Before choosing roof or wall insulation, establish which legal basis applies to your project rather than looking for a single universal U-value. Check whether the design falls under the transitional provisions as at 1 November 2026, and verify the current regulations if you order later. Compare lambda and thermal resistance R, but remember that U describes the whole building envelope, not the material alone. Assess fire, moisture, acoustics and continuity of layers too. Leave the decision on thickness and build-up to the designer.