Resistance and transmittance describe opposite views
Thermal resistance describes how strongly a layer or assembly resists heat flow. Thermal transmittance describes how readily heat passes through the complete assembly. Higher R-values indicate more resistance; lower U-values indicate less heat transfer. SI RSI and US customary R-values use different unit systems and must not be compared as if they were the same number.
For a homogeneous layer, resistance is based on thickness divided by thermal conductivity. The surface and layer resistances are added before the total is inverted to obtain the U-value.
Parallel paths change the answer
Studs, rails or repeating structural zones create paths with different resistance. A simplified parallel-path method calculates each path and combines their transmittance by area fraction. The fractions must total one, and the geometry should represent the real repeating assembly rather than a convenient guess.
- Use product-specific conductivity values with a recorded source.
- Keep thickness and conductivity units explicit.
- Model repeating paths only when their area fractions are defensible.
- Record corrections separately rather than hiding them in a layer value.
Know when the simplified method stops
Point bridges, metal fasteners, air cavities, ground contact and complex junctions may require standard-specific corrections or numerical modeling. Regulations also introduce conventions, approved data and project-specific evidence beyond the arithmetic shown by a simple calculator.
Use the Architect Assets builder to compare options and expose the calculation trace, then move to the appropriate verified method for the technical design and compliance submission.