Dow Protected Membrane Roof EN - Manual (Page 16)

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1 6
Moisture
Absorption
THE ISSUE
STYROFOAM ™ Brand Extruded
Polystyrene Foam 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
™ Brand Extruded
Polystyrene Foam 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 vapour
barrier (e.g., pavers) and the roof
deck, vapour 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" (5 mm)
minimum air space. See “Pavers”
on page 12 for additional
details. In addition, if the wear-
ing 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 ™ Brand Extruded
Polystyrene Foam Insulation
offers demonstrated long-term
performance in a PMR assembly.
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