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A Protected Roof Membrane Assembly (PMRA) differs
from a conventional roof design in that the membrane is placed under
the insulation layer, maximizing membrane life by protecting it
from temperature extremes, freeze/thaw cycles, UV degradation, and
traffic wear. A PMRA begins with the application of the EPDM membrane,
followed by the XPS boards, the protective scrim, and finally the
ballast. Extruded polystyrene is the only type of insulation recommended
for use in a PRMA system because it so thoroughly resists moisture
— a critical property in the selection of insulation.
The protected roof membrane assembly (PRMA) has represented the
best in flat roof technology since it was invented in 1970. The
PRMA concept incorporates an “upside down” approach
to insulating the roof: placing the insulation on top of the waterproof
membrane. Some of the original roof membranes installed using this
concept remain in exceptional condition, more than 30 years after
installation.
If this method of installing a roof membrane assembly
is so effective, why did it take so long to happen? Easy, no insulation
had ever proven durable enough to withstand exposure to the environment
like extruded polystyrene foam insulation (XPS). Once it began to
be used as a roofing insulation, building experts saw the possibilities,
and the PRMA was born.
The PRMA is a cost effective assembly that extends
the life of the waterproofing membrane by protecting it from UV
exposure, large temperature swings, and physical abuse caused by
rooftop traffic. The longer a roof can be kept in service the more
it contributes to the overall sustainability of the building by
delaying the impact of eventual tear-off. Increasing the sustainability
of a building is one of the underlying concepts of the Leadership
in Energy and Environmental Design (LEED) program.
Building owners must consider the time value of
money over the life of the building – not just the up front
construction costs. Using a PRMA can reduce annual maintenance costs
and increase the life of the roof. Deferring a roof replacement
by 10 years, which is often the difference between the life spans
of a conventional roof and a PRMA, can save hundreds of thousands
of dollars.
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This
cross-section drawing can be seen on the CREEL website ...
just click this link to go there for more information (CREEL).
Drawing to the left from
CREEL website. |
Membrane Field Temperature
Test
Membrane protection (PMRA vs. conventional): Field
studies have shown that a constant temperature can extend the life
of the roof membrane. XPS Type X insulation products offer superior
temperature control.
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PMR roof (blue line)
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the membrane temperature remains relatively constant
Conventional roof (yellow line)
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the membrane temperature fluctuates widely |
–Case Study–
McDonald's Corporate Headquarters,
Oak Brook, IL
When McDonald’s Corporation constructed a new
127,000 square feet office building at their 80 acre Oak Brook,
Illinois corporate headquarters complex, they selected a protected
roof membrane assembly (PRMA) incorporating Type VI extruded polystyrene
insulation (XPS), as specified in ASTM C 578 2001 (Type VI XPS).
Type VI XPS precisely meets the stringent requirements of this type
of roofing system and is the only insulation of its kind to provide
rain channels on all four sides of the panels. By placing Type VI
XPS over the membrane, the design team specified a PRMA system because
XPS protects it from both puncture and the damaging effects of UV
and weather.
In the McDonald’s application, a Carlisle
EPDM Membrane is nestled against the building protected by Type
VI XPS which contributes warmth in the winter and cooling in the
summer. In addition, the insulation protects the membrane from thermal
shock because it helps slow the thermal cycling to low curves rather
than abrupt ones thereby greatly contributing to the membrane’s
life expectancy.
The PRMA “sandwich” begins with the
application of the EPDM membrane, followed by the 2’ x 8’
XPS 404 panels, the protective scrim, and finally the ballast. Extruded
polystyrene is the only type of insulation recommended for use in
a PRMA system because it so thoroughly resists moisture —
a critical property in the selection of insulation.
Other properties that weighed heavily in favor of
Type VI XPS are its superior R-Value of 5 per inch of product thickness
(providing the McDonald’s building with a total of 12.5),
minimum compressive resistance of 40 psi, lightweight strength,
ease of hoisting, cutting and installing, and long lasting thermal
performance afforded by the product’s imperviousness to moisture
of all kinds.
The architectural firm for this project was Lohan
Associates of Chicago. They incorporated approximately seven truck
loads of 2½” XPS 404 to cover the 120,000 square foot
roof.
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