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Overcoming Vapor Drive Issues in Cool Roofing

May 15, 2015

With an increase in the use of reflective
coatings in North America, we are experiencing
more and more installations of
reflective roofing surfaces in cooler climates
as well as warm climates. As a result,
observations indicate that there are concerns
with subsurface condensation where
reflective surfaces are employed. This paper
will explore the net effect of vapor drive on
reflective roofs and will offer proper system
design and product selection remedies,
including insulation and the need for an
installed vapor control layer.
The benefits of cool roofing are abundant
and extend well beyond the thermal
impact they can provide to a facility. It is
proven that through the use of reflective
coatings and membranes, the resulting
surface temperature of the roof is dramatically
decreased. Reduction of surface temperature
and thermal shock are recognized
to extend the overall useful life of a roof.
Further, the additional layer of protection
reduces the UV exposure to the waterproofing
system and can eliminate exposure to
harmful chemicals, if present.
It is also recognized that lower surface
temperature reduces cooling costs in the
summer. In unconditioned spaces, the comfort
of the inhabitants/occupants of the
building is increased. How does one put a
price on comfort?
Finally, studies have indicated that
through the use of reflective surfaces, cities
can lower the urban heat island effect. This
also reduces the net effect of additional
cooling needed to lower conditioned temperatures
because of retained heat in large
urban structures.
Simply put, coatings provide benefits in
both warm and cool climates (although in
cooler climates, the energy savings related
to cooling the building may be limited). They
extend the life of roof systems and reduce
the energy required to cool a facility (Figure
1). However, understanding the net effect
that using a reflective product may have in
different climates is critical to the long-term
success of any given facility.
Specifically, problems can arise from
cool (reflective) roofing, especially when
installed on roof systems that previously
had darker surfaces. These problems are
observed as drips and unexplained leaks
that occur when no evidence of a roof system
failure can be found, especially in the
spring. It is well understood that dew point
1 8 • I n t e r f a c e J u l y 2 0 1 5
Overcoming Vapor
Drive Issues in
Cool Roofing
Figure 1 – Reflective roof surfaces can provide many benefits, but the roof must be designed and installed properly.
By Joseph W. Mellott
and Thomas G. Diamond, PE
temperature location changes within the
roof assembly throughout the year. Vapor
drive and dew point temperature locations
are directly related to external and internal
environmental conditions. Vapor drive can
become more severe in the winter with the
cooler exterior roof assembly. As a result,
higher amounts of moisture can condense
within the roof assembly. This is a result of
air leakage from the interior of the building
into the roof system structure.
In cases where a darker roof is installed,
the dark roof will have a warmer surface
than a comparable light-colored or cool
roof. The dark roof will heat up the entire
roof assembly and cause downward drying
to occur. The cooler roof surface does not
allow the roof assembly to become as warm,
reducing the drying effect; and, as a result,
the subsurface condensation remains and
Acceptable and typical construction
types vary across the United States.
Assemblies in the Midwest and East usually
consist of some type of rigid board insulation
above a structural deck such as steel
or concrete. The South and the Gulf Coast
are similar, with the exception that a lightweight
concrete pour over a form deck is as
common as a steel or concrete deck. West
Coast construction typically consists of a
wood deck, below which fiberglass insulation
with vapor retarder facers are installed.
Each system has its own advantages and
challenges and is selected for performance,
life cycle, and economic reasons. Each type
presents different thermal conditions and
must be understood and handled correctly
to maintain long-term performance.
Several facts are omnipresent when considering
thermal and moisture conditions:
• Moisture is always present, in either
vapor or liquid form. It is unavoidable.
• Water vapor will occur on both the
exterior and the interior of a roof
• Warm air rises.
• Water vapor, in general, is driven
from warm spaces to cold spaces.
• Pressure wants to be equalized. High
pressure moves to low-pressure
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J u l y 2 0 1 5 I n t e r f a c e • 1 9
Figure 2 – Careful consideration must be given to transitions such as this. Notice the air
barrier tying into the wall.
The dew point temperature is the temperature
at which water vapor condenses
into liquid. Within a roof assembly, water
vapor condenses into liquid when it contacts
a surface that is at or below this dew point
temperature. Understanding these basic
principles and terms of physics (and the
unavoidable presence of water) is necessary
to control reflectance-related condensation.
