R-value measures thermal resistance—an insulation material's ability to resist heat flow. Higher R-values indicate better insulating performance. Building codes specify minimum R-values for walls, ceilings, and foundations based on climate zones, with colder regions requiring higher values. Different insulation materials (fibreglass, cellulose, spray foam, rigid foam) achieve different R-values per inch of thickness. When building or renovating, meeting or exceeding code R-values reduces heating and cooling costs and improves comfort. Building inspectors verify insulation installation meets code requirements. Climate change and energy efficiency goals have led to increasing R-value requirements in building codes over time.