How to insulate a flat while preserving its historic façade

Inspect the façade, cornices and junctions at the roof or balcony. Defective flashing on an upper floor can affect the flat below. Agree the scope with those responsible for the building and the other parties whose involvement is required. Preserving the façade’s appearance still requires the technical conditions of the work to be established.

Where timber joists bear in the masonry, their condition is part of the assessment. Internal insulation reduces heat flow into the wall and may change moisture conditions at the bearings. A satisfactory result for the brickwork between windows says little about conditions at a joist end.

Plan inspections and any local opening-up to give a reliable picture of the structure. Do not seal in damp or unexamined timber on the strength of a general guide. The designer may require a detailed junction model or a different solution locally from that used across the wall.

Assess both heat and moisture for the wall as surveyed. A simple diffusion calculation cannot capture all water transport from rain, capillary rise or leaks. These effects can matter greatly in masonry that stores substantial moisture. Select the calculation method to suit the actual problem.

PIR with a vapour-tight facing can be considered in a suitably designed assembly. It is not a requirement for every historic building. Consider the substrate, expected benefit, drying potential and finish. If the assessment identifies limitations, revise the design rather than concealing them behind a thicker board.

Windows may sit deep within a thick wall, with little space at hinges or decorative plasterwork. Measure all sides of the reveal. Check how the frame, sill and insulation will meet. A narrow uninsulated strip may remain the coldest spot after renovation even if the main wall improves markedly.

Allow for air movement, drainage and sash opening. Do not reduce insulation thickness locally without checking the surface temperature. If historic interior features are to remain, specify how the new work terminates against them. Agree a sample junction before repeating it throughout a storey.

In an occupied flat, choose a work area that can be prepared, insulated and sealed around its perimeter. A small trial area may help assess flatness, adhesion and the finished appearance. It cannot replace calculations or structural inspection. Define the purpose of the trial first.

Plan service relocations and fixing points in advance. Check clearances for radiators, curtains and furniture. Detail every penetration through the airtight layer. Photograph finished junctions before covering them. In an older building these pictures may become the only accurate record of the new layers.

After completion, the flat still needs effective ventilation and heating suited to its use. List the points to watch: reveals, floor junctions, corners and areas behind fittings. If moisture monitoring is specified, establish the measurement locations and how readings will be interpreted.

Keep the specifications, drawings and conditions under which the solution was accepted. Future owners should know that new holes and enclosed fittings can affect performance. If damp returns, inspect both the interior and the shared façade. Renovating one flat does not repair defects elsewhere in the building.