There are several issues that can arise when
improper construction meets physics.
Insulation installed above the roof deck
often provides the best opportunity to design
a high-performance roof system. Many
times, the insulation is attached directly to
the deck. While the deck itself can act as a
reasonable air barrier, it does not always
stop the migration of air and water vapor
through the deck (from the interior of the
building). This can occur through penetrations
in the deck and where the deck meets
transitions in the building, such as walls,
elevation changes, and differing deck types
(Figure 2). Vapor can also migrate through
the vapor-retarding facer on the fiberglass
batt insulation where it is not perfectly
sealed against all penetrations and walls.
Inconsistencies in the design with respect
to the “air barrier” will allow rising air into
the roof assembly, which can transport very
large amounts of water vapor. Air movement
itself can “transport” over ten times more
water vapor than diffusion alone will.
Moisture migration (vapor drive) from
the interior to the exterior of the building
usually occurs in winter months when the
interior relative humidity is higher than the
drier exterior environment. The interior air
is typically warmer than the outside air,
compounding the issue. Vapor drive can
also be caused by the overall differential in
pressure between the interior and the exterior
of the building. Wind is an additional
factor in the mix. When wind whips across
the roof surface, it creates a pressure differential
(negative and positive, depending
on location). The resultant airflow through
the structure can result in the deposition of
moisture within the roof assembly.
The combination of these factors creates
a very strong moisture drive in the winter. If
the roof system is not designed with an airtight
air barrier, or if a vapor retarder installation
is not done properly, large amounts
of moisture can become trapped within the
roof assembly. The moisture, in vapor form,
will loop up until it meets a condensing
surface. If the surface is below dew point
temperature, such as the bottom of a roof
membrane in a cold winter environment,
the moisture in the air will condense on it.
When the air and moisture loops back down
into the roof assembly, it will be in the form
of condensed liquid.
Another factor to consider is the facility’s
use of air conditioning. There are many
cases where a reflective roof brought the
internal temperature of the building down
considerably in the summer. This created a
scenario where the air conditioning within
the building did not run nearly as often
as before. As a result, the interior relative
humidity of the building rose considerably,
causing condensation within the facility
that had not occurred previously. Once this
moisture-laden air enters the roof assembly,
it will continue to migrate upwards
until it reaches the dew point temperature
within the insulation at the membrane surface.
This can even occur on the bottom of
the roof deck in West Coast construction
assemblies. By definition, once water vapor
hits the dew point temperature, it condenses
into liquid.
In typical, nonreflective roof assemblies
in summer months, some of the moisture
within the assembly will be able to “dry out.”
The rooftop surface temperature can rise to
between 160° and 170° Fahrenheit. At that
temperature, it will “reverse the moisture
drive” back into the building. The high heat
will then drive any moisture trapped in
the roof assembly into vapor and equalize
into the building. However, when the roof
surface temperature is lower (around 110°
Fahrenheit), the drying action is significantly
reduced. Much of the trapped moisture
will start to condense.
The condensed moisture can result in
many undesirable effects. In many cases,
the “leaks” are misdiagnosed as roof leaks.
In any case, the trapped moisture can cause
degradation of the roofing components and
eventually lead to structural damage to the
deck. Further, moisture present in the insulation
will lower thermal protection, since
2 0 • I n t e r f a c e J u l y 2 0 1 5
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March 20116 Design issues December 15, 2015
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saturated insulation causes the thermal
resistance of insulation (R-value) to plummet
to near-zero levels. Finally, moisture
in trapped spaces can lead to mold growth.
Models were performed to analyze the
temperature profiles of a roof assembly
with a dark surface and a reflective surface.
The dew point temperatures were then calculated
to see where condensation might
occur within the assembly in winter design
conditions when the vapor drive is strongest
from the interior to the exterior of the building.
The temperatures of these “condensing
zones” within the roof assembly were then
monitored in summer conditions to determine
how warm this portion of the roof
assembly became.
One model analyzed a facility in northern
Minnesota with conditioned interior air
space that included the following assembly:
• Metal deck with the interior surface
exposed to the interior of the facility
• R-20-equivalent polyisocyanurate
insulation board
• ½-inch, high-density wood fiberboard
• 2 plies of felts adhered in hot asphalt
• Dark mineral-surfaced modifiedbituminous
cap sheet adhered in hot
A second model was then performed
using the same exact assembly, only with a
white reflective roof surface. Typical interior
conditions were chosen to be 70° Fahrenheit
and 20% relative humidity for both models.
