Q. I built a lot of homes in upstate New York in the 1980s using 2x6 studs, fiberglass batts, and a poly vapor retarder for the shell. Given the rising cost of heating and cooling, the owners are asking about energy upgrades to their walls. I’d like to suggest adding 1 or 2 inches of rigid foam on top of the existing OSB or plywood sheathing, followed by new siding. Would the presence of polyethylene vapor retarders make this a risky retrofit strategy?

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.