|
COVER
BOARDS
Across Canada and the United States, the
incorporation of cover boards in low slope roofing
applications is common practice. Cover boards, or overlay
boards, are thin, normally homogeneous materials, formed
into boards and used over roof insulation to provide
protection to the insulation during installation and service
and to enhance the performance of the roofing assembly.
Traditional materials that have been used as cover boards
are wood fibre, perlite, mineral and glass fibre. Today,
however, we see a wide array of other board materials being
promoted for use as cover boards. These include cementitious
board, asphaltic core board, and glass-faced gypsum board.
Even some of the more traditional materials now come with
specially factory applied facers intended for use in
torch-on membranes or to enhance adhesion under other
membrane systems.
The increased use of cover boards has,
to a large extent, corresponded with the increased use of
cellular plastic foam roof insulations in low slope roofing.
Cover boards, when applied properly, can mitigate some of
the problems associated with the application of built-up,
modified bitumen and other hot process roofing such as
blistering and ridging of the membrane system, and crushing,
melting or delamination of the facers of the underlying
insulation. However, there is some evidence that even in
non-traditional roofing, such as loosely laid and ballasted,
mechanically attached, and cold adhesive applied systems,
the use of cover boards may help avoid potential performance
problems.
Benefits of
Cover boards
a)
During installation, and in service, the roof may be
subject to a variety of loads from traffic, snow, rain, hail
and wind. Depending on the properties of the insulation
below the membrane, damage may occur from these loads as a
result of the deformation or even crushing of the
insulation. This may lead to the formation of blisters and
ridges, voids and vapour traps below the membrane, membrane
and facer delamination and a reduction of the insulation’s
thermal resistance. A cover board acts to protect the
insulation from mechanical damage by distributing the loads
over a wider area thereby reducing the potential for such
damage.
b)
Insulations such as polystyrene are susceptible to
damage from the high temperatures associated with hot
asphalt or torch applications. To protect such heat
sensitive materials, a cover board is usually applied over
the insulation prior to the application of the membrane.
c)
Cover boards can also reduce the risk of performance
problems associated with condensation within the roof
assembly. By offsetting the joints of the cover board over
the joints of the underlying insulation, the risk of
condensation occurring at joints can be virtually
eliminated. When adhered over insulation that is
mechanically fastened to the deck, a suitable cover board
can also prevent the potential of condensation at the
fasteners due to thermal bridging.
d)
Some insulations are adversely affected by the
solvents in the adhesives used to adhere various membranes.
A suitable cover board can protect the insulation below from
damage due to solvent attack or chemical incompatibility.
e)
Cover boards can act as a moisture buffer between the
insulation and the membrane. It has been shown that one of
the causes of blistering of built-up membranes applied over
cellular foam insulations is a result of the vapour
impermeability of these insulating materials. Surface
moisture on the insulation can be trapped between the
impermeable membrane and the relatively impermeable
insulation. A porous, permeable cover board will allow this
moisture, turned into steam by the heat of the asphalt, to
dissipate through the cover board material, thereby reducing
the vapour pressures necessary for blister formation.
f
)
Depending on the nature of the cover board, it may
also provide a temporary moisture storage mechanism.
Moisture that accumulates from the outward flow of water
vapour in heated buildings due to the absence of, or flaws
in the vapour retarder may be effectively stored in the
cover board during the heating season and released during
the summer when the vapour flow is reversed. To be
effective, such materials must suffer no harmful
consequences from moisture stored and the annual
accumulation must not be greater than the desorption.
g) A suitable cover board may
facilitate the recovering of an existing roof when the
membrane has reached the end of its useful service life.
However, this is dependent upon the mechanical properties of
the cover board material and the existing roofing system’s
configuration and design.
Although the benefits of cover
boards are numerous, these benefits assume that there is an
ideal material that can be used in all applications.
Unfortunately, there is no such product and each cover board
material must be looked at in the context of its own unique
physical properties and how and where the board is to be
used.