Results of the study showed that the
dew point temperature of 28° Fahrenheit
occurred in the middle of the roof assembly
for the reflective roof surface in the winter,
and near the top of the darker-surface roof
assembly. The moisture present in the
air will likely not condense exactly at this
location, but at the first condensing surface
that is colder than the dew point temperature.
This likely occurs at the roof’s cover
board, or just below the membrane. The
moisture-laden air may not actually contact
or get to this location in the majority of
the roof. However, it can get to this point
through any areas of the roof that contain
mechanical units, pipes, duct work, or any
other areas that allow for disruptions in the
roof assembly. Moisture can travel through
the roof assembly at these penetrations
and may condense as soon as it comes into
contact with a condensing surface below the
dew point temperature.
When these same roof assemblies
were analyzed in the summer months,
it was determined that the “condensing
zone” for the reflective membrane reached
an average temperature of approximately
80° Fahrenheit. However, the temperature
of the “condensing zone” in the darkersurfaced
roof assembly reached an average
temperature of 150° Fahrenheit. This difference
of 70° provides for a much larger
drying potential for the darker roof than the
reflective roof, if moisture did accumulate
within the assembly.
The results of the thermal models are
shown in Figures 3A-D. Notice the difference
in dew point temperature locations and the
temperatures of these locations in summer
months. It should be noted that these
models are only “snapshots” of the roof
assembly. To gain a better understanding of
the conditions that a specific roof assembly
will experience throughout the entire year,
including heat gain and moisture accu-
J u l y 2 0 1 5 I n t e r f a c e • 2 1
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Why are we still debating
the merits of cool roofs?
Thermoplastic white roofs have proven
performance in all climates. Bust the myths:
2 2 • I n t e r f a c e J u l y 2 0 1 5
Figures 3A-D – The results of solar effects on thermal models in Minneapolis, Minnesota, at different times of the year and with both light
and dark roof surfaces.
mulation within it, a more in-depth model
should be performed. Doing so will allow
one to determine not only if moisture accumulates
within the roof assembly, but also if
it will have the ability to dry back out.
How can we overcome these potential
issues so we can still enjoy the benefits of
cool roofing? Of course the solution for new
construction is much easier than in reroofing.
Existing conditions may be much more
expensive to fix due to reconstruction and
redesign costs, which is why doing it right
the first time is critical.
With new construction, the following
steps must be considered.
Include Air Barriers
Always ensure that an effective air barrier
is installed above the deck. Redundancies
in air barriers are very advantageous. Highperformance
building enclosures cannot
afford even moderate amounts of air leakage.
While staggered insulation boards
can act as an air barrier, a membrane
installed below the insulation performs better.
Continuity is key! Air barriers must be
sealed around penetrations and must tie
into the wall’s air barriers. Even minor gaps
in the air barrier can lead to large quantities
of moisture migration, as well as heat transfer
into and out of the roof assembly.
Include Vapor Retarders/Barriers
Vapor barriers are not to be confused
with air barriers. Air barriers can be:
• Vapor-open
• Vapor retarders
• Vapor barriers
The type of vapor profile needed is highly
dependent on the functionality of the
building, as well as the climate. Each building
should be handled uniquely. Generally,
warmer climates can afford (and typically
prefer) a vapor retarder or a vapor-open
profile. An open vapor profile will allow any
moisture that finds its way into the roof
assembly to eventually dry out through
diffusion. If a moderate amount of moisture
finds its way into the assembly via diffusion
through the air barrier, it will, in theory,
be able to diffuse back out in the summer
months. This small amount of moisture is
usually not enough to create condensation
issues. (See Figure 4.)
In more severe climates and in buildings
where even moderate amounts of moisture
are prohibited (such as museums and data
centers), and in buildings where the interior
relative humidity levels are extremely high
(such as swimming pools or paper mills), a
vapor-closed profile is needed.
For metal decks, fasten a gypsum board
J u l y 2 0 1 5 I n t e r f a c e • 2 3
Figure 4 – Common commercial or industrial
roof system in a warm climate with low
relative humidity.
