Use And Care Manual
Q: How do XPS and polyiso water absorption properties compare?
A: The material standard that defines properties for all XPS and EPS is ASTM C578.₆ It requires that polystyrene insulation
be tested for water absorption in accordance with ASTM C272.₇ C272 requires the polystyrene sample to be immersed in
water for 24 hours, and weighed immediately upon removal from immersion to determine the amount of absorbed water. The
material standard for polyiso is ASTM C1289.₈ It requires that polyiso be tested for water absorption in accordance with ASTM
C209.₉ C209 requires the polyiso sample to be immersed in water 2 hours, and drained for 10 minutes before weighing for
water absorption. Figure 4 shows the signicant differences in XPS and polyiso water absorption that result from using different
measuring techniques. Note that the water absorption level for polyiso increases greatly when tested by the same method used
for XPS. Glass faced polyiso goes from absorbing 14x more water than XPS, to absorbing 30x more water than XPS, when
measured using the same method. Foil faced polyiso goes from absorbing 1.5x more water than XPS, to absorbing over 7x more
water than XPS. Because the presence of foil makes such a difference in water absorption, the long-term durability of the foil is
critical. If the foil is punctured or corrodes while in service, the polyiso core is unprotected from water, and is even more prone to
higher water absorption, like the condition with glass facers.
Q: Why does water absorption resistance matter when it comes to selecting a rigid insulation?
A: Moisture gets into all types of buildings. Unless the building insulation is highly resistant to water absorption, moisture can
degrade insulation R-value, structural integrity and provide an essential ingredient to support mold growth. Water is a good
conductor of energy, so if insulation is water soaked, R-value is lost. Also, compressive strength may be reduced by water
absorption depending on the make-up of the foam plastic insulation, hydrophobic/closed cell XPS or hydrophilic/more open cell
polyiso. Therefore, absorbed moisture is to be avoided to achieve sustainable quality construction. One of the greatest attributes
of XPS is its ability to retain R-value and compressive strength even when exposed to water.
Figure 4. Water Absorption, XPS vs. Polyiso
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
XPS Method, ASTM C 272
24 Hour Immersed, No Drain
XPS
Type VI, VII, V
XPS
Type X, IV
Polyiso
Glass Faced
Polyiso
Foil Faced
Polyiso Method, ASTM C 209
2 Hour Immersed, 10 Min. Drain
% Volume