Calculate R-value from material and thickness, then check compliance with NZS 4218 minimum requirements for your climate zone.
NZS 4218:2009 requires a minimum of R6.6 for ceilings in all three climate zones across New Zealand. This is significantly higher than older standards — most existing homes only have R1.8–R2.5 and would benefit from a ceiling insulation upgrade.
Standard glasswool batt has an R-value of approximately 0.022 per mm of thickness. To achieve R6.6, you need 6.6 / 0.022 = 300 mm of glasswool. In practice, a combination of 200 mm batts plus 100 mm top-up is common for ceiling upgrades.
R-value measures thermal resistance — higher is better. U-value measures thermal transmittance (heat loss) — lower is better. They are reciprocals: U = 1/R. New Zealand building code uses R-values; European and UK standards typically specify U-values.