how thick should loft insulation be

The recommended thickness for loft insulation is generally around 270mm for materials like mineral wool or glass wool to achieve optimal thermal efficiency and meet UK building regulations. This thickness corresponds to a U-value of about 0.16 W/m²K, which effectively prevents heat loss through the roof. For other materials like rigid insulation boards (e.g., PIR boards), a thinner layer of about 120-150mm can provide equivalent insulation due to their higher thermal efficiency. Adding more than 270mm typically does not bring significant benefits and could cause issues such as condensation if overdone. The thickness can be achieved by layering insulation, for example, 100mm between joists and 170mm on top. This thickness is recommended for new builds and major renovations but can also be used for upgrading existing loft insulation. Adequate ventilation should be maintained alongside insulation installation to prevent moisture problems.