context, there
are many products in a home that contain foam, including carpet pads,
sofa's, and mattresses, just to name a few. The vast majority of fires
start inside the house and not in attics. The foam we are installing is
also separated from livable areas, people and ignition sources by sheet
The codes intention is designed to keep people and ignition sources
away from the foam. To keep people away from the foam, the attic space
must not have a walk in door and storage areas. In an attic there must
not be any open source flame appliances such as an eighty-percent
efficient furnace. We tell our customers, who are using gas heating, to
only use sealed combustion 90% efficient or better furnaces.
Unvented Attics in Cold Climates
In cold climates unvented attics with cathedralized insulation
require special attention to detail with the insulation due to the risk
of moisture condensation. Insulation defects can create cold spots, and
cold spots can lead to moisture condensation if the relative humidity in
the building is not properly controlled. Warm moisture laden air will
rise into an attic space and if there are any surface temperatures in
the attic that are too low due to insulation defects, condensation will
Building codes now require us to insulate at the underside of roof
sheathing in commercial buildings over five thousand square feet because
of the requirement for a sprinkler system in the attic. To prevent the
fire sprinkler systems from freezing, it must be insulated at the
underside of the roof sheathing. Based on our recent research, we
believe it is a mistake to insulate these attics in cold climates with
As we stated earlier, it is almost impossible to eliminate all of the
defects in thermal pressure envelope in an attic with wired-up
fiberglass batts. Even though we are proud of our work at Advanced
Insulation, in cold climates, we do not believe this method
satisfactorily reduces the potential for condensation to a low enough
A recent article titled
Roofs, stipulated “if any insulation other than foam is used, it
will be required that one inch of rigid insulation be installed on top
of the roof in order to reduce the risk of condensation in the attic
space. In extremely cold climates a vapor barrier paint will have to be
sprayed over the half-pound foam.”
When insulation is installed at the underside of roof sheathing, the
roofing material and roof sheathing will experience slightly higher
operating temperatures. In extremely hot climates this impacts the
choice of products that can be installed on the roof. Roofs that have
tiles, cement shingles, or three-pound roofing foam on the roof are not
a concern. Asphalt shingles and metal roofs have special requirements.
Tile, cement shingles and three-pound foam are all acceptable, since
the tile is back vented so there is a reduced conduction path into the
roof sheathing and a flat roof with three-pound roofing foam prevents
conduction. These types of roofing systems protect the roof sheathing of
There are also different types of metal roofs. In extremely hot
climates like the Phoenix metropolitan market, we only want to install
half-pound foam on backvented metal roofs, since the airspace breaks the
conduction path between the metal and roof sheathing. In other words we
do not want to see metal roofs in which the metal sits directly on top
of the roof sheathing.
We only recommend asphalt shingles that are guaranteed by the shingle
manufacture for unvented roofs with cathedralized insulation. Elk
Corporation has provided a guarantee for their premium roofing shingles
as long as they are installed correctly.
Foam Costs More
When you make the choice to go to an unvented attic with
cathedralized insulation you will pay more for insulation than you would
have with a more conventional insulation package. The same holds true
when you upgrade your window package from aluminum frame to wood frame.
You pay considerably more for the superior product.
Foam insulation costs more for a variety of reasons. Installation
labor and the material costs for traditional blown-in attic are
considerably less than wired-up batts or foam. Foam insulation is a
highly refined petro-chemical based product while cellulose is recycled
newsprint treated with a fire retardant.
The minute you go from insulating at the ceiling, where the sheet
rock is installed, to installing insulation at the underside of the roof
sheathing, the area you need to insulate increases and so does the cost
to insulate regardless of the type of insulation you select. The area
increases due to the slope of the roof and separation walls that must be
sprayed when trusses extend out over porches.
So the question that needs to be asked, is do the benefits of the
foam outweigh the additional expense? We believe so, when you consider
the lower utility bills, increased comfort reduced amount of dust being
driven through attic vents and into the house, and a healthier home.
Advanced Insulation is committed to installing half-pound foam
insulation on the underside of roof sheathing (cathedralized insulation)
because it represents the future of insulation in attics in the
Southwest. Our commitment to foam is based on our experience installing
both fiberglass and cellulose to the underside of roof sheathing and
realizing their shortcomings. Our perspective has been enhanced because
we have the benefit of performing infrared scanning and blower door
testing on homes insulated with foam versus conventional insulation.
Based on these experiences coupled with anecdotal feedback from
customers, we know that foam does a superior job.
The benefits of eliminating attic vents, bringing the ductwork into
the thermal pressure envelope are too significant to ignore. It makes no
sense to run conditioned air through the hottest part of the house in
ductwork that only has an R-value of 4.
When you opt to incorporate an unvented attic with cathedralized
insulation into your building design, we hope you will consider using
Advanced Insulation. Foam insulation will only be as good as the
products used and the installation itself. Advanced Insulation has an
outstanding reputation for doing the highest quality insulation work in
the State. Our prices are competitive and our commitment to quality is
www.buildingscience.com for articles on unvented roofs and
Spray Foam Insulation Uses
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