Material resistance and construction resistance
In Dutch product documents, Rd is the declared thermal resistance of an insulation product at a stated thickness. Rc relates to the complete construction, including the effect of its layers and repeating thermal bridges such as timber. A quotation saying only “Rc 4.0 insulation” leaves unclear whether it describes a product or the finished roof.
Worked example: thickness divided by lambda
For an illustrative homogeneous insulation layer, assume a thickness of 0.140 m and a thermal conductivity of 0.035 W/(m·K). Its calculated resistance is 0.140 ÷ 0.035 = 4.0 m²·K/W. This is an example, not a specification for a named product. Use the manufacturer’s declared Rd for the exact product and thickness in the quote.
That 4.0 is not automatically the Rc of a timber roof. Heat also passes through timber and junctions, and the completed build-up needs a construction calculation. Avoid simply adding a board’s advertised value to an unverified value for the existing roof.
What should the quotation record?
Ask for the product name, thickness, declared conductivity or Rd, the intended Rc of the complete build-up and the assumptions used. Record how rafters, eaves, party walls and roof windows are treated. Thermal resistance does not describe airtightness, moisture safety or summer comfort by itself; those details require their own assessment.
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Technical sources
Checked on 23 September 2026.
Apply this to your roof
Record your preferences in the roof plan. Final dimensions, product suitability and the complete build-up are assessed for the actual roof; keep the agreed details and manufacturer instructions with your project documents.