The Rc value describes the thermal resistance of a complete construction, such as a roof assembly. It differs from the Rd value of an individual material layer.
What does this mean on your roof?
An insulation board with a stated product rating is not the same as a supported rating for the complete roof. Existing layers, structural components and the calculation method belong in the assessment.
Do not compare a material value with a construction value in quotations as though they describe identical performance.
Ask which value is intended and which assembly supports it. Establish how retained components are included and what still needs investigation. General insulation advice is not a project-specific calculation. Keep the reasoning with the agreed scope so a later change of material or thickness can be checked against the original assumptions rather than being treated as automatically equivalent.
Frequently asked questions
Are Rc and Rd interchangeable?
No. Rc concerns the construction and Rd an individual material layer.
Does a higher Rc predict an energy bill?
No. Energy use also depends on the entire building and how it is used.
Consulted 19 September 2026. Check requirements for your own address and selected system.