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We have all heard claims that the "effective" R-value of walls insulated with fiberglass batts may be drastically reduced by installation defects such as gaps, voids, compression, etc. I've seen advertising brochures, for example, that state the "effective" R-value of an installed R-19 fiberglass batt may actually be as low as R-8. Like many such assertions, the brochure claimed there was government research data to support its statements. How real are those claims? Is it possible that installation defects can cause a 50% or 60% reduction in wall R-value? And are there really research studies that prove it? When your insulation contractor smilingly stuffs that fluffy 6-inch batt into wall cavities, are you getting an R-19 wall or an