Does insulation foam board provide adequate moisture barrier when installed behind Hardie siding?
James Comeaux
Bailey Engineering Services
San Antonio TX
Depends. Does it have a foil facing like Johns-Manville products? Yes? All seams must be taped with appropriate tape.
No, just Dow Styrofoam blue board. Dow list a water vapor permeance of 1.5 per inch. Not sure if that means anything though for vapor barriers.
This should help… http://www.finehomebuilding.com/item/5090/whats-the-difference-vapor-barriers-and-vapor-retarders
Expanded polystyrene (EPS)
EPS is the insulation used most widely in insulated concrete forms and structural
insulated panels. EPS has the lowest average R-value of the three types of rigid-foam insulation, around R-4 per inch. At about 19¢ per sq. ft. for a 1-in.-thick 4x8 sheet, it also costs the least. Although EPS is acceptable for ground contact and can be treated to resist insects, it does absorb water. When applied as sheathing, EPS should be used over housewrap. Most EPS is unfaced, which means it is fragile to work with and is considered semipermeable, so it does not create a vapor barrier.
**Extruded polystyrene (XPS) **
Easily recognized by its blue, green, or pink color, XPS falls in the middle of the three types of rigid-foam insulation in both cost and R-value. At about R-5 per inch, XPS costs around 42¢ per sq. ft. for a 1-in.-thick 4x8 panel. XPS comes unfaced or with a number of different plastic facings. Unfaced 1-in.-thick XPS has a perm rating around 1, making it semipermeable. Thicker and faced XPS is stronger and can have a lower perm rating, but either way, it is considered a vapor retarder, not a vapor barrier.
**Polyisocyanurate (ISO) **
ISO panels are expensive, costing as much as 70¢ per sq. ft. for a 1-in.-thick panel, and they pay off with (aged) R-values as high as R-6.5 per inch. (R-values start around R-8 and degrade slightly over time.) Because ISO starts as liquid foam and has to be sprayed against a substrate to form a rigid panel, all ISO panels are faced. A few different facings used on ISO affect the performance of the panel in both durability and perm rating. Foil-faced ISO panels are considered impermeable. Because applying these products as sheathing creates an exterior vapor barrier, they never should be used with an interior vapor barrier. More permeable ISO panels are faced with fiberglass and can be used without creating a vapor barrier.
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It is XPS in 1/2" nom. I take that to mean even with the film face the material does not meet IRC requirements as a moisture barrier installed behind siding. An additional wrap is recommended.
Thanks
James Comeaux
Bailey Engineering Services
San Antonio, TX