Although there are standards in
place for the materials being promoted for use as cover
boards there are, at present, no performance standards for
cover boards themselves.
The following discusses some of
the products available for use as cover boards, their
applicable standards and their physical properties and
features.
Fibreboard
In
Canada, cellulosic fibreboard is the most widely used cover
board material used in roofing. In North America most of the
material is manufactured from wood fibre or bagassi (sugar
cane). Practically any cellulosic material can be used to
manufacture fibreboard. The porous nature of fibreboard, its
compressive and flexural strength and its affinity for
moisture have made it a satisfactory material for use as a
cover board under many membrane systems. However, the last
feature may also be the source of problems under some
conditions. It is susceptible to absorption of moisture in
the field during storage and installation. If exposed to
moisture for extended periods of time, these materials can
lose their mechanical strength or decay and rot. Also,
excessively porous materials can absorb the asphalt or
adhesives used to adhere membranes, compromising the
securement of the membrane.
Fibreboard
is available in a wide variety of thicknesses and board
size. It is available with butt edges, and in materials with
a thickness over 12.7 mm, with ship lapped (offset) edges.
To enhance the moisture resistant properties of the board,
the fibers are treated with emulsions of asphalt, clay or
paraffin. In addition, some manufacturers produce fibreboard
which is treated with fire retardants to enhance their fire
resistive properties. Fibreboard also comes in a range of
densities which may be indicative of their compressive
strength. To reduce absorption of asphalt during
application, the boards may be coated with an asphalt
emulsion. However, these coatings and emulsions may lead to
compatibility problems with certain types of membranes.
Perlite
Roof Board
Perlite
roof insulation has been widely used in the US since the
1950’s, and is finding increased usage in Canada.
Perlite boards are composed of expanded perlite ore,
cellulose fibres, asphalt and starch binders.
Traditionally, the boards have been coated on the top side
surface with an emulsion coating to inhibit excessive
absorption of the mopping asphalt. In 1982, 12.7 mm boards
were introduced for use as cover boards. Perlitic roof
board provides a dimensionally stable and thermally
resistant surface for the application of hot applied
roofing membranes. It exhibits excellent flame spread
characteristics, which may be important in the design and
installation of some roof assemblies. It has also been
demonstrated that perlitic insulation can rapidly gain and
lose moisture from ambient air, similar to fibreboard.
A
distinction must be made between perlite board used as
roof insulation and that used as cover board. The thinner
12.7mm board, commonly used in roofing and referred to as
recover board, contains more organic or cellulosic
material to provide more strength to the thinner boards.
As a result, this type of insulation may, due to the
higher cellulose content, retain more process moisture
than regular perlite insulation and may be more
susceptible to absorption of moisture in the field during
installation in hot and humid weather. Insulation with
higher moisture contents at the time of installation may
provide a greater risk of releasing sufficient moisture to
induce blistering in hot applied installations, especially
if the board is to be encapsulated in hot asphalt during
the application process. There have been reports of
“bubbling of asphalt” during installation and
blistering of the membrane after installation when thin
boards have been used. For boards set in hot asphalt, the
temperature of the asphalt, the ambient humidity, as well
as the moisture content of the board itself have been
shown to be factors contributing to frothing of asphalt.
The hotter the asphalt, the greater the potential to drive
the moisture out of the boards. In addition, the greater
length of time allowed for the moisture to be released
(prior to covering with the membrane) the less the
potential for trapping the moisture. To avoid the
development of blisters, the industry recommends that the
hot membrane not be applied for a minimum of 10 minutes
after the perlite has been applied.
Perlitic
cover board, like cellulosic fibreboard, is moisture
sensitive. All materials that are susceptible to retaining
moisture or that may be damaged by moisture should be
stored in a dry location and protected with properly
secured water resistant coverings.
There
is no current Canadian standard for perlitic roof
insulation. The appropriate standard for this material is
ASTM C 728. it should be noted that the standard
distinguishes between Type 1 perlite, used to produce both
thicker and composite board material, and Type 2 perlite
insulation used primarily as a cover board material.
