A.Patrick Dundon, an insulation contractor in Windsor, N.Y., responds: To perform at maximum R-value, fiberglass batt insulation must be installed perfectly. It has to be of consistent density throughout the wall cavity, which means that every piece must be cut to fill the cavity completely in all three dimensions. If you can do that in every building cavity, fiberglass batts will work as labeled.

But installation perfection is hard to achieve. For example, it’s common to see fiberglass batts compressed in a wall cavity that’s slightly too narrow. Most installers, rather than trim a small amount of material along the edges, will simply stuff the insulation into the cavity, causing it to pinch at the edges and curl away from the studs at...

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