The Rd value is the declared thermal resistance of a material layer at a specified thickness. For insulation it is product information, not automatically the rating of the entire roof.
What does this mean on your roof?
The selected thickness and product belong together. Naming the material alone is insufficient for comparing proposals on the same basis.
A different board thickness may also affect junctions, available space and finishes. Those construction consequences are not summarised by a single product rating or a broad statement about insulation quality.
Request the exact product specification and intended build-up. Clarify whether a quoted figure concerns only the new insulation or a calculated construction performance. If another product is proposed during the work, make the comparison again. Keep the final product records so later reviews can establish what was actually installed and how it relates to the assumptions in the original proposal.
Frequently asked questions
Does every PIR product have the same Rd?
No. Product properties and thickness determine the declared value.
Can Rd replace junction design?
No. It does not describe junctions or execution of the complete assembly.
Consulted 19 September 2026. Check requirements for your own address and selected system.