Conforms to
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Air barriers
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Optional test chambers: 12” wall to base, 48” roof seam, and inside and outside corner!
to the deck and then install an air/vapor
control membrane.
Concrete decks require an air/vapor
control membrane to be installed directly
over the deck.
For wood and other nailable decks,
mechanically fasten a base sheet to the
deck, and then install an air/vapor control
membrane directly over the base sheet.
Next, install the rigid insulation board
above the air/vapor control membrane.
The required amount of R-value is dictated
by the International Energy Conservation
Code, ASHRAE, or local code requirements.
Hygrothermal Models
Have a design professional perform a
hygrothermal model of the building to see
how much moisture will be present within
the roof assembly and how quickly it will be
able to dry out. Material type, thickness, and
location can all be varied within the model to
determine the optimum roof assembly.
Ventilation is critical. Ensure that the
proper amount of ventilation is provided
for the building. Moving moisture-laden
air out of the facility or treating it with air
conditioning will help reduce the risk of its
finding its way into the roof assembly.
The most critical component is proper
application. No matter how well designed
the roof system may be, it is dependent on
proper installation. Continuity is critical
with respect to the integration of the air/
vapor control membrane, as well as with the
insulation, roof membrane, and coating. If
an air barrier is not sealed perfectly against
any and all penetrations, it presents a point
where air can seep into the roof assembly.
Ensure that highly skilled contractors are
installing the roof and its components. Also
consider having a Registered Roof Observer
present during the installation to ensure
that the roof system is being installed as is
Reroofing existing buildings can be more
challenging. Adding ventilation to a building
is typically not very cost-effective. Also,
if insulation is already present below the
roof deck, installing an air barrier/vapor
retarder below it can be nearly impossible
to do properly, depending on the ceiling
finish. In these scenarios, adding additional
insulation above the roof deck is almost
always beneficial. Even a moderate amount
of R-value added above the roof deck will
bring the deck’s surface temperature above
the dew point temperature, moving the dew
point above the first condensing surface.
In any case, it is recommended to perform
a hygrothermal model on the building
to see how the roof assembly will perform
with different materials and insulation
Simply put, reflective roofing can be very
beneficial. Reducing cooling-based energy
demand and protecting the waterproofing
substrate are valuable performance characteristics
of coatings and membranes.
Understanding how thermal changes to rooftop
conditions can affect moisture drive is
critical in building roofs to perform to and
beyond their life cycle expectation. When in
doubt, perform the necessary calculations
with the assistance of trained professionals
to protect assets and to eliminate failure.
2 4 • I n t e r f a c e J u l y 2 0 1 5
Joe Mellott is VP
of Innovative
Metal Company,
Inc. (IMETCO).
He holds multiple
patents for roofrelated
and received
the 2006 Industry
S t a t e s m a n
Award from the
Roof Coatings
Association (RCMA) for his work in advancing
roof coatings technology. He has also
served on the board of the Cool Roofs Rating
Council (CRRC) and is a past president and
technical chair of RCMA. Mellott has been a
member of RCI, NRCA, and ARMA. A graduate
of Case Western Reserve University, Joe
holds a BS in polymer engineering.
Joe Mellott
Tom Diamond,
PE, is a product
engineer for The
Garland Company,
Inc., a manufacturer
and distributor
of building enclosure
solutions for
commercial and
industrial applications.
Tom’s primary
focus involves
designing optimum
performance roof and wall systems while
assuring compliance with building code.
He frequently delivers seminars and AIAapproved
classes on installation techniques,
building enclosure design, and roofing and
wall system technology.
Tom Diamond, PE
Pinnacle Roofing Contractors, Inc., Jacksonville, Florida, has been fined $154,000 stemming from a 24-foot fall that
killed John W. Miles III. Miles—working without fall protection—fell through an unprotected skylight in a warehouse
roof in November 2014, according to the citation by the Occupational Safety and Health Administration (OSHA).
Pinnacle Roofing, founded in 1994, had been cited for a serious safety hazard relating to fall protection in July 2012.
In the current case, Pinnacle allegedly allowed employees to work at heights greater than 6 ft. without guardrails or
fall protection and had not installed protective systems on the skylights. Citations were also issued for failing to ensure
the edge of the roof was marked.
Roofer Fined for Skylight Death