Planning permission was obtained to retrofit this 17th-century mill in Buckinghamshire. Originally converted and extended in the 1980s and 1990s, the mill has an inefficient envelope and windows, leading to high heating bills and a cold interior in winter. To accommodate the owners’ preference to avoid internal disruption while continuing to live in the property, a strategy was developed to add high-performing natural insulation and airtight, waterproof breather membranes. By removing and reinstalling the roof tiles and timber shiplap cladding and replacing the windows from external scaffolding, the retrofit was achieved without impacting the interior. New Steico flex woodfibre insulation is being installed between the rafters, along with Steico special dry. During the works, some of the roof’s structural rafters were found to be rotten, so new rafters were installed alongside the old ones, which were treated, to avoid any disruption to the bedroom below.