Glass/Mineral
Fibre
Glass
and mineral fibre insulations are produced from fiberized
glass or basalt rock bonded with resinous binders and
compressed to various densities. To prevent absorption of
adhesives or asphalt into the porous fibrous core, a facer
is applied at the factory. Some manufacturers provide
boards with special facers designed for the direct
application of torched on modified bitumen membranes.
These boards are generally compressible making then
unsuitable for some applications, such as mechanically
fastened single ply systems.
Although
generally unaffected by moisture, they will lose their
structural integrity if they remain wet for long periods
of time. In addition, their facers may be adversely
affected from exposure to moisture. Due to the inherent
absorptive nature of their bottom surfaces, they cannot be
back mopped with hot asphalt for securement over heat
sensitive insulations. These products are dimensionally
stable and have a high degree of heat resistance. Due to
their nonhygroscopic nature, they do not absorb moisture
readily. Their fibrous structure allows the dispersion of
moisture vapour with minimum potential for the formation
of vapour traps, thereby minimizing blister potential in
hot applied systems.
Asphaltic
Core Boards
These
cover boards are relatively new in roofing applications.
They are typically fabricated with an
asphaltic
core sandwiched between glass fibre reinforcement.
Designed for use primarily with hot applied asphalt
membranes and torch-on systems, they are highly resistant
to moisture, and obviously compatible with all asphaltic
systems. However, their asphalt content renders them
unsuitable for use under single ply membranes that may be
adversely affected by contact with asphalt. Thinner boards
(3-6 mm) due to their low thermal resistance may provide
insufficient protection for use over heat sensitive
insulations. The impermeability of the asphalt core may
increase the potential of blister formation during the
application of hot asphalt applied systems. Currently
there are no national standards for these materials.
Therefore, each product should be looked at on the basis
of their own testing and test results.
Glass-faced
Gypsum Board
Glass-faced
gypsum board is a proprietary product that has recently
been introduced as a cover board for roofing. This
material is highly resistant to roof top traffic, having a
compressive strength almost twenty times greater than that
of wood fibreboard or perlite. As it contains no organic
material it should not rot when subjected to moisture. Due
to its non-combustibility, it is classified as a thermal
barrier, which may be required under certain membrane
systems to achieve specific fire resistance ratings. The
product is generally resistant to attack from the solvent
adhesives used to adhere many single ply membranes.
There
have been reports of blister formation and frothing of
asphalt when hot process roofing systems (BUR and modified
bitumen) have been applied over glass-faced gypsum board.
As a result, the manufacturer now recommends the priming
of the surface of the board prior to application, special
torching techniques, or the use of a venting base sheet as
the first layer of the membrane system in hot applied or
torched-on assemblies.
Cementitious
Board
Fibre
cement roof boards are relatively new to the industry and
are being promoted as overlay boards primarily under
built-up and modified bitumen
membrane
applications. They are noncombustible, highly moisture
resistant and have a high degree of structural strength.
Priming of the board surface is required prior to the
application of asphaltic roof membrane systems. Currently,
there is no specific standard for this class of product.
Due to its limited usage to date, there is little
information on field performance.
Conclusion
The
inclusion of a cover board in a compact low slope roofing
system may provide many benefits. It may protect the
underlying insulation from mechanical damage, chemical and
thermal attack. in service, it may alleviate the stress
imposed from thermal, wind, seismic and traffic loads. It
may also prevent the problems from condensation resulting
from thermal bridging at fastener locations and joints in
insulation. In addition, it may provide a moisture buffer
between the membrane and the primary insulation.
However,
these features are a function of the physical properties
of the cover board material, the other components in the
roof assembly and the roof’s configuration. Performance
of the roofing system may be compromised by using the
wrong product for a given application. When the need for a
cover board has been identified, the membrane manufacturer
should be consulted with respect to identifying the
appropriate product that will serve as the substrate for
the membrane in a specific application and the unique
characteristics required of the board.
|
