A.Martin Holladay, editor
of Energy Design Update, responds: You may
safely install exterior foam on most houses with a
polyethylene vapor barrier, as long as the foam
does not include aluminum-foil facing. In fact,
exterior foam is a great idea, since it
significantly improves the energy performance of
walls.
As most builders now realize, polyethylene is a
double-edged sword. Its ability to limit the
outward migration of water vapor into a wall comes
with a downside, since poly also prevents the
useful inward drying of damp walls. In very cold
climates — including your region, upstate
New York — many builders still use
interior
poly.‚
However, in warmer regions — Ohio and
Connecticut, for instance — most walls
perform better without any interior
polyethylene.?
Back in the ’80s, when building
scientists did not fully understand the
disadvantages of interior polyethylene, its use was
encouraged from North Carolina to Oregon. In most
of the U.S., the routine use of interior poly was
probably a mistake. Fortunately for builders, most
older homes with interior polyethylene have not
experienced moisture problems.
The installation of exterior foam is not advised
on any home that has suffered wet-wall problems
like leaking windows, condensation in stud
cavities, or mold. If you plan to install exterior
foam during a siding replacement job, keep an eye
out for any signs of moisture problems when
stripping the old siding from the walls.
Investigate any water stains on housewrap or
sheathing to determine whether the existing
flashing was adequate.
Dry and unstained sheathing may safely be
covered with 1 or 2 inches of extruded polystyrene
foam (XPS) or expanded polystyrene foam (EPS). One
inch of XPS has a permeance of 0.4 to 1.6, while 1
inch of EPS has a permeance of 2 to 6; that means
that walls sheathed with EPS have more ability to
dry to the exterior than walls sheathed with XPS.
Since aluminum foil is completely impermeable, the
use of foil-faced foam is not recommended on walls
with interior polyethylene.
Walls sheathed with exterior foam perform better
when they include a rain-screen drainage gap
beneath the siding — for example, vertical
1x3 strapping or a product like Cedar Breather. Of
course, a foam retrofit job will require
adjustments to window trim, door trim, and wall
flashing, so be sure to research these topics
carefully before tackling such a project.