Table of Contents
What Is Thermal Resistance?
Thermal resistance opposes heat flow, analogous to electrical resistance. Higher R means less heat flow. R-value (per unit area) is the standard insulation metric. In the US, measured in ft2·degF·h/BTU; SI: m2·K/W. Building codes specify minimum R-values for energy efficiency.
For composite walls, resistances in series add directly. For parallel paths, use 1/R_total = sum(1/Ri). Include convective film resistances at interior and exterior surfaces for complete thermal analysis.
Formulas
R = L / (k × A)
Q = ΔT / R
Insulation R-Values
| Material | R per inch (US) |
|---|---|
| Polyiso foam | R-6.5 |
| Spray foam (closed) | R-6.0 |
| XPS | R-5.0 |
| Fiberglass batt | R-3.2 |
FAQ
How to calculate composite walls?
Add R-values for layers in series: R_total = R1+R2+R3. Include air film resistances (~R-0.7 inside, ~R-0.2 outside).