Dow Protected Membrane Roof EN - Manual (Page 17)

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1 6
Moisture
Absorption
THE ISSUE
STYROFOAM ™ extruded poly-
styrene insulation will absorb
water and the insulation value
will be reduced.
DISCUSSION
In a PMR design, it is critical
that any insulation installed
above the membrane can per-
form in a wet environment
without any detrimental effects
on its long-term performance.
STYROFOAM
™ extruded poly-
styrene insulation has a unique
closed-cell structure that provides
excellent moisture resistance
and long-term R-value.
Nine PMR systems were moni-
tored over a period of 22 years
and the insulation properties
assessed. The average moisture
content of the insulation was
0.9 percent on a percent by
volume basis, with a retained
R-value of 96 percent.
In plaza deck designs, it is
important that a drainage layer
be created above the insulation,
allowing precipitation to drain
off the top surface of the insula-
tion, creating a “diffusion open”
assembly. If the insulation is
sandwiched between a vapor
barrier (e.g., pavers) and the roof
deck, vapor cannot escape so it
is driven back into the insula-
tion. To create a “diffusion
open” layer, ensure imperme-
able roof coverings (such as
pavers) have a ventilating air
space. This could be a layer of
fine-free gravel or a 3/16" mini-
mum air space. See “Pavers” on
page 12 for additional details. In
addition, if the wearing surface
is installed in direct contact
with the insulation, moisture
may become trapped and freeze-
thaw cycling could cause
spalling on the bottom of the
wearing surface.
Always ensure that the roof
deck has proper drainage; if the
PMR system has significant
ponding (e.g., standing water), the
insulation will not be “diffusion
open.” Follow roofing association
guidelines for drainage recom-
mendations.
CONCLUSION
STYROFOAM ™ extruded
polystyrene insulation offers
demonstrated long-term perform-
ance in a PMR assembly.
Dimensional
Stability
THE ISSUE
STYROFOAM ™ extruded poly-
styrene insulation “shrinks”
over time, leading to increased
heat loss.
DISCUSSION
All building materials will
experience dimensional change
due to temperature fluctuations.
STYROFOAM
™ extruded poly-
styrene insulation is no different.
For example, the coefficient of
expansion of STYROFOAM
extruded polystyrene insulation
is 3.5 x 10
-5 in/in/°F. A 2' x 8'
sheet of insulation exposed to a
temperature swing of 75°F could
result in a maximum change of
just 1/4" in the 8' direction.
Once the temperature is
reduced, the insulation will
return to its original cut dimension.
In addition, this theoretical
change does not account for the
temperature profile across the
insulation. For example, while
one side may see a large temper-
ature swing, the underside may
see only a small change.
This relatively small gap
between the boards does not sig-
nificantly increase the heat loss
through the board joints. In
heat loss studies comparing PMR
versus conventional roofs, there
was no significant difference
between the two systems. The
findings showed that the PMR
system used 3 percent more
energy per year.
In addition to addressing the
coefficient of expansion, another
consideration is the “creep” of
materials. Creep is the permanent
deformation resulting from
continuous, long-term dead
(or non-moving) loads. Creep
is generally only an issue for
STYROFOAM insulation used in
pavements, airport runways,
parking decks, floors, etc. –
installations where the insulation
is used to carry a significant
load for a long time. In these
applications, higher compressive
strength insulation may be
required.
CONCLUSION
All building materials have a
coefficient of expansion that
results in dimensional change
with temperature fluctuations.
The dimensional change that
occurs in STYROFOAM
™ insula-
tion in a PMR assembly does not
significantly impact the system’s
thermal performance.
Conditions, Issues and Ratings
S P E C I A L C O N D I T I O N S A N D I S S U E S