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,...
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