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Extruded polystyrene foam is the best insulation
you can choose when it comes to resisting water moisture absorption.
Without the need for a facer or laminate, XPS products only absorb
0.3% by weight. [Cite: ASTM C 578 — 03a] With its closed-cell
structure, XPS foam products have a natural "skin" surface
which is not conducive to moisture absorbancy. When installed in
walls, XPS shifts damaging dew points, thus minimizing the potential
for condensation to occur within a wall.
This compares to molded, expanded polystyrene foam
products, whose same property ranges from 2%-4%, depending on the
density of the product.
When properly installed, XPS weather-resistant,
insulating sheathing provides a continuous layer of protection against
inside and outside moisture. When moisture gets into a wall assembly
it compromises wood and as a result can reduce the overall R-value
of the building envelope.
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