Where more than 25 per cent of a ceiling below a cold loft space or flat roof is being replaced then building regulations would normally apply, and the thermal insulation of that ceiling would normally have to be improved.
However, it will be necessary to assess the condensation risk in the roof space and make appropriate provision in line with the requirements of Part C (resistance to moisture) relating to the control of condensation.
There are different requirements for ceilings below pitched and flat roofs. For a pitched roof of a dwelling this would normally involve the provision of new or additional loft insulation so that there is at least 250 mm mineral fibre or cellulose fibre as quilt laid between and across ceiling joists or loose fill or equivalent. But this may be inappropriate if the loft is already boarded out and the boarding is not to be removed as part of the work.
For a flat roof of a dwelling the insulation should be placed between and over joists as required to achieve the target U-value set out in the Approved Document. Where ceiling height would be adversely affected, a lower performance target may be appropriate.
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For technical guidance please see the relevant approved documents: