Basement insulation strategy

A homeowner planning to insulate a basement asked for advice on choosing between materials he found cheap or as factory seconds: 1-inch and 2-inch paper-faced polyiso foam board, unfaced 2.25-inch EPS foam, and 1-inch XPS foam. He wanted to stop moisture from working its way in through the basement walls (sometimes called inward vapor drive) and was weighing whether to add a fluid-applied vapor barrier on top of the foam.

A building science expert responding in the forum recommended polyiso for its higher insulating value per inch, suggesting a layered approach: a thinner layer of foam board first, then a thicker layer over it with the seams offset, to reach the R-value likely required by code (often around R-15 in colder regions, though it depends on climate zone). For blocking moisture, he suggested using plain sheet plastic against the basement's block wall before adding the foam, rather than paying for a fluid-applied membrane, since the plastic is cheaper, installs faster, and works just as well for typical situations. He noted that whether moisture is a real concern depends on the home's humidity levels and surroundings, such as nearby water sources. He also flagged that a basement insulation job like this usually needs a fire-safety layer on the interior side, such as drywall or a thin hardboard panel, to meet local building rules.