Effective R-Value
Effective R-value is the real-world insulating performance of a wall, roof, or floor assembly after accounting for thermal bridges through framing, fasteners, and structural members. It is always lower than nominal R-value because studs, plates, and headers conduct heat much faster than the cavity insulation around them. A nominal R-19 wood-framed wall typically has an effective R-value of R-13 to R-15 — and a steel-framed wall can drop to R-7.