In this guide
4. Loft insulation
The installation of insulation in your loft must meet the minimum energy efficiency values set out in the Approved Documents.
However, if such an upgrade is not technically or functionally feasible, the element should be upgraded to the best standard which can be achieved within a simple payback of no greater than 15 years.
If you are installing loft insulation as part of a roof renovation project, where more than 25 per cent of the roof is being renewed, then the level of insulation should meet the standards required by building regulations Approved Documents. Care should be taken not to block any ventilation at the edges (eaves).
Read more about roofs (including information about thermal resistance)