Cost effective U-value targets for flat roof renovation works

Flat roof constructions Renewal of roof covering – existing insulation, if any, less than 100 mm, mineral fibre (or equivalent resistance) or in poor condition and likely to be significantly disturbed or removed as part of the planned work: 0.18 W/m²·K. Warm structure – insulation placed between and over joists as required to achieve the target U-value. Assess condensation risk and make appropriate provision in accordance with the requirements of Part C. Also see BS 6229 for design guidance. Consider combustibility of insulation (if relevant) or other fire safety risks - ref Part E. Renewal of the ceiling to flat roof area. Existing insulation removed as part of the works: 0.18 W/m²·K. Insulation placed between and to underside of joists to achieve target U-value. Assess condensation risk and make appropriate provision in accordance with the requirements of Part C. Also see BS 6229: for design guidance. Where ceiling height would be adversely affected, a lower performance target may be appropriate.