Thermal Conductivity of Fibreglass Batt Insulation

Convert with care: R-value in North America is expressed in h·ft²·°F/BTU per inch, and 0.040 W/(m·K) corresponds to about R-3.6/in, so a nominal 3.5 in cavity batt is the familiar R-13. SI R-values (m²·K/W) are 5.68 times smaller for the same product, so an R-13 batt is RSI-2.3 — a factor that has caused more than one badly under-insulated wall.

The laboratory number is measured on a flat, uncompressed sample at a mean temperature of 24 °C. Installed performance is usually worse. Compressing an R-19 batt into a 2×4 cavity gives roughly R-13, not R-19. Gaps, voids and wiring cavities that leave even 5 % of the area uninsulated can cost 20–30 % of the assembly performance, and framing members bridging the insulation typically drop a nominal R-19 wall to an effective R-13 or so. Air movement through low-density batt degrades it further. Treat this figure as the material's best case and compute assembly U-values with the framing factor included.