Nominal R-Value
Nominal R-value is the rated insulating performance of an insulation product as printed on its label — the R-value of the cavity insulation by itself, before considering any framing, fasteners, or installation effects. A nominal R-19 fiberglass batt is rated at R-19 in laboratory conditions; the wall it ends up in usually performs at a lower effective R-value because of thermal bridges through the studs.