R-Value
R-value measures a material's resistance to heat flow per inch of thickness, expressed in ft²·°F·hr/Btu. Higher numbers mean better insulating performance. R-value is reported per inch (e.g. R-3.1 per inch for fiberglass batt) or as a total (e.g. R-19 for a 6-inch batt). Real-world wall performance also depends on installation quality, framing thermal bridges, and air sealing — which is why "effective R-value" is usually 15-25% lower than nominal